Project contract no. 036851

ESONET

European Seas Observatory Network

Instrument: Network of Excellence (NoE) Thematic Priority: 1.1.6.3 – Climate Change and Ecosystems

Sub Priority: III – Global Change and Ecosystems

D13 - Science Modules of the European Seas Observatory NETwork (ESONET)

Due date of deliverable: Month 48 Actual submission date: Month 88

Start date of project: March 2007 Duration: 48 months Organisation name of lead contractor for this deliverable: NOCS, H. A. Ruhl Authors for this deliverable: H. A. Ruhl, L. Géli, Y. Auffret, J. Grienert Other contributors: A. Colaço, J. Karstensen, P. M. Sarradin, D. De Beer, M. André, R. Person, L. Menot, A. Khripounoff, P.-M. Sarradin, J. Galéron, . J. Blandin, P. Bagley, P. Favali, J. Mienert, L. Thomsen, H. Villinger, N. Sultan, O. Pfannküche, E. Delory, J.M. Strout, C. Floquet, L. Beranzoli, N. Rothe.

Revision [26, April 2011]: yearly update, 2011 version Project co-funded by the European Commission within the Sixth Framework Programme (2002-2006) Dissemination Level PU Public PP Restricted to other programme participants (including the Commission Services X RE Restricted to a group specified by the consortium (including the Commission

Services)

CO Confidential, only for members of the consortium (including the Commission Services)

 

 

TABLE OF CONTENTS

Summary 4

1. Introduction 5

2. Generic Sensor Module 8

a. Provisional generic module parameters 9

b. Generic Sensors Impact 12

3. Science Specific Sensor Modules 16

a. Geosciences 16

b. Physical Oceanography 25

c. Biogeochemistry 26

d. Marine Ecology 31

4. Instrumentation module architecture 36

5. References: 40

6. APPENDIX 47

a. Demonstration Mission sensors. 47

b. California Cooperative Oceanic Fisheries Investigations (CalCOFI) variables: 53

c. US Ocean Observing Initiative (OOI) Coastal and Global Scale Nodes (CGSN) 53

d. IOOS core variables: 55

e. NEPTUNE Canada: 56

f. Seacycler: 57

g. POSEIDON-II seabed platform and PYLOS Buoy 58

h. GEOSTAR (and derived single-frame observatories) 59

i. SEAMON: 62

j. TEMPO: 65

k. DELOS: 66

7. Annex I – Ocean Acoustic Observatories report 69

8. Annex II – Sea Water Electrodes 180

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Summary

The European Seas Observatory NETwork Network of Excellence (ESONET NoE) has

documented here several types of sensor modules that can be used in an ocean observatory

setting. These modules include a provisional specification that we recommend for use at all

sites and several other rather specialised specifications that can be used on an ad hoc basis

to meet the science objectives relevant to a particular location. This modular nature will

allow for a wide variety of configurations while also making progress towards

standardization and interoperability. Examples of operating modules and systems of

modules are also provided in an appendix.

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1. Introduction

The European Seas Observatory NETwork - Network of Excellence (ESONET NoE) has suggested a set of parameters to be collected at all European observatory sites, as well as a set of rather specific parameters that may only be measured at some sites. For practical purposes these sensors have therefore been divided into those that might be included in a generic module and those that might be part of science specific modules. Outlining provisional module specifications has allowed progress with studies of observatory design and operation. These specifications are provisional and can be changed to meet scientific priorities. For example, the configuration of the generic or one of the specific modules can be updated as science needs and feasibility change.

The generic sensor module and the science-specific modules are both envisioned to address the key questions that have been identified in the four main areas covered by ESONET NoE: geosciences, physical oceanography, marine biogeochemistry, and marine ecology. These questions are fully listed in the ESONET NoE Deliverable D11 report. We will not repeat all of the key questions here, but will discuss many of the sensors that can be used to address them. In practice, many of the scientific questions addressed are interdisciplinary. Major questions that presently arise in geosciences, for example, are linked to fluid circulation, which, in turn, can control cold-seep bacterial activity, biogenic gas emissions, and deep-sea benthic ecology. A functional split not only comes between those modules that are considered generic or specific, but also the infrastructure setting in which those systems are used, such as seabed and mooring systems. Maybe rephrase: Functional differences exist between generic sensor modules and science-specific modules. They further depend on the infrastructure setting in which these systems are used, be it seadbed-fixed structures or mooring systems.

The concept of recommending a specific set of parameters for collection at all European observatory sites was brought up at the All Regions Workshop in Barcelona (September 2007) and has also been discussed in detail at the Best Practices Workshop in Bremen, Germany (January 2008), at the General Assembly meeting in Faro, Portugal (October 2008), and was further reviewed at an Implementation Strategies meeting in Paris, France (January 2009). Initial suggestions for parameters to include at each node (e.g. Arctic, Marmara, etc.) were provided in the Global Change session report from the 2007 All Regions Workshop. Most recently the readiness of various sensors was addressed at the 2009 Best practices Workshop in Brest, France. Of the ‘core’ water-column or benthic parameters discussed in the Global Change session, the variables determined to continue to be considered included currents, temperature and salinity changes, oxygen, nutrients, biogeochemical quantities (e.g. C, N, P, etc.), pH, Eh, and CO2. A survey was also circulated to the ESONET NoE General Assembly to ask for suggestions on what parameters to include in a generic module, however, only limited input was provided. Since then a variety of potential instruments have been suggested for a generic module (i.e. a standard set of instruments to collect data parameters to be used at the nodes for local measurements, between-site comparisons, and hypothesis testing). The present discussion is also based on the experience that has been gained from the Demonstration Missions (DM) that were funded through ESONET NoE (Table 1). The sensors used in these DMs are summarized in tables included at the end of this document.

Sensor module development activities in other observatory and research programmes are also considered here with some examples given in the Appendix. Most prominently this includes ongoing efforts within the EuroSITES and HERMIONE (Hotspot Ecosystem Research and Man’s Impact On European Seas) programmes, as well as programmes in the US, Canada, Japan and elsewhere. Specific input came from a report on the use of biogeochemical sensors in the EuroSITES network (Coppola et al. 2009). The EuroSITES deliverable 1.1.3 outlines the existing platforms and sensors used throughout the network. This report includes descriptions of the observatory design in terms of mooring sensor arrangements. Additional experience is also being gained through the activities of KM3NeT, which has three sites in common with ESONET NoE. A great amount of effort by the US Ocean Observatories Initiative (OOI) and the NorthEast Pacific

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Time-Series Undersea Networked Experiments (NEPTUNE Canada) has contributed to the diversity of sensors reviewed here. Indeed, NEPTUNE Canada is now in full operation and the OOI is fully into construction with calls for tender issued.

The Alliance for Coastal Technologies (ACT) is an organization that is funded by the US National Oceanic and Atmospheric Administration (NOAA) to evaluate sensors and provide a source for information on sensors in partnership with the US Integrated Ocean Observing System (IOOS), a US contribution to GOOS. ACT efforts are resulting in detailed reporting of controlled assessments of sensor performance. Contributions to the OceanObs’09 Conference held in Venice Italy have also highlighted both observational needs and consensus on solutions through peer-reviewed community white papers (www.oceanobs09.net). These include documents that review ocean observatory needs from a wide range of perspectives covering natural and anthropogenic changes, potential influences of climate change, and geo-hazard early warning (e.g. Favali et al., 2010; Larkin et al., 2010; Meldrum et al., 2010; Merrifield et al., 2010; Send et al., 2010). ESONET NoE and the Institute of Electrical and Electronics Engineers (IEEE) co-convened the Global Earth Observing System of Systems (GEOSS) Workshop XXVII on “Understanding the Integrated Ocean Observation Systems, including sub-surface sensors” in May 2009 to further garner discussion and prioritisation of observational needs. The recommendations of the GEOSS Workshop XXVII included further consideration of sea glider and AUV capacity building, continued progression of sensors for critical parameters such as CO2 and pH, as well as rather complex sensors like those using microbial probes. The value-added aspects of examining systems from the surface to the sub-seafloor over long time periods were presented as not only enhancing scientific capability but as an attractor for additional investigators and sensors through synergy. EuroGOOS, a contributor to GEOSS, has also conducted substantial research into observational requirements for various societal and industrial needs (e.g. Flemming et al., 2007), something that is also considered here.

In order to move forward with recommending sensors for a generic sensor module and make

progress on integration, interoperability, standardization, and data management aspects of generic sensor module development and use, we have outlined a minimal list of parameters and sensors that can serve as a basis for generic module development. The selection of these sensors includes inputs from several meetings and reports and focuses on broad applicability to fixed-point open ocean observatory science objectives, simplicity of deployment, endurance, commercial availability, and depth rating. Technology readiness is an important factor for the deployment of sensors and ranges from operational to research and development applications (Waldman, 2008; Brasseur and Tamburri, 2009; Table 2). All instruments should be evaluated using accepted definitions of performance. In practice, however, instruments using European observatory infrastructures will include proof of concept because of technological evolutions expected over the anticipated multi-decadal lifetime of ocean observatory activities. As standards develop for new methods they should consider their tractability to SI units, standards, and international reference standards for transition to operation use (Waldman et al., 2009).

Table 1. ESONET NoE Demonstration Missions and locations. Acronym Title Location AOEM Arctic Observatory ESONET Mission Frame Straight LIDO LIstening to the Deep Ocean Offshore East Sicily, Gulf of Cadiz LOOME Long-term Observations On Mud-

volcano Eruptions Haakon Mosby Mud Volcano Site, Norwegian Sea

MARMARA-DM Marmara Demonstration Mission North Anatolian Fault, Sea of Marmara

MOMAR-D MOnitoring the Mid Atlantic Ridge-Demonstration

Hydrothermal Site, Mid-Atlantic Ridge

MODOO Modular Deep-Ocean Observatory Porcupine Abyssal Plain

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Table 2: OOI Recommended Technology Readiness Levels (from Brasseur and Tamburri, 2009) based on US National Aeronautics and Space Administration (NASA) guidelines (Mankins, 1995). OOI TRL Description

1. Proof of Concept / Development

Lowest level of technology readiness. Scientific research begins to be translated into applied research and development. The application is speculative and there is no proof or detailed analysis to support the assumption. Includes analytical studies and laboratory studies to physically validate analytical predictions of separate elements of the technology.

2. Research: Prototype

The basic technological components are integrated with reasonably realistic supporting elements so that the technology can be tested in a simulated or relevant environment. Prototype instrument packages have been used to collect data in research studies of technology or environmental parameter.

3. Research: Proven

Technology has not been commercialized, but is clearly beyond prototype stage. Multiple instrument packages have been fabricated and deployed for extended periods under expected environmental conditions. Publications exist which demonstrate scientific utility of data.

4. Commercial Technology has been proven to work in its final form and under expected environmental conditions. Instruments are in commercial production with appropriate supporting materials (replacement parts, operations manual, etc.)

5. Operational Actual application of commercial or research-proven technology in its final form and under sustained operational conditions. Independent, third-party evaluation or application that demonstrates reliable long-term field operations.

Although the scope is limited here, the ultimate use of sensors not included in the initial

specification will be possible wherever reasonable. The modular nature of the systems also allows them to be readily modified to add rather specialized systems, such as seafloor seismometers, biogeochemical sensors, or cameras. The provisional sensors will provide a variety of data types and volumes and will serve to test module development and use in several ways including standard oceanographic, acoustics, and geo-hazard warning capability.

The sensor modules considered here are envisioned to be utilized either on an observatory network that has a deep-sea telecommunication cable for transmitting power and data, or in standalone systems utilizing moorings with a buoy system to transmit data via satellite. ESONET NoE has worked to create infrastructure specifications and cost evaluations for both of these cabled and standalone solutions that include the generic models (ESONET NoE Deliverable 5). Also, for a large number of applications, continuous observations from seafloor observatories must be complemented by specific data obtained via sampling and maintenance cruises. These additional efforts could include site studies to calibrate or validate continuous observations, to enhance the diversity of perspectives studied and to increase the spatial scale of the study. Occasional or regular site visits also offer useful opportunities to deploy autonomous equipment.

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2. Generic Sensor Module

Taking into account only those sensors that are rated to operate at the deepest ESONET sites, have an established endurance of approximately a year or more, and are commercially available, only a rather small subset of sensors remains. This set of instruments has been widely reviewed by the General Assembly, ESONET Work Package 3 and 5 members, Best Practice workshops, and the ESONET Science Council.

Defining a list of ‘generic sensors’ or ‘the’ generic sensor immediately gives rise to discussions between members of different research disciplines (biology, geophysics, microbiology, oceanography) and working areas (shallow or deep, open ocean or coastal) about what a generic sensor should be able to measure. However, defining a list of generic sensors and variables is worthwhile if a consistent set of data is to be acquired at open ocean observatory sites. Moreover, it will help in setting up accuracy, calibration, and data handling standards for specific purposes.

Generic sensors should be available at a reasonable resource allocation with respect to purchasing and operating the sensor. Thus, commercially available sensors that can take reliable measurements over year-long periods are the most suitable sensors to be used in a generic sense. In this respect, a platinum resistance thermometer (PRT) linked to a simple logging unit is possibly the most generic sensor. Systems that combine these thermometers with other basic sensors are already available from several companies in several countries and are frequently used in the scientific community. They are multi-probe, CTD-type (conductivity temperature, and depth [pressure]) systems, which often come with a basic set of sensors and the option to add a wide variety of other sensors. The advantage of such systems is that data capture and power supply units already have some integration and interoperability standardization and can be used for quite different sensors and parameters.

The generic variables cover several of the Global Climate Observing System (GCOS) Essential Climate Variables to contribute to the UN Framework Convention on Climate Change (UNFCCC) and the IPCC (Table 3). Continued interest for these variables is noted in the proceedings of the OceanObs’09 Conference. To account for the different research areas and needs outlined in the scientific objectives of ESONET, a first list of variables has been compiled based on the criteria “most commonly needed", "availability", "ease of use", “deep-sea compatible” and "capability for long-term monitoring (corrosion, calibration periodicity, stability…)" of the water column (Table 4). The presented list shows a basic CTD configuration plus a few additional variables. Operation of these sensors will need to meet some basic criteria (Table 5), which are currently met by a variety of manufacturers (Table 6).

Table 3. GCOS Essential Climate Variables. Surface Sub-surface Sea-surface temperature Temperature Sea-surface salinity Salinity Sea level Current Sea state Nutrients Sea ice Carbon Current Ocean tracers Ocean colour Phytoplankton Carbon dioxide partial pressure

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Table 4. Generic ESONET variables in the water column and at the seafloor surface. Variable Geosciences Physical

Oceanography Biogeochemistry Marine Ecology

Temperature X X X X Conductivity X X X X Pressure X X X X Dissolved O2 X X X X Turbidity X X X X Ocean currents X X X X Passive acoustics X X Table 5. Overview of specifications under consideration for generic sensor modules that may be used across European ocean observatory sites. Type of sensor Range† Accuracy† Conductivity 0 to 9 S/m 0.001 S/m Temperature -5 to +35°C 0.01 K Pressure 0 to 600 bar 0.1 % FSR Dissolved oxygen 0 to 500μM 5% Turbidity 0 to 150 NTU 10% Currents 0 to 2 m/s 2% Passive acoustics 50 - 180 dB re 1 μPa +/-3dB †Range and accuracy given are often adjustable through calibration and given here as suggestions.

a. Provisional generic module parameters Conductivity & Temperature: CTDs can already be deployed as standalone or networked long-term monitoring devices, (e.g. on moorings or landers). All three parameters are the basis for many other derived parameters, which can be calculated (e.g. potential temperature, sound velocity, salinity) and are used in all four of the major science areas of ESONET. The logging of temperature and conductivity should be considered compulsory for long-term monitoring, independently of the prime purpose of the observatory set-up. Collecting both data sets will add to the global understanding of the status and change of the ocean. Pressure (Depth): There has also been considerable interest expressed, in part, by the Global Ocean Observing System (GOOS) and the Northeast Atlantic and Mediterranean Tsunami Warning System (NEAMTWS) groups to have offshore sensors capable of detecting tsunami waves. So the inclusion of a pressure sensor that can detect depth and tidal variation, as well as passing tsunami waves will be of broad interest in the scientific and geo-hazard warning communities. Specifications for the US Deep-Ocean Assessment and Reporting of Tsunamis (DART II) system require an effective accuracy of 0.5 cm (depth) or less, sampling at 1 min or less. These sensors should be deployed in a system that can process collected data in 2 min or less, and deliver data to shore in 5 min or less (Meinig et al., 2005). The German-Indonesian Tsunami Early Warning System (GITEWS) has now preliminary systems operating (http://www.gitews.org). These systems can also measure tides (Spencer and Vassie, 1997; IOC 2006) and in the observatory setting can help evaluate how anomalies in tides occur including the passing of seiches and storm surges in the open ocean. Longer-term monitoring systems, such as Myrtle X, have also been developed to observe variations in sea level, a key objective of the Global Sea Level Observing System (GLOSS). Dissolved Oxygen: Oxygen measurements are very useful for physical oceanography, biogeochemistry, and marine ecology. Oxygen is an additional proxy to define water mass character

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and is also a key indicator of environmental change. A wide variety of oxygen sensors exist for stationary and profiling CTDs. Two different methods are commonly used: optodes (luminescence quenching principle) and Clark electrodes. Unlike the electrochemical sensors, which can have relatively large drift after a few weeks to months, optodes have been shown to have minimal drift and make a preferred tool to measure dissolved O2 in an observatory setting. Optodes are more stable for long-term deployment and have a predictable pressure effect. Both sensors should be oriented in such a way that water flows relatively freely around the sensor. Pump systems are sometimes used to help secure a steady flow and to increase the accuracy of the measurement. They are especially recommended for use with non-optode sensors. Testing by the US-ACT and others shows that optodes deployed in shallower depths will require biofouling protection (Martini, 2007; Delauney and Compère, 2008). Turbidity: Turbidity measurements in the water column aim to define, for example, the pollution stage of an area, the amount of transported material from the sea surface towards the seafloor, or the amount and periodicity of re-suspended material transport from the shelf towards the abyssal plain. Two widely used methods exist. Firstly, optical backscatter sensors that emit light and measure the amount of backscattered light, which is correlated to the amount of particles in the water; secondly, light transmissiometers, which measure the absorption of a light source through a certain distance of water. Both systems are commonly used with profiling and stationary CTD systems. These optical sensors will also need biofouling protection at shallower depths (Delauney and Compère, 2008). Ocean currents: Water motion measurements at the seabed and in the water column are fundamental for understanding the transfer of energy, heat, and chemical and biological variables in the ocean. Using acoustic Doppler current meters is now an established technique for measuring ocean currents. Depending on the orientation, acoustic and sampling frequency, current meters can collect data on ocean currents over a variety of scales. Some systems are adapted for sampling a single volume of water, while Acoustic Doppler Current Profiler (ADCP) and Recording Current Meter (RCM) systems can collect information on currents in binned depth ranges that vary, in part, according to acoustic frequency used by the sensor. Given the wide applicability of ocean current data and effectiveness of acoustic current meters, it is recommended that each generic module contains a current meter suitable for its location be it on the seafloor or in the water column, at abyssal, bathyal, mesopelagic depth, or near the surface. These systems are now available from a variety of vendors including RDI/Teledyne, Nortek, Annderaa Data Instruments, and LinkQuest. Passive acoustics: Passive acoustics using hydrophones are used to study marine mammals (Nystuen, 2006) and geophysical processes (e.g. Favali et al., 2006). Recent investigations have also used these systems to measure rainfall at the sea surface (Ma and Nystuen, 2005). In the field of bioacoustics, systems are now capable of evaluating the reaction of mammals to the increasing noise produced by ships and their migratory behaviour. The main differences between hydrophones for seismic and mammal monitoring are bandwidth, sampling frequency, cut-off filter, gain, and timing requirements. Hydrophones selected for the generic package should be compatible with cabled and non-cabled networks. These sensors would preferably be connected through a smart sensor interface, for calibration, position and identification tracking. This will prove particularly useful in the context of globally distributed passive acoustic monitoring (Delory et al., 2008). Possible additional generic module parameters Carbon dioxide: In recognition of the importance of understanding carbon cycle dynamics, CO2 should also be considered as a potential generic parameter as soon as the relevant sensors have become established research tools. These sensors help understand the capacity of the ocean to take up and store anthropogenic carbon. Several commercial systems are now available to measure pCO2 autonomously. However, most are new technology which has yet to have a proven track record. Nonetheless, the adoption of these sensors is a scientific priority and considered an ESV. Therefore,

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we recommend that consideration is given to include these sensors across all European ocean observatory sites, even if some systems do not have a long track record. Thus in selecting systems for use, one should be aware of the latest developments in pCO2 best practices.

Within EuroSITES there are ongoing efforts to test the viability of a particular pCO2 sensor for use in ocean observatories manufactured by Pro-Oceanus. The trials have included programming for low power consumption routines in long-term recording. Comparison tests with a General Oceanic shipboard system have shown that the two systems have agreement approaching 1.5 μatm. Also in use at several EuroSITES locations is the Submersible Autonomous Moored Instrument (SAMI)-CO2 system from Sunburst Sensors (DeGrandpre, 1995). The new SAMI2-CO2 system is now under evaluation by the US-ACT. Studies carried out by EuroSITES partners have indicated that pressure variation related to variable positions in the water column as a result of mooring dynamics could be related to the operation of its pump. JAMSTEC have developed a spectrophotometer-based pCO2 sensor. Battelle has developed, in collaboration with NOAA and the Monterey Bay Aquarium Research Institute (MBARI), a long-term optical system for measuring pCO2. Contros also has a pCO2 system which uses similar optical methods that rely on non-dispersive infrared (NDIR) technology. The recent wide adoption of the NDIR system suggests it may ultimately gain similar levels of adoption to that of optode systems in oxygen sensors.

Chlorophyll-a: Chlorophyll-a can be easily measured by fluorometers on profiling or stationary systems. Such sensors are commonly used on CTDs and are able to take measurements over long periods of time. They can capture local variations not visible to space-bourn colour sensors in both horizontal and vertical dimensions depending how they are used (Hartmen et al., 2010). These systems provide critical data to help interpret in situ observations and calibrate satellite ocean colour data (O’Reilly et al., 1998; Kahru and Mitchell, 1999).

There are however several known complications of using fluorometers in long-term deployments. Influences of changes in community structure, chlorophyll to carbon ratios and other dynamics are only accounted for through careful calibrations (Mowlem et al., 2008). There are several fluorometer systems now available commercially, but their use in long-term deployments of up to a year or more has still not been well demonstrated, particularly because of biofouling. There are ongoing efforts to reduce or eliminate biofouling (see biofouling section). Within the EuroSITES network several fluorometers are being used in long-term deployments including the Wetlabs FLNTUSB, Turner Cyclops, and Chelsea Aqua Tracka MKIII (with HPLC) usually with trials on biofouling protection.

Time-lapse cameras (benthic only): Monitoring of benthic biological activity involves not only the study of microbial and biochemical processes in sediments, but also time-lapse cameras. Wide angle cameras mounted obliquely at about 2 m above the seafloor can effectively view several square meters of seafloor (sensu Smith et al., 1993). Conversely, systems mounted closer to the seafloor, such as the Bathysnap system (Bett et al., 2001; Lampitt and Burnham, 1983), can view a smaller area with greater detail. Most systems now use hourly or daily imagery, but this can vary depending on power and data storage. Still photograph cameras have substantially greater resolution than video images, so it is recommended that for most applications still images be used. Careful consideration is also needed with regard to the placement of the light source relative to the camera so as to avoid high backscatter, which is typically at least 1 m apart (Jensson et al., 2009; Swartz, 1994).

Biofouling protection: Biofouling remains a substantial issue for the deployment of sensors in situ. It typically decreases with depth and cursory knowledge of biofouling potential is known from many long-term research sites around European seas. Mechanical and chemical solutions exist with varying levels of effectiveness (EuroSITES D1.1.1). Mechanical solutions include sensor wipers that operate intermittently and coatings which are physically resistant to biofouling formation from their exceptional smoothness. Chemical solutions include those which use either copper, or other

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chemical coatings, movable copper shutters (Drakopoulos et al., 2004), as well as those that use electrically produced local chlorination (Delauney and Compère, 2008). Illumination with ultra violet light has also been used where a power source is available (Kunitomo and Obo, 2003). Several vendors now have options for biofouling protection.

Biofouling protection of a enviroFlu-DS fluorometer by localized seawater electro-chlorination (©Ifremer) – Protection system under Ifremer licence – NKE, Hennebont (56), France (photo: Ifremer, France)

b. Generic Sensors Impact These generic sensors can be used to directly address a wide range of geo-hazard warning and scientific applications related to understanding natural and anthropogenic variation and the possible impacts of climate change. They will also provide supporting data to a large set of additional uses. Firstly, these systems will be able to detect passing tsunami waves and associated low frequency sounds related to earth motions. In the observatory setting these data can then be relayed back to shore via seafloor cable satellite telemetry within minutes. Because nearly all tide gauges are along shorelines, offshore data can improve warning time. The system will also be able to detect storm and tide wave loading, sedimentation dynamics that influence turbidity, such as resuspension and benthic boundary layer (BBL) dynamics. By linking tide, turbidity, and current meter readings, interaction strength and thresholds for resuspension and sediment transport can be further described. Furthermore, the measurement of these parameters on the seabed and in the water column can help determine how seabed processes interact with ocean circulation, biogeochemistry, and ecological parameters. Combining generic sensors with specific sensors such as seismometers, geodesy, bubble flux observing systems, hydrothermal flow meters, and piezometers the remaining key questions outlined in Deliverable 11 can be addressed such as how are seismic activity, fluid pore chemistry and pressure, gas-hydrate stability, and slope failure related? And what are the feedbacks between deformation, volcanism, seismic, and hydrothermal activity? Generic sensors can also address questions related to physical oceanography. However, a generic sensor module at the surface, midwater and/or at the seafloor can only answer these questions partially. The use of salinity and conductivity sensors spaced regularly along strings and additional ADCP coverage can, however, capture the themes related to ocean physics. These include understanding wind-driven and deep-ocean circulation, planetary waves, and interactions between the BBL and the seabed. Mobile systems, such as gliders, used in conjunction with the fixed infrastructures can also augment the impact of generic sensors.

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The oxygen sensor in the generic specification can address several aspects of biogeochemistry. Oxygen itself is important for aerobic life in the oceans which includes all metazoans (e.g. zooplankton, fish, and benthic invertebrates). Oxygen in the oceans is replenished primarily by inputs related to photosynthesis and equilibration at the air-sea interface. By making some crude assumptions one can estimate how much oxygen has been utilized by measuring how much remains compared to saturation levels (apparent oxygen utilisation [AOU]) (Garcia et al., 2006). So, variations in oxygen minimum zones (OMZs), as well as oxygen dynamics in the rest of the water column are of interest. Generic modules will also be able to make sensitive measurements of how oxygen concentration relates to turbidity and temperature, which have both connections to time variant respiration and/or remineralisation.

Carbon dioxide is an abundant greenhouse gas and is a key molecule in the oceans’ biological pump. It is transferred from the atmosphere into the ocean and incorporated into phytoplankton production during photosynthesis. Some of this photosynthetic production is exported out of sunlit surface waters and sequestered for extended periods of time. There remains, however, much uncertainty in the transfer rates and dynamics of CO2 uptake.

Measuring chlorophyll-a as an indication for the amount of primary production through the water column has many implications for biogeochemistry and marine ecology. These include sedimentation processes from the sea surface to the seabed, the input amount and seasonality of organic material, and the latter’s role as food supply and the resulting implications for the existing fauna in different habitats. Chlorophyll-a also provides an insight into the importance of other parameters that trigger plankton blooms, as well as their seasonality/periodicity.

As sensor technology develops biogeochemical sensors will likely transition from specialized to generic instruments in the coming months and years, including pCH4 and pH sensors. Moreover, the more specialized measurements of particulate fluxes greatly augment the breadth of biogeochemical themes that can be addressed. The most elemental of these themes is oceanic carbon and greenhouse gas uptake, storage dynamics, and estimating how anthropogenic change might alter the efficiency of the biological pump.

With pictures taken hourly, the activity of benthic vent communities and the growth of chimneys and fluid venting may be recorded on a time-series basis. Similarly, the behaviour, diversity, activity rates, and size distributions of many fauna can be determined (Lampitt et al., 1983; Smith et al., 1993) for slope, canyon and abyssal plain habitats. In addition, photogrammetric techniques can determine the coverage and lifetime of visible phytodetritus on the seafloor and thus quantify biogeochemical fluxes (Smith et al., 1998; Bett et al., 2001; Smith et al., 2008). Another sensor with generic specification is the hydrophone, which is capable of detecting marine mammal sounds. Currently, there are hydrophone-based systems that can detect the position and identity of mammal sounds and thereby come up with estimates of density and distribution. Other sounds can also be detected, including anthropogenic sounds like those of passing ships, as well as rain, and the sounds of certain plankton and fish. Combining these systems with other ecological measurements will provide verification data that is needed to improve the detection of even more sounds. ADCP systems are sensitive to zooplankton and fish distributions, as well as currents. For example, the relative density variations associated with diurnal vertical migrations and their variation from hours to decades can be quantified and calibrated (Flagg and Smith, 1989; Kaufmann et al., 1995). The addition of cameras and active acoustic systems like scanning sonar or synthetic aperture systems can greatly enhance the quantification of abundances. Fluorometers, zooplankton samplers, and advanced microbial sensing systems also add to the impact of the generic observing system in order to address the diverse set of ecological question in Deliverable 11. Chemical sensors (H2S, pH, Eh, hydrocarbons): Measurements of pH are of great interest, for example, to monitor ocean acidification and hydrothermal vent and methane discharging seep chemistry. Sensors that can be added onto CTDs are available and are commonly used with an accuracy of +/- 0.05 pH and calibration periods of nine or more months. But here, too, a long-term

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track record is still developing. Hydrocarbon sensors that use fluorescence techniques are now commercially available. Industry partners have already indicated that observatory systems capable of alerting industry managers to spills would be very useful. Because these sensors are still in development for long-term use, additional quality assurance methods for longer-term data may be necessary. Special calibrations or long-term endurance testing may be needed to ensure that drift and depth effects are within allowable limits. Improving understanding of long-term stability should be an area of particular focus when collaborating with sensor developers.

Combining different sensors into one sensor package, as is commonly done for ‘CTD-plus’ systems, has the great advantage of only one data logging and power-supply unit. This simplifies setup and maintenance and direct time synchronization between attached sensors. Timing can be particularly important for geo-hazard applications, but also requires careful consideration when monitoring stations where short-term events (e.g. gas discharge, earthquakes, turbulent currents) are logged. A disadvantage is that if such an integrated system fails, some or all sensor data might be lost depending on the telemetry arrangement. To prevent this, redundant systems are recommended for critical data. Furthermore, because certain types of failures can be dependant to some extent on the instrument model, it is recommended that redundant systems use alternate sensor models and/or manufactures. Each generic module can be attached to a sensor network using a single wet-mate plug and all data and data protocol handling can be routed through a single smart sensor interface (see Instrumentation Module Architecture). These sensors can be mounted on a modular frame that can be modified for use on the seafloor, or attached to a lander, junction box, or mooring.

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Table 6. List of potential generic parameters and examples of available systems. Most systems are available to 6000 m depth. Variable Model Manufacturer Internet link Temperature SBE MicroCat 37 w/o pressure Seabird http://www.seabird.com NXIC-CTD, Micro CTD Falmouth Scientific Inc. http://www.falmouth.com CTD48, 60, 90 Sea & Sun Technology http://www.sea-sun-tech.com Conductivity SBE MicroCat 37 w/o pressure Seabird http://www.seabird.com NXIC-CTD, Micro CTD Falmouth Scientific Inc. http://www.falmouth.com CTD48, 60, 90 Sea & Sun Technology http://www.sea-sun-tech.com Pressure SBE 54 Tsunameter Seabird http://www.seabird.com Digiquartz series 8CB Paroscientific http://www.paroscientific.com Differential pressure gauge (DPG) Scripps Institution of Oceanography http://marineemlab.ucsd.edu Dissolved O2 Oxygen Optode 3830 or 3975 Aanderaa http://www.aadi.no Turbidity Turbidity Meter Seapoint Sensors http://www.seapoint.com MST-AutoCal Trios http://www.trios.de C-star Wetlabs http://www.wetlabs.com ECO-FLNTU Wetlabs http://www.wetlabs.com Ocean currents 300kHz workhorse Teledyne RDI http://www.rdinstruments.com 75kHz workhorse Teledyne RDI http://www.rdinstruments.com RCM series Aanderaa http://www.aadi.no FlowQuest, FlowScout LinkQuest http://www.link-quest.com Aquadopp series Nortek AS http://www.nortek-as.com Passive acoustics HTI-96-MIN + Data logger High Tech http://www.hightechincusa.com HTI-99-PCFD2 + Data logger High Tech http://www.hightechincusa.com OAS E-2PD + Data logger Optimum Applied Solutions http://www.oas-inc.com Ethernet hydrofon Bjørge Naxys AS http://www.bjorge.no Possible additions Chl-a fluorescence ECO-FLNTU Wetlabs http://www.wetlabs.com C3 Submersible Fuorometer Turner Designs http://www.turnerdesigns.com AQUAtracka III Chelsea Technologies Group http://www.chelsea.co.uk Chlorophyll Fluorometer Seapoint Sensors http://www.seapoint.com enviroFlu-DS or microFlu-DS TriOS GmbH http://www.trios.de Time-lapse camera OE14-208, OE11-242 + controller Kongsberg Maritime http://www.km.kongsberg.com SharkEye series Desert Star Systems http://www.desertstar.com SDS 1210 + high power flash unit Imenco AS http://www.imenco.no pCO2 SAMI2-CO2 Sunburst Systems http://www.sunburstsensors.com HydroC Contros http://www.contros.eu CO2-Pro Pro-Oceanus http://www.pro-oceanus.com

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3. Science Specific Sensor Modules

Here we outline many of the rather specific parameters that will likely be of interests at particular ESONET sites. We highlight the basic science objectives associated with the parameters along with some fundamental requirements, sensor types, availability, and the ways in which these instruments might be used in an ocean observing network. It is important to point out that while most researchers would always favour having the most accurate measurements that are reasonably possible, some minimum requirements must be met so that the sensors will indeed be capable of addressing the science objectives adequately.

a. Geosciences Key questions in the field of geosciences include: How can monitoring of factors such as seismic activity, fluid pore chemistry and pressure, improve seismic, slope failure, and tsunami warning? And to what extent do seabed processes influence ocean physics, biogeochemistry, and marine ecosystems? Because the opportunities to obtain networked measurements required to address these fundamental questions are very scarce in marine environments, synchronous measurements of seismic motion, gravity, magnetism, seafloor deformation, sedimentation, pore water properties, and fluid and gas hydrate dynamics will provide a great opportunity to make advancements in the scientific understanding of geosciences and geo-hazard early-warning capabilities. Solid-earth dynamics can release large amounts of destructive energy and lead to major impacts on society, such as mass fatalities and civic infrastructure damage. The seabed also acts as an important boundary for fluid dynamics and vent and seep processes have important implications on ocean physics as well as marine chemistry and biology. Seismic motion: Monitoring seismic motion, a fundamental measurement for observing solid earth dynamics, requires ocean-bottom seismometers (OBSs) with 4 components: one vertical and two horizontal geophones, defining the 3-dimensional motion of the ground, as well as a hydrophone sensitive to pressure variations in the water column. A number of public research institutions in the USA, in Asia (Japan, China, Taiwan) and Europe have recognized the need for a broadband-based approach and acquired autonomous, portable broadband OBSs that can be deployed for several months (up to 18 - 24 months) on the seafloor. In the USA, the Scripps Institution of Oceanography, the Woods Hole Oceanographic Institution, and the Lamont Doherty Earth Observatory maintain the US National Ocean Bottom Seismograph Instrument Pool (http://www.obsip.org/inst.html), which includes Broad Band (BB) OBSs. In Europe, many marine research institutions have acquired their own fleet of short-period OBSs. In contrast, broad-band OBSs are less commonly found, the two main pools being in Germany at AWI (www.awi.de) and in France, at the Institut National des Sciences de l’Univers (INSU).

There are two leading manufactures of BB sensors used in ocean bottom seismology: Guralp (www.guralp.com) and Kinemetrics (www.kinemetrics.com). We recommend the use of broadband OBSs (between 0.03 and 30 Hz) wherever feasible, most particularly when communication is available for data transmission because broadband seismology covers the majority of needs for passive source seismology. Short period OBSs (> 1 Hz) are useful for studying local micro-seismicity (they can provide hypocenter location and focal mechanism), but they are unable to provide the seismological information contained in the “coda” signal generated by earthquakes. Functionally these seismometers can exist as single instruments attached to a seafloor network or be included as part of a module. In order to be useful for long-term deployments the systems need to have better than 10-8 local clock or continuous time synchronization. Cabled BB-OBSs may also be complemented by numerous, short period (> 1 Hz), autonomous OBSs which are suited for local applications. These systems frequently require careful seabed installation to ensure that earth

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motions are indeed effectively transferred to the sensor. The spatial arrangement of the OBSs depends on the problem that is addressed. For instance, studying fluids and seismicity relations requires precise localization, within a few hundred meters. Here, the ideal design could consist of one broadband OBS at the ESONET site and a cluster of short-period OBSs, spaced a few kilometres apart. (see also NERIES-ESONET OBS Marine Seismology Workshop, 11-12 February 2010, IPG Paris; www.ipgp.fr/~rai/)

Geodesy and Seafloor deformation: Changes in surface deformation and mass movements

are powerful constraints on models of tectonic, magmatic or subsidence processes. In submarine environments large-scale experiments based on surface vessels and a combination of GPS and acoustic measurements have proved useful, for instance, in determining the plate deformation during an earthquake cycle at subduction zones (Gagnon et al., 2005). Such shipboard and GPS-based systems can capture the 10-8 time accuracy needed to effectively evaluate deformation for geoscience and geo-hazard applications, but are less feasible with seafloor stations, due to the difficulties in determining variations in sound speed over large time periods. Fabian and Villinger (2007) have hence proposed an alternate solution to measure seafloor deformation that uses a specially designed Ocean Bottom Tiltmeter, combining biaxial tilt sensors and a high-resolution vertical accelerometer. Interestingly, Polster et al. (2008) have investigated the long-term drift behaviour of high-resolution pressure sensors as tools for detection of seafloor subsidence and uplift. This system can therefore be placed on the seafloor and capture deformation events on the order of 1 µrad resolution. Right: The Bremen Ocean Bottom Tiltmeter (Fabian and Villinger, 2007), deployed at the Logatchev Vent Field.

Optical methods can also detect variation in seafloor features such as seabed wave forms

and these can be carried out either on fixed or mobile platforms. In particular laser scanning methods can map bathymetry at a variety of scales depending on the deployment scenario ranging from 10 cm or more at further distances to sub cm scales for objects within about 10 m (Moore et al., 2000; Moore and Jaffe, 2002). At closer scales a deployment using structured laser lighting was able to measure variations in sand wave morphology of less than 1 mm (Moor and Jaffe, 2002) and such a system is well suited for adaptation to deep-sea and observatory settings for monitoring not only bed forms, but processes driven by volcanism or hydrothermal venting. Gravity: Changes in the local and regional gravity (i.e. physical geodesy) can be influenced by motions of the Earth’s crust, polar motion, and tides. Repeated surveys conducted on land with absolute gravimeters providing an accuracy of ~ 1 µGal (e.g. FG5 constructed by Micro-g solutions (Niebauer et al., 1995) have allowed the detection of absolute vertical uplift of less than 0.5 mm per year (Djamour et al., 2007). However, this type of repeated measurement is not adapted for deep seafloor monitoring. On the other hand, relative gravity meters provide continuous measurements (e. g. Lacoste-Romberg or Scintrex), but are affected by important temporal drifts (~ 500 µGal/month). Hence, deep seafloor gravity measurements are not yet mature for operational purposes. Although interesting experiments are presently ongoing, such as at the Nemo Site off Sicily, where a prototype gravity meter is being tested as part of the NEutrino Mediterranean Observatory—Submarine Network-1 (NEMO SN-1, [Iafolla et al. 2006]).

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Magnetism: Like gravity, changes in local magnetism can also be related to other solid earth dynamics, but magnetic temporal variation is not well known, especially in relation to geo-hazard events. The GEOSTAR system (see appendix) achieved long-term, seafloor magnetic measurements using 2 magnetometers: a 3-axis suspended magnetometer designed and built by INGV providing a resolution of 0.1 nT and an absolute accuracy of 5-10 nT, and a scalar Overhauser GSM-19L proton magnetometer from GEM System Inc. (De Santis et al., 2006). Fluid related processes monitoring: Fluid flow within and between the sub-surface relates to many physical, biogeochemical, and ecological processes (Tryon et al., 2001). The additional study of gas hydrate dynamics, sediment pore water pressure and chemistry, and subsurface fluid flows is important for studies of seismology, slope instabilities, as well as heat flux, biogeochemistry, and ecology. It is thus of critical importance to measure the geophysical properties of the pore fluid, as well as the flows and chemical fluxes across the sediment-water interface. To meet the versatile demands of fluid and chemical fluxes at the seabed, the Scripps Institution of Oceanography has developed the CAT (Chemical and Aqueous Transfers) meters (shown below). These instruments have been in use since 1998 and have been very successful in monitoring long-term fluid flow in both seep and non-seep environments. They are designed to quantify inflow and outflow rates on the order of 0.01 cm yr-1 to 100 m yr-1. At high outflow rates, a time series record of the outflow fluid chemistry for tracer and major ion concentrations (Na, Ca, Mg, S, K, Sr, B, Li) can be obtained, with a typical resolution of 2 days for flow and 3-5 days for chemistry. The instruments are also equipped with an auxiliary osmotic pump connected to copper coils and high pressure valves so that they can be returned to the surface at ambient pressure, maintaining the gas composition of the fluids for time series analysis of 3He/4He, CO2/3He, δ13C and CO2 and He concentration. Important sections of the records are additionally analysed for B and Li isotopes and δD and δ18O.

Chemical and Aqueous Transport (CAT) meters deployed on the seafloor (above left) and schematic (above right), after Tryon et al. (2001a). The CAT meter uses the dilution of a chemical tracer to measure flow through the outlet tubing exciting the top of a collection chamber. The pump contains two osmotic membranes that separate the chambers containing pure water from the saline side that is held at saturation levels by an excess of NaCl. Due to the constant gradient, distilled water is drawn from the freshwater chamber through the osmotic membrane into the saline chamber at a rate that is constant for a given temperature. The saline output side of the pump system is rigged to inject the tracer while the distilled input side of the two pumps are connected to separate sample coils into which they draw fluid from either side of the tracer injection point. Each sample coil is initially filled with de-ionized water. Having two sample coils allows both inflow and outflow to be measured. A unique pattern of chemical tracer distribution is recorded in the sample coils allowing a serial record of the flow rates to be determined. Upon recovery of the instruments the sample coils are subsampled at appropriate intervals and analysed. Both tracer concentration and major ion concentration (Na, Ca, Mg, S, K, Sr, B, Li) are determined simultaneously (Mike Tryon, pers. comm.).

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Pore pressure: Obtaining differential pore pressure measurements at different depths within the sediment requires specific marine piezometers. Despite the technical difficulties, many efforts have been made in recent years to monitor pore pressure, due to the promising possibility that such instruments may offer important geotechnical and geophysical applications like geo-hazard early warning. Theoretically, slip at active fault motion results in a change of mean stress and induces volumetric strain in the sediment. It is therefore hypothesized that strain events may be recorded as pressure variations. Thus, pore pressure variations on the order of a few kPa might be used for monitoring background strain within active fault zones. To be fully efficient, however, piezometers need to be installed in the deformation zone, very close to active faults, and hence very detailed site surveys are needed together with advanced hydrological studies prior to long-term deployment. Piezometers are also very useful for assessing the failure potential of submarine slopes (e.g. Sultan et al., 2007; Strout and Tjelta, 2007). Infrastructures for deploying sensors in open boreholes have been developed by academic research institutions, including the Circulation Obviation Retrofit Kit (CORK) system, for deploying at ocean drilling sites (Davis and Becker, 2007). The MeBoTECH (Meeresboden-Bohrgerät, seafloor drill rig) system is also capable of drilling 50 to 70 m into the seabed. The associated SMART sensor string (Subseafoor Small Autonomous Recording Tools) can be deployed in the borehole to collect data on pore pressure as well as temperature and other biogeochemical quantities (Freudenthal and Wefer, 2007). Different types of piezometers for shallow penetration (less than 15 m within sediments) have been developed by academic research institutions, including members of ESONET NoE, particularly the “Push”-piezometers (e. g. Flemings et al., 2008; Strout and Tjelta, 2005) and the free-fall piezometers (e.g. Schultheiss and McPhail, 1986; Sultan et al., 2009; Strout and Tjelta, 2005). As part of the EuroSITES programme and the AOEM demonstration mission a Fluid flow Seabed Observatory (FluSO) system has been developed to simultaneously examine seismic, pore pressure, fluid flow, and slope stability. By examining these parameters together the relationships between seismic motions and slope stability can be established with the aim of improving geo-hazard early warning.

NGI has performed a field test for its latest autonomous piezometer (pore pressure) design. The units were deployed in 1300 m water depth, monitoring pore pressures at depths of 18 m, 20 m, and 65 m beneath the seabed (single depth at each location). The units successfully monitored pressure for 4 months prior to the first data download. The equipment remains in place and will continue operating for up to 5 additional years or until the site owner (StatoilHydro) decides to stop the monitoring programme and retrieve the equipment. NGI has completed the design of the seabed template and hook-up for cabled connections between the template and the Troll A platform. The design is complete and the template has been procured and constructed, including all associated hardware, underwater stab connectors, and the cabling connection to Troll A. However, due to logistical limitations, the plan to install the template and to hook up the equipment to the Troll A platform has been placed on hold indefinitely. The equipment remains staged at the NGI warehouse site. The template is specifically designed for another monitoring task required by the Troll A platform, but additional connectivity points, power supply and communication capacity has specifically been supplied for potential use by ESONET/EMSO. The seabed around Troll A is covered in pockmarks, indications of possible seeps, as well as an abundant marine life in the pockmarks that has apparent differences to what is found on the seabed outside of the pockmarks.

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Illustration of active acoustic system quantifyingbubble fluxes (Jens Greinert, pers. comm.)

A)

B)

FluSO: Synoptic raw A) flow meter and B) seismometer data (z-axis) from the Mediterranean Sea (courtesy V. Hühnerbach and C. Berndt, www.eurosites.info/fluso.php) Gas hydrate monitoring: Monitoring of gas hydrates and seeps is an important science objective of ESONET because the related methane is an important green house gas that under global warming could manifest a positive feedback on warming, as well as being related to unique chemosynthetic communities. Monitoring of gas seeps and

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methane fluxes is also likely to become highly valuable for a large variety of previously unsuspected applications in geosciences, such as 1) understanding of the evolution of the fluid-fault coupling processes during the earthquake cycle (Géli et al., 2007); 2) assessing the amount of gas released from the seafloor, either from biogenic or thermogenic produced methane, decomposing gas hydrate or magmatic sources.

Recent observations of massive methane release in the Arctic by ship-based single-beam echosounders again shows the great use of hydroacoustics for bubble detection (Westbrook et al., 2008) monitoring hydrothermal vent dynamics, particularly in relation to gas (CH4 or H2) production at ultramafic outcrops (e.g. Charlou et al., 1991).

Above: Scheme of a hydroacoustic system swath system detecting rising gas bubbles (top) and an example of data showing the current-induced movement of bubbles 3 m above the bottom and the periodic and episodic release (from Greinert, 2008).

For monitoring purposes it is recommended to take advantage of standard and well known acoustic technology such as high directivity single beam or multibeam echo-sounders to map and quantify the sea gas emissions and monitor their temporal variability (e.g. Greinert et al., 2006, Greinert, 2008). These echo-sounders are ideally combined with 70 to 300 KhZ ADCPs systems to determine the horizontal and vertical velocity of the bubbles. This helps in identifying different seeps in the data sets. Under specific circumstances it might even allow for determining the rise velocity of the gas bubble plume. A Fluid Flux Observatory (FLUFO) system used in situ takes seabed methane flux measurements along with water column backscatter data from ADCPs to quantify methane fluxes and their relationship to BBL dynamics (Linke et al., 2009). Specifically the system used a 300 kHz Workhorse Sentinel ADCP by Teledyne RD Instruments, for water column currents, bubble backscatter, and benthic chambers to quantify methane flux at the seabed. This FLUFO system could be modified for long-term use by either being deployed on a mobile

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lander or rover, or with an alternate methane flux measurement system using optical or electrode sensors and eddy flux estimation techniques.

Acoustic detection of gas emissions in the Sea of Marmara was for the fist time extensively and successfully investigated during the Marmesonet expedition in November 2009 with the use of 1) the EM302 multibeam echosounder and 2) the deployment of the standalone acoustic observation module BOB (Bubble Observatory Module). Gas echoes were very well detected and flare 3D visualisation was performed by Movies3D (Ifremer software). BOB is a standalone observation module equipped with a Simrad ER60 echosounder and a 120 kHz split-beam transducer mounted on a pan & tilt for horizontal insonification. The objective of monitoring bubbles is to study the variations in bubble activity, microseismicity and pore pressure.

For this reason, BOB was deployed in the eastern part of the Sea of Marmara, ~ 60 m away from a well identified gas source, at 1270 m of water depth and together with 5 piezometers and 6 OBSs. The instruments were all concentrated in this area, because it has persistent crustal microseismicity, including one cluster localized immediately beneath the zone where gas seeps are observed. In addition, geodetic data indicate that the 1999 Izmit earthquake still has an important post-seismic motion, corresponding to 50% of plate motion 6 years after the earthquake (Ergintav et al., 2009). Hence, the Eastern part of the Cinarcik basin was considered to be the most appropriate test site for the MARMARA-DM demonstration mission in order to investigate the consequences of deformation occurring at the crustal level on seafloor fluid expulsion and pore pressure.

Above: BOB on deck of R/V Le Suroit ready for deployment on November 7th, 2009. The red cylinder on top of the structure is the 120 kHz split-beam transducer mounted on a pan & tilt for horizontal insonification. Three cylinders below the structure contain Lithium batteries for energy supply. The fourth cyclinder contains the data logger and electronics. Floatability is provided by the yellow blocks made of syntatic foam.

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Above: Shaded bathymetry map of the Sea of Marmara with spatial distribution of gas flares inferred from the EM320 multibeam echosounder surveys during the Marmesonet Leg1.

Above: BOB standalone observation module in the tank facility of Ifremer.

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Dissolved Fe, Mn and sulfide species: During the circulation of seawater through fractured rocks at mid-ocean ridges, a range of geochemical processes lead to profound alterations in the composition and physical properties of the seawater with implications for the geochemical cycle of a series of elements. Water emanating from high temperature black smokers is acidic, contains free sulphide and high concentrations of dissolved manganese and iron II amongst other elements. On a global scale, hydrothermal plumes are both a source for some elements and a sink for others. The kinetics of iron oxidation within the plume can be used to estimate the plume’s age and the variation of manganese can be used as a plume signature. For hydrothermal vent fauna, the hydrothermal fluid provides both necessary energy sources, such as methane and hydrogen sulphide, and potential stressors or toxic compounds such as heavy metals.

In situ chemical analysis can perform high-frequency measurements (30 to 60 analysis per hour) when using FIA (flow injection analysis) resulting in a better resolution of chemical gradients occurring both in time and space. Two chemical analysers based on similar FIA / spectrophotometric detection techniques are available for plume studies (Fe/Mn analyser, Statham et al. 2005) and for ecosystem studies (CHEMINI, Fe and H2S, Vuillemin et al., in press, see Appendix). Based on the equipment that was initially developed by Statham PJ et al. (2005), the CHEMINI analyser provides enhanced autonomous operation (including self-calibration, reagent stability enhancement, improved in situ calibration and fouling reduction techniques) for long time spans. This analyser has been successfully tested in hydrothermal environments for more than 6 months on the TEMPO module (Sarrazin et al., 2007) (see Appendix). Particle flux: In seismically active areas, sediment traps are useful to characterize vertically settling material from hemipelagic sedimentation and from possible turbidity flows of different origin (e.g., seismic and flood events). Sediment trap systems are commercially available and have been used commonly in the deep sea. Near hydrothermal vents at the mid-ocean ridge crest, sediment traps allow the study of the possible influence of the vent fluid on the oceans’ chemical balance and to tackle the question of reproduction timing in hydrothermal organisms. These systems can collect material in up to 21 cups over periods of a few hours to several months per cup. The ability to detect particular flux quantities are limited by accumulation rates and laboratory detection limits for each biogeochemical quantity examined. For example, at low fluxes integration times (number of days per cup) may be longer and at higher fluxes shorter integration times might be used. When returned to the lab, the samples collected can be analysed for organic and inorganic carbon, for mineralogical analysis using X-ray powder diffraction and for biological analyses. Therefore, data collected by this type of system is not yet available in near real-time, but the data returned is of importance for many of the ESONET science priorities.

Left: Sediment trap systems developed by A) Technicap (Model: PPS 6), and B) MacLane 78H-21PARFLUX Mark. These traps are specifically for use in deeper waters or in lower flux areas.

A B

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Systems for optical detection of particles have been in various stages of development for nearly two decades (Lampitt et al., 1993a,1993b; Sherman and Smith, 2009). Recently, a commercially available holographic system from Sequoia Scientific, with the capability of detecting particles from 1.25µm to several mm in size, was introduced (www.sequoiasci.com). This system, originally developed by A.N. Smith at the University of Plymouth, can work in a time-series mode and detect the settling distance and velocities of particles. CNRS has developed an Underwater Video Profiler (UVP) that can be calibrated using sedimentation traps to measure sinking particle fluxes (Guidi et al., 2008). The UVP is being developed to ultimately work down to abyssal depths. Another system utilizes the MacLane PARFLUX sediment trap mechanics that uses a camera and fluorometer system to quantify flux material rather than preserving the material in cups (Sherman and Smith, 2009). The digital flux data can then be transmitted via the observatory network telemetry system. Further discussion of holographic methods can be found below.

b. Physical Oceanography One of the key questions in physical oceanography is how the many described physical dynamics and processes that occur at different spatial and temporal scales are related? Specific observatory applications in physical oceanography require vertically instrumented moorings providing long-term high resolution time-series data on the whole water column that can be used as a reference at fixed sites. The sensors for physical oceanographic measurements are mostly generic (e.g. CTDs, dissolved oxygen, turbidity, ADCP). Hydro-acoustic sensors are also recommended to assess the ecology of marine species, most particularly at those sites where marine mammals or other migrating species are expected along their migratory routes. Hydrophones deployed in the sound fixing and ranging (SOFAR) channel, will also be very useful to monitor the global micro-seismicity at distances of thousands of kilometres. Acoustic tomography: Acoustic tomography offers an alternative method whereby integrated measurements of ocean temperature can be taken over long distances. Such systems have been used in oceanographic research for decades and are now well established. A recent system developed to measure temperature using acoustic tomography in conjunction with autonomous glider system is the ACOBAR system (Acoustic technology for observing the interior of the Arctic Ocean). A recent review of acoustic tomography can be found in Munk (2006). More details on acoustic observatory techniques for ESONET can be found in Annex I of this document. Photosynthetically Active Radiation (PAR): Phytoplankton rely on PAR for energy. Light also drives warming and thus ocean physics. In situ sensors for PAR are widely available and have been integrated into many multiparameter senor modules. These sensors are indeed worth considering for use across European seas. Light attenuation can be calculated by comparing surface PAR with PAR at depth providing a key indicator for a limiting factor in phytoplankton productivity. YSI, Satlantic, and LI-COR produce sensors, some of which are available as spherical or directional sensors. Specialized CTD equipment for hydrothermal vents: Temporal variation in hydrothermal vent fluids can not only influence local physical oceanographic conditions, but can also be related to seismic activity and the abundance of animal communities that depend on the often energy-rich fluids being emitted. Because the temperatures of these fluids can approach 400°C, specialized thermometer equipment is needed. Several sensors are now available and the careful placement of sensors and logging units can lead to useful time-series records of temperature, as well as conductivity.

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c. Biogeochemistry Prominent questions in the field of biogeochemistry include: how are physical and biogeochemical processes that occur at differing scales related? What aspects of physical oceanography and biogeochemical cycling will be most sensitive to climate change? And, what will the important feedbacks of potential ecological change be on biogeochemical cycles? Many sensors for studying biogeochemistry are included in the generic specifications (e.g. turbidity, dissolved O2, etc). The case of dissolved Fe, Mn and H2S has been described above. This section covers how some generic sensors need to be situated in an observatory setting to address specific biogeochemical questions. In this section, we will also consider some of the more specialized sensors that are likely to be useful. Methane: Long-term measurements of methane concentration in the water column are a critical challenge for many research applications, such as fluid related processes near the seabed (gas hydrates, seismogenic zone processes, hydrothermal vent systems, deep-sea microbial communities), and global climate change (e.g. changes in ocean geochemistry, modification of CO2 dynamics). However, to date there is no autonomous sensor available on the shelf that meets the requirements in sensitivity, reliability and accuracy needed to address the scientific questions within ESONET. No sensor has the sensitivity that may correspond to the background methane concentration of ~ 10-2 nano-liter per l in the open ocean. In addition, the commercial sensors do not allow continuous measurement for periods longer than a few weeks, due to drift, corrosion, or bio-fouling problems that strongly affect accuracy. New perspectives are expected from infra-red (IR) detection techniques. Contros, for instance, has developed a new sensor (HydroC™/ CH4, Hydrocarbon & Methane Sensor) with substantially improved performance. The IR-based methane sensor (Boulart et al., 2007) developed at NOCS (in collaboration Laboratoire de Chimie, Ecole Normale Superieure, Lyon as part of the MoMARnet) may also open interesting perspectives (still subject to further trials). Efforts to improve the sensitivity and the capacity of such sensors to function autonomously over periods longer than a few weeks must be strongly improved. Further updates will revisit progress in methane sensor development. pH, Eh, and alkalinity: Understanding the impacts of ocean acidification has emerged as one of the highest international science priorities as the recently observed decreases in ocean pH are projected to continue as CO2 accumulates in the atmosphere. Updates of this document will include more details on the sensitivities required for various applications. Sensors that can measure pH have been commercially available for years, however, most are not suited for long-term deployments. Some, though, are now suitable for use on ocean observatories and specified for deployments of six months to depths of 6000 m or more with an accuracy of +/-0.01 pH. But experience with these sensors is relatively minimal. A spectroscopy-based pH sensor is also being investigated by the EuroSITES programme (M. González Dávila) and is being developed by the Universidad de Las Palmas de Gran Canaria. Sunburst Sensors has also introduced a pH sensor, the SAMI-pH, which has also been deployed in the EuroSITES network in 2009. Efforts are also currently underway to update SAMI-pH designs (DeGrandpre et al., 2010). Recent advances in field effect transistor (FET) pH sensors (e.g. Honeywell durafet pH sensor) have been made. See also the developing documents on ocean acidification research methods (www.epoca-project.eu/index.php/Home/Guide-to-OA-Research). FOCE: The need to understand how changing CO2 levels in the ocean will influence factors like pH, physiology, ecology and overall function of marine life is of high scientific priority internationally. MBARI is currently developing and deploying a system to experimentally alter

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ambient CO2 of an area of seabed and to quantify the ecosystem responses to varying CO2 and pH. The Free-Ocean Carbon dioxide Enrichment (FOCE) system is an example of how deep-sea observing systems can be used to address not only monitoring needs but also conduct important experimental manipulations in situ. Nutrient analysers: The availability of nutrients required for phytoplankton production, (e.g. nitrogen, phosphorous, and iron) is perhaps the most limiting factor in primary productivity rates. Sensors for detection of nutrients are rapidly changing. Optically based systems for some nutrient quantities are now commercially available. The In Situ Ultraviolet Spectrophotometer (ISUS) and its successor the Submersible Ultraviolet Nitrate Analyzer (SUNA) (Johnson and Colletti, 2002) are nitrate sensors, for example, that are now available from Satlantic. Other systems like the EcoLAB Pro from EnviroTech Instruments can analyse water samples in situ for nitrate, silicate, phosphate, and ammonium with an endurance of between 2 to 6 months. Other in-situ analysers like the NAS-3X are suited for nitrate in particular. The further development of spectrophotometer and mass spectrometer equipment for in situ use should further facilitate the measurement of nutrients in a sensor network. Several EuroSITES partners are using the ISUS and NAS-3X systems with one using the newer Microlab Envirotech (fifth generation) system. While in-situ analysers have been commercially available for a while, optical sensors offer freedom from malfunctions related to the moving parts of the sample processing systems. Fe and Mn analysers: The NOCS Iron and Manganese Sensors (IronManS) are a new low cost generation of colorimetric analysers developed in house and aimed at measuring a wide dynamic range (from 10nM to 10µM). They carry onboard standard to compensate for potential drift and validate in-situ measurements. IronManS relies on the colour development induced by the reaction between a reagent and the sample to analyse. For Fe2+, the Ferrozine reagent (Stookey 1970) was chosen and tested. Dissolved Manganese measurements were performed with the PAN assay (Chin et al. 1992).

To minimise the power and reagent consumption, a new microfluidic chip with built-in absorption cells was developed and validated (Ogilvie et al. 2010, Sieben et al. 2010). The low internal volume of the microfluidic chip (250µL) allows for very low reagent consumption (25µL for each actuation for a mixing ratio of 10:1). All fluids, including waste, are stored in gas tight nutrition bags. Each analyser is housed in an oil-filled housing of 150mm diameter and 230mm

height (pressure compensated design) and accommodates the fluidic and electronics (below). Left: Illustration of microfluidic chemical analyser. The piston pump withdraws fluids from the reagent and the sampling or standards lines. The sample is then injected in the microfluidic chip where a blank measurement is taken before being mixed with the reagent while travelling through a serpentine mixer at 750µL/min. The intensity of the colour development is measured through two different absorption cells (100mm and 25mm) with an LED and photodiode centred on the wavelength of interested. The two different cell

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lengths allow for the wide dynamic range.

Above right: IronManS module fitted on the SEAMON EAST station and floatation (Photo: C. Floquet). Above left: IronManS on the top of the diffused vent site. The sampling point is located in the middle of the module, underneath the base, therefore protecting the inlet. (Photo: Ifremer)    In situ mass spectrometer: More comprehensive chemical analysis is becoming possible through the development of in situ mass spectrometers. These systems have can monitor chemicals such as CO2, CH4, gas hydrates, and many other compounds. Several prototype systems have been developed and tested including TETHYS (TETHered Yearlong Spectrometer) In situ laser Raman spectrometer DORISS (Deep Ocean Raman In Situ Spectrometer) II. TEHTYS has also been shown to be directly useful in hydrocarbon exploration (Camilli et al. 2006). Osmosampler: Long-term chemical sampling through osmotic pumps has taken place for more than a decade (Jannasch et al., 2004). NOC has implemented this type of system for water sampling for nutrients. The system, which has been trialled as part of EuroSITES, can collect samples every 2-3 days. Depending on the size of the samples collected during these time-series, the samples can be used to analyse for macronutrients, as well as trace metals such as Fe, Mn, and Zn. Pigment and hydrocarbon fluorescence: In addition to measuring chl-a fluorescence, important insights into phytoplankton dynamics can come from quantifying secondary, and phaeopigments. The interpretation of satellite ocean colour and primary production data also benefits from in situ evaluation of chromophoric dissolved organic matter (CDOM). As noted previously, optical sensor development is now rapidly progressing. Several sensors are now capable of not only measuring chl-a fluorescence but also quantifying phaopigment and CDOM. These sensors will be most useful in the upper ocean where mesoplagic nutrient-repleted waters interact with more productive and often nutrient limited waters nearer the surface. However, our understanding of nutrient reservoir dynamics at deeper depths is also poor. Many commercially available sensors in this category can also now quantify lipids and other hydrocarbon compounds which are likely to not only be useful in quantifying carbon budgets for example, but also for use in environmental studies related to oil and gas extraction activities.  

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Particle flux: In addition to the geoscience sediment applications mentioned above, particle flux traps are also standard equipment for monitoring the downward flux of important biogeochemical quantities, such as total particulate carbon, organic carbon, nitrogen, opal (silicate), as well as biochemical qualities like pigments, lipids and amino acids. Important considerations must be taken into account for the projected range of sample accumulation and the volumes of the cups, as well as the quantities needed for each parameter analysis and any replicate measurements.

In order to collect information on particle flux that can be digitally transmitted, a prototype system is being developed by MBARI that photographically quantifies flux material and measures its fluorescence (Sherman and Smith, 2009). The system uses a commercially available funnel as is used in the system outlined above, but instead of collecting the material in a cup, the sample accumulates on a glass window and is photographed at a programmable interval. After photographing each sample a specialized fluorometer quantifies the chl-a in each sample. The window is then cleaned and set in place for another collection interval. It is expected that along with empirical verification the system will provide important information on flux variation and quality and do so through a network without the sample processing needed with conventional traps. Benthic Rovers: Several mobile lander systems with biogeochemical objectives have been developed by MARUM, NIOZ, MBARI and Jacobs University.

Above right: The moving lander system being developed by MARUM. Above left: The Jacobs University crawler on a gas hydrate outcrop in the Northeast Pacific. The ‘Wally’ system picture is connected to the NEPTUNE Canada observatory in real time.

The Jacobs University crawler systems Wally I and II have successfully been deployed in the NEPTUNE Canada observatory system. These rovers have focused on studying the biogeochemistry and ecology of cold seep and gas hydrate outcrop areas. The systems measure temperature, pressure, currents, salinity, methane concentration and turbidity. The Wally systems are also linked to shore via real time connections and can be operated from the lab. This connectivity has allowed for fine-scale manipulation of measurements and survey patterns to allow researchers to navigate to smaller-scale habitat hotspots.

The Royal NIOZ has been developing a movable in situ observatory called MOVE! The system is large enough to host more than one benthic chamber with a water sampling unit that can collect 5 samples each at 30 stations over a period of 9 months. MOVE! was developed in cooperation with Dutch and German partners and holds instruments to measure current direction and speed, tilt and compass direction for navigation, conductivity and temperature, optical turbidity (OBS), fluorescence, oxygen, currents with an upward looking ADCP, as well as recording time lapse images and videos. The available scientific payloads include a benthic chamber module, a micro-optode profiling unit, a resuspension chamber, and a sediment injection module.

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The size of MOVE! allows for more, heavy payload. When deployed MOVE! is

programmed to always travel upstream so that sediment clouds that result from moving (5cm/sec) are always drifting behind the vehicle before another station is reached. Navigation at the current stage is based on wheel-rotations. Coming developments will include ADCP-based bottom-track and maybe inertial navigation for better navigation and an acoustic data link for status reports, data upload and re-programming during a mission.

Above left: MOVE! being recovered after a test deployment. Above right: The MOVE! benthic chamber (left) and the 150 bag water sampling array (right).

MBARI has also developed a benthic rover with a focus on measuring benthic respiration. This system can also be configured for a variety of sensors and deployment scenarios. It can importantly not only be operated as an autonomous vehicle, but it can be operated by acoustic modem and via telecommunications cable. In a cabled arrangement the rover system can also be operated in real time. The system is rated to 6000 m depth and can be deployed for months at a time. In addition to respiration the system can collect line-transect photography and estimate fluorescence of photographed material.

Left: Illustration of key Benthic Rover components. Right: Benthic Rover deployed on the MARS observatory system. The cable reel at the back is used to connect the rover to the observatory network in real time. Images: © 2008 MBARI

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d. Marine Ecology The most fundamental question in marine ecology relates to the factors that control the distribution and abundance of marine life and the influence of anthropogenic change on marine communities. Ecology is by nature interdisciplinary and input from all of the above science areas and sensors will play some role in helping to determine dominant factors and observe and forecast possible ecological shifts. Deep biosphere: Monitoring of sub-seafloor environmental conditions will be important in understanding deep-biosphere ecology. The MEBOTECH and CORK systems described above will also be useful for monitoring deep biosphere environmental dynamics which are poorly known. As more specialized biological probes are developed they could be added to the existing SMART or CORK type systems described in the geosciences section. Time-lapse cameras: In addition to the basic time-lapse photography concepts introduced above, several variants are now available which can be adopted depending on the application. The degree of separation of camera and flash depends on water clarity. Other considerations include effective resolution, colour temperature of the illumination, and the distance the light will travel through the water which can influence the amount of potential red light returning to the camera. Another method of backscatter reduction uses polarized lighting and lenses to filter the image and create a 3-D effect (Schechner and Karpel, 2005). Structured light (SL) can also enhance image character by laying out a grid or other pattern of light on a scene and measuring distortions in the pattern because of scene topography. Stereo, still, or video SL imagery has already been used in the offshore industry (Negahdaripour and Firoozfam, 2006). Hybrids between scanning and snapshot imagery can capture effective imagery in a manner similar to that used in LIght Detection And Ranging (LIDAR) for terrestrial surface mapping.

Although film is generally no longer used, a variety of digital sensors are now commercially available including the Charged Coupled Devices (CCD) and Complementary Metal Oxide Semiconductor (CMOS). These are available in most retail cameras with the CCD systems preferred for lower image noise and light sensitivity, while CMOS systems often have higher resolution and image capture rates. Systems adapted for low light or high-speed applications are also available including those using Silicon Intensified Target (SIT) and Intensified CCD and CMOS sensors. Low light imaging systems have already been adapted for use in the KM3NET programme, where bioluminescent and neutrino particles are imaged in the deep sea (Priede et al., 2008).

It has been suggested that over long time periods these camera systems can impact their surroundings leading to biased results such as increases in abundance related to a reef effect and that this effect would decrease with increasing depth. It is difficult to independently verify these ideas with existing data. An initial investigation into potential biases related to reefing at a time-series site in the abyssal Northeast Pacific found that there was no observed trend associated with the length of deployment of a time-lapse camera system for periods of about four months (Vardaro et al., 2005). The DELOS programme will also be examining the impact of long-term infrastructure deployments on time-series photographic techniques. Line-scan imaging: A plankton imaging system has been developed that uses the same technology used in flat-bed scanning devices, which have an extraordinary resolution. A successful application of this method is the In situ Ichthyoplankton Imaging System (ISIIS, Cowen and Guigand, 2008). The ISIIS system is towed with a fixed line rate so that variations in ship speed can result in smearing. Efforts are underway, however, to set the line-scan rate relative to the movement of the sensor. Additionally, the system could be reconfigured to operate in a profiling mode or on and AUV. The remarkable resolution facilitates the identification of phytoplankton, zooplankton and ichthyplankton, and has great potential for fisheries research.

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Fig. 4. (from Cowen and Guigand, 2008). In situ invertebrate zooplankton. 0–40 m depth, Florida Current. Selected images of invertebrate plankton captured via ISIIS. Organisms are not scaled to each other in this composite image; sizes range from a few millimeters to several centimeters. A. Larvacean (Oikopleura sp.). B. Scyllarid lobster larva. C. Unidentified larval crustacean. D. Chaetognath. E. Copepod with eggs. F. Ctenophore. G. Ctenophore with feeding tentacles extended. H. Aggregate phase Thaliacean salp with reproductive buds. I. Ctenophore (Velamen sp.). J. Pterotracheid heteropod.

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Fig. 5. In situ cyanobacteria. Close-up of image of the cyanobacteria Tricodesmium. Although very small (ca. 2–4 mm), its unique shape renders this organism easily discerned by the system. Holographic imaging: Holographic imaging is a process whereby two light beams are scattered off an object and the scattered light creates a hologram image. These holograms are rendered using the scattered light and form an image where the dimensions and relative positions of the observed objects can be determined and the viewing position changed such that a 3-D rendering is made (a volumetric display). Several systems have been developed for ocean research including those to examine plankton particle dynamics (Hobson and Watson, 2002) and fisheries (Jericho et al., 2006, Garcia-Sucerquia et al., 2006). These systems can measure turbulence and the motions of small particles (Katz et al., 1999). The HoloCam system (Watson, 2003) has been designed for operation to 2000 m depth and is under commercial development by CDL Underwater Engineering of Aberdeen. A Digital Holographic Imaging (DHI) has also been developed with a resolution as low as 9 μm for deployment by AUV or similar system (Loomis et al., 2007). The progress in holographic imaging has produced extraordinary images of particles and plankton and their transition into the commercial domain is underway. Video: In cases were video is required several scenarios can be applied. Video can either be statically mounted or have pan and tilt capability with adequate network connections. Video can now readily be taken in standard format and high-definition (HD) format. Some applications, such as burst swimming performance measurements of fish, require high-speed cameras. Special care should be used in using video lighting because the lights can be a major factor in attracting animals to the observing system. Some systems, though, can use light intermittently such as the Sprint system (Bailey et al., 2007) and others can be used in low- or ultraviolet light, such as the Eye in the Sea (Raymond and Widder, 2007; Widder, 2007). Super-Hi Vision (SHV) is also now available with a resolution of 7680×4320 pixels (Takayanagi et al., 2005) but is not yet in use by the oceanographic research community. Planar lasers and thin light sheets: For more than a decade a suite of systems have been in development that use thin laser sheets to illuminate objects down to bacterial scales (Fuchs and Jaffe, 2002), while variants can detect fluorescence of imaged objects (Karaköylü et al., 2009). These systems have been designed with the idea of quantifying biological abundances at smaller scales including densities across a wide spectrum of size classes from zooplankton and phytoplankton to microbial scales. They can also detect either chl-a abundance in phytoplankton or in the guts of zooplankton. A planar laser imaging fluorescence (PLIF) system developed by Franks and Jaffe (2008) has been deployed in a profiling mode to determine micro-scale variability in fluorescence particles and has observed highly structured distributions of plankton at scales of meters and less. As far as we are aware, these systems have not been commercialised or adapted for deep-sea deployments, but they do show promise for observatory applications. Active acoustics: Several existing deep-sea observatory systems now use active acoustics to not only scan the water column for properties such as gas bubble flux, but also to examine the dynamics of zooplankton and fish. The systems can be deployed on the seafloor fixed in the water column or on profiling devices. Recent advances in scanning sonar and signal processing now allow for greater detail in acoustic imaging. With adequate ground truth verification these systems can be

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used to estimate, for example, fish and zooplankton abundances at ranges not achievable with photographic techniques. When used in approximate synchrony acoustic and photographic systems could prove to be a useful tool in estimating fish abundance. Active acoustics can take two main forms, those examining reflection intensities, such as a sonar, or those that use more sophisticated arrays to emulate video or photography (Jensson et al., 2009). These higher resolution systems include the Dual-frequency IDentification SONar (DIDSON) (Soundmetrics 2008), Echoscope II (CodaOctopus 2008), and BlueView (BlueView 2008). For example, the Echoscope II is capable of imaging a 3-D volume 50 x 50 degrees and 200 m range with cm scale accuracy at rates of 20 images per second. Many such systems now have the capability to resolve larger fish profiles, swimming speeds, and other parameters at distances greater than is possible for light imaging. With sufficient calibration many different groups of fauna from copepods to fish can be discerned in acoustic backscatter data (Griffith, 2002). The abundance, velocities, and trajectories of observed zooplankton can be determined (De Robertis, 2001; De Robertis et al., 2003) down to a size of 1-2 mm (Genin et al., 2005). The BIo-Optical Multiple-frequency Acoustic Platform and Physical Environmental Remote-sensing system (BIOMAPPER II) uses optics and multiple frequencies and has been used successfully to map plankton distributions (Wiebe et al., 2002). Science Fishery Systems Inc. has also developed a broadband system. BioSonics is another commercial vendor of echosounding equipment that has a range of products for fish and marine life, as well as seafloor mapping applications. Zooplankton sampling: Long-term sampling of zooplankton dynamics can also be achieved by direct sampling. Variations on the continuous plankton recorder (CPS) have been adapted for use in observatory frameworks. The rather mechanical nature of the samplers does suggest that careful consideration will be needed for reliability and endurance. These systems can also provide samples for identification and more detailed study of zooplankton community structure. Zooplankton dynamics is an area being addressed by the EuroSITES programme and updates on autonomous zooplankton sampling systems will soon be available. McLane Research Laboratories has introduced a commercial system that is rated to 5000 m depth and can collect up to 50 time-series samples (Morrison et al., 2000). This system was also designed to minimise the biases associated with escape responses. This system is being trialled in the EuroSITES programme. Flow imaging: Fluid imaging Technologies has now commercialised the FlowCAM system which can collect high-resolution images of plankton and particles (e.g. Reynolds et al., 2010). The system combines two channel fluorescence, a high-speed camera, and automated image analysis tools. With development proceeding for over a decade the system has now been adopted for use in moorings, profilers, and AUVs. The Flow Cytobot is a flow cytometer which has been adapted for observatory use (Olson and Sosik, 2007; Sosik and Olson, 2007). Both systems can provide indications of harmful algal blooms and quantify plankton and particle dynamics. Molecular probes: The synchronous development of molecular probes along with the technical development for in situ sampling has allowed for great advances in using molecular probes in an observatory setting. The Environmental Sample Processor (ESP), developed at MBARI, has been demonstrated to be capable of collecting probe-based information on harmful algal blooms, fisheries or other larvae, and sending the digitized molecular probe sampling results back to shore without the need to return the sample to a lab. Although this system is not yet commercially available it has strong potential for many applications to monitor biogeochemical and ecological quantities through time and space. Specifically it can run real-time DNA, RNA, and protein analysis, qPCR reactions, whole cell microscopy, Fluorescent In Situ Hybridisation (FISH), and phytotoxin quantification (Haywood et al., 2007; Jones et al., 2008; Mikulski et al., 2008; Preston et al., 2009). Another system developed at the University of South Florida, the Autonomous Microbial Genosensor (Fries et al. 2007) uses Nucleic Acid Sequence-Based Amplification (NASBA) to detect molecular targets using probes (Davey and Malek, 1989). The AMG system can report data

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back in minutes effectively providing real-time monitoring of events such as harmful algal blooms. Although the AMG sensor was developed primarily for coastal work it could be adapted for open-ocean and perhaps deep-sea deployment. Systems that can capitalize on the advancing molecular techniques like the Genome Sequencer™ FLX System 454 (Droege and Hill, 2008) are already making major advances microbial diversity research. Although basic themes and methods are emerging (Bowler et al., 2009; Delong et al., 2009), cooperation with MarBEF NoE and Marine Genomics Europe (MGE) NoE is needed to further explore the potential of these methods to make transformative advances in molecular and microbial observations. Systems that can further clarify metabolic rates and their underlying mechanisms will be particularly useful.

In-situ respiration: As marine life consumes oxygen under aerobic respiration, energy is utilized, with CO2 as a by-product. Respiration is therefore a fundamental measure of metabolism, food demand, ecosystem function, and carbon flow in marine systems. Several systems for studying respiration at and near the seafloor have now been developed to measure oxygen consumption in both pelagic settings and at the sediment-water interface. The choice of materials and experimental design can impact respiration system results because some plastics and metals can interact with oxygen concentrations in the chambers and thereby bias results. In a system where there is concern about the influence of materials on measurements, a control can be used in most cases. In some systems glass has been adopted as a possible alternative because of its inertness. The benthic rover systems described above are capable of conducting time-series respiration measurements in undisturbed seabed areas for months at a time. Some systems have also been adapted to run sophisticated enrichment and manipulation experiments to, for example, study the effect of increased food availability. Others have used planar optode systems that can capture two dimensional images of oxygen concentrations in the top few centimetres of the sediment in time-series measurements. A system is now in commercial development that measures oxygen and can photograph the subsurface conditions through time and relay the images through a network.

There are several systems based on the principal of eddy flux measurements, some of which look at scales on the order of less than a m2 (Berg et al., 2003, 2007), and more experimental approaches that address fluxes over larger areas by looking at differences in the gradients of oxygen concentration (e.g. McGillis et al., 2009). This gradient method is being used at the Arctic Hausgarten site. Detailed comparisons of benthic oxygen flux measurements can be found in Glud (2008) and Tengberg et al. (2005). Commercial solutions for a variety of these developing techniques are emerging, including small lander systems that offer flexibility in sampling design.

The Biogeochemical Observatory (BIGO) system is a lander-based biogeochemical flux measurement system that uses a compensation system so that fluxes such as oxygen can be obtained without actually drawing down chamber oxygen (Pfannkuche and Linke, 2003). The in situ Deep Oxygen Dynamic Auto-Sampler (Deep-IODA) is a system which is being deployed at several EuroSITES locations. These research deployments are helping to validate the IODA system, which was designed to supply near-real time water column respiration data through deep-sea observatory telemetry. The system has undergone several iterative developments and has achieved a year-long time series of respiration data.

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4. Instrumentation module architecture

In the last two decades significant experience has been gained in Europe in deep-sea monitoring systems (see appendix). However, the existing systems mostly concern autonomous observatories. The instrumentation modules need to be compatible with cabled and non-cabled networks, hence module architecture must be specified and designed with consideration to interoperability and standardization principles. ESONET has created a General Reference Model, a design framework that will provide a solution, independent of the network infrastructure, to be compatible with the proposed data management model for Esonet. It will also be open to future developments in terms of bandwidth and power management for integrating new state-of-the-art sensors and instruments. This ongoing design process includes several partners from ESONET, a working group is led by C. Waldmann in WP2. One of the most important challenges is to provide the same generic interface (hardware/software) for all the different infrastructures in ESONET: cabled observatories, autonomous observatories, buoy based observatories.

Above: Example of a possible Ethernet architecture based on smart services and direct connect

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Above: Example of smart board services

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Conceptual observatory designs: ESONET has conducted a design study which outlined three concepts including a cabled observatory (below), a simplified cable observatory, and a standalone design which uses satellite telemetry instead of a cable (ESONET Deliverable 5). Cabled observatory design concept

Legend: 3 - Technical supervision infrastructure 4 - Onshore network 5 - Land Base termination of sea infrastructure 6 - Land sea communication segment 7 - Node from branching unit to node/extension xx 8 - Branch extension of the network 9 - Junction box 10 - Link to instruments 11 - Individual instrument              

 Standalone Observatory design concept.

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The instrumentation module specifications may include:

• A scalable architecture based on Ethernet protocol providing a high bandwidth for the integration of new equipment and also low power consumption, allowing compatibility with cabled and non-cabled networks.

• Scientific instruments as defined in this document and compatible with long-term deployment (titanium housing, titanium connectors, sensor protection).

• A mechanical frame including cables equipped with dry mate and wet mate connectors as needed.

• A smart interface (one smart board for one instrument) between the network infrastructure and the scientific instrument can provide additional services to improve the compatibly and interoperability between cabled and non-cabled architecture.

Example of core services provided with a smart interface:

• Data recording system allowing local data storage (temporary storage in the case of a cabled network or permanent storage in all other cases)

• Ethernet protocol for time synchronization (NTP / PTP) on cabled network or with a time base (local clock) for non-cabled networks. This includes time synchronization NTP, PTP, PPS+NMEA and time stamping services

Peer-to-peer communication between instrument drivers, this allows for example:

• Mutual exclusion (synchronization) between acoustic instruments to avoid any perturbations or interference

• Mutual exclusion (synchronization) between local chlorination devices for instruments with optical sensor to avoid any perturbations between data sampling and the local chlorination

• Remotely upgradeable instrument driver through the network. This local driver allows for connecting plug-and-play devices by providing a generic software layer between the network and the instrument.

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5. References:

Auffret, Y., Pelleau, P., Klingelhoefer, P., Géli, L., Crozon, J., Lin, J.Y., Sibuet, J-C., 2007.

MicrOBS: A new generation of ocean bottom seismometer, Journal: First Break, ISSN: 0263-5046, Vol: 22.

Bailey, D.M., King, N.J., Priede, I.G. 2007. Cameras and carcasses: historical and current methods for using artificial food falls to study deep-water animals. Mar. Ecol. Prog. Ser.: 179-191.

Béjà, O., et al., 2000. Bacterial rhodopsin: evidence for a new type of phototrophy in the sea. Science 289, 1902–1906.

Berg, P., et al., 2003. Oxygen uptake by aquatic sediments measured with a novel non-invasive eddy-correlation technique. Mar. Ecol. Prog. Ser. 261: 75–83.

Berg, P, Røy, H., Wiberg, P.L., 2007. Eddy correlation flux measurements: The sediment surface area that contributes to the flux. Limnol. Oceanog. 52: 1672-1684.

Bett, B.J., Malzone, M.G., Narayanaswamy, B.E., Wigham, B.D., 2001. Temporal variability in phytodetritus and megabenthic activity at the seabed in the deep northeast Atlantic. Progress in Oceanography 50: 349-368.

Blandin, J., Rolin, J.F., 2005. An Array of Sensors for the Seabed Monitoring of Geohazards: A Versatile Solution for the Long-Term Real-Time Monitoring of Distributed Seabed Parameters. Sea Technology 46 (12), 33.

Boulart, C., Mowlem, M.C., Connelly, D.P., Dutasta, J.-P., German, C.R., 2008. A novel, low-cost, high performance dissolved methane sensor for aqueous environments. Opt. Express, 16 (17), 12607.

Brasseur, L., Tamburri, M., 2010. Sensor needs and readiness levels for ocean observing: An example from the Ocean Observatories Initiative (OOI), OceanObs’09 Conference Community White Paper, Venice Italy.

Camilli, R., Duryea, A., Buchner, J., Whelan, J.K., “TETHYS: an in-situ mass spectrometer for Cabled Observatories” accepted to the Fourth International Workshop on Scientific Use of Submarine Cables and Related Technologies. Dublin, Ireland February 2006.

Campell, L., et al., 2010.First harmful Dinophysis (Dinophycae, Dinophysiales) bloom in the U.S. is revealed by automated imaging flow cytometry. J. Phycol. 46, 66–75.

Chin, C.S., Johnson, K.S., Coale, K.H., 1992. Marine Chemistry, 37 65-82. Davis, E. E., Becker, K., 2007. On the Fidelity of “CORK” Borehole Hydrologic Observatory

Pressure Records, Scientific Drilling, doi:10.22 04/iodp.sd.5.09.2007, 5, 54-59. DeGrandpre, M.D., Beck, J., Dickson, A., 2010. An autonomous indicator-based pH sensor for

oceanographic research and monitoring. Annual rept. 1 Dec 2009-30 Nov 2010. DeGrandpre, M.D., Hammar, T.R., Smith, S.P., Sayles, F.L., 1995. In situ measurements of

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6. APPENDIX

a. Demonstration Mission sensors.

Sensor Objective Owner Institution

Validated for deep sea

Validated for long term deployment

Autonomous Connected to the SEAMON node Site Studies

Short-period OBS

To record local seismicity, determine hypocenter locations and to model magnitude and focal mechanisms

IPGP & FFCUL/CGU yes A

C

NKE T sensors

To study (associated with seismicity) the permeability distribution (associated with seismicity) and characterize faunal microhabitats. Local oceanography

Purchased by Ifremer and IPGP

yes yes A SS

Pressure gauge, tilt, GPS

To measure deformation of the seafloor IPGP yes A, C & SS

Ocean Bottom Motion Meter

To measure deformation of the seafloor Bremen U. yes C

In situ Fe and Mn analyser

Chemical evolution of hydrothermal fluids NOCS yes C

In situ ΣdFe analyser

Chemical evolution of hydrothermal microhabitats Ifremer yes yes C

Aanderaa O2 optode

Chemical evolution of hydrothermal microhabitats

Purchased by Ifremer C

Methane sensor

to measure dissolved methane concentrations in the water column and near the seabed

NOCS no

CTD/ADCP Local hydrodynamics MARUM yes C Video imagery

Temporal dynamics of a mussel assemblage Ifremer yes

yes C

Fibre Bragg Grating T

Fine-scale variations of fluid temperature NOCS no SS

In situ ΣS Chemical evolution of hydrothermal microhabitats Ifremer yes

yes SS

Flow meter Fluid flow measurements Ifremer yes SS

Fluid sampling

Geochemical characterisation of the fluid

Ifremer NOCS OMP/LMTG

yes SS

List of sensors to be deployed during the MOMAR-D Demonstration Mission. Sensors will be either connected to a Sea Monitoring Node (C), autonomous (A) or used to acquire complementary data during the “site studies” part of the demonstration mission (SS).

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Sensor Objective Owner Institution

Validated for deep sea

Validated for long term deployment

Short-period OBS (> 1 Hz)

to detect mud and fluid eruptions, and its precursory phenomena, like mud movement at depth

UiT yes

Piezometers lance to record sediment pore pressure dynamics over the top 15 m

Ifremer yes yes

Sub-surface Thermistors

A string of 24 temperature sensors to measure seafloor and surface temperature every 15 minutes. A string of 12 thermistors on a 1m lance to measure a vertical T-profile in the sediment every 15 minutes

purchased by IFM-Geomar yes yes

Pressure sensor Measures tidal pressure changes

purchased by IFM-Geomar yes yes

Oxygen, pH and OPR sensors

to measure oxygen, pH and OPR at 6 positions at the sediment surface

MPI yes yes

Acoustic gas bubble detector and ADCP

Acoustic sensors to record the plume, and ADCP that scans vertically over a distance of 100 m, and a scanning sonar that can scan over a horizontal distance of ca 50 m

MARUM yes yes

Digital lapse camera

To capture eruptive events of the Hakon Moxby Mud Volcano

Ifremer yes yes

CTD To measure temperature, conductivity, pressure AWI yes yes

CTD To measure temperature, conductivity, turbidity, DO MPI yes yes

List of sensors to be deployed in the LOOME Demonstration Mission.

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Sensor Objective Owner Institution

Validated for deep sea

Validated for long term deployment

10 Short-period OBS (> 1 Hz)

to record micro-seismicity, particularly in response to fluid transfers

Ifremer yes Not yet, but should be compatible

1 BB-OBS (Guralp CGM-3 on SN-4)

to record micro-seismicity and fault dynamics parameters near the 1999 earthquake rupture tip

INGV yes

Piezometers

lance to measure sediment pore pressure dynamics over the top 15 m : record the response to earthquakes and monitor pressure variations during the seismic cycle

Ifremer yes yes

Oxygen, CTD and three axis, single-point current meter

Sensors installed on SN-4 Observatory to be deployed near the 1999 earthquake rupture tip

INGV yes

Flowmeter to measure pore water outflow through the sediment surface

Scripps Inst. of Oceanog. yes

Methane sensor on SN-4

to measure dissolved methane concentrations in the water column near the seabed near the 1999 earthquake rupture tip

INGV

No. Development and testing efforts needed before long-term deployment

No. Development and testing efforts needed before long-term deployment

Acoustic gas bubble detector

Acoustic transducer to record the plume ovder a scanning distance of ~ 200 m

Ifremer yes

List of sensors deployed in the Marmara Demonstration Mission.

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Sensor Objective Owner Institution

Validated for deep sea

Validated for long term deployment

Array of high perfomance hydrophones with high frequency sampling (192 Hz)

to characterize noise induced by marine mammals (Bioacoustics) and anthropogenic noise

UPC yes

1 BB-OBS (Guralp CGM-3 on SN-4)

to monitor seismicity (geo-hazards) INGV yes

Seabottom pressure sensor (“tsunamimeter”)

Tsunami Early Warning System INGV yes

Oxygen, CTD and three axis, single-point current meter

Sensors installed on SN-4 Observatory to characterize the physical oceanography at fixed point

INGV yes

List of sensors deployed in the LIDO Demonstration Mission.

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Measured parameters Depth

rating Sampling/storage/acquisition frequency

Current Speed 2000 Once per hour, 1 year of data

Current direction 2000 Once per hour, 1 year of data

Oxygen 2000 Every 30 seconds 1 year of data storage

Turbidity 2000 Every 30 seconds 1 year of data storage

Salinity 2000 Every 30 seconds 1 year of data storage

Depth 2000 Once per hour 1 year of data storage

Seawater temperature 2000 Every 2 minutes 1 year of data storage

Sediment temperature 2000 Every 15 minutes at every 10cm depth through 90 cm

Fluid flow flux 2000 Every 2 minutes 1 year of data storage

Seismicity 2000 Continuous recording with 1 year of data storage

List of sensors deployed in the AOEM Demonstration Mission.

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Measured parameters

Depth range of measurements (water dpeth4850 m)

Sampling/storage/acquisition frequency

Comments

Temperature Several depths, water column & lander

Real time: 24hours; Delayed mode: 60 minutes

Salinity (Conductivity)

Several depths, water column & lander

Real time: 24hours; Delayed mode: 60 minutes

Pressure Several depths, water column & lander

Real time: 24hours; Delayed mode: 60 minutes

Oxygen Several depths, water column & lander

Real time: 24hours; Delayed mode: 120 minutes

Real time data available via EuroSITES website – input to GEOSS via Coriolis data centre

Ocean currents Near seafloor Real time: 24hours; Delayed mode: 120 minutes

In part real time (lander)

Turbidity 40m & Sea floor Real time: 24hours; Delayed mode: 120 minutes

Nitrate 40m

delayed mode: 120 minutes

pCO2 40m delayed mode: 120 minutes Fluorescence 40 m

delayed mode: 120 minutes

Real time data available via EuroSITES website – input to GEOSS via Coriolis data centre

Passive acoustic

Positioned at sea floor, sources unknown

240h available, sampling may be synchronized with photo

Delayed mode only

Seismicity Sub-sea floor 450 Hz – delayed mode only Delayed mode only Photography Sea floor Not deployed due to technical

problems with the camera List of Sensors deployed in the MODOO Demonstration Mission.

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b. California Cooperative Oceanic Fisheries Investigations (CalCOFI) variables:

The period of observation varies: temperature, salinity, oxygen, zoo/ichthyoplankton in 1949, other CalCOFI measurements ~1984; CCE LTER, marine mammals: 2004; acoustics: 2009. Data methods and access to the data are available at: www.calcofi.org. (quoted from Koslow et al., 2009). Variable Investigator/Programme Method Temperature, salinity, Chl a fluorescence

CalCOFI CTD, fluorometer

Irradiance (in situ profiles & daily PAR)

CalCOFI PAR

Light transmission @ 660 nm CalCOFI Transmissometer Oxygen CalCOFI Auto-Winckler, CTD Nutrients (N, P, Si) CalCOFI Auto analyzer, CTD Primary production CalCOFI C-14 uptake Chl a extracted CalCOFI Fluorometer Sea surface pCO2 CalCOFI IR absorbance Zooplankton, ichythyoplankton CalCOFI Bongo net tows Particulate C&N CCE LTER Dry combustion Dissolved organics (DOC, DON) CCE LTER Combustion Upper ocean currents Chereskin/CCE LTER ADCP Taxon-specific pigments CCE LTER HPLC Bacteria & picoautotrophs CCE LTER Flow cytometry Nano- & microplankton CCE LTER Microscopy, FlowCAM Mesozooplankton, optical size classes

CCE LTER OPC, LOPC

Mesozooplankton, sentinel species CCE LTER Microscopy, ZOOSCAN Acoustics: krill, micronekton, pelagics

Koslow Multi-frequency EK-60

Seabirds Pt Reyes Bird Observatory/Farallon Institute

Observer

Marine mammals Hildebrand Observers, passive acoustics

c. US Ocean Observing Initiative (OOI) Coastal and Global Scale Nodes (CGSN)

 

OOI - CGSN Core Sensors (Heidi Sosik (hsosik@whoi.edu) Derived from community input (RFIs), Advisory Groups (iOSC, BRP), Trace Matrices and SUR (x) = number of sensor types in measurement class Air-sea fluxes (3) Turbulent velocity (1) Nitrate (1) CO2 flux (2) Dissolved oxygen (2) Nutrient 4 Chan (1) Surface waves (1) pH (1) ZP sonar (2) Temp/cond/press (5) PAR (1) Digital still camera (1) Pressure (1)

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Spectral irradiance (1) Hydrophone (1) Mean currents (point and profile) (5) Optical attenuation and absorption (1) Chl-a, CDOM, and turbidity (2)

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d. IOOS core variables: Two tables indicating the IOOS core variables and the associated socioeconomic impact areas (from Malone 2007).

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e. NEPTUNE Canada: Autonomous systems used in NEPTUNE Canada (including Landers and benthic): Neptune Observatory Vertical Profiler •CTD •Oxygen sensor •Fluorometer •Transmissometer •Nitrate sensor •CO2 sensor •Upwelling/downwelling radiometer •Broadband hydrophone •ADCP •Bottom pressure sensor

Neptune Benthic System •Acoustic Doppler Profiler •Rotary SONAR •Multi-Beam SONAR •CTD •Microbial package •Sediment trap •Plankton pump •Fluorometer •Hydrophone •Video cameras •High resolution still camera

Above: Example of generic module implementation for a cabled network (courtesy of NEPTUNE Canada). This frame includes a junction box and several generic scientific instruments (left side) such as ADCP, hydrophone, echo sounder, bottom pressure recorder and a specific science module called Tempo-Mini from Ifremer (white frame on the right side).

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f. Seacycler: This system is being developed by several academic institutions and ODIM Brooke Ocean and is likely to be a system that is particularly well suited for areas were both cables and surface buoys prove challenging to implement. A specialised version of the system, ICYCLER, is now being developed for use under ice.

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g. POSEIDON-II seabed platform and PYLOS Buoy

The involvement of HCMR in the WP3 is related to the operation of the stand alone observatory of Poseidon multi-parametric buoy system (water column and seabed platform) that has been deployed on the SE Ionian margin (Pylos site, 1680 m depth). This is the first step of implementation of the Hellenic node towards a future cabled observatory through the EMSO and KM3NeT ESFRI projects. Within the frame of this system, a couple of specific activities are directly related to observatory design. The key scientific objectives of the Hellenic node will be to monitor geohazards, geological dynamics, including seismic activity and sediment processes, benthic ecosystems, long-term seabed water mass characteristics for potential climate change, sea water level trends, ambient noise, tsunami detection and early warning. The scientific significance of the greater area of the western sector of the Hellenic Arc is evidenced by numerous EU and international projects within the FP6-FP7 frameworks (such as HERMES-IP, HERMIONE, EUROCEANS-NoE, CoralFish, SESAME-IP [marine ecosystem research], 3-HAZ, SEHELLARC, TRANSFER (geohazards) and EuroSITES, Poseidon I, II) that are implemented in the region , regarding water column monitoring, sea-state forecast and modelling.

The standalone observatory has been in operation since 2007 and includes a deep mooring equipped with a surface buoy and sensors in the upper 1000 m, as well as an autonomous seabed platform that delivers data in near real time using underwater acoustic technology (POSEIDON-Pylos). The system was developed in the framework of the POSEIDON-II project. The seabed component of the observatory (the autonomous platform) operated in a pilot phase between November 2008 and March 2009 and has been on a regular operational basis since early December 2009.

Further module background: The platform is equipped with a high accuracy pressure sensor (Paroscientific 43K-101) for tsunami detection, as part of a future early warning system. The sampling rate for the pressure sensor is set to 15 s but in standard mode data are transferred to the surface buoy every 3 hours. In case a tsunami event is detected, the platform switches into an alarm mode sending data every 15 s to the surface buoy and from there to the operational centre. The software for on-board pressure data analysis and tsunami detection is based on the NOAA DART system of the Pacific Ocean (http://nctr.pmel.noaa.gov/tda_documentation.html). The platform is also equipped with a CTD probe (SBE 16) and a dissolved oxygen sensor (Aanderaa Optode) for near-bed physical hydrological measurements. The sampling interval for these sensors is set to 1 hour. These near-bed measurements allow monitoring of the evolution of deep-water properties (EMDW of either Adriatic of Aegean origin) and detection of possible dense water formation events in the adjacent seas. The communication with the surface buoy is done through an acoustic modem (Benthos SR-100) at a baud rate of 1200 bps. The battery pack of the central system (pressure sensors, processing and data transmission units) allows an autonomous operation of the platform for approximately 12 months, while the peripheral sensors (CTD, DO) have their own energy system of higher autonomy.

The autonomous seabed platform was deployed in November 2008 for a pilot-operation period of 4 months. During this period erroneous pressure data led to false Tsunami alarms and events of unsuccessful communication between the platform and the surface buoy. The seabed platform was recovered in March 2009 in order to undergo laboratory tests and to resolve these problems, as well as to make software upgrades (new bios etc). Following communication with the pressure sensor manufacturer, the erroneous data have been attributed to gas bubbles trapped in the instrument (in the tubing between the pressure port in the lid to the pressure transducer). The problem was solved by gas removal before the next deployment and this has become a standard maintenance procedure. The communication problems between the platform and the buoy were attributed to shadowing effects of seabed anomalies. A more appropriate area with smaller topographic anomalies was selected during the next deployment of the seabed platform, which took

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place in early December 2009. The operation of the platform after the second deployment indicates that the problems were indeed solved.

Another effort has been devoted to ambient noise monitoring (including acoustic monitoring for mammals). The mooring line is hosting a Passive Aquatic Listener (PAL) at 500 m depth for rainfall and wind estimates, as well as for marine mammal detection. A PAL consists of a broadband, low noise omni-directional (zenith angle) hydrophone (Hi-Tech-92WB), a signal processing board, a low-power microprocessor (Tattletale-8) with a 100 kHz A/D digitizer, a 2 GB memory card and a 60 Amp-hour battery pack. The sampling strategy can be designed to allow autonomous operations for up to one year. These systems have been recently evaluated against X-band radar measurements in this area and were found to provide realistic estimates of precipitation.

h. GEOSTAR (and derived single-frame observatories) : The original GEOSTAR (GEophysical and Oceanographic STation for Abyssal Research) system, developed within the EC projects GEOSTAR (1995-1998) and GEOSTAR-2 (1999-2001), was designed as a standalone autonomous modular seafloor observatory based on three main sub-systems: a) the Bottom Station (BS), which is the frame equipped with sensors, power and communication systems (the underwater sub-system); b) the Communication Systems (CS); c) MODUS (MObile Docker for Underwater Sciences). It operates like a simplified ROV and was specifically designed to handle the BS from the sea surface along with other heavy loads during deployment/recovery operations. GEOSTAR is capable of long-term (over 1.5 years) multidisciplinary monitoring at abyssal depth. At present, the maximum operative depth is 4,000 m.

Above: The original GEOSTAR conceptual scheme. The BS is managed by the dedicated vehicle Mobile Docker (called MODUS) and communicates through a) data capsules (Messengers); b) an underwater acoustic link with: i) a ship of opportunity, or ii) a surface buoy and then through radio/satellite links to the shore

The BS has a four-leg marine aluminium frame, hosts a wide range of sensors, able to collect multidisciplinary data on the same spot. It also contains the battery pack (primary lithium), electronics mounted inside titanium vessels, hard disks for data storage and the underwater part of the communication systems.

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Above: GEOSTAR system (left): BS (bottom) and MODUS (top); surface relay buoy for the communications (right). The BS operation is driven and controlled by a central Data Acquisition and Control System unit (DACS) to allow the management of a complete scientific mission with the acquisition of a wide set of data streams and the tagging of each measure according to a unique reference time provided by a central high-precision clock. The BS was designed by Tecnomare S.p.A. (ENI Group) following the scientific requirements specified by INGV. The solutions adopted have been based on the offshore engineering practice deriving from oil and gas industry know-how and experience.

Accurate and safe positioning on the seafloor, re-entry and recovery capabilities of the BS are ensured by the dedicated cable suspended module MODUS, developed and built at the Technische Universität Berlin (TUB) and the Technische FachHochschule Berlin (TFH). MODUS is a sub-sea intervention shuttle operating in deep seas, while it is connected to a surface vessel with a umbilical cable, which provides power, bi-directional data-transfer via F/O telemetry and carries the system load during operation. MODUS is conceived to be driven by a ship-board operator and can be moved as needed by means of thrusters during BS deployments/recoveries. For deep-sea missions the MODUS has been enhanced with a transponder and an altimeter to check its location from the sea surface. It is also equipped with pitch and roll sensors for positioning in space, sonar to identify the BS location during recovery, and video cameras and lighting for visual seabed inspection, and different sensors for load and acceleration. MODUS has a unique latch/release device for remote coupling or decoupling of the BS, which is equipped with a correspondent docking pin, and it is able to carry up to 30 kN at abyssal depth. MODUS can manage different seafloor platforms if mechanically compatible with the vehicle.

The complete system comprises a winch with an electro-optical-mechanical cable, the vehicle and an integrated control unit on the ship. Also needed are a support ship of medium size with dGPS and dynamic positioning (dP), an appropriate A-Frame for the envisaged loads and dimensions, and available space on board.

Two independent Communication Systems (CS) based on different principles were originally developed for GEOSTAR,. The first one consists of buoyant data capsules named Messengers (MES) and releasable upon surface command or automatically, when filled with data or

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in case of emergency. Two types of MES are available: a) expendable (data storage capacity 64 Kbytes); b) storage (data storage capacity larger than the expandable, 40 Mbytes). The capsules can transmit their position at the sea surface and small quantities of data via ARGOS satellites. The second CS is based on a bi-directional vertical acoustic link to a ship of opportunity or a moored buoy. A surface relay buoy, equipped with a telemetry unit and radio/satellite transmitters, assures the near real-time communication between a shore station and the observatory on the seafloor. The most recent communication link implemented on the GEOSTAR-class observatories is the cabling through interfaces between benthic platforms and electro-optical cables.

Two paths were followed after the GEOSTAR experience: the development of other single-frame observatories devoted to specific applications and the enhancement of GEOSTAR as a principal node of a network of seafloor observatories. These paths have led to the availability of other five GEOSTAR-class observatories and to a European prototype of a deep seafloor observatory network.

The SN1 (Submarine Network 1) observatory has taken seismological, oceanographic and environmental measurements, and was initially developed between 2000 and 2002 as part of an Italian project. In 2005, it became part of the cabled underwater infrastructure presently operating off Eastern Sicily (NEMO-SN1 seafloor multidisciplinary observatory). The GMM (Gas Monitoring Module), built as part of the EC ASSEM project (2002-2004), is devoted to seafloor gas monitoring. Another single-frame system, MABEL (Multidisciplinary Antarctic BEnthic Laboratory, now SN2), was developed for polar sea applications within the framework of the Italian PNRA (National Programme for Antarctic Research).

GEOSTAR, within the framework of the EC ORION-GEOSTAR-3 project (2002-2005), was implemented to be the main node of an underwater network of deep-sea observatories of GEOSTAR-class with the capability of near real-time communication. The conceptual scheme is shown in Figure 6C. In addition to this main node, two more observatories, with the function of satellite nodes (SN3 and SN4), were built and equipped with geophysical and oceanographic sensors.

Many experiments have been performed in the last 11 years. The first was a shallow-water demonstration mission in the Adriatic Sea (1998) and the last one in the Weddell Sea, Antarctica, concluded with the recovery of SN2 (MABEL) from 1874 m depth using the R/V Polarstern (December 2008). The depths of the experiments ranged from 40 to 3320 m and the geographical areas extended over the Mediterranean Sea, the Atlantic Ocean, and the Southern Ocean (Weddell Sea). These experiments also included the cabling of SN1 off-shore Eastern Sicily (2060 m depth) in January 2005 as part of the underwater infrastructure NEMO-SN1. SN1 is the first real-time seafloor observatory in Europe and one of only a few in the world. It is also the first operative seafloor observatory in one of the sites planned in ESONET and EMSO. At the end of April 2008, SN1 was recovered after 3 years and 3 months of operations.

Many different instruments were used in the experiments described above, and, being the platforms modular, they can host different sets of sensors depending on the scientific goals. All the instruments have a unique time reference given by the use of a single high-precision clock and therefore datasets can be easily compared. Particular attention was paid to the deployment of the seismometer, the de-coupling of its housing from the frame and coupling the instrument to the seabed. The high quality of the acquired data has definitively validated this procedure.

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List of sensors and typical sampling rates (from Favali et al. 2006, in press)

Sensor Typical sampling rates 3-C broad-band seismometer 100 Hz hydrophone (geophysics) 100 ÷ 200 Hz hydrophone (bio-acoustics) 96 kHz gravity meter (1) 0.1 ÷ 1 Hz scalar magnetometer 1 sample/min 3-C fluxgate magnetometer (2) 1 sample/s absolute pressure gauge 1 ÷ 15 s differential pressure gauge 1 ÷ 15 s precision tilt meter (X, Y) 10 Hz 3-C single-point current meter 2 ÷ 20 Hz ADCP (300 kHz) 1 profile/hour transmissometer 1 sample/hour turbidity meter 1 sample/hour CTD 1 sample/10 min (or /hour) nuclear spectrometer 1sample/4, 6, 8 hours (stand-alone)

1 sample/30 s (real-time) CH4 sensor 1 Hz H2S sensor 1 sample/10 min O2 sensor 1 sample/10 min (or /hour) chemical analyser (pH/eH) (3) 1 sample/6 hours water sampler (off-line) 1 sample/500 s ÷ 1week (48 bottles)

(1) prototype developed by IFSI-INAF, see reference [18] (2) prototype developed by INGV, see reference [8] (3) prototype developed by INGV and Tecnomare S.p.A., see references [11] and [15]

i. SEAMON: The SEAMON system was developed by Ifremer through the European projects ASSEM and EXOCET, and the French projects ROSE and FRAME. It’s a modular system that was designed as a light (less than 250 kg) and deep (4000 m depth rated) platform. An observatory could be constituted of one or many nodes. Each node can be autonomous, autonomous and acoustically linked, or cabled. Generalized by SEAMON, ASSEM developed a new concept of seafloor observatories dedicated to long-term monitoring of seabed parameters concerning an area of several km2 and based on a cost effective and light platform where sensors could be installed in a standardized way and would be able to share a common data and communication infrastructure. Deployment and recovery of a node was thought to be accomplished using available ROV or submersible facilities.

The design of ASSEM as described below was made as modular as possible with standard connecting and easy installation interfaces, thereby allowing for the adaptation of the system to the site of interest by adding new sensors and replace components for maintenance. In this sense, SEAMON is understood as an array of nodes that can be deployed almost independently, and which interact in such a way that they can cooperate in a large monitoring multi-parameter operation. A two-way communication link between sensors and between the sensors and the shore is built on either an acoustic network or wired links to allow for a large diversity of connection devices, whether local (e.g. ROV) or remote. Local raw data storage in each node with local analysis

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resources able to generate alarms is also a key element, adding reliability and redundancy, which is critical for warning systems.

SEAMON is designed as an array composed of several nodes. Each node includes an electronic unit, named COSTOF (for COmmunication and STOrage Front-end), through which all available sensors (pore pressure, methane, geodesy, tiltmeter, CTD, turbidity, currents, oxygen, transmissometer, fluorimeter, etc.) can communicate. The architecture is organized around an internal CAN/CAN open bus, hosting sensors, communication and data storage resources on a common transmission backbone. All the modules connected to the bus include the same "kernel" card and a specific extension card. Each "kernel" card includes an Atmega129L processor, a 512Kbytes flash memory, a real time clock and 2 RS232 links.

Above: Basic design of the SEAMON node. The software resources needed to enable a monitoring node to act as a network node (routing algorithms throughout the network, network configuration management, data transmission protocol and other network layers) are implemented in every COSTOF unit. Warnings can be generated for example if a critical parameter, or a group of parameters, comes above a preset threshold for a given time length. This distributed architecture allows configuration of a monitoring node very easily and adds new functions without modifying the existing ones.

The same modularity concept is applied to the mechanical design. The usual deployment and maintenance procedures require the use of a submersible or an ROV, but free-fall launching is possible if needed. The design includes some innovations:

• A low cost underwater connection system is used to replace in situ power packs, to install or

to replace sensors and to establish cabled links between nodes if needed; Ifremer is transferring the system to an industrial company.

• A contact-less serial interface (CLSI) allows node testing before launching and during maintenance operations;

• The acoustic array with bell-shape protection is mounted at the top of a flexible mast to provide protection against trawlers.

• Lithium cells are used as power source. Two voltage, 12V and 24V, are available on the platform with a capacity up to 12kWh. Power packs are replaceable by ROV or submersible. Protection devices against trawling are used and the acoustic transmitter is installed on a special flexible arm. Power packs are installed, even if the node is cabled to assume operations, and even in case of failure of the remote power source. However, the capacity is reduced.

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Above: ASSEM node is deployed on the bottom. An ROV (or submersible) is required for recovery. A variety of sensors can be installed on a SEAMON node. The configuration is chosen depending of the scientific objectives. Usually, temperature sensors are implemented on all nodes. At least, one node is equipped with a current meter. Turbidity sensor, current profiler, chemical analyser, stills camera, transmissometer, and fluorometer have already been implemented. Pore pressure is an important parameter for the modifications of the soil before and during any geo-hazard event. It is possible to measure pore pressure at several levels, in boreholes down to 200 m and in tubes of CPT probes inserted in the sediment layer down to 30 m. The natural occurrence and emission of gas on the seafloor (methane seeps) are increasingly recognized as an important marine process for its environmental and geo-hazard implications. A methane sensor from CAPSUM was adapted for long-term deployment and an acoustic detector of methane bubbles has been developed (BOB). The Following sensors are already connected to SEAMON:

• Seabird Micro Cat CTD SBE 37-SM and SI; • Teledyne RDI 300kHz ADCP • Aanderra Current Meter 3820R • Aanderaa Oxygen Sensor 3975: • Wet labs C star Transmissometer • Wet labs combination (ECO FLNTU) • TRIOS enviroFlu-HC • NKE piezometer 2 • Stills camera • Chemical analyser.

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j. TEMPO: A specific instrument has been designed to monitor faunal assemblages at hydrothermal vents (Sarrazin et al., 2007). TEMPO is an autonomous long-term imaging module equipped with a deep-sea video camera, a digital video recorder, adequate lighting and sufficient energy storage. The system is able to pilot the projectors and to record digital video sequences on a hard disk. Biofouling protection was set on the camera port hole and on the lights. TEMPO was tested and deployed during the Momareto cruise (2006) and recovered in 2008. Experience was gained during several cruises that repeatedly explored and monitored the Momar site by using the TEMPO module (Sarrazin et al., 2007), which includes a long-term deep-sea video camera with two LED lights and efficient biofouling protection. The TEMPO module was successfully deployed near hydrothermal vents on the mid-ocean ridge in order to monitor environmental changes along with community dynamics. The whole system is powered by a SEAMON node (Blandin et al., 2005). TEMPO-Mini:

Fig. 1: Above: TEMPO-Mini during the test deployment in Saanich Inlet, Canada

Above: A view of the scene in Saanich Inlet, BC, Canada TEMPO-Mini is a new custom-designed instrument package created by IFREMER for real-time monitoring of hydrothermal faunal assemblages (J. Sarrazin et al. 2007).

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TEMPO-Mini integrates a 2 megapixel streaming video camera with embedded event detection, 4 LED lights, an oxygen sensor, and a 10m-long, 10-sensor temperature probe. An efficient and innovative biofouling protection system is set on the camera porthole, on the lights, and on the optical oxygen sensor (Delauney and Compère, 2008). IFREMER is collaborating with the NEPTUNE Canada and VENUS Canada networks to acquire live data from the seafloor of the Saanich Inlet near Sidney, BC, Canada. VENUS has provided the cabled network and node connections for an instrument platform including TEMPO-Mini, which was tested, deployed and connected in late September 2008. NEPTUNE has provided a junction box to allow the connection to the VENUS network. After this test in shallow water TEMPO-Mini was recovered in February 2009.

In summer 2009, TEMPO-Mini and CHEMINI Fe, a new generation of analysers for in situ sea-water chemical parameters, descended to 2300 m, where the camera, lights, sensors and probes helped scientists observe deep-sea ecosystems at the main hydrothermal vent field on the Endeavour Ridge in the Northeast Pacific. In addition to the Fe in situ analyser, three temperature probes (NKE) were coupled to the imagery module to monitor environmental changes along with community dynamics. The whole system is powered by a Sea-Monitoring Node (SEAMON, Blandin and Rolin, 2005).

k. DELOS: The Deep-ocean Environmental Long-term Observatory System (DELOS) was developed by the University of Aberdeen (Oceanlab) for BP, in collaboration with a number of international research organizations. It is a fully autonomous system. A cabled system could be derived from the first system that could provide interactive experimental control and real-time data feedback. The DELOS system, as it was initially specified for deployment in Block 18 off the Angolan Coast, comprises two environmental monitoring platforms: one in the far field (5 miles from the seafloor infrastructure), and one within 50 m of a seafloor well. Each platform has a seafloor docking station that is deployed on the seafloor at the start of the monitoring programme and remains there. There are a number of observatory modules that are designed to perform specific environmental monitoring functions with one of each module available to each platform, as field conditions allow. DELOS was installed in February 2009.

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Above: The DELOS observatory platform (left) with an empty seafloor docking station and a seafloor docking station with five observatory science modules and one guest module area in the centre (right).

Once deployed each observatory module will have enough battery and storage capacity to operate autonomously for at least 6 months. Towards the end of the 6 months deployment period each platform requires ROV intervention to recover the observatory modules to the surface for service, calibration and data offload. During this service period (days) no monitoring at the seafloor will be possible. However, the scientific steering committee concluded that this interruption to continuous monitoring would not significantly compromise the overall scientific objectives.

The seafloor docking station is designed to be deployed at the start of the programme and to be left on the seafloor for approximately 25 years. It consists of a robust triangular glass fibre construction designed to withstand long-term seafloor deployment and periodic service by industrial ROV. Glass fibre is used to eliminate any corrosion effects which may affect biological processes on the seafloor. A major research project was conducted to determine the long-term effects of deep-water immersion of glass fibre and the final design encompasses this research.

To minimise disturbance to seafloor animals from current eddying effects due to the seafloor structure, the docking station is raised off the seafloor on legs with minimal hydrodynamic impact. This design enables the observatory modules to sample both the water column above the docking station as well as seafloor processes below it. Each observatory module contains a suite of related instrumentation housed within either a standard size or an over-sized observatory module frame. To facilitate data offload and instrument programming a controller is also situated within each frame. The controller deals with different interface issues for each of the sensors and instruments enabling a standard programming interface to be presented to the user. It also acts as a backup, storing a copy dataset in a central location.

The observatory modules are designed to be placed onto the seafloor docking module by ROV. Each module will have sufficient battery and memory capacity to operate for a maximum of 6 months. All instrumentation, including batteries and memory, were confined within the dimensions of the standard observatory module frame design. During transportation or storage the frame protects the instruments within and also allows easy access for crane or forklift operations. Modules include an oceanographic module with a suite of standard oceanographic instruments that are used to record local environmental parameters as a background for the remaining experiments on the platform. Each instrument is controlled by a central controller that takes readings against a pre-programmed schedule of benthic currents and ADCP, turbidity, fluorescence, pressure, conductivity, and dissolved oxygen. A camera module with a close view and wide view camera system are controlled by a single controller. Pictures are taken at intervals specified in the program loaded into the controller prior to deployment and an acoustic module with a passive and active sonar system is controlled from a central controller to schedule module activity at pre-programmed intervals as well as a passive acoustic system. Furthermore, there is a sediment trap module with a single sediment trap and winch system to raise the sediment trap 100 m above the sea floor.

DELOS system is a low power system. Every 6 months the observatory modules must be removed from the seafloor platform by ROV and returned to the surface for data offload, instrument calibration, and battery charging. When cabled, higher sampling rates will probably be used increasing the consumption, but the peak current required will remain the same at a low 1kW per module. The camera and the acoustic modules are the more energy-consuming units.

It should be noted that the DELOS system deployed in Angola followed a set of design requirements that were substantially different from most marine observatory systems. These requirements included the need for a 25-year lifetime of the frame, the ability to park an ROV on the frame, and the deployment, placement, and recovery of the system using hydrocarbon industry facilities. Furthermore, each module was fully independent, in part to prevent system-wide faults. Future DELOS systems are expected to have variable requirements, especially when there is an

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opportunity for the systems to be designed into the hydrocarbon production infrastructure in advance, allowing data and power transmission in real time.

Above: Photo from the DELOS system. Such photos can be used to determine abundance and biodiversity, as well as seabed sediment dynamics and coverage of phytodetritus and POC.

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7. Annex I – Ocean Acoustic Observatories report

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Ocean Acoustic Observatories Concept and specifications 

   

The ESONET ACOUSTIC TOMOGRAPHY GROUP  

Foundation for Research and Technology-Hellas, Institute of Applied and Computational Mathematics, Greece

Nansen Environmental and Remote Sensing Center, Norway

University of Algarve, Centro de Investigação Tecnológica do

Algarve, Portugal February 2011

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This report has been prepared for ESONET NoE – Workpackage 3. February 2011

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PREFACE  

This report is a first attempt to present a brief overview of the European experience in the observation of the marine environment using acoustics. It has been prepared in the framework of the ESONET Network of Excellence and in particular within Workpackage 3 on scientific objectives of the Ocean Observatories. The systematic use of acoustics was introduced in physical oceanography a few decades ago and resulted in the formation of a special area in Oceanography, under the term “Acoustical Oceanography”. Under this term we include scientific aspects on the use of sound for oceanographic purposes and in particular for the monitoring of the marine environment. Acoustic waves being efficient carriers of information on the domain through which they have propagated especially in water, had been extensively used for many applications related to target localization and communication in the marine environment but only recently their potential for the observation and monitoring of the marine environment has been widely recognized. Acoustic observatories can be considered as additional modules in the network of marine observatories and they can play an important role in the continuous monitoring of the oceanographic processes and the quality of the marine environment in both shallow and deep water areas and long ranges. The aim of this report is to present the concept and the specifications of ocean acoustic observatories based on the scientific research performed over the last 25 years in three European research groups which are members of the ESONET NoE and which have participated together or individually in many European projects related to Ocean Acoustic Tomography and Geoacoustic Inversions. These projects which included theoretical studies, field work (experiments in the marine environment) and extensive analysis of data, contributed in the building of the necessary expertise which can be exploited at the European level in the design and operation of future integrated ocean acoustic observatories. As a matter of fact Ocean Acoustic Tomography concept has reached an advanced stage of maturity, such that it can be implemented as an operative system. This is clearly illustrated in this report. The structure of the report is as following: Chapter 1 presents a general overview of the use of sound for the observation of the marine environment.. Chapter 2 is devoted to the classification of the inverse problems which are associated with the monitoring applications using acoustics. In Chapter 3, the theoretical background of the basic mathematical methods applied for the processing analysis and exploitation of the acoustic data collected at an acoustic observatory are presented. Chapter 4 gives a brief description of some of the experiments carried out in Europe for the validation of the concept of Ocean Acoustic Tomography. More experiments along with detailed description of the instrumentation used in ocean acoustic tomography in shallow or deep water areas are presented in Chapter 5. Chapter 6 discusses the differences among mobile and stationary acoustic observatories. Finally Chapter 7 presents a summary of the issues

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presented in the report and some ideas on the basic modulus of future acoustic observatories. The Groups participated in this report under the general label “Ocean Acoustic Tomography Group” are: Foundation for Research and Technology-Hellas, Institute of Applied and Computational Mathematics (Heraklion, Greece). Principal Scientists : Professor Michael Taroudakis, Dr Emmanuel Skarsoulis, Dr Panagiotis Papadakis. Nansen Environmental and Remote Sensing Center, (Bergen, Norway). Principal Scientists : Dr Hanne Sagen, Dr Stein Sandven. University of Algarve, Centro de Investigação Tecnológica do Algarve, (Faro Portugal) Principal Scientists: Prof. Sergio Jesus, Dr. Cristiano Soares, Dr. Paulo Felisberto Acknowledgements We would like to thank the ESONET NoE and in particular the Coordinator Dr Roland Person, and the WP-3 Leader Dr Henry Ruhl, for the opportunity given to the Ocean Acoustic Tomography Group to present their experience within the Network.

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CONTENTS

PREFACE ............................................................................................................................................... 3 

CHAPTER 1. .......................................................................................................................................... 7 

The role of sound in the observation of the ocean. .............................................................................. 7 

1.1  Introduction ............................................................................................................................... 7 

1.2  Lagrangian observing systems. ................................................................................................ 7 

1.3  Gliders and AUVs. ..................................................................................................................... 9 

1.4  Acoustic networks for navigation and tracking. ................................................................... 10 

1.5  Acoustic tomography. ............................................................................................................. 13 

1.6  Acoustic communication. ........................................................................................................ 14 

1.7   Passive acoustics ...................................................................................................................... 15 

1.8  Integrated acoustic systems .................................................................................................... 16 

1.9  Conclusion ................................................................................................................................ 17 

1.10  References ................................................................................................................................ 17 

CHAPTER 2 ......................................................................................................................................... 21 

Inverse Problems in Underwater Acoustics ....................................................................................... 21 

2.1   Introduction ............................................................................................................................. 21 

2.2   Formulation of the inverse problems ..................................................................................... 22 

2.3   Ocean acoustic tomography .................................................................................................... 23 

2.4   Bottom classification ............................................................................................................... 26 

2.5   Source localization ................................................................................................................... 26 

2.6  References ................................................................................................................................ 27 

CHAPTER 3 ......................................................................................................................................... 29 

Ocean Acoustic Tomography and Geoacoustic Inversions ............................................................... 29 

3.1   Introduction ............................................................................................................................. 29 

3.2   Forward Propagation Modeling ............................................................................................. 30 

3.3   Methods of Ocean Acoustic Tomography ............................................................................. 33 

3.3.1  Ray Inversions ...................................................................................................................... 33 3.3.2  Modal travel time inversions ................................................................................................. 36 3.3.3  Peak inversions ..................................................................................................................... 37 3.3.4  Modal-phase inversions ........................................................................................................ 38 3.3.5  Matched-field inversions ....................................................................................................... 38 3.3.6   Inversions using statistical characterization of the acoustic signal. ...................................... 40 

3.4.   Bottom classification ............................................................................................................... 40 

3.5   Source Localization ................................................................................................................. 41 

3.5   References ................................................................................................................................ 42 

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CHAPTER 4 ......................................................................................................................................... 45 

Typical Experiments related to Ocean Acoustic Tomography ......................................................... 45 

4.1  Introduction ............................................................................................................................. 45 

4.2  The THETIS Experiment ....................................................................................................... 45 

4.3  The THETIS-2 Experiment .................................................................................................... 46 

4.4  The FRAM Strait Experiment (DAMOCLES project) ........................................................ 49 

4.5  Software for the analysis of the tomography data ................................................................ 51 

4.5  References ................................................................................................................................ 54 

CHAPTER 5 ........................................................................................................................................ 57 

Instrumentation for Ocean Acoustic Tomography ............................................................................ 57 

5.1   Introduction ............................................................................................................................. 57 

5.2  An overview ............................................................................................................................. 61 

5.2.1  Deep water tomography ........................................................................................................ 61 5.2.2  Shallow water tomography ................................................................................................... 62 5.2.3  Towards three-dimensional shallow-water tomography ....................................................... 67 

5.3  Acoustic instruments: description and technical specifications .......................................... 72 

5.3.1  Acoustic receiver for permanent observatories ................................................................... 72 5.3.2  Acoustic receiver for rapid response observatories ............................................................. 74 5.3.3  Acoustic emitters .................................................................................................................. 76 5.3.4  An emitter system for a permanent observatory .................................................................. 78 5.3.5  Mobile emitter systems ......................................................................................................... 79 

5.4   Summary .................................................................................................................................. 81 

5.5   References ................................................................................................................................ 82 

CHAPTER 6 ......................................................................................................................................... 89 

Stationary versus Mobile Observatories ............................................................................................ 89 

6.1   Introduction ............................................................................................................................. 89 

6.2  Permanent acoustic tomographic networks .......................................................................... 92 

6.2.1  A simple geometry: a line network ...................................................................................... 94 6.2.2  Volume observation with a two branch network ................................................................ 96 

6.3   Mobile acoustic tomographic networks ............................................................................... 100 

6.4  Summary ................................................................................................................................ 101 

6.5   References .............................................................................................................................. 102 

CHAPTER 7 ....................................................................................................................................... 103 

Concluding Remarks .......................................................................................................................... 103 

7.1  General ................................................................................................................................... 103 

7.2   Location .................................................................................................................................. 104 

7.2   Instrumentation ..................................................................................................................... 104 

7.3   Integration .............................................................................................................................. 106 

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CHAPTER 1.

The role of sound in the observation of the ocean.

HANNE SAGEN and STEIN SANDVEN,

Nansen Environmental and Remote Sensing Center, Thormøhlensgt. 47, 5006 Bergen, Norway

e-mail: Hanne.Sagen@nersc.no, Stein.Sandven@nersc.no

1.1 Introduction OceanObs 09 defines ships, moored moorings, satellite remote sensing, Lagrangian Buoys, and ocean acoustic to be the major elements of the future Global Integrated Ocean Information System (see http://www.oceanobs09.net/documents/OceanObs09-Conference_Summary-draft_24OCT10.pdf. Active and passive acoustic technologies contribute to physical/chemical oceanography and marine biology. Acoustic Doppler current profilers, inverted echosounders and hydrophones are obvious instrumentation for a sea floor observatory and enable observations of small-scale physical oceanography, biology and sea floor processes. Furthermore, low frequency acoustic concepts are used for acoustic tomography [1.1] and positioning/navigation/tracking of Lagrangian observing systems [1.2]. The lack of ocean observing systems in polar regions and increased focus at the enhanced impact of global warming in polar regions has lead to an increased used of acoustic networks in Polar and Sub polar Regions, [1.3], [1.4], [1.5]. For more detailed description of what acoustics and global acoustic networks can be used for in marine sciences see for example [1.2], [1.6], [1.7].

The emphasis of this chapter is to describe briefly acoustic applications within tracking, navigation, tomography and communication. Furthermore, we describe how the inclusion of low frequency acoustic into the cabled networks will increase the influence of the cabled network within physical oceanography and climate monitoring.

1.2 Lagrangian observing systems.

The ARGO float program is the most extensive oceanographic program ever. Through international collaboration the net of 3000 ARGO floats provide oceanographic profiles from the world oceans (http://www.argo.ucsd.edu/), except from the high latitudes. Each ARGO float has a life-time of 2-5 years depending on the number and kind of sensors integrated into the float. Ocean model systems, use ARGO profile data for assimilation and validation, see for example http://topaz.nersc.no. In ice-free waters ARGO systems will surface on a regular schedule, usually every 10 day, to send data and update their position and internal timing via satellite communication see illustration in Figure 1. In ice-infested regions this is not possible. Furthermore, the ARGO floats are untracked during the drifting phase between each profiling event,

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this makes it unable to resolve several meso-scale phenomena. If the ARGO float is equipped with a hydrophone and accurate clocks, they can be tracked using acoustic signals from an acoustic network. By introducing acoustic network in the world ocean the benefit of the drifting phase of the ARGO floats would increase, and make it possible to study meso-scale ocean features also in ice infested regions.

Figure 1.1 ARGO floats are able to profile the water column (typically down to 1000 or 2000m) for up to 4 years. Schematic drawing to the left shows a typical ten-day cycle for a Navigating European Marine Observer (NEMO) profiling float. The floats in the drawing were programmed to drift at 650 m depth for ten days before descending to 1000 m and subsequently rise to the surface while measuring T, S, and O2. At the surface they determine their position by onboard GPS, before transmitting data through Iridium satellite communication. This configuration of the NEMO float is used by Gisle Nondal in his PhD work at NERSC/UiB. The drawing is inspired by http://www.argo.ucsd.edu/pnp.html.

Tracking of floats was pioneered by Doug Webb and Thomas Rossby, and have been done for several decades. Currently, sources are produced by Teledyne Webb Research, http://www.webbresearch.com/rafos.aspx . RAFOS floats are drifting floats used to track and map ocean currents (see http://www.whoi.edu/instruments/viewInstrument.do?id=1061). RAFOS floats do not profile as they seek to maximize the Lagrangian content of a float track and focus on revealing the structure and statistics of fluid motion. Figure 2 show the measurement cycle of the RAFOS floats from deployment till final data transmission. The floats are used to get a clear picture of how fluid moves around in the Oceans, to what extent bathymetry defines pathways and the corresponding transit times, at depth where control is likely to be strong, and near the surface where stratification weakens bathymetric control. In particular, floats are the only tool for elucidating meso-scale processes in the Arctic and Antarctica. Acoustically tracked floats are a way to go for new and improved knowledge of ocean circulation patterns, as is done for example in the ongoing float program in the Nordic Seas ([1.8], personal communication Rossby, 2010). The capability of floats to reveal new knowledge about ocean circulation was clearly demonstrated by the results emerging from the measurements collected by 76 special Range and Fixing of Sound (RAFOS) which were launched into the current south of the Labrador Sea from 2003 to 2006 (see http://www.sciencedaily.com/releases/2009/05/090513130942.htm). Furthermore, Argo and acoustic floats in the Antarctica have been traced at distances of up to 600 km under the ice by use of RAFOS signals at 260 Hz, and in combination with ice-sensing algorithms the floats have spent up to 7 years in seasonally ice-covered regions ([1.3], [1.5]).

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Figure 1.2 RAFOS floats are designed to take measurements of temperature, salinity, and pressure in layers of ocean water at any depth. They are deployed using one of two methods. Some floats are attached to a small anchor and dropped over the side of a research vessel (left). They sink to the bottom and remain there, in a dormant state, until a pre-programmed time when they release the anchor and rise up to their target depth to start their drifting mission. After completing their drifting mission, these floats release a second weight and rise to the sea surface, where they transmit all their stored data to orbiting satellites which then rebroadcast the information to ground stations (right). Most floats however begin their drifting missions immediately after deployment from a research vessel or ship of opportunity (middle). At the end of their drifting mission, they also transmit the data they collected to satellites that relay the information to the scientist. (Illustration by Jayne Doucette, Woods Hole Oceanographic Institution). This figure and explaining text is downloaded from http://www.whoi.edu/oceanus/viewArticle.do?id=60586.

1.3 Gliders and AUVs. While floats drift passively with the current, gliders and AUVs can be remotely steered by an operator via satellite communication at the surface. Gliders can operate for more than 9 months at a horizontal speed of around 0.21 cm/s. The maximum diving depth is currently 1000 m, but this will increase with new types of gliders (http://iop.apl.washington.edu/seaglider/map.php). In Arctic areas gliders have been tested with positive results in the Labrador Sea (eg. [1.9]) and in the ice free area of the Fram Strait (http://acobar.nersc.no) see figure 4.

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AUVs are vehicles with propulsion capability, allowing controlled measurements, but operation time is limited to a few days due to battery capacity. Advanced AUVs has very sophisticated instrumentation for navigation, autonomous control and communication. The current operation time of an AUV is about 60 hours, the maximum diving depth is 6000 m and the range is 100-200 km at a speed of 1.5-2 m/s, depending on payload. Navigation systems for AUVs are based on a Doppler velocity log (DVL) aided by inertial navigation system and GPS updates at the surface. In ice covered regions AUVs will need a supplement of navigation data due to the lack of GPS updates.

An acoustic navigation network provide a replacement of GPS updates (timing and positioning) where this is missing such as in ice covered regions and under ice shelves. In particular in the Polar Regions the use of Lagragian systems will depend on installation of an acoustic system for navigation and tracking (see [1.10], [1.11], [1.12]).

1.4 Acoustic networks for navigation and tracking.

Long-range acoustic navigation systems consist of a network of low to mid frequency sources. Acoustic networks are currently implemented and used at several locations, such as in the Lofoten basin (Søyland, 2010, personal communication), in the Davis Strait ([1.4]), Antarctica (Klatt & Fahrbach 2010, personal communication, Figure 3), and in the Fram strait (http://acobar.nersc.no, http://arctic-roos.org/advanced-underwater-ocean-monitoring-system-deployed-in-the-fram-strait , Figure 4). Sources in those networks send out narrow banded signals centred at 260 Hz, except for the system used in the Davis Strait, which transmit at 780 Hz. The transmissions go off 2-4 times a day depending on the design of the regional navigation system, and the transmissions from the different sources are separated in time. The areas covered by an acoustic network at 260 Hz are large. For example it is found that 600 hundreds kilometres in the Antarctica during winter conditions, and 900 km in summer time under ice free conditions (Klatt and Fahrbach, 2010, personal communication).

Advanced Ice Tethered Platforms (AITP) equipped with 1560-Hz and 780-Hz sound sources are used for regional acoustic navigation and communication of gliders and floats operating under ice. Main reason to go up in frequency is to reduce the size and weight of the sources; however, higher frequencies will increase attenuation of the acoustic signal due to reflection and scattering losses caused by interaction with sea ice, surface waves and small scale ocean structures. Depending on ice conditions clusters of 3 – 5 AITPs the 780 signal can cover areas of 100-200 km (Gascard, 2010 personal communication; [1.4], [1.5]). In such regional systems the navigation signal is transmitted every hour to allow for detailed navigation of the gliders.

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Figure 1.3 Within the Weddell Sea the Alfred Wegener Institute have installed an array of ten RAFOS sound sources @ 260 Hz. The range of the sound signals is about 500 km during winter and 900 km in summer. The circles in the figure correspond to 500 km. Each sound source emits one pong per day (Klatt and Fahrbach, 2010, personal communication; [1.2].

Fig 1.4 The Sea glider used by AWI in the Fram Strait under the ACOBAR project. The orange rod is the antenna communicating with the IRIDIUM. To the left a map of the glider surfacing summer 2008. (Photo/Map: Agnieszka Agnieszka Beszczynska-Moeller)

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Figure 1.5 The Fram Strait acoustic system for navigation and tomography, implemented summer 2010, consists of a triangle of multipurpose acoustic sources (yellow dots), which provide RAFOS navigation signals @ 260 Hz and sweep signals for tomography (200-300 Hz). The yellow star in the middle is a 700 m long vertical listening array. Two RAFOS sources @ 260 Hz are deployed westward of the triangle and they produce standard navigation signal. The propagation conditions in this region are different from the Antarctica due to different ice conditions (mix of first year and multiyear ice), and strong horizontal oceanographic gradients related to the warm Atlantic water and the cold East Greenland current. The propagation length is expected to be above 300 km but needs to be verified under the ice. (Figure from [1.5])

Briefly, the Navigation algorithms, builds on positioning of the vehicles or float, which again is based on

1. Assumptions of the sound speed characteristics between the source and the vehicles or float.

2. Accurate timing is required in the source, and for the acoustic receiver onboard the floats or the glider.

3. Receptions are in general needed from 3 or more sources to triangulate.

After the position is obtained, the deviation from the target position from actual position is calculated and the direction and distance to target is obtained. The accuracy in positioning is limited by timing, the bandwidth of the acoustic signal, signal to noise ratio, and the environmental impact on the acoustic propagation.

The clocks used in RAFOS source may drift off by 1-3 seconds a year causing an error of at least 1.5 km. For a long time accurate clocks meant high power consumption. However, atomic clocks with significantly lower power consumption are now available, and are under incorporation into for example the Webb sweeper sources (A. Morozov, 2010, personal communication). The standard 1.5 Hz wide RAFOS navigation signal adds on an error of another 1.5 km. Today more efficient source technology exists to provide wideband (100 Hz) sweeps, with a better signal to noise ratio, and improved accuracy in detecting the arrival time ([1.5], [1.12]).

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However; the more unknown errors introduced due to a dynamic and range dependent ocean environment, including seasonal changes, internal waves, tidal currents, eddies and sea ice, is a limiting factor, which cannot be eliminated, but better understood.

Low frequency acoustic sources set up to cover a large geographical area need significant energy. This impacts the maintenance cycle and costs. If the sources were cabled they would have access to unlimited amount of power, as in a cabled system, the maintenance costs would be reduced. However, the number of cabled observatories is must likely less then the required number of acoustic nodes. Most likely an acoustic network will consist of both cabled and un-cabled acoustic nodes.

1.5 Acoustic tomography.

In the 1970s the existing ship borne and mooring based observing techniques were incapable to properly study the ocean which is dominated by mesoscale processes. To meet this problem Walter Munk at Scripps Oceanographic Institution got the idea of using acoustic travel time measurements to measure ocean temperature and current (e.g. [1.1], [1.13]). The approach is to measure acoustic travel time between acoustic sources and receivers at a few millisecond accuracy, at any predefined repetition rate. The sound speed is strongly dependent on the ocean temperature. Through inversion techniques, the travel times provide spatially integrated temperature, where meso- and small-scale variability is suppressed, at an accuracy of 0.01°C over a 200 km distance ([1.1], [1.14]). In the same way, precise measurements of average current can be determined from the difference between reciprocal travel times produced by simultaneous transmission (reciprocal transmissions) of acoustic pulses in opposite directions along an acoustic path ([1.1]). Ocean acoustic tomogaphy is a cost effective method for obtaining time series of unique, synoptic and horizontally averaged oceanographic depth profiles of sound speed at high temporal resolution. Today acoustic tomography is a unique underwater remote sensing technique complementary to satellite remote sensing of the ocean surface, point measurements from fixed moorings, floats, and drifting and moored profiling instruments.

The ultimate use of acoustic travel time measurements or acoustically derived parameters are to integrate them with ocean circulation models through data assimilation techniques. Assimilation of integrated ocean parameters derived from acoustic thermometry and tomography into ocean circulation models was originally recommended by [1.1], and a few studies have been carried out; eg. [1.15], [1.16], [1.17],[1.18]). Assimilation of acoustically derived ocean parameters and travel times into operational ocean monitoring systems depends on data in near real time. Raw acoustic data are very extensive and it would not be feasible to send by acoustic modems or by satellite communication. Pre-processing of the acoustic recordings would be necessary within the buoys in compromise with long term monitoring and available battery power. A fully cabled tomography system would be the optimal solution for obtaining acoustic data in near real time.

During the last 30 years acoustic tomography has been successfully implemented and operated in a large number of experiments including the Pacific Ocean, Arctic Ocean, Atlantic Ocean, and Mediterranean. (e.g. [1.1], [1.7]). The most extensive acoustic experiment took place in the North Pacific Ocean from 1996 till 2006 as part of the Acoustic Thermometry of Ocean Climate (ATOC) project and the North Pacific Acoustic Laboratory (NPAL) project (e.g. [1.14]). In the Arctic a 6 six year long

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experiment was carred out in the Labrador Sea ([1.19]). Both experiments show the strength of acoustic monitoring of ocean climate change. Most often the tomographic systems have been un-cabled systems due to lack of cables, during the ATOC/NPAL projects one of the sources and receivers was cabled to shore, providing near-real time data ([1.7], http://aog.ucsd.edu/publications/po_posters/CLIVARPoster_ATOC.pdf ).

In ice- free oceans cabled networks can be combined with moorings with surface units, which provide the real time capability, only limited by the data transfer capacity of the satellite communication. A moored system, with surface unit is not a possibility in the Arctic Ocean due to a drifting sea ice field. Without real time capability for underwater moorings the data cannot be used for assimilation into operational monitoring systems. Real time capability in the Arctic is therefore the main bottleneck in underwater observing systems. The Arctic drifting ice-tethered platforms has real time capability, and can carry smaller sources and/or light receiver arrays for navigation of gliders and regional tomography (Gascard, 2010, personal communication, [1.19], [1.5], [1.7]).

1.6 Acoustic communication.

Simple acoustic communication is already extensively used for example acoustic releases in bottom-mounted moorings, and when we interrogate acoustic transponders placed at the sea floor. Furthermore, a new generation of acoustic communication devices, acoustic modems, has been developed the last 5-10 years. The benefit of using acoustic communication within ocean observatories or between different observatories is that the need of cables can be significantly reduced. However, acoustic communication has its clear limitations in particular to the amount of data to send due to the highly variable properties of the ocean waveguide properties in space and time. In 2007 a report was prepared by IFREMER for the ESONET project where several modems from different providers were compared [1.20]. It was detected large differences in the capacities of the modems. In this report the modems were tested in a temperate environment. Here we just summarize a few experiences reported by our partners in ACOBAR, AOEM, and AWAKE project to illustrate that acoustic communication is not yet “plug and play”. Propagation of acoustic signals (amplitude and travel time) is strongly influenced by spatial and temporal variability in the oceanography (internal waves, tidal current, mesoscale features), in addition to strong bathymetric effects. This will of course influence the acoustic communication. The most difficult area to get modems to work seems to be in the ice-covered regions, this is due to the strong stratification of the ocean causing a bunch of multi paths with arrivals that overlaps in time such that the information is not always obtained. Therefore, even if a communication set-up h is developed and implemented in a particular area, for a particular use, works; do not necessarily imply that it will work in another season or in any another area. The communication has to be designed for the problem on hand including ranges, typical oceanographic characteristics, modem platform (mooring, glider) and amount of data to be sent. Typical set ups are data transmissions between instruments and surface units within a mooring (ranges of some 1000s meters), between different moorings (ranges of 10s of km), between a moored unit and for example a glider, and commands to very distant moorings.

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University of Bergen uses modems in moorings in the Weddell Sea, Barents Sea and Færøy channel. So far only results from the Weddel Sea is available. In this case standard instruments from Aanderaa Data were interfaced with a commercially available modem. Instruments and modems were deployed within a mooring deployed in a partly ice covered region (Svein Østerhus, 2010, personal communication). During data download the data transmission was delayed by poor signal to noise and propellers had to be stopped and the surface unit was lowered between the ship and ice floe to be protected from drifting ice floes. Range limitations were significant [1.21]. However, a large amount of the oceanographic data was successfully downloaded. Long-range horizontal transmission of oceanographic data between mooring in the Fram Strait has been tested by AWI within ACOBAR and DAMOCLES. It showed that it is very hard to get the modems to work; however some data was transferred. The instability in data delivery is claimed to be due to variable tilt of the modem due to mooring motion. Attempts to solve this have been made by the manufacturer, and results of this are currently under evaluation (Beszczynska-Moeller, 2010, personal communication) Under the DAMOCLES project, NERSC used acoustic modems in acoustic data from tomography moorings in the Fram Strait for data download with ship born acoustic modem ([1.22], [1.23]). These modems suffered from hardware and software bugs, and signal to noise ration was reduced due to noise ships. Furthermore, the amount of data from a tomography mooring is very high, and the acoustic data has to be internally processed in the mooring. With the bit rates available in this environment with this specific modem, the ship had to stay on position for at least 78 hours to download data, corresponding to engineering data and a few days of pre-processed receptions from the 8 hydrophones. Based on the experience of current modems capabilities in data transfer rates it was concluded that acoustic modem for transfer of acoustic receptions is not yet practical. A cabled system provides the best data transfer both within passive and active acoustics. Other alternatives are to use pop-ups, winches, or gliders/AUVs equipped with modems. To summarize, acoustic communication is not a easy task; and success depends on careful testing of instruments in the laboratory to check both hardware but also check the interface between the observing sensors/instruments and the modem it self. In addition the sound propagation conditions in the area of deployment should be investigated to choose the optimal configuration of the system and to be aware of the environmental limitations of the communication.

1.7 Passive acoustics

Ambient noise is generated by natural physical sources such as sea ice, ocean waves, seismic activity, rain, gas bubbles in the ocean; marine mammals and by human activities. The ambient noise after it is generated is flavoured by the propagation conditions determined by the ocean stratification and the boundary conditions at the sea surface and the sea floor. The ambient noise therefore, contains a lot of information about the marine geophysical environment, which we would like to extract ([1.7], [1.24]). Furthermore, Mounting autonomous acoustic recorders on fixed

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moorings, floats, gliders, and AUVs, permits systematic measure of the seasonal occurrence of vocal cetaceans ([1.25]), and provides a window into the large-scale seasonal movements and habitat selection of Arctic marine mammals [1.26]).

Passive acoustics comprise a few (or many) hydrophones and a small controller/processing unit with modest power consumption and is easily integrated into platforms such as oceanographic moorings, floats, gliders, and in cabled systems. The acoustic recordings, in particular from large arrays of hydrophones, will produce a large amount of data, which will be demanding to obtain in real time unless the system is connected to a cabled network. The applications of passive acoustics is manifold:

Monitoring and detection of seismic activities, tsunamis and explosions.

Passive acoustics can be used to monitor the impact of human activities on marine environment. Examples are the monitoring of the impact of wind farms on marine life where the sound level was estimated to be 228 dB// 1 micro Pa at 1 m during pile driving ([1.27]); seismic exploration, shipping activities, drilling activities and seismic exploration.

Clusters of acoustic arrays can be used to localize and track marine mammals. Marine biologists have underlined the importance of integration of acoustic recorders into emerging ocean observatories in different parts of the world ocean, as this will significantly increase our monitoring capability with respect to cetaceans, walruses and ice seals ([1.26], [1.5], [1.7],). This can be used for assessing the impact of increased acoustic noise generated by human activities on marine mammals in for example the Polar Regions.

In the interior Arctic thermal cracking and ridging process are significant sources of acoustic energy. This can be used to monitor long-term changes in the sea ice dynamics in the interior Arctic. Ambient noise can be used in the marginal ice zone to monitor sea ice dynamics and wave propagation into the ice pack (see http://msc.nersc.no/?q=wifar).

1.8 Integrated acoustic systems

While tomography is superior to the Lagrangian systems in temporal resolution the glider and floats provide ocean data at a much higher spatial resolution. A region with strong dynamic activity requires both high temporal and spatial resolution of the observations. Therefore, the ultimate goal is an integrated data and model system combining acoustic tomography, gliders, floats, and fixed-point measurements with high-resolution ice-ocean modeling through data assimilation techniques (eg. [1.23], [1.7]) .

Currently there are two significant tomography experiments ongoing; one in the Pacific Ocean lead by Scripps Oceanographic Institution, USA and one in the Fram Strait between the Svalbard and Greenland lead by Nansen Environmental and Remote Sensing, Norway. Those experiments are in totally different regions of the world oceans, but use the same source and receiver technology, and demonstrate the flexibility in the acoustic technology. The acoustic technology used in these two experiments are mature and facilitating both acoustic tomography, acoustic navigation, and passive acoustic, description of the instruments are provided in [1.23].

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1.9 Conclusion

Acoustic multipurpose systems provide instantaneous measurements of the heat content in large ocean volumes, acoustic travel times for constraining ocean circulation models through assimilation, and signals to track floats and navigate gliders. Furthermore, a cluster of hydrophone arrays in a cabled network would enable advanced detection, localization and tracking of sources of sound for example marine mammals and seismic events. Technically there is “no problem” to integrate the existing acoustic sources and receivers into cabled networks. Incorporating components of an acoustic network for positioning, navigation and tracking of Lagrangian systems, passive acoustics and for tomography into a cabled network would increase the area of impact of a geographically very local cabled network out to ranges around 1000 kilometers. Furthermore, acoustic communication within a cabled observatory can reduce the amount of cabling and junctions. An acoustic network will provide edge-cutting observations within the fields of physical/chemical oceanography, geophysics and biologic sciences. We strongly support the implementation of a global acoustic network following the recommendations in the community paper by [1.7]. In particular we support the implementation of a cabled network in the Arctic, which can facilitate components of an acoustic network covering the Fram Strait and the Arctic Basin. Cables in the Arctic are not a particularly easy task either technically or financially, and this can only be solved at the international level and through interdisciplinary collaboration.

1.10 References

1.1 Munk, Worcester, Wunsch (1995). Ocean Acoustic Tomography. Cambridge Monographs on Mechanics.

1.2 Duda, Howe, Cornuelle. Acoustic Systems for Global Observatory Studies. IEEE, 2006.

1.3 Klatt, Boebel, Fahrbach. A profiling float’s sense of ice. Journal of atmosphereic

and oceanic technology, Vol. 24, 1301-1308. 1.4 Lee, C.M., H. Melling, H. Eicken, P. Schlosser, J.C. Gascard, A. Proshutinsky,

E. Fahrbach, C. Mauritzen, J. Morison, and I. Polykov, 2010. Autonomous Platforms in the Arctic Observing Network. In Proceedings of OceanObs’09: Sustained Ocean Observations and Information for Society (Vol. 2), Venice, Italy, 21-25 September 2009, Hall, J., Harrison, D.E. & Stammer, D., Eds., ESA Publication WPP-306.

1.5 Sagen, H., Sandven, S., Beszczynska-Moeller, A., Boebel, O., Duda T. F.,

Freitag, L., Gascard, J. C., Gavrilov, A., Lee, C. M., Mellinger, D.K., Mikhalevsky, P., Moore, S., Morozov, A., Rixen, M., Skarsoulis, E., Stafford, K., Tveit, E., Worcester P.F. (2009). Acoustic technologies for observing the interior of the Arctic Ocean. In Proceedings of the “Ocean Obs´09: Sustained Ocean Observations and Information for Society” Conference (Annex), Venice,

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Italy, 21-25 September 2009, Hall.D.E. and Stammer, D., Eds., ESA Publication WPP-306, 2010.

1.6 Howe and Miller “ Acoustic sensing for Ocean Research”, Marine Technology Society Journal, Volume 38, No. 2, 2004.

1.7 Dushaw, B.D. (In alphabetic order) W. W. L. Au , A. Beszczynska–Möller, R. E. Brainard, B. D. Cornuelle, T. F. Duda, M. A. Dzieciuch, E. Fahrbach, A. Forbes, L. Freitag, J.–C. Gascard, . A. N. Gavrilov, J. Gould, B. M. Howe, S. Jayne , O. M. Johannessen , J. Lynch, D. Martin, D. Menemenlis, P. N. Mikhalevsky, J. H. Miller, W. H. Munk, J. Nystuen, B. Odom, J. Orcutt, T. Rossby, H. Sagen, S. Sandven, J. Simmen, E. Skarsoulis, R. Stephen , S. Vinogradov , K. B. Wong, P. F. Worcester, C. Wunsch (2009). ” A Global Ocean Acoustic Observing Network.” In Proceedings of the “Ocean Obs´09: Sustained Ocean Observations and Information for Society” Conference (Vol 2), Venice, Italy, 21-25 September 2009, Hall.D.E. and Stammer, D., Eds., ESA Publication WPP-306, 2010.

1.8 Søiland, H., M.D. Prater and T. Rossby. Rigid topographic control of currents

in the Nordic Seas. Geophysical Research Letters, Vol. 35., 2008. 1.9 Hatun, Eriksen and Rhines (2007). 'Buoyant Eddies Entering the Labrador Sea

Observed with Gliders and Altimetry', JPO, vol. 37, DOI: 10.1175/2007JPO3567.1.

1.10 Lee and Gobat, (2006). Acoustic Navigation and Communication for high-

latitude ocean Research Workshop, EOS, Vol. 87, No.27. 1.11 Duda, Morozov, Howe, Brown, Speer, Lazarevich, Worcester, Cornuelle,

(2006). Evaluation of a long-range joint acoustic navigation/thermometry system, IEEE Oceans'06 Conf. Proc.

1.12 Sagen, Sandven, Fahrbach, Beszczynska-Möller, Klatt, Worcester, Morozov

(2011).ESONET AOEM report D9, Final report on design of the acoustic network for acoustic tomography, underwater navigation and passive listening in the Fram Strait.

1.13 Munk (2011). The sound of climate change. Tellus. DOI:10.1111/j.1600-

0870.2010.00494. 1.14 Dushaw, B. D., P. F. Worcester, R. K. Andrew, B. M. Howe, J. A. Mercer, R. C.

Spindel, B. D.Cornuelle, M. A. Dzieciuch, W. H. Munk, T. G. Birdsall, K. Metzger, D. Menemenlis, and C.Wunsch, 2009. A decade of acoustic thermometry in the North Pacific Ocean, J. Geophys. Res., 114, C07021, doi:10.1029/2008JC005124.

1.15 Park and Kaneko (2000). Assimilation of coastal acoustic tomography data into

a barotropic ocean model. Geophysical Research Letters.

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1.16 Remy, Gaillard, Verron (2002) Variational assimilation of ocean tomographic data: Twin experiments in a quasi-geostrophic model, Q. J. R. Meteorol. Soc. (2002), 128, pp. 1739–1758

1.17 Lewis, Rudzinsky, Rajan, Stein, Vandiver (2005). Model oriented ocean tomography using higher frequency, bottom mounted hydrophones. J.Acoust. Soc. Am. 117(6), June

1.18 Carriere, Hermand, Candy (2009). “Inversion for time-evolving sound speed field in a shallow ocean by ensemble Kalman filtering”, IEEE Journal of Oceanic Engineering, Vol.34, No.4

1.19 Avsic, Send and Skarsoulis, “Six years of tomography observation in the central

Labrador sea,” Proc. Int. Conf. Underwater Acoustic Measurements: Technologies & Results, Heraklion 2005.

1.20 Skarsoulis and Piperakis, (2009). Use of acoustic navigation signals for

simultaneous localization and sound-speed estimation, J. Acoust. Soc. Am., 125 (3).

1.21 ESONET D57. Intermediate report from underwater acoustic modems inter-

comparison experiment. March 2007. 1.22 Strand, 2010. Rapport fra BIAC Felttokt i Weddellhavet, Antarktis. University

of Bergen. 1.23 Sandven, Sagen et al "THE FRAM STRAIT TOMOGRAPHY EXPERIMENT

2008", NERSC Technical report No. 298, 19 December 2008. 1.24 Sagen, Sandven, Worcester, Dzieciuch and Skarsoulis. “The Fram Strait

acoustic tomography system” In the proceeding for “Acoustics´08, Paris” (2008 a).

1.25 Roel Snieder and Kees Wapenaar, Imaging with ambient noise, Physics Today, September 2010.

1.26 Mellinger, Stafford, Moore, Dziak, and Matsumoto, (2007). An overview of fixed passive acoustic observation methods for cetaceans, Oceanography 20(4), 36-45.

1.27 Moore and Huntington (2008). Arctic marine mammals and climate change:

impacts and resilience. Ecological Applications 18:S157-S165.

1.28 Thomsen, Lüdeman, Kafeman, Piper. “Effects of offshore wind farm noise on marine mammals and fish. Copies available from: www.ofshorewind.co.uk.

 

 

 

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CHAPTER 2

Inverse Problems in Underwater Acoustics

MICHAEL I. TAROUDAKIS

Department of Mathematics, University of Crete, 714 09 Heraklion, Crete, HELLAS

Institute of Applied and Computational Mathematics, P.O.Box 1527, 711 10

Heraklion, Crete, HELLAS

e-mail: taroud@iacm.forth.gr 2.1 Introduction Inverse problems in underwater acoustics have recently drawn the attention of scientists working in underwater technology, due to the relatively high efficiency of the sound waves as carriers of information related to the environmental and operational parameters in the sea environment, including acoustic source characteristics and shapes of objects in the water or the bottom layers. The recovery of these parameters using measurements of the acoustic field is the main objective of the inverse problems in underwater acoustics. Thus, inverse problems are in the core of the acoustical methods applied for the monitoring of the marine environment and they are defined according to the acoustic technology adopted for the ocean observation. According to the classification proposed by Collins and Kuperman [2.1], the inverse problems in underwater acoustics fall in two main categories: Remote sensing of the sea environment and localization. In the first category we define inverse problems for the estimation of the sound speed structure and the current field in the water column (acoustic tomography), as well as the basic parameters of the sea-bed (sound and shear speed profiles, density and thickness of the bottom layers). Within the second category we define problems of source recognition and localization as well as source path estimation. In this chapter, we will make reference to problems of ocean acoustic tomography, bottom identification, and source localization. These problems are encountered in applications related to the monitoring of the marine environment using the technology of stationary or mobile ocean observatories. The structure of the chapter is as following: In the next section, general remarks on the formulation of inverse problems in underwater acoustics will be made. The subsequent chapters will be devoted to applications of ocean acoustic tomography, bottom recognition and sound source identification.

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2.2 Formulation of the inverse problems Conceptually, the formulation of the inverse problems is simple. Practically, all the inverse problems we encounter in underwater acoustics are discrete. A set of model parameters

T

Mmmm ],....,,[ 21m is to be recovered from a set of measured data d.

T

Nddd ],....,,[ 21d Model parameters and data are related through a generally non-linear vector equation of the form

0d)f(m, (2.1)

The equation is determined by appropriate modelling of the forward acoustic propagation problem and in general the function f is complicated and not amenable to linearization (See Chapter 1). Thus, the problem is non-linear and very difficult to be solved. In general, optimization schemes are used to treat the problem as described in next Chapter. In some limited cases though, the problem can be linearized, and is reduced to the solution of a linear system of equations of the form:

Gmd (2.2)

written analytically as

1

, 1,...,M

i ij jj

d Q m i N

(2.3)

where ijQ is a kernel calculated on the basis of the forward propagation model and some reference condition. In summary, the key factor governing the solvability of an inverse problem is the determination of the function f, given the recoverable parameters m and the data set d. Although the recoverable parameters are dictated by the physics of the problem and the specific application, the data are dictated by the specifications of the experiment which in turn will lead to the definition of the “observables”, that is the set of measurements that can be exploited by the inversion procedure

In the subsequent sections we will refer to some standard inverse problems in underwater acoustics while in the next Chapter we will present a selection of typical methods that have been proposed so far for their treatment. The presentation of the

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methods is far from being complete, but it will give an overview of the subjects of current research in the field. It should be noted that a general continuous problem of the form ( , ) 0f m d can also be formulated, where m and d are considered continuous functions of the spatial and temporal co-ordinates. However, in most cases such a problem has limited applicability in realistic situations. Even in cases where discretization of the model parameters is not desirable, mathematical concepts such as the use of orthogonal basis for the projection of the continuous functions would lead to the association of unknown physical parameters with discrete coefficients associated with the basis functions. A final note on the type of data is necessary. This is dictated by the specific application and the method for solving the inverse problem. In general they are of deterministic character, and they are used to characterize the acoustic signal and through this characterization to derive the parameters to be recovered. They are also associated with the type of the acoustic observatory and the availability of the measured elements, which in our case they are the hydrophones. A specific sub-section will be devoted in the next chapter to define the appropriate observables for several classes of applications

2.3 Ocean acoustic tomography

Ocean acoustic tomography was introduced by Munk and Wunsch in 1979 following a demonstration in the ‘70s that about 99% of the kinetic energy of the ocean circulation is associated with mesoscale features, that is features that are about 100 km in diameter [2.2]. Monitoring the changes of the mesoscale and larger-scale features is therefore a useful process on the way of understanding global changes. As the continuous monitoring of these features by traditional in-situ sampling tools may be proven extremely expensive and non practical, while at the same time the other alternative tool available at that time, that is the remote sensing by satellite, could not give depth resolving information for deep water, the sound propagating in the water between moorings was proposed as a possible carrier of information and techniques for extracting and exploiting this information started to be studied. The term “tomography” was well known in medical and seismic applications and reflects the fact that the carrier of information on a specific medium penetrates the area under investigation. The processing method is based on the definition of several slices (τομές – tomes in the Greek language) on which an inverse problem is solved. The integration of the solutions obtained in each one of the slices provides the “image” required by the specific application. In the case of ocean acoustic tomography, the actual information required by the scientists is the temperature structure of the ocean, sometimes associated with the current structure of the same area. This type of information is generally what the oceanographers need in order to either directly derive the necessary information on the changes of the oceanographic processes, or feed appropriate numerical models that would make the prediction. It should be noted that the concept of “temporal change” introduces the necessity for repeated measurements at different time spots. The temperature is typically derived from the sound speed through some semi-

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empirical relationship and this is due to the fact that the sound speed structure is more easily derived from inverse problems due to its immediate relationship with the acoustic field. Ocean acoustic tomography takes advantage of the fact that measurable acoustic properties such as travel-time, phase or even the full-field are related to the sound and current velocities in the ocean. The derivation of the sound speed (temperature) and current velocity profiles from the sound field measurements is the main goal of ocean acoustic tomography. An additional feature of the ocean is that low frequency sound propagates at long distances in the water column, and thus long acoustic propagation paths can be exploited. Experimental procedures, forward propagation modelling and inversion schemes are all interrelated and constitute the ingredients for the development of the ocean acoustic tomography methods. Going back to the definition of the inverse problem, in the case of ocean acoustic tomography, model parameters are the sound speed c and current velocity v in the water column. In general, both parameters are functions of the spatial and temporal co-ordinates ( , ), ( , )c x t v x t

To simplify the problem, it will be assumed that model

parameters are functions of the spatial co-ordinates only, the time dependence being retrieved by successive experiments. An ocean acoustic tomography experiment involves sound sources and receivers. A single source emitting a known signal and a single receiving station define the tomographic pair. In most cases the receiving station consists of a single hydrophone or a vertical array of hydrophones and recording devices (Figure 2.1). Thus, vertical slices (τομές) in water are considered. The recoverable parameters are estimated within each slice either as range averages or in a range-dependent sense. In any case the inverse problem within each slice is 2-D. The 3-D image of the environment is obtained by combining results from multiple slices. In some (rare) cases, horizontal arrays of hydrophones are also used as measuring devices. The original geometry in these cases is also 2-D, with the sound source located at the vertical plane defined by the horizontal array. Thus an observation system related to ocean acoustic tomography applications involves moorings, in each one of which sound sources and receivers are mounted. Additional measuring devices such as current meters could also be mounted to help establishing a multidisciplinary ocean observatory (Figure 2.2). Ocean tomography stations have been deployed in different areas of the oceans in the globe to demonstrate the concept of ocean acoustic tomography. The design of tomography stations and the analysis of the measurements have been the subject of big scientific projects in the thematic area of marine technology. More details will be presented in the next Chapter 3.

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Figure 2.1 A typical configuration for Ocean Acoustic Tomography

Figure 2.2 A Conceptual design of a pair of moorings intended for ocean acoustic observatories.

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2.4 Bottom classification

The classification of the sea-bottom by acoustic means has been a subject of research since a long time. The reasons are related to the fact that bottom sounding and bathymetry estimation has been among the first applications of underwater acoustics. Noting that acoustic waves penetrate in the bottom was led to a straightforward application of bottom properties recovery using acoustics. Also, the research in this area was supported by the fact that the nature of the sea bottom is an important factor governing all applications of underwater-acoustics especially in shallow water areas. Two main classes of inverse methods related to bottom classification could be mentioned at this point: Ray theoretic approaches and full wave techniques. In the first class belong all methods based on the decomposition of the acoustic field into plane-wave components and use the concept of reflection coefficient for performing the necessary inversions. All the methods using measurements of the acoustic field without making reference to its ray composition fall in the second class.

It should be noted that ambient noise is an alternative source of information for imaging the sea environment which of course includes the estimation of the bottom parameters [2.3]. It is well known that the spatial characteristics of the ambient noise are correlated with the ocean sound speed structure in both the water column and the bottom. Accordingly, if there is a good knowledge of the water parameters at a specific region, which is often the case, bottom parameters can be estimated by exploiting the ambient noise information obtained using arrays of hydrophones. Talking about ocean acoustic observatories, the related applications concern mainly monitoring of the quality and of course the changes of the sea-bed sediments. Thus the inversion procedures to be adopted will be based in static measurements of the acoustic field. In view of this, typical sob-bottom profilers involving mobile side-scan sonars will be excluded from the subsequent analysis. This type of technology can be used when specific applications (such as the detection of buried objects in some area) are under consideration. 2.5 Source localization

Source localization is referred to the problem of identifying the position of a sound source. Most of the methods that have been proposed so far exploit matched-field or matched-mode processing algorithms. As a matter of fact matched-field algorithms have been introduced as a tool for source localization. Necessity for source localization problems emerges in practically all the applications of underwater acoustics. This is due to the fact that the exact position of the source is indispensable information for solving large classes of inverse problems, including ocean acoustic tomography, bottom recognition and identification of marine mammals. The matching techniques have the advantage that been non-linear by their nature could be used for the recovery of a multparameter space, which of course could in principle include the information on the location of the sound source. Thus, source localization may be a sub-product of the bottom recognition mentioned above. Recent applications of Marine Bioacoustics dictate the development of methods specifically tuned for the localization and tracking of marine mammals, using

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stationary or mobile acoustic stations. In addition, the necessity for using simple stations with small number of hydrophones (to avoid high cost), has driven the development of acoustic methods based on ray theory. More details on methods for source localization will be outlined in a subsequent chapter. 2.6 References 2.1 M.D. Collins and W.A. Kuperman "Inverse problems in ocean acoustics"

Inverse Problems 10, pp 1023-1040, (1994). 2.2 W. Munk, P. Worcester and C. Wunsch Ocean Acoustic Tomography,

Cambridge University Press, Cambridge, (1995). 2.3 M.J. Buckingham, "Theory of acoustic imaging in the ocean with ambient

noise," J.Comput.Acoust. Vol 1, pp 117-140 (1992).

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CHAPTER 3

Ocean Acoustic Tomography and Geoacoustic Inversions

MICHAEL I. TAROUDAKIS

Department of Mathematics, University of Crete, 714 09 Heraklion, Crete, HELLAS

Institute of Applied and Computational Mathematics, P.O.Box 1527, 711 10

Heraklion, Crete, HELLAS

e-mail: taroud@iacm.forth.gr 3.1 Introduction As mentioned in the previous Chapters, ocean acoustic tomography is referred to the problem of the estimation of the parameters of the water column (typically the temperature and current structure) using measurements of the acoustic field. The inversion approaches are based on characteristic observables which are dictated by the specifications of the ocean acoustic observatory with respect to source/receiver system and the inversion method to be applied. The temperature profile is determined from sound speed profiles which can be directly estimated by inversion of the acoustic data. Typically, inverse problems are formulated as discrete problems, the invertible quantities considered as discrete values, In order that the a sound speed profile which is by nature a continuous function of the spatial variables is associated with discrete values, a projection in Empirical Orthogonal Functions (EOFs) is done. Since a general 3-D problem is difficult to be solved, even for the forward stage (See Section 3.2), most of the practical inversion problems of ocean acoustic tomography are formulated as 2-D. The 2-D character of the problem is anyway compatible with the concept of ocean acoustic tomography as presented in the previous chapter. The ocean acoustic observatories can be used for bottom classification as well. The inversion methods are similar as they are based on same type of signals and therefore of the same observables. This Chapter presents a summary of the most known inversion methods applied for Ocean Acoustic Tomography, Geoacoustic Inversions and Source localization. Only 2-D cases will be examined and the formulation of the inverse problems will be done in the discrete sense. The structure of the Chapter is as following: Section 3.2 presents an introduction to the forward propagation modeling which is the core of the formulation of the inverse problems. Section 3.3 is devoted to methods of Ocean Acoustic Tomography. Typical methods applied so far in real experiments or have been developed as potential tools for inversion are presented. Section 3.4 presents the methods applied for geoacoustic inversions when the technology of a typical ocean

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acoustic observatory is adopted and the final Section 3.5 presents one method for source localization. Additional methods can be found in the literature accompanying the text of this Chapter. 3.2 Forward Propagation Modeling The measurement exploited by ocean acoustic tomography methods are signals recorded in the time domain. (Figure 3.1). The sound source emits signals of short duration and relatively narrow bandwidth. The reception is made at a single hydrophone (single reception) or at an array of hydrophones (multiple receptions) (Figure 3.2).

Figure 3.1 A typical tomography signal (simulation)

Figure 3.2 The environment at a vertical slice.

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Ocean acoustic tomography methods are dictated by the type of the processing applied to this signal after reception which leads to characteristics of the acoustic field (observables) that can be used for inversions. The measured characteristics of the signal form the set of data involved in equation 2.3. The following characteristics are among the most frequently used in problems of ocean acoustic tomography and geoacoustic inversions:

a. A single signal in the time domain a. Ray arrivals b. Modal arrivals c. Peak arrivals d. Dispersion curves e. Statistical characterization

b. Signals at an array of hydrophones a. The full field b. The modal phase c. Ray arrivals applied in multiple receivers.

On the other hand, the way an inverse problem is defined, depends on the modeling of the forward problem. The structure of the arrival pattern of the signal can be explained by alternative ways. It is of critical importance to isolate and exploit in the inversion procedure, the characteristics which are associated with the forward propagation model. The starting point for the forward acoustic propagation modelling is the definition of the elliptic problem governing propagation of monochromatic signals in the ocean environment. The core of the problem is the Helmholtz equation for the acoustic pressure, ( )p x

written in the form

2

202( ) ( ) ( )

( )p x p x x x

c x

(3.1)

Where ( )c x

the sound speed, ω is is the circular frequency and 0x is the position

vector of a point source. The problem is completed by assigning appropriate boundary conditions. When a broadband source is considered, as it is the case in the problems of ocean acoustic tomography, a Fourier transform from the frequency to the time domain enables the representation of the acoustic field in the time domain.

1( , ) [ ( ; )]; ]p x t p x t

(3.2)

where ( ; )p x is the solution of equation (3.1) for the frequency ω. When ray acoustics is considered, the signal is described as a superposition of ray arrivals. In other words the acoustic energy is considered as propagating along distinct rays. This multipath propagation is mathematically reflected in the calculation of the acoustic field by means of a series expansion of the acoustic pressure, which takes the form:

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( )

1 1

( )( ; ) ( ; )( )

N Ni x n

n nn n

A xp x p x e

i

(3.3)

where, pn is the acoustic pressure corresponding to the nth eigenray and N is the total number of eigenrays reaching a specific receiving location. Solutions for ( )x and

( )nA x are obtained through the eikonal and transport equations respectively [3.1].

The pressure field in the time domain can be written in the form

1( , ) ( ) ( ( ))

N

n nn

p x t a x t x

(3.4)

where n is the arrival time of the nth ray (eigenray) and na is the corresponding amplitude. Using ray acoustics, the inverse problem is defined on the basis of the ray arrivals. The arrival time of a specific eigenray is related to the sound and current velocity profiles along the propagation direction through a relationship of the form

( ) ( ) cosnn

ds

c x v x

(3.5)

for a transmission in the positive x direction. ds is the infinitesimal ray path and n

is the ray-path for the eigenray of order n and corresponding to a specific angle of reception. is the angle between the ray and the horizontal. It is well known that the sound speed profile determines the ray-paths and therefore the forward problem of determining the ray arrival times from the sound speed and current velocity profiles is uniquely solved. In the ray-theoretic approach of ocean acoustic tomography the actual value of the acoustic pressure has no particular use. Another alternative is to apply normal-mode theory and consider the acoustic field as a superposition of normal modes that is distinct ways of energy distribution in depth. The normal mode solution of the acoustic field is of the form

1 1

( ) ( ) ( ) ( )N N

n n nn n

p x p x B x u z

(3.6)

where un(z) is the eigenfunction of order n, being the solution of a Sturm-Liouville type problem for an Ordinary Differential equation defined in depth z (depth problem). ( )nB x

is defined by solving a problem derived from the Helmholtz equation when substituting expression (3.6) for the sound pressure. Note that for range-dependent environments ( )nu x

, is defined for each range step. Each one of the terms in the sum corresponds to a modal component. Given the sound speed profile the geometry of the environment and the acoustic frequency, propagating modes are uniquely defined. Normal-mode solution can be exploited for inversion purposes in various ways to be analyzed below.

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Finally, alternative ways of modeling acoustic propagation (Parabolic Approximation, Hankel transform) lead to a solution for the acoustic pressure, which is not easily exploited for inversion purposes, unless the full field is considered. 3.3 Methods of Ocean Acoustic Tomography

3.3.1 Ray Inversions Originally, ocean acoustic tomography was based on ray theory, since the vehicle, carrying the information on the environment was the acoustic ray [3.2]. It was the ray travel time, which gave the necessary information for the calculation of the sound speed. This approach is still very popular among acoustical oceanographers due to its simplicity and its inherent physical meaning. Broadband sources are used and the signal is recorded at a single hydrophone. The original idea for solving the inverse problem of ocean acoustic tomography was based on the assumption that a reference environment (background state) is always known and that the actual one differs from the background very little. The background environment normally corresponds to a historical mean. Thus, a relationship of the form

0( ) ( ) ( )c x c x c x

(3.7)

with δc very small, can be written. Linearizing the expression (3.5) with respect to the known background state, and assuming that there is no current in the area of the tomographic experiment, the travel time variation δτn along a certain ray Γn defined for the reference environment is associated with sound speed variation through the formula

20

( ) , 1, 2,....,( )

n

n

c xds n N

c x

(3.8)

Provided that ray arrivals can be identified at the receiver location, the use of N measurements (N eigenray arrivals) could result in the extraction of the sound speed along the ray path and finally at discrete points in the water column. The problem is normally solved by discretization of the ray path and the use of orthogonal functions to describe the sound speed variations in depth (see below). The method has been extensively used especially in deep-water areas with good results. It should be noted that the identification of the peaks, that is relating the peaks of the signal at the background environment with those of the actual measurements, is essential for the application of the method. When current is added, the problem is solved by performing reciprocal transmissions from the source to the receiver. By subtracting the reciprocal times we come up with an additional relationship of the form

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20

( ) , 1, 2,......( )

n

n

v xd ds n N

c x

(3.9)

where 1 ( )2n n nd , the times n

and n being travel times for eigenray n in

the reciprocal directions. Linear ray inversion has been for several years the traditional way for implementing ocean acoustic tomography and several techniques have been developed for treating the problem defined by equations (3.8) or (3.9). Typically, this problem, falls within the class of inverse problems following continuous inverse theory defined by expressions of the form

( ) ( )i id G x m x dx (3.10)

In general, traditional ocean acoustic tomography using ray theory is applied in range-independent environments or for the range average of the sound speed variations. Thus, model parameters (sound speed differences) are functions of depth only ( ( ) ( )m z c z ). Discretization of the sound speed differences in the water column and the description of the acoustic field in terms of empirical orthogonal functions, expressing sound speed differences as

( ) ( )c z f z

(3.11)

where ( )lf z is the EOF of order l and θl is its amplitude, lead to the formulation of a discrete inverse problem of the form

1, 1,2,.......

J

i ij jj

d G m i I

(3.12)

solved by appropriate methods of linear algebra as ijG is a known matrix calculated at the background environment. Here J is the total number of unknowns and I is the total number of discrete data values.

EOF's are functions determined by statistical analysis of historical data and are frequently used to describe the depth variation of the sound speed Figure 3.3 presents an example of EOFs (three orders) determined for the environment of the Gulf of Lions in France. They are scaled for simulation reasons to the depth of 400 m. They correspond to the sound speed variation with respect to a linear reference profile.

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Figure 3.3 An example of Empirical Orthogonal Functions. This is the form of EOFs

describing typical sound speed profiles in the Gulf of Lions

Figure 3.4 The concept of ocean acoustic tomography using ray theory

The concept of ray tomography is schematically represented in Figure 3.4. The multipath propagation in the ocean environment (a) results in an arrival pattern of the signal as shown in (b). Association of the ray-paths with the peaks of the signal followed by subtracting the actual arrival times for the eigenrays from those of the reference environment, result in the formulation of equation (3.8).

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The method has been extensively used especially in deep-water areas with good results. It should be noted that tracking of the peaks, that is relating the peaks of the signal of the background environment with them of the actual measurements, is essential for the application of the method.

3.3.2 Modal travel time inversions

An alternative approach is to identify modal arrivals instead of rays. This approach could be applied in shallow water areas where there is better modal resolvability with respect to the deep water case [3.3]. Modal travel time is defined as the travel time of a modal packet propagating in the water column. The modal velocity is defined as :

0

gnn

vk

(3.13)

where kn is the eigenvalue of order n, when the problem is solved using normal-mode theory

The formula associating modal travel time variations with respect to a background environment is of the form :

0

( )nn

S

Qc x dx

(3.14)

and the integration is over the area of the sound speed variation. The function Qn is calculated for the parameters of the background environment. Figure 3.5 presents the magnitude of the signal presented in Figure 3.1, where the identification of the modal arrivals is indicated. It should be mentioned that identification of the modal arrivals is a difficult task and should be done with reliability prior to the application of the inversion procedure [3.4]. By appropriate discretization of the water environment, the application of this formula for N measurements of the modal travel time, result in the linear system

δτ Gδc (3.15)

The method works well for the recovery of range-average sound speed profiles or structures where the recoverable parameters are local variations described by means of empirical orthogonal functions (EOFs) as introduced in (3.11).

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Figure 3.5 The identification of modes of the signal presented in Fig. 3.1

It has been shown that the use of EOFs has a twofold effect as regards the inverse problem. First it leads in a decrease of the number of the unknowns (normally 2 or 3 orders of EOFs are enough to determine the field), thus rendering the problem more easy and second ensures the physical meaning of the results by forcing them to be smooth functions.

3.3.3 Peak inversions When neither ray arrivals nor modal arrivals are identifiable, an alternative approach has been introduced allowing inverting for the local maxima of the arrival pattern of the tomographic signal [3.5 and 3.6]. If the amplitude of the pressure at the receiver's location is

( ; ( )) ( ; )a t c x a t

(3.16) the arrival times of the local maxima satisfy the equation

0'( ; ) 0a

(3.17) where prime denotes differentiation. Using this approach, a linear relationship between travel time variations of the peaks and associated sound speed variations in the same way as in equation (3.7) can be defined. This approach has been used extensively for processing the data of the THETIS I and II experiment in the Mediterranean Sea with excellent results. Recently same method has been applied in Fram strait experiment of the DAMOCLES project (see next Chapter) with good results as well. Range average sound speed profiles have been obtained so far, but progress is underway to assess the conditions of applicability in range-dependent environments.

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3.3.4 Modal-phase inversions A similar approach is to invert for the amplitudes of the EOFs or the sound speed values using measurements of the modal-phase differences, defined as the phase of each normal mode filtered at the receiver's location [3.7-3.8]. However, in order that this approach is applied, an array of hydrophones is needed to determine the modal structure in the frequency domain. Although simulations for the inversion of the sound speed profiles even in the case of range-dependent media have been reported, the author knows no results using real data. This is probably to the difficulty in obtaining noise-free data which are necessary for an accurate determination of the modal phase

3.3.5 Matched-field inversions All these approaches are linear, and only variations from a known background date can be obtained. This means that a-priori information for the sound speed structure at the area of interest exists and that it is enough to define the background environment. When this is not the case, non-linear schemes have to be applied. So far, inversions based on matched-field processing are very popular. The sound speed structure at a specific area is determined by "matching" replica fields with measured ones, the replica fields determined by means of a suitable direct propagation model. The matching over a generally wide search space is controlled by means of an objective function, which has its maximum when its input set consists of the real model parameters [3.9]. Thus, the solution of the problem is in principle obtained by looking for the model parameter vector m that maximizes the objective function P(m). The approach is very simple in its concept but it is computationally expensive, due to the great number of estimations for the replica fields that have to be performed. Of course, the process is controlled by a suitable algorithm aiming at reducing the number of the required calculations. This can be done by directing the search towards most probable solutions or to a population of acceptable solutions. Simulated Annealing [3.10], Genetic Algorithms are typical examples of algorithms developed to control the directive search [3.11-3.12]. Hybrid approaches combining non-linear and linear techniques have also been applied [3.11]. Figures 3.6 and 3.7 present examples of the use of such a hybrid approach using simulated data. The approach is based on the use of a background environment for a modal-phase approach, which has been the result of the application of a matched-field scheme. Figure 3.6 (a) is the actual environment corresponding to a cold eddy, described by means of EOFs of the type of Figure 3.2. Figure 3.6 (b) presents the recovered structure, when a matched-field processing scheme with a Genetic Algorithm is applied and Figure 3.7 corresponds to same environment recovered by a hybrid scheme. The color scale is referred to the sound speed in the environment.

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a b Figure 3.6 Simulation of a cold eddy (a) and its recovery by Matched-Field processing (b).

Figure 3.7 The structure of the cold eddy recovered by the hybrid approach

The objective function used in this matched-field processing was a standard Bartlett processor defined as

( )P Cm w w (3.18)

where w(m) is referred to the calculated pressure field, C = αα , and α is referred to the measured pressure field (complex quantities). This processor takes its maximum values when the calculated pressure field matches the measured field. Alternative approaches in the context of non-linear schemes include matched-mode schemes [3.13] in which the sound speed structure is calculated by matching the modal structure rather that the pressure field, and neural networks in which the field matching is preceded by a learning phase, aiming at teaching the network to understand the differences in the fields and a possible reason for these [3.14].

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3.3.6 Inversions using statistical characterization of the acoustic signal. Recently, new methods for ocean acoustic tomography and bottom classification have been developed at FORTH [3.15-3.17]. The new concept is that the acoustic signal is characterized using the statistics of the wavelet sub-band coefficients, which obey a certain statistical law, described by an Alpha-Stable distribution. The inversion procedure is based on an optimization scheme, which may be associated with any suitable directive search within a prescribed search space. In the methods developed at FORTH either a Neural Network or a Genetic Algorithm has been used in connection with a suitable norm such as the Kullback-Leibler divergence (KLD) of the wavelet sub-band coefficient distributions, which measures in an appropriate way the difference, between the measured and simulated acoustic signals involved in the optimization process. It has been shown that the method is effectively applicable for both ocean acoustic tomography and bottom classification applications, in both nois-free and noisy environments. Of course, when the measurements are performed in a noisy environment, the performance of the signal characterization and subsequently of the inversion deteriorates substantially 3.4. Bottom classification When an acoustic observatory is considered, bottom classification can be done by using same data as in the case of ocean acoustic tomography. Therefore, the methods applied for ocean acoustic tomography are also applicable for classifying the bottom or monitoring its changes. It should be noted, that if the receivers are positioned away from the source, local reflection phenomena that could in principle be involved in the inversion procedure, cannot be considered, due to the multiple-path propagation that makes it difficult to isolate single rays that interfere with the bottom and apply techniques similar to the one described in the previous section. It is therefore desirable to use the full-field measurement at the hydrophone array or at a single array to invert for the geoacoustic parameters. A straightforward approach is therefore matched-field processing [3.18]. The technique is vastly applied for geoacoustic inversions as it was noted during a benchmark exercise for bottom geoacoustic inversions (Vancouver 1997 [3.19]), where most of the presentations were based on this technique, the only differences being in the search algorithm utilized. In this context, genetic algorithms were proven to attract the interest of the majority of the participants. Working with matched-field processing, all the mathematical tools related to ocean acoustic tomography are also usable for bottom classification. This is in any case the main reason that the methods are often presented together. The bottom properties can be retrieved simultaneously with the water parameters. Typically classification of the bottom means determination of the number of the various sediment layers, their thicknesses, the compressional and shear velocities at the various layers, their densities and possibly attenuation coefficients. Thus, the problem of bottom classification dictates the use of a multidimensional search space when non-linear optimization inversion problems are formulated and possible restraints or relationships among the various parameters should be exploited to minimize the possibility of trapping to wrong solutions and/or accelerate the convergence. Moreover, one should take into account that some of the

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bottom parameters, such as the density, play minor role in acoustic propagation and therefore they cannot easily be retrieved. 3.5 Source Localization The matching techniques for source localization mentioned in a previous chapter, have the advantage that been non-linear by their nature could be used for the recovery of a multparameter space, which of course could in principle include the information on the location of the sound source. Thus, source localization was a sub-product of the bottom recognition mentioned above or source location is considered as an additional unknown. Here, we will present a method of source localization based on matched-mode processing. In order that this method is applicable, a vertical array of hydrophones is required at the receiving location and recordings from all the hydrophones are used. In brief, matched-mode approach as applied at FORTH is based on the recovery of the eigenforms 0( , )nA r z of the modal expansion of the acoustic field in the frequency domain, written in cylindrical co-ordinates:

01

( , ) ( , ) ( )N

n nn

p r z A r z u z

, (3.19)

where ( )nu z is the eigenfunction of order n.

Performing discrete measurements of the acoustic field in J hydrophones of a vertical array, we get the data vector:

1 2[ , ,..... ]TJp p pd , (3.20)

where,

1

N

j jn nn

p u A

, j=1,2,…,J

The "model" vector m, is defined as

1 2[ , ,..... ]NA A Am (3.21) and can be obtained by solving the system defined in by least square method.

1( )T TU U Um d , (3.22)

provided that the matrix (UTU)-1 is invertible.

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Figure 3.8 The Matched-Mode processor for a typical source localization experiment. To proceed, we need a means to "match" the eigenforms nA . This can be achieved by defining an appropriate cost function in the same way as in the case of matched-field processing. Again, we have to apply suitable search algorithms for accelerating the convergence of the procedure.

In the example presented in Figure 3.8, we have used a typical environment of a benchmark exercise described in [3.17] An array with just 25 hydrophones spaced 4 m apart have been used covering the first 100 m in the water depth at the nominal distance of 4 km. A simple search algorithm was applied with a cost function being an adaptation of the Bartlett processor used for matched-field processing. In doing so, w and α are referred to the complex eigenform instead of the complex pressure.

The search space (20-35 m in depth and 3000-4500 in range) was discretized in small range and depth increments and all possible values were checked through the program MODE1 developed at FORTH. The results showing good source localization appear in figure 9. The figure presents the cost function calculated using the corresponding range and depth for the sound source. The actual source range is 3790 and the actual source depth is 28.23 m. The inversion results obviously are excellent. 3.5 References 3.1 F.B.Jensen, W.A.Kuperman, M.B.Porter, H.Schmidt, Computational Ocean

Acoustics American Institute of Physics, New York, 1994 3.2 W. Munk, P. Worcester and C. Wunsch : Ocean Acoustic Tomography,

Cambridge University Press, Cambridge, 1995. 3.3 M.I. Taroudakis "A comparison of modal-phase and modal-travel time

approaches for ocean acoustic tomography" in Proceedings of the 2nd European Conference on Underwater Acoustics, edited by Leif Bjorno, pp 1057-1062 (1994).

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3.4 M.I. Taroudakis "Identifying modal arrivals in shallow water for bottom geoacoustic inversions" Journal of Computational Acoustics, Vol 8, No 2, pp 307-324 (2000).

3.5 E.K. Skarsoulis, G.A. Athanassoulis and U.Send. "Ocean acoustic tomography

based on peak arrivals" J.Acoust.Soc.Am. 100, pp 797-813 (1996).

3.6 G.A. Athanassoulis, J.S. Papadakis. E.K. Skarsoulis and M.I. Taroudakis: "A comparative study of modal and correlation arrival inversion in ocean acoustic tomography" in Full Field Inversion Methods in Ocean and Seismic Acoustics edited by O.Diachok, A Caiti, P. Gerstoft and H Schmidt, Kluwer Academic Publishers, pp 127-132 (1995).

3.7 E.C. Shang, "Ocean acoustic tomography based on adiabatic mode theory"

J.Acoust.Soc.Am. 85, pp 1531-1537 (1989). 3.8 M.I. Taroudakis and J.S. Papadakis "Modal inversion schemes for ocean

acoustic tomography" J. Comput. Acoust., 1, pp 395-421 (1993). 3.9 A. Tolstoy, Matched Field Processing for Underwater Acoustics, World

Scientific, Singapore, 1993. 3.10 A.Basu and L.N. Frazer, "Rapid determination of the critical temperature in

simmulated annealing inversion" Science 249, pp 1409-1412 (1990). 3.11 M.I. Taroudakis and M.G. Markaki "On the use of matched-field processing and

hybrid algorithms for vertical slice tomography" J. Acoust. Soc. Am 102, pp 885-895 (1997).

3.12 P. Gerstoft, “Inversion of seismo–acoustic data using genetic algorithms and a

posteriori probability distributions” J.Acoust.Soc.Am. 95, pp. 770-782. (1994). 3.13 T.C. Yang "Effectiveness of mode filtering: A comparison of matched-field and

matched-mode processing", J.Acoust.Soc.Am. 87, pp 2072-2084 (1990) 3.14 Y. Stephan, S. Thiria, F. Badran and F.R. Martin-Lauzer "Three neural inverse

methods for ocean acoustic tomography" in Theoretical and Computational Acoustics '95 edited by D.Lee, et al. World Scientific, Singapore, (1996).

3.15 Μ.Ι. Taroudakis, G. Tzagkarakis and P.Tsakalidis: “Classification of acoustic

signals using the statistics of the 1-D wavelet transform coefficients” J.Acoust.Soc.Am. 119, pp 1396-1405 (2006).

3.16 G. Tzagkarakis, M.I. Taroudakis, and P. Tsakalides: «A statistical geoacoustic

inversion scheme based on a modified radial basis functions neural network» J.Acoust.Soc.Am. 122, pp 1959-1968 (2007).

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3.17 M.I. Taroudakis and C. Smaragdakis: "On the use of Genetic Algorithms and a statistical characterization of the acoustic signal for tomographic and bottom geoacoustic inversions” Acta Acustica united with Acustica Vol. 95, No 5, pp 814-822 (2009).

3.18 M.I. Taroudakis and M. Markaki “Bottom geoacoustic inversion by broadband

matched-field processing”. J. Comput. Acoust. 6, Nos 1 & 2, pp 167-183 (1998).

3.19 A. Tolstoy, N.R. Chapman and G. Brooke "Workshop '97: Benchmarking for

geoacoustic inversion in shallow water" J.Comput. Acoust. Vol 6, pp 1-28 (1988).

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CHAPTER 4

Typical Experiments related to Ocean Acoustic Tomography

MICHAEL I. TAROUDAKIS1,2 and EMMANUEL SKARSOULIS2

1 Department of Mathematics, University of Crete, 714 09 Heraklion, Crete, HELLAS

2Institute of Applied and Computational Mathematics, P.O.Box 1527, 711 10

Heraklion, Crete, HELLAS

e-mail: {taroud, eskars}@iacm.forth.gr

4.1 Introduction In this chapter a brief presentation of typical tomography experiments performed in the past to validate the concept of Ocean Acoustic Tomography and test the methods implemented to handle tomography data is done. The first two experiments used similar concepts with respect to instrumentation, and inversion methods, while the third one was more advanced in the sense that additional tools were tested and the technological background was different due to the location requirements. The common factor to all these experiments was the participation of FORTH in the application of inversion tools and the analysis of the tomography data. Chapter 5, will present more details on the instrumentation used in other experiments related to Ocean Acoustic Tomography. Of course, full details on the design of the related networks, the configuration and the instrumentation used in all cases mentioned can be found in the references. 4.2 The THETIS Experiment This experiment was conducted between November 1991-April 1992 in the Gulf of Lions in France and was intended to demonstrate the potential of ocean acoustic tomography as a tool for monitoring the marine environment. It was among the first tomography experiments ever conducted in Europe and the outcome was very encouraging. The schematic layout of the location of the six (6) transceivers is shown in Figure 4.1. The acoustic sources of center frequency 400 Hz were positioned at a depth of 150 meters. The emissions were scheduled at regular intervals and recorded on a storage medium that took power from a battery. The receivers were hydrophones hanged from the same moorings at lower depths. The full recordings were exploited after the recovery of the moorings and the monitoring of the station was performed by passive listening of the emitted signals made at a shore-based station. The experiment involved measurements of the sound speed profiles at regular intervals with traditional oceanographic devises in order to obtain “ground truth” and compare the parameters

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to be recovered by acoustic means with them directly measured in the environment. It is understood that there was no possibility for acoustic data processing in real time. The participants in the program besides FORTH were the Kiel Institute of Oceanography and IFREMER, France. The analysis of tomographic data was performed using the peak-arrivals technique. [4.1]

Figure 4.1 Schematic layout of the THETIS experiment 4.3 The THETIS-2 Experiment This experiment was conducted in the period January 1994 - October 1994 in the whole western Mediterranean basin. It was actually a follow-up of the THETIS experiment with more Institutes involved. The main characteristic of the THETIS-2 experiment was the long acoustic propagation paths between source and receiver. The schematic layout of the location of the seven (7) moorings are shown in Figure 4.2. Now, lower frequency sources (center frequency 250 Hz, were added to the THETIS acoustic sources. Again the sources were positioned at a depth of 150 m. The emissions were scheduled at regular intervals (3-6 broadcasts a day) and recorded on a storage medium that took power from battery as in the case of the experiment THETIS. The receivers were hydrophones hanged from the same moorings at lower depths. The full recordings were exploited after the recovery of the moorings, just as in the case of THETIS experiment [4.2-4.3].

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Figure 4.2 The schematic layout of the THETIS-2 experiment

Again, ground truth was obtained by regular XBT shots made during regular Ferry cruises on the way from Marseille to Algiers and special oceanographic surveys. The analysis of the measurements taken, was made using special software produced for this purpose and based on tomographic inversion methods based on ray arrivals and peak arrivals (See previous chapter). The inversion results showed that the recovery of the average temperature structure of the sea over the distance between the transceivers, and as a function of depth can be done efficiently using these techniques. Figure 4.3 Examples of processed signals and inversion results showing average temperatures over depth and distance between transceivers, as a function of year-day.

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Figure 4.4 Estimated temperature profiles averaged over distance along the WR-H path of the THETIS-2 experiment (upper figure) compared with true profiles along the same path (lower figure). Figure 4.3 shows results from measurements along the path W3-H. A set of signals is shown on the left and the inversion results in the form of average temperature values over distance and depth are shown on the right. The results are superimposed to actual temperature measurements, showing a very good comparison between actual and recovered values. Figure 4.4 shows schematically the temperature profiles, averaged over distance, as obtained from the analysis of the results of the experiment THETIS-2 along the path W3-H, compared with profiles calculated from field measurements at the same time. The excellent comparison once again supports the potential of ocean acoustic tomography as a tool for ocean observation and monitoring [4.2, 4.3].

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4.4 The FRAM Strait Experiment (DAMOCLES project) The deep and wide Fram Strait, between Greenland and Spitzbergen, is the main passage through which the mass and heat exchange between the Atlantic and Arctic Ocean takes place: on the eastern side of the strait the northward West Spitzbergen Current (WSC) brings Atlantic water to the Arctic Ocean whereas on the western side the southward East Greenland Current (EGC) brings cold water and ice from the Arctic back to the Atlantic ocean. Between these two current systems the Return Atlantic Current (RAC) recirculates water masses from the western flank of the WSC back into the Atlantic (see Fig. 1). The RAC is responsible to a large extent for the variability in the heat flux through the strait [4.5]. A 11.5-month long tomography experiment was conducted by NERSC, in collaboration with Scripps Ocenaographic Institution, USA, from 16 August 2008 until 31 July 2009 in the framework of DAMOCLES EU/FP6 project to monitor the average heat content along a 130-km transect across the eastern part of the Fram Strait and contribute to improved estimation of heat fluxes through the strait by assimilation in fine-scale oceanographic models. The experiment involved a sweep-frequency source (Morozov and Webb, 2006) (S in Fig. 1) at 78o30.6'N, 8o15.1'E, at a depth of around 400 m, and a vertical receiver array (R in Fig. 1) at 78o25.5'N, 2o26.5'E with 8 hydrophones spanning the depths between 300 m and 1000 m. The source-receiver range corresponding to the above locations is 130.01 km. The source emitted 60s long sweeps every 3 hours between 190 and 290 Hz at a maximum level of 190 dB re 1μPa@1m. The receiver array was setup by combining two extended Simple Tomographic Acoustic Receiver (STAR) arrays, (4 hydrophones each), developed by Scripps Oceanographic Institution in a tail to tail configuration [4.6], [4.7]. Short baseline systems were used to monitor mooring motion and correct for displacements of the source and the receiver during the experiment. In the case of the receiver the short baseline system monitored the position of the two STAR control units at the upper and lower part of the array and the correction for each hydrophone was estimated by interpolation [4.8].

Fig. 4.5 Experiment area, source (S), receiver (R) location.

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The temperature distribution ( , )T r z on the vertical plane between source and receiver, as a function of range (r) and depth (z), was parameterized as a modal sum

0( , ) ( , ) ( , )L

T r z T r z f r z

(4.1)

where 0 ( , )T r z is the background (reference) temperature distribution, ( , )lf r z are the temperature modes (e.g. empirical orthogonal functions - EOFs) and are the modal amplitudes. The parameterization for the Fram Strait experiment relied on EOF analysis of moored-array data collected by the Alfred Wegener Institute (AWI) in the period from January 2006 to June 2008 along 78o50′N. On the basis of the previous parameterization the vector 1 2, , , L

of EOF amplitudes describes a model

state, i.e. a possible temperature distribution. The matched-peak solution to the inversion problem adopted for the analysis of the tomography data obtained during the experiment consists in finding the population of model states that interpret (identify) the maximum number of peaks in each reception [4.9]. For this purpose the parameter domain was discretized into a finite set of model states. Using the typical model relations, arrival times were predicted for each model state and compared with the observed ones seeking to maximize the number of matches. For the inversion of the measured travel-time data the matched-peak approach with Markov Chain Monte Carlo (MCMC) sampling was applied. The first 5 EOFs were used for the parameterization of the temperature (sound-speed) distribution. The inversion results were once again very good. The average temperatures estimated by inversion, matched very well the ones estimated by traditional oceanographic surveys. Fig. 8 shows the heat content evolution represented by the temperature average between the surface and the depth of 2000 m resulting from the inversion of the travel-time data over the 10-month duration of the experiment. The error bars represent the rms variability of the selected population of model states for each reception. The average temperature from the KV Svalbard (KVS) and Håkon Mosby (HM) oceanographic surveys along the tomographic section, also shown in the figure, are in agreement with the inversion results. Similar results for average temperatures in the 150-500-m layer have been obtained and are presented in Fig. 4.7 [4.10]. This layer is the most important one in the area since it represents the Atlantic water. Previous inversion studies have shown that, due to the particular propagation conditions in the Fram Strait area, ocean acoustic tomography has maximum resolution in that layer. The 150-500-m average temperature from the KV Svalbard (KVS) and Håkon Mosby (HM) surveys are in agreement with the inversion results.

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Fig. 4.6 Evolution of range-average temperature over 0-2000-m layer (entire water column) resulting from inversion and comparison with KV Svalbard (KVS) and Håkon Mosby (HM) data.

Fig. 4.7 Evolution of range-average temperature over 150-500-m layer (Atlantic water layer) resulting from inversion and comparison with KV Svalbard (KVS) and Håkon Mosby (HM) data.

4.5 Software for the analysis of the tomography data Based on the experience gained through the participation of the ESONET Ocean Acoustic Tomography Group in International projects, special computational tools have been developed aiming at the handling and subsequent inversion of the acoustical data collected at acoustic observatories for specific tomographic applications.

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Some of these tools which are in principle usable by non experts (but of course scientists aware of the ocean acoustic tomography features) are described below. Of course, these tools are not the only ones available in the scientific community, but are mentioned here as an example of what the members of the ESONET Ocean Acoustic Tomography group have developed to support the handling and exploitation of the acoustic data measured at the Ocean Acoustic Observatories.

TOMOLAB

Tomolab is an integrated software package, developed in the framework of the OCTOPUS project (EU/MAST-3), for the design, processing and analysis of ocean acoustic tomography experiments. It contains toolboxes for experiment and instrument description, ocean data analysis, forward and inversion-related acoustic calculations, transceiver navigation, tomography data pre-processing, travel-time estimation, peak identification and slice inversion. Tomolab includes the following toolboxes:

The experiment and instrument setup toolboxes are used to define/describe the geographical and geometrical parameters and data of tomography experiments as well as the parameters and data referring to the moored instruments and their components (controllers, clocks, transducers, receiving arrays, navigators).

The ocean data toolbox handles historical and experiment-specific oceanographic information (data bases) and provides sound-speed and bathymetry sections, mean parameters for temperature and salinity, sound-speed modes and their variances, as well as sound-speed to temperature conversion relations to be used for tomographic inversions.

The acoustic toolboxes (Forward acoustic and Inversion-related acoustic) are used to perform forward and inversion-related acoustic calculations for a particular experiment and using ocean data defined/provided by the previous toolboxes The forward calculations refer to arrival patterns, arrival times and related quantities using ray-trace and normal-mode codes. The inversion-related calculations refer in addition to the derivatives of arrival times with respect to sound-speed modes (influence/observation matrices) for a set of background states.

The navigation toolbox is used for the estimation of the tranceiver position using the time series of interrogation data between the transceiver and the bottom transponders. It is also used for the estimation of the clock drifts during the experiment exploiting endpoint or intermediate calibration data.

The pre-processing toolbox contains tools to perform correlation processing and Doppler analysis of the raw tomography data. It can be further used for normalization, windowing and oversampling, as well as for navigation (mooring-motion) and clock-drift correction of the acoustic data.

The estimation-identification toolbox addresses the problems of arrival-time estimation, peak tracking and identification in the pre-processed acoustic data, using manual and statistical methods, as well as the offset calibration problem to account for the uncertainty in the horizontal distance between source and

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receiver.This toolbox is used to convert the pre-processed acoustic data into travel-time data and identified peak tracks.

Finally, the inversion toolbox is used to perform slice inversions and estimate averaged temperature distributions along particular sections. It covers a broad range of inversion methods, from traditional inversions of the identified peak tracks to simultaneous identification-inversion of the estimated arrival times and matched-peak inversions.

The above toolboxes have been developed by 4 different groups (IfM Univ. Kiel, FORTH/IACM, LIS and IFREMER), exploiting the available expertise of each group in the different fields. The interaction of the user with the toolboxes takes place through a graphical user interface under MATLAB. An interactive geographical map gives an overview of the experiment geometry on a geographical background and facilitates the definition, selection, and control of moorings and sections to be processed.

Figure 4.8 A typical screen from TOMOLAB

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SMTAS SMTAS (Streaming Mode Tomographic Analysis System) is an integrated software system developed by FORTH/IACM in the framework of the Damocles IP (EU/FP6) for asynchronous reception, validation, processing and inversion of acoustic tomography data. SMTAS expects the tomographic data to be transmitted in near real time directly from the experiment location or sent in batches at a later time whenever data are recovered from the moorings. A first level of preprocessing is expected to be carried out beforehand, such that the transmitted data are free of clock-drift errors and mooring motion effects. All dataflow to and from SMTAS is performed in near-real time through the internet using standard communication protocols (SMTP and POP3). The inversion results produced by SMTAS -in the form of horizontally averaged temperature profiles or heat contents of particular depth layers and associated error estimates- are archived locally. A selected fraction of the results is uploaded in near-real time to a web site for further dissemination. Besides on-line near-real-time analysis of incoming data, SMTAS also allows for off-line batch mode analysis of already stored data. In that case the user is able to select one or several sets of input data and manually invoke an inversion code to produce new inversion results, combine the archived inversion results (average temperatures) to plot the temperature variability over time, post the produced inversion results and combined plots on the web for dissemination.

4.5 References 4.1 The THETIS Group, "Open-ocean deep convection explored in the

Mediterranean" EOS, Transactions, American Geophysical Union 75, pp.217-221 (1994).

4.2 The THETIS-2 Group, "Acoustic observations of heat content across the

Mediterranean Sea" Nature 38, pp 615-617 (1997). 4.3 E. Skarsoulis, U. Send, G. Piperakis and M. Testor, “Acoustic thermometry of

the western Mediterranean basin”, J. Acoust. Soc. Am. 116, pp. 790-798, (2004).

4.4 E. Skarsoulis and G. Piperakis, “Use of acoustic navigation signals for

simultaneous localization and sound-speed estimation”, J. Acoust. Soc. Am., 125 (3), (2009).

4.5 U. Schauer, A. Beszczynska-Möller, W. Walczowski, E. Fahrbach, J. Piechura, E. Hansen, “Variation of Measured Heat Flow Through the Fram Strait Between 1997 and 2006,, in Arctic-Subarctic Ocean Fluxes: Defining the Role of the Northern Seas” in Climate, R. Dickson et al. (Ed.), Springer, 65-85, (2008).

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4.6 H. Sagen, S. Sandven, P. Worcester, M. Dzieciuch and E. Skarsoulis, The Fram Strait acoustic tomography system, Proc. 9th Eur. Conf. on Underwater Acoustics, Paris, 13-18, (2008).

4.7 S. Sandven, H. Sagen, S.A. Haugen, J. Wåhlin, S.L. Johansen, S. Myking, P.

Worcester, A. Morozov, C. Hubbard, A. Smerdon, J. Abrahamsen, K. Bruserud, J. Johansen, P. Wieczorek, A. Beszczynska-Moeller, O. Strothmann, H. Legoff, H. Hobæk, and V. Rosello, The Fram strait tomography experiment 2008, NERSC Technical report No. 298 (2008).

4.8 Svein Arild Haugen "Acoustic Tomography in the Fram Strait: - Predicted and measured travel times" Department of Physics and Technology University of Bergen/Nansen Environmental and Remote Sensing Center.

4.9 C-T. Chen and F.J. Millero, Speed of sound in seawater at high pressures, J. Acoust. Soc. Am., 62, 1129-1135 (1977).

4.10 E. Skarsoulis, G. Piperakis, M. Kalogerakis, H. Sagen, Ocean acoustic tomography: Travel-time inversions in the Eastern Fram Strait, Proc. 9th Eur. Conf. on Underwater Acoustics, Paris, 19-23 (2008).

4.11 E. Skarsoulis, G. Piperakis, M. Kalogerakis, H. Sagen, S. Haugen, A. Beszczynska-Möller, P. Worcester, Tomographic inversions from the Fram Strait 2008-9 experiment. ECUA conference, July 2010.

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CHAPTER 5

Instrumentation for Ocean Acoustic Tomography

CRISTIANO SOARES and PAULO FELISBERTO

Institute for Systems and Robotics

University of Algarve Campus de Gambelas

8005-139 Faro Portugal

e-mail: {csoares, pfelis}@ualg.pt

http://www.siplab.fct.ualg.pt/

5.1 Introduction

At a full operational stage, Ocean Acoustic Tomography (OAT) is based on the integration of acoustic and environmental instruments into a network that will provide the means for acoustic and non-acoustic data storage and analysis, in order to produce meaningful tomographic estimates of the water column traversed by the acoustic waves: such a network has been named tomographic network in several occasions [1]1. A variety of concepts with increasing complexity can be considered, according to the scenario of appli- cation and underlying objectives.

Three decades ago, the OAT concept was proposed as a tool to map ocean mesoscale variability, with spatial scales of order 100 km and time scales of order 100 days using many crossing acoustic paths [2, 3]. OAT was also recognized to be advantageous to measure variability on the scale of the great wind-driven ocean gyres, which have spatial scales of thousands of kilometers. This is considered a branch of acoustic tomography in which a much sparser set of acoustic paths is used to obtain the average temperature of the intervening ocean. The most significant experimental activities in large OAT have been undertaken in the North Pacific, in order to study the low frequency fluctuations over long ranges [4], and the Acoustic Thermometry of Ocean Climate (ATOC) project [3]. In a different context, the end of the Cold War, the attention of research in underwater acoustics shifted to littoral regions. While deep-water tomog- raphy aims mainly to measure the variability of heat content, shallow-water tomography aims at the observation small-scale oceanographic features such as internal tides, solitons or other frontal structures. During the 1990s a num- ber of studies were made in order to adapt deep ocean methods to coastal environments, for the observation of small-scale ocenographic phenomena, in which shallow water propagation and high environmental variability make the inversion significantly more difficult. Over the 1980s significant efforts in understanding the acoustic propagation in shallow-water were already made with regard to Anti-Submarine Warfare (ASW) activities [5]. A 1 The references in this and the next chapter will be referred in the text without the chapter number.

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number of reliable acoustic propagation models based on the description of physical quantities (water temparature, seafloor parameters, etc.) of the propagation channel were available, which allowed for taking on research of model-based estimation problems involving physical quantities of an experimental scenario [5, 6]. Initially most of these studies were on source localisation problems using the Matched-Field Processing technique (MFP) [7, 8]. MFP is a model-based inversion technique that uses replica fields generated by a computer code for a set of model candidate parameters to be matched with the observed acoustic data. The success of this technique is highly dependent on the accuracy of the environmental knowledge, and therefore intensive environmental data acquisition was required. If a certain degree of accuracy is not achieved the inversion may fail to pinpoint the source at the right location due to model mismatch. To mitigate model mismatch, the focalization processor [9] and the uncertain OFUP processors [10] emerged during the first half of the 1990s decade the latter with lower degree of success. Collins et al. have demonstrated that it is possible to overcome mismatch and accurately estimate source location with limited a priori environmental information by expanding the parameter search space of MFP to include environmental parameters. Focalization has the primary goal of determining source location and perhaps the secondary goal of determining effective ocean acoustic parameters. While a linear growth of the number of parameters implies an exponential growth of the size of the search space, the implementation of this technique was possible thanks to the simultaneous emergence of very efficient computational algorithms such as genetic algorithms (GA) and simulated annealing (SA), and a rapid development of powerful computers at the beginning of the 1990. Environmental focalization provides a powerful solution for the lack of accurate measurements of the environmental parameters, and to overcome mismatch to allow proper source localisation. This technique clearly allowed enhancing source localisation since little success on source localisation with real data was achieved before it was employed [11,12,13,14]. The only successful shallow water continuous source localisation results with real data were reported by Jesus [15]. Since then, there has been a number of papers reporting on successful source localisation results [16,17,18,19,20,21]. Soares et al. [19,22] have shown with experimental data collected in well con- trolled experimental conditions that the impact of environmental mismatch on source localisation performance can vary with range and frequency. It was also demonstrated how effective focalization for source localisation can be at frequencies up to 1500 Hz and source ranges up to 10 km. Today it must be recognized that although many of these studies had in mind the objective of enhancing performance on source localisation, these developments were seen as a generalisation of the environmental focalisation methods, this fact has, with no doubt, boosted the rapid development of tomographic and geoacoustic inversion in shallow waters. These problems have been called Matched-Field Tomography (MFT) and Matched-Field In- version. This class of methods for environmental inversion was successful because they exploit amplitude

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and phase information, in space or time, received by an array of hydrophones [16, 23,24,25,26,27,28,29,30]. While in deep water, the inversion may be based upon acoustic travel times of identifiable multipaths, this class of approaches has little chances of success in shallow coastal waters due to complex propagation effects incurred by boundary interactions and due to ray wandering, which often makes arrival identifiably impossible. While the introduction of physical models in underwater acoustic signal processing has been one of the most significant advances in this field ever, the experimental studies on shallow water source localisation, during the 1980s, have demonstrated the challenge that shallow water acoustic propagation poses in terms of modelling of the physical scenario. It soon became obvious that basing inverse problems on the physical description of the propagation channel physical, implies that environmental knowledge must be systematically exploited as a priori knowledge to be incorporated in most estimation problems, even if the environmental parameter search becomes as broad as in a tomographic inversion problem. The point is that an effective tomographic network cannot consist only of acoustic instruments. Its performance will benefit from a variety of direct environmental observation tools, in order to collect useful environmental information for the physical description of the underlying environment. A tomographic network may range from a single pair of acoustic source acoustic receiver array, to more complex tomographic networks consisting of multiple moored and mobile acoustic instruments, and moored and mo- bile environmental instruments. Historically, these degrees of complexity are respectively associated to deep-water large-scale tomography and shallow- water small-scale tomography. For example, meaningful observations of large-scale processes can be obtained by simply measuring the travel times of pulsed acoustic signals propagating from a source to a distant receiver over multiple propagation paths. This objective can be achieved with a pair of a single acoustic source and single receiver at each propagation path. On the other hand, the observation of small-scale processes in shallow-waters usually goes further than just observing the temperature variability through changes in travel-time over long observation periods. Often the ocean interior, e.g., watercolumn and seafloor descriptors are to be determined, either on a long term basis or just within a few days or weeks, e.g., in the case of man-made disasters. In that case, it has been demonstrated in many occasions that in order to consistently track environmental properties such as the watercolumn temperature or geoacoustic properties, a combination of relatively well populated acoustic vertical arrays are required to provide sufficient acoustic discrimination, and a diversity of environmental instrumentation are required in order to provide a priori input data for the inherent acoustic inversion problem. It is an important issue to determine to what extent it makes sense to perform environmental surveys and continuous acquisition of environmental ground truth data of water column and seafloor properties, in and to deter- mine the which means are most efficient in providing that information, for feeding acoustic inverse modelling with input data. Conceptually, this extent should be small enough to keep OAT advantageous in comparison to the traditional data acquisition tools

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used in oceanography, and large enough to allow compatible acoustic modelling to take place and the objectives of a tomographic network to be achieved. In practice, this will contribute to the dimension of the network in terms of the number of observation points and amount of instruments that need to be employed. Concerning the Acoustic Observatories under discussion herein, perhaps a precise answer to that question can not be found within this study, but at least minimal specifications must be achieved in order to start a network. It is clear that the main objective of the future acoustic observatories is to aid in the oceanographic observation of certain spots of the ocean on a permanent basis, for a long time-interval (let’s assume several decades). Generally, oceanographic observations serve the purpose of developing ocean models for oceanographic forecasting and for feeding these ocean models with in-situ data for continuous update. However, the high spatial and temporal variabiltity of the littoral oceanography severely limit the predictive capabilities of oceanographic models, which are essential in topics such as ecossystem management and coastal protection, spacially in the case of environmental hazard. Past studies on frontal structures in regions of mixing or upwelling have focused either on high-resolution volume measurements directly in the frontal region, or larger scale synoptic surface observations. To accomplish both objectives with traditional methods can not be practiced. In order to overcome this limitation, several studies made significant efforts in understanding whether the inclusion of environmental observations by means of acoustic tomography concepts would bring a contribution in im- proving the predictive capabilities of oceanographic models [31, 32, 33, 34]. Instead of using only local observations provided by direct measurement instruments, the use of acoustic fields provides synoptic observations of integral nature over the propagation path with high time-resolution. This represents an important advantage in terms of spatial coverage. Spatial coverage can be significantly increased if multiple emitters or receiving devices are employed, and even further, if moving platforms are used for emitting or receiving acoustic signals [35]. Such characteristics are important for the problem at hand, as usually, in applications regarding small-scale phenomena, as for example in Rapid Environmental Assessment (REA), it is hypothesized that such tomographic network should cover at least a shallow-water area in the order of 10×10 km2 in order to appropriately sample phenomena in frontal structures such as mixing and upwelling. Although it is in general established that the environ- mental properties sensed over horizontal propagation paths are of integral nature (which is often associated to range-independence), it is conceptually possible to use an arrangement of acoustic instruments (emitters and receiver arrays) such that range-dependent features become observable, at least to a certain degree, if the acoustic data received at multiple acoustic arrays is combined for acoustic inversion. The idea is based on the division of a given area into cells, and set up emitters and receivers in such positions that those cells are crossed by multiple transmission paths [36, 37]. Once a working area of interest is established, one needs to determine the minimum number of platforms of acoustic emitters and receivers necessary to be able to resolve the spatial structure of oceanographic features, and the required

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environmental observation instrumentaton necessary in order for feeding the acoustic tomography. One can anticipate that the spatial structure will be resolved with resolution depending on the number and relative positions of emitter and receiver platforms. Also at the level of each individual platform issues related to minimal requisites will be posed. Basic questions are, for example, operating frequencies, minimum array aperture or minimum number of acoustic receivers in each acoustic array; or issues related to the experimental configuration, such as, moored platforms versus mobile platforms, active mode or passive mode, whether using sources of opportunity are an issue to be taken under consideration. Increasing scientific requisites will tend to require a more complex acoustic system. The reciprocal is also true, as one must accept loss in performance when simplifications in the acoustic system are operated. A significant part of the scientific effort has gone into the development or adaptation of acoustic inversion techniques for acoustic hardware with decreasing complexity (e.g less acoustic receivers), as it was clear to the community that most of the existing acoustic apparatus were too complex and bulky for operational use. Over the years the quest was inevitably to reduce the acoustic hardware without significantly losing the overall performance of the acoustic system. The objective of this chapter is to provide a description of the instrumentation required in a tomographic observatory. The underlying hypothesis is to design an acoustic observatory called small-scale tomographic network for a baseline shallow water area of 10× 10 km2. First an overview of a number of experimental studies on tomogrpahy and geoacoustic inversion is provided, with a main focuses on the instrumentation employed to achieve the underlying scientific objectives. This study allows for obtaining an insight on the instrumentation required for shallow water topographic networks.

5.2 An overview This section provides an overview on previous experimental studies in ocean acoustic tomography. This review focuses mainly on the instruments used in each experiment, with the objective of retrieving their technical specifications in the framework of state-of-the-art or future scientific objectives.

5.2.1 Deep water tomography Three decades ago, the OAT concept was proposed as a tool to map ocean mesoscale variability, with spatial scales of order 100 km and time scales of order 100 days using many crossing acoustic paths [2, 3]. OAT was also recognized to be advantageous to measure variability on the scale of the great wind-driven ocean gyres, which have spatial scales of thousands of kilometers. This is considered a branch of acoustic tomography in which a much sparser set of acoustic paths is used to obtain the average temperature of the intervening ocean. Beginning 1989 a series of experimental activities have been undertaken in the North Pacific in order to study the low frequency fluctuations over long ranges [4]. The first

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experiment combined a broad- band transmitter designed to measure travel times with 1 ms precision with a 3 km long vertical receiving array. The transmissions consisted of a 250 Hz continuous wave over a 1001 km range. Another opportunity for further testings was an engineering test to support the Acoustic Thermometry of Ocean Climate (ATOC) project [3], where transmissions of 75 Hz broadband transmissions over a 3252 km range took place near California to a 20-hydrophone vertical array located near Hawaii, in order to demonstrate the feasibility of using long, ocean-basin-scale paths for acoustic thermometry. Results showed early ray-like time fronts that were resolvable, identifiable, and stable. These arrivals were actually more stable than predicted, with relatively small values of travel-time wander and pulse spread. This ATOC project was to determine the precision with which acoustic methods can measure large-scale changes in ocean temperature, with the ex- pectation that the ATOC project would lead to a long-term, global program for measuring the changing ocean heat content. After the successful test early arrivals were clearly shown to be useful for thermometry - two 75 Hz ATOC transmitters were installed. One, off California (1995), and another off Kauai (1997). Also in conjunction with ATOC, in 1995 two vertical array receivers were installed near Hawaii and Kiritimati Islands. Each operated for about a year. In 1998 another vertical array was installed at Ocean Weather Station Papa in the Gulf of Alaska, which operated for nine months. There were also bottom mounted arrays. These arrays were moored at waterdpeths of approximately 1800 m.

5.2.2 Shallow water tomography Early experimental work Until the beginning of the 1990 the impact of internal waves upon acoustic transmission was reasonably well understood in deep water, while in shallow- water this problem had not received significant attention due to the lack of priority in anti-submarine warfare efforts, and due to its grater difficulty. In shallow-water this problem is more complicated due to bottom interaction and complicated range-dependent oceanography, and the need for vertical acoustic arrays to resolve the modal structure. Before 1990 only a few shallow-water experiments took place in order to address this problem. The first effort was by Essen , et al. [38]. In an experiment in the North Sea using a single CW source transmitting to a single hydrophone, they measured total field fluctuations due to internal waves in the North Sea, and reasonably reproduced the observed data features with a theoretic model using a modal wavenumber perturbation approach. Another major effort was the Yellow Sea experiment by Zhou , et al., to examine the scattering of acoustic waves by internal waves[39]. Using shot sources and single hydrophone receivers, Zhou noticed large frequency and azimuth-dependent propagation losses.

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The Barents Sea Polar Front Experiment In 1992, Lynch et al. went on a larger effort on understanding the impact of oceanographic features on acoustic propagation on the acoustic propagation [40]. They conducted three extensive hydrographic surveys over a 70×80 km area along the front. Acoustic transmissions were performed using a 224-Hz tomographic source and a vertical line receiving array for the observation of shallow-water internal waves. They were able to model the internal-wave- induced travel time fluctuations seen in the data using adiabatic mode theory and a perturbation approach. The SWARM Experiment Later, in 1995, several institutions joined for another large-scale experiment- the SWARM Experiment, that took place in the Mid-Atlantic Bight off the coast of New Jersey [41]. The primary objective of this experiment was to quantify the interaction of the acoustic field in the 10-1000 Hz band with linear and non-linear waves in a shallow-water waveguide. In addition to acoustic data, physical oceanographic data and bottom geoacoustic data were collected during a 3-weeks cruise. To achieve this, a variety of acoustic transmissions were employed, and up to 25 oceanographic/acoustic moorings were deployed and recovered successfully. These instruments were placed on a straight line with a length of 42 km. Several moored and towed acoustic emitters and moored receiver arrays were used during the SWARM Experiment. The moored sources consisted of: 1) a 224-Hz Webb Research Corporation organ pipe tomography source; 2) a 400-Hz Webb Research Corporation organ pipe tomography source; and 3) a 300-Hz linear frequency modulation (LFM) source. The 224-Hz source was moored in a water depth of 54.5 m, with the source 1.5 m above bottom. It had a bandwidth of 16 Hz and a nominal source level of 181 dB re 1µPa at 1 m. The 400-Hz source was moored in a water depth of 54 m, 25 m above the bottom, 800 m to the SE of the 224-Hz mooring. It had a bandwidth of 100 Hz and a source level of 181 dB re 1 µPa at 1 m. The 400-Hz source was also navigated to a nominal 1-2 m accuracy and carried a Siemens temperature sensor near the source. Two J-15-3 towed acoustic sources were deployed during the experiment. These sent CW tones for both Hankel transform inverse bottom property studies and shelf propagation studies, including transmissions: 1) out of the plane of the moored source/receiver line and 2) across the shelf break front. Some LFM sweeps over the band of 50-600 Hz were also transmitted. These sources were generally placed just below the surface mixed layer, producing a peak pressure level of 160 dB re 1 µPa at 1 m. A 20-cubic-in Bolt air gun was deployed from a ship to generate broad-band acoustic pulses. These pulses has the energy peak at 75 Hz, with usable bandwidth

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from 50 to 1000 Hz. The source level of the air gun was 210 dB re 1µPa at 1 m. Its repeatability was checked in situ by a source-monitoring hydrophone for each pulse. An automatic triggering system enabled differential Global Positioning System (DGPS) time and position to be recorded for each shot. The main purpose of the air gun experiments was to measure the anisotropy in propagation induced by the coastal internal wave field. Three verical line arrays were moored on a line. The distance of these instruments to the moored acoustic sources was, respectively, 23, 33, and42 km. The vertical line array (VLA) receivers were positioned so as to look at the acoustic field and its fluctuations versus range from the sources: • A 16-element internally recording VLA was placed at 23 km from the

sources, but unfortunately failed due to an O-ring leak. The autonomously recording thermistors placed on this mooring did function, giving us some useful environmental information.

• Another VLA placed on the shelfward line of VLAs was deployed at 33 km from

the sources in 70.5 m of water. The received band was generally 25-500 Hz, though this could be adjusted in real time by the operator. The array had both internal recording and highspeed local area network (LAN) data and control telemetry capabilities, making it quite flexible for data acquisition. The array also had five temperature sensors attached to it, recording temperature at 30 s internals.

• A 32-element radio-telemetered array was deployed on the shelf ward line of

VLAs at 42 km from the sources in 88 m of water. This array spanned the water column from 21 m to 85 m, with a hydrophone spacing of 2 m. Its working bandwidth was from 1 to 1000 Hz. The array had five autonomous temperature sensors attached to it.

All the arrays transferred data via radio telemetry, thus allowing very high data rates. A large number of instruments for environmental survey/observation was used from a research vessel, satellites and an aircraft were employed: • A CTD was operated on a tow-yo line back and forth between the source and

receiver arrays (and often as far out as the shelf break), thus pro- viding the range- and time-dependent sound-speed profiles needed for acoustic propagation and scattering studies, as well as physical oceanographic information. Also XBTs were deployed, particularly toward the end of the cruise when a gale and associated high sea state prevented normal CTD operations.

• ADCP current profile records were continually made as part of the background

operations of the ship, as were meteorological and wavefield measurements. • To visualize the vertical and horizontal structure of the internal wave field, two

highfrequency backscatter sonars were employed, operating at 200 and 300 kHz, respectively. These sonars had beamwidths of 3.5◦ and 1.5◦, respectively, vertical resolutions of 15 cm, and sampled at a 4 Hz rate.

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• The surface signatures of the solitons over a circular region of 10.8 km diameter

was mapped out with the ships radar. • In addition to the ship’s radar observations of the surface reflectivity disturbance

due to the soliton field in the region, ERS-1 SAR images of the region were obtained.

• Advanced very-high-resolution radiometer (AVHRR) radiometry and other

satellite images were obtained. • Aircraft radar was also employed, but unfortunately did not produce any

acceptable data. The Yellow Shark Experiments The Yellow Shark Experiments were a set of two broadband inversion exper- iments carried out by the NATO Undersea Research Centre (NURC) in a shallow water area south of Elba Island, off the West Coast of Italy [42, 25]. One took place during the Summer of 1994 (YS94)[42], and the other dur- ing Spring of 1995 (YS95) [25]. The objective of these experiments was to investigate the possibility of determining the representative integral environ- mental properties of shallow water media from the inversion of waterborne acoustic propagation data. The YS94 experiment was carried out over a 15 km transect with mild range-dependence and approximately 110 m water depth, northwest of Formiche di Grosseto islands. A 32-element vertical line array with an acoustic aperture of 62 m was moored in a site with 112 m water depth, spanning the water column below the thermocline. The acoustic source, emitting 7 tones in the 200-800 Hz band was moored at ranges 15, 9, 6, and 4.5 km from the VLA over 2 experimental days, at mid-depth. The choice of broadband emissions was seen as an important step to increase the observability of the environmental properties, since it was already well established that propagation in a shallow water channel is frequency-dependent and sensitive to the boundary conditions, and that time dispersion characteristics are intrinsically related to the frequency-dependent geoacoustic properties of the sea bottom. The YS95 was a much larger experiment, as the experimental configuration was based on a 40×41×55 km triangle, with the three acoustic transects connecting the Elba Island, the Formiche di Grosseto Islands, and the Montecristo Island, with variable degree of range-dependence and bathymetry varying between 65 to approximately 400 m. Extensive acoustic and environ- mental measurements were performed along the three transects. Large time- bandwidth-product signals spanning the frequency band 200-1600 Hz were transmitted for a period of 12 days, over propagation distances ranging from 8 to 55 km. Fixed and ship-towed acoustic projectors were used. Most of the receiver arrays used were portable easy-to-deploy 4-element VLAs with sub- surface floats. As an example, the experimental configuration over the Elba Island-Formiche di Grosseto Islands consisted of an acoustic source moored on the Elba Island platform with water 65 m and deployed at 32.5 m depth, and five 4-element portable VLAs at ranges of 8, 16, 24, 32, and 40 km. The acoustic transmissions with this configuration served the purpose of estimat-

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ing range-dependent geoacoustic properties. The experimental results for geoacoustic inversions obtained with this data set were of good quality according to the authors of references [25,42]. However over propation ranges of the magnitude of the Yellow Shark Experiments, the successful geoacoustic inversion was possible only with the inclusion of range-dependent oceanography in the inversion process, either using in situ sound-speed data, or by incorporating free parameters for sound- speed modeling in the inversion problem. The INTIMATE Experiments The objective of the INTIMATE Project was a partnership between the University of Algarve, Faro, Portugal, and CMO-SHOM, Brest, France. The objective of this project was to conduct a theoretical/parametric study for determining the performance of a source-receiver system for the inversion of oceanographic parameters in shallow water, and to determine the characteristics of the acoustic system (frequency, resolution, array aperture, depth, etc.,...) for an optimal performance on typical coastal scenarios. This included tests of complete systems and validation with ground truth data, in order to establish the capabilities of the tomographic system to study internal tides on the continental shelf. The INTIMATE project included two sea trials, one in June 1996 in Nazare [43, 44, 45, 46, 47, 48], off the Portuguese West Coast, for testing the feasibility of the scientific objectives, and a full scale sea trial that took place off the coast of France in the Gulf of Biscay in1998 [49, 48, 50, 51, 52, 53]. During the INTIMATE’96 sea trial took place in an area with waterdepth varying between 130 and 180 m. The acoustic system cosnsisted of an acous- tic source and 4-element vertical array (VA). The acoustic source was either stationnary or being towed by Research Vessel Bo D’Entrecasteaux along predetermined paths, both with range-dependent and range-independent bathymetry. The VA was deployed in a position with waterdepth of 130 m, in a U- configuration. One side of the U was the VA, completely submerged, with a sub-surface float keeping the array vertical, the 4 receiving elements spanning a 85 m of watercolumn, and an electronic module for acoustic data acquisition and digital-to-analog conversion. The other side of the U was an RF-transmitter. The acquired data was sent real-time to the data reception system on board of the second Research Vessel NRP Andromeda, which held a fixed position. During the INTIMATE’96 sea trial also employed an extensive number of measurements were performed simultaneously with the acoustic data trans- missions: • bathymetry surveys of the area and along transmission lines; • bottom moored ADCP data at the vertical array location; • thermistor chains at the vertical array location; • CTD data at the vertical array location;

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• hull mounted ADCP on the source ship; • XBT data from the source ship; • CTD data over night and along the area; • seismic survey with Uniboom; • cores along source tracks. These ground truth data play an important role in the acoustic inversion problem, as they provide great deal of a priori information for describing the environment where the acoustic experiment takes place. In the case of the INTIMATE’96 sea trial, the experimental design focused on the internal tides. In terms of the inverse problem the focus was mainly on internal tides, and therefore an accurate description of bathymetry and seafloor were required in order minimise model mismatch for forward modelling purposes. The CTD and XBT data sets are required for the initialisation of the inversion process. Usually, the temperature data collected during the experimental period is used in regularization approaches, data are based on the extraction of mean profiles, Empirical Orthogonal Functions (EOF) - a description of the temperature variability, or hydrostatic modes. The INTIMATE’98 experiment was dedicated to internal wave tomography in 4 areas of the Gulf of Biscay. The observations were carried out in the period of 25 June until 30 July 1998. The experiment consisted of acoustic transmissions using a low-frequency acoustic source and two 8-element receiver arrays. The emitted acoustic signals covered a band in the inter- val 300 to 1000 Hz. Acoustic transmission paths spanned a ranges of up to 10.45 km. Also several environmental observation instruments were used: CTD and temperature sensor arrays for the watercolumn survey; for the sur- vey of seafloor characteristics, an ADCP mounted on the hull of the source ship and an uniboom were used, and cores along transmission tracks were taken. These environmental data were incorporated as environmental knowledge in order to support acoustic data inversion for the acoustic observation of internal tides. The INTIMATE98 data set allowed to demonstrate that with a reduced number of hydrophones, sound speed perturbations can be tracked in challenging environments where internal waves activity is very significant. At the time, the achieved results were remarkable, since they were achieved in an environment with important sound speed fluctuations, with scarce information on geometric parameters, source-array synchronization was not available, and the array had a small number of hydrophones.

5.2.3 Towards three-dimensional shallow-water tomography The past sections made an overview on several experimental studies that at- tempted to develop concepts for remote sensing of oceanographic and geoa- coustic features of the ocean by means acoustic waves using fixed configurations of acoustic emitters

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and receivers. During the late 1990s and the 2000 decade, with a rapid advances in wireless communications and development of miniturized computer devices, the attention focused on concepts with the objective of observing simultaneously multiple ocean transects, or even to attempt to produce three-dimensional observations of the interior of an ocean volume, rather than two-dimensional planes. This requires using easy-to-deploy acoustic emitter and receiver instrumentation. This is due to operational issues, because in some cases it is interesting to perform deployments of equipments that are not moored any- more. For example, in Rapid Environmental Assessment, complicated and risky operations should be avoided in order to quickly obtain an operational acoustic observatory network. In such a case, a temporary survey with the duration of hours, a set of free-drifting receiver arrays and moving emitters is an interesting option. This section provides examples of experimental studies where light easy- to-deploy acoustic instruments were used. The Haro Strait Experiment Schmidt et al. proposed an acoustic oceanographic approach for improving the predictability of oceanographic features which combined the coverage ca- pability of acoustic tomography with the high resolution afforded by mobile platforms (e.g. AUVs carrying acoustic and environmental sensors) called Acoustically Focused Oceanographic Sampling (AFOS) [31,35]. The idea is to use real-time integral estimates of temperature or current provided by acoustic tomographic inversions to adaptively direct AUVs towards regions where high resolution is required due to large gradients or large uncertainties. In order to demonstrate the feasibility of this concept, the Haro Strait Experiment was performed in June-July 1996 during 5 weeks in the Haro Strait, British Columbia [31]. The main objectives of this experiment was to test the integration of the available technology into a single network, and the demonstration of the AFOS concept in the highly active zone of the Haro Strait. This was a large-scale experiment that employed multiple acoustic and environmental moorings, and mobile equipments. Figure 5.1 shows the sampling geometry in the zone where the acoustic experiment took place taken from Ref. [31]. The experiment was designed with four moored vertical acoustic arrays, each with 16 receivers, a 1.5 kHz tomographic source, and a 15 kHz communication source, but one of the moorings was lost at an early stage of the experiment. Also four environmental moorings were deployed along the Haro Strait channel for measuring local temperature, salinity, current magnitude and direction at discrete depths (only two in the region shown in Figure 5.1). A moving source towed by a ship consisting of a light bulb was used in conjunction with local non acoustic measurements in order to image the three-dimensional sound speed and current fields within the water mass en- closed by the moored arrays. This source generated signals of approximately 160 to 170 dB re 1µPa, with a 3 dB band of approximately 300 Hz centered at 500 Hz.

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Figure 5.1: Acoustic and environmental sampling geometry during the Haro Strait experiment (1996). Circles: acoustic moorings with emitters and receivers. Stars: moving source consisting of light-bulb deployments. Squares: environmental moorings [31]. The stars in Figure 5.1 show the positions where light-bulb trans- missions were performed. The moored arrays and moving acoustic source were located within short relative ranges of less than 3 to 4 km. The experimental results obtained with the Haro Strait data set in Ref. [31] used the transmissions indicated by the stars furthest to the left of the panel in Figure 5.1, between the to acoustic moorings. These results clearly demonstrated that the estimation error of temperature and current could be significantly lowered within and around the regions containing acoustic moorings when acoustic data was combined with local oceanographic measurements. The estimation of the oceanographic distribution also benefited from an increased spatial coverage not achievable by non-acoustic data only. The Acoustic Oceanographic Buoy-Joint Research Project Over the years 2001 to 2005, several institutions (CINTAL, ULB, NURC) joined for another initiative called Acoustic Oceanographic Buoy-Joint Re- search Project for research on a similar concept aiming at the conjunction of acoustic data with direct oceanographic measurements, however, with emphasis on military operations [54, 55, 56, 57, 58]. The objective was to proceed with studies on an Acoustic Rapid Environmental Assessment (AREA) concept able to produce acoustic tomographic inversions in regions without access to man and ship for feeding oceanographic forecast models. In a real scenario, such operations, would necessarily require the deployment of un- manned underwater vehicles and/or air deployments of acoustic equipments, both for emission and reception of acoustic signals. The main focus of this project was the development of a new hardware philosophy for the acquisition of acoustic data that would be able to respond to aspects related to operation in such demanding scenarios. In particular, the development of a new acoustic receiver

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device - the Acoustic Oceanographic Buoy (AOB) which named the initiative. The AOB was a new generation device with a reduced overall size, which was expected to clearly contribute for easing deployment and recovery operations, and also with a significant reduction in production cost [59]. This device integrated a computer for controlling acquisition and local data storage, and was set up with TCP/IP protocol for data download and upload, or other ways of interaction, and a GPS system for coordinates recording and real-time tracking of its position. These features were considered key features for easy operation and for real-time access of acoustic and non-acoustic data, as all these would be telemetered to a base station using standard wireless communication protocols. The AOB version 1 was first equipped with a vertical array of 4 receivers, and first deployments took place during MREA’03 sea trial in the North Elba area in June 2003. Then the AOB version 1 was equipped with a 8-element vertical array, and deployed during the MREA’04 sea trial off the Portuguese West Coast near Setu´bal in April 2004 [60, 61, 62]. All deployments over these experiments were in free-drifting mode, which brought advantages in deployment and recovery by eliminating the difficult operation associated to a mooring, and in terms of data quality, as the buoy/vertical array body did not opposed to currents, possibly reducing array tilt and bending, as well as sources of acoustic self-noise that may potentially be generated in the case of a vertical array moored in a moving mass of water. During the MREA’03 sea trial a local area network (LAN) was set up on board for monitoring the AOB using a laptop and a computational node. The laptop served both as a workstation and gateway to the wireless component of the LAN. During deployments the buoy was monitored and drifting was followed by means of a MATLAB software running on the computational node, that regularly downloaded the on-buoy generated GPS log-file to update the AOB’s position. During that experiment the AOB was deployed during two days, one for tomography where chirp signals in two different bands within 500 to 1200 Hz were trans- mitted, and the other for underwater communications. Figure 5.2 shows the sampling geometry during the chirp transmissions for tomography. The source was towed by Research Vessel Alliance over most of the experiment, achieving a maximum source-receiver range of approximately 9 km (black curve). The AOB drifted about 3 km away from the deployment position. After AOB recovery the data was retrieved and copied onto the computational node for inversion attempts during the remaining experimental days. The inversion results were considered of reasonable quality, since they were obtained on board shortly after data collection using an inversion software previously set up for that purpose, and for the reduced number of receivers [55]. Later that data set was processed in the scope of a study on matched-field processors, in an attempt to adapt the acoustic inversion to the characteristics of the receiver device (e.g. reduced number of receivers, free drifting configuration, frequency) which brought significant improvements in terms of inversion stability [57]. In that study only 3 hydrophones were used due to reduced data quality in the fourth receiver. His study demonstrated that meaningful joint inversions of water column temperature and seafloor characteristics can be consistently achieved with an acoustic reception system with these characteristics, even with source-receiver of 9 km (approximately 80 water

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depths).

Figure 5.2: Acoustic sampling geometry during the Maritime Rapid Envi- ronmental Assessment 2003. Black curve: tow performed by Research Vessel Alliance. Blue curve: AOB drift. The development of the AOB continued towards a new version with an even smaller container and the inclusion of a DSP for running the acquisition software and for performing data pre-processing tasks. A first exemplar of AOB version 2 equipped with a 8-element vertical array was constructed, and first deployed during the High-Frequency Initiative in Hawaii in September 2005 [63, 64, 65]. After, a second exemplar of AOB version 2 equipped with a 16-element array was constructed. One of the AOBs was also equipped with a thermistor string. These devices were jointly deployed during the RADAR’07 sea trial, an experiment that took place off the West coast of Portugal near Setu´bal, with water depth over the working box varying between 50 and 140 m, in a region with strong oceanographic activity [66, 67]. Also extensive oceanographic data was collected with moored termistor strings and CTD measurements were made over the experimental area. One of the objectives was to collect acoustic data to support research on networked tomography. The idea would be at least to simultaneously observe multiple ocean transects. Individual acoustic inversion may be produced for each ar- ray data, or at a more advanced stage, processing schemes allow for joint acoustic data invsersion, or two step inversions schemes should be considered. During the RADAR’07 sea trial both AOBs were deployed and recovered during several days in a free drifting configuration. The acoustic source was towed according to pre-defined geometries, over range-dependent and range-independent transects. For this experiment, a computer LAN was set up on board consisting of a server, two dual-processor processing nodes, and several laptops (see Fig. 5.3).

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Figure 5.3: Simplified scheme describing the local area network set up at the research vessel. This network consists of data acquisition nodes, data server, computation nodes, and user workstations. The wireless component consisted of the two AOBs drifting freely over the experimental area. In this occasion, the inversion software was tuned to automatically download acoustic data and GPS data from the AOBs, in order to perform acoustic inversions of the water column and seafloor, alternating one AOB at the time. The inversion program downloaded the last time-series available for inversion and updated the GPS coordinates for determining segment ship-to-buoy bathymetry accordingly. The water column temperature compared favorably against the temperature collected with the thermistor chain equipping one of the AOBs.

5.3 Acoustic instruments: description and technical specifications

This section will provide a compilation of the instrumentation that is used in acoustic tomographic observatories. This is based on the literature review.

5.3.1 Acoustic receiver for permanent observatories

An acoustic receiver module for inclusion in a permanent topographic observatory in a shallow water coastal region is moored at a given position and linked to the land junction box via a sea cable providing energy supply and high-rate data link (IP protocols). This device will be deployed in a submersed configuration for security reasons (navigation and stealth). Its body will consist of a container, usually called telemetry unit, which consists of electronics performing data acquisition. An array of hydrophones and other non-acoustic sensors is connected to the telemetry unit [59]. The body of such a receiver system will be submerged and will consist of a telemetry unit with a an hydrophone array attached. For mooring such a device, a ballast with an acoustic release is used together with a sub surface float attached to the opposite end in order to maintain the hydrophone array vertically straight. The acoustic data acquisition module should be designed in order to allow the following functions:

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• acquisition of acoustic and non-acoustic data; • network operation for real-time data transmission; • continuous streaming of acquired data; • local data storage; • dedicated signal-processing; Such a system is implemented using a specialized PC for system management. The computer manages an acquisition board or a digital signal processor for data acquisition and peripherals such as disk, LAN communication, and timming. The data may be stored in situ or transmitted to a remote data storage and analysis platform. Each acoustic channel is a channel consisting of a pre-amplified hydrophone followed by a programmable gain amplifier, a dedicated analog-to-digital converter, and a digital acquisition board or a digital signal processor (DSP). For TCP/IP data exchange, standard LAN network devices are used, since the data volume generated by an acoustic array may be below 100 Mbit, for an adequate trade-off between number of acoustic channels and sapling rate. An acoustic module for a permanent tomographic network should be de- signed to be moored on a sea floor with a depth of up to 150 m depths. This module is a telemetry unit (a container), a vertical array of hydrophones and other sensors, such as temperature and pressure sensors, and a float. A shore cable supplies power and network connection to land station. Some of these nodes may also include an underwater acoustic modem. The acoustic channels The vertical array is suspended by a directional subsurface float with a swivel as to maintain a minimum of drag. Each acoustic sensor element has an overall sensitivity (sensitivity before analog-to-digital conversion) from -170 to -140 dB re 1 V / 1µPa. For tomography a transducer with pass-band from 1 Hz to 28 kHz is sufficient. Pre-amplifiers should provide a flat response with large bandwith, up to 50 kHz. A high-pass filter of 1 or 2 poles should be included for the removal of very low frequency - with start frequency at 100 Hz. Acquisition of acoustic data should be performed at a rate of 60000 samples per seconds, and an analog-to-digital conversion of 24 bits. The electronics The electronics is suitably based on an embedded computer standard, such as for example a PC/104. This standard is intended for specialized embedded computing environments where applications depend on reliable data acquisition despite an often extreme environment. The advantage of this standard is that it allows modules to stack together like building blocks. This allows for the construction of systems with

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relatively small size. A typical system of this type includes motherboard, analog-to-digital converter, and digital I/O (data acquisition) module, or even peripherals such as GPS receivers, IEEE 802.11 controllers, and USB controllers. The stack of such a system includes a controllable power supply, a processor board with standard interfaces; acquisition cards - usually 24-bit sigma delta with sampling frequency of up to 200 kHz per channel; non-acoustic sensor acquisition boards. Temperature and pressure sensors can be interfaced a 1-wire interface. Also inclinometers and electronic compass may be included. The controllable power supply is usually a DC-DC converter able to convert high voltage to the voltage supplied to the system. Time synchronization of the operating system is performed through the Ethernet network using the standard (Network Time Protocol) NTP protocol. The surface base station which supplies the time to the underwater module is GPS synchronized. The network bandwidth Concerning the network and bandwidth, at 100Mbps a maximum of 24 channels at a sampling frequency of 120 kHz can be streamed in real-time. At 60 kHz sampling frequency per channel, all data can be streamed in real-time to base station. The software Several software modules are useful in an acoustic module of this type: • an acoustic acquisition program, which reads the data from ADC cards and

presents the acquired data as a continuous stream in a shared memory location; • multiple simultaneous reader processes can be attached to a shared memory

location:

a) a local file writer;

b) a remote network streamer;

c) a selectable single channel extractor for monitoring;

d) a signal pre-processing module. Data can be stream and stored locally. The operating system may be a Linux kernel. The telemetry unit body The telemetry unit is a submersible cylindrical container made of anodysed aluminum in 5xxx and 6xxx grades, in order to resist against corrosion caused by salty water. A stainless-steel cage can be attached around the container to provide shock protection and fastening points. The container must provide a variety of connectors, as for hydrophones, RS-232 port for non-acoustic data, power supply, fiber optic connection, or other. The container must consider the way the mooring is going to be performed.

5.3.2 Acoustic receiver for rapid response observatories

A rapid response observatory should be based on easy to deploy acoustic receiver devices. Such an observatory may consist of a field air dropped receivers with characteristics of a sonobuoy, at least in terms of deployment ease and size. In

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terms of functionalities, this device should provide the following functions: • acquisition of multi-channel acoustic and non-acoustic data; • local data storage; • wireless network operation data transmission; • GPS for timming and positioning; • dedicated signal-processing; The acoustic channels The vertical array is suspended on the surface body and should be designed such that drag is minimized, in order to reduce array tilt and drift. Each acoustic sensor element has an overall sensitivity (sensitivity before analog- to-digital conversion) from -170 to -140 dB re 1 V / 1µPa. For tomography a transducer with pass-band from 1 Hz to 28 kHz is sufficient. Pre-amplifiers should provide a flat response with large bandwidth, up to 25 kHz. A high- pass filter of 1 or 2 poles should be included for the removal of very low frequency - with start frequency at 100 Hz. Acquisition of acoustic data should be performed at a maximum rate of 60000 samples per seconds or less, and an analog-to-digital conversion of 16 or 24 bits. In such a system, it would be useful to have variable rates of data acquisition, or dedicated signal processing resources for data down sampling, or other computations aiming the reduction data volume. This is important regarding data transfer to a remote station.

The electronics The electronics is suitably based on an embedded computer standard, such as for example a PC/104. The advantage of this standard is that it allows mod- ules to stack together like building blocks. This allows for the construction of systems with relatively small size. A typical system of this type includes motherboard, analog-to-digital converter, and digital I/O (data acquisition) module, or even peripherals such as GPS receivers, IEEE 802.11 controllers, and USB controllers. The stack of such a system includes a controlable power supply, a processor board with standard interfaces; acquisition cards - usually 24-bit sigma delta with sampling frequency of up to 200 kHz per channel; non-acoustic sensor acquisition boards. Temperature and pressure sensors can be inter- faced a 1-wire interface. Also inclinometers and electronic compass may be included. Time synchronization of the operating system is performed by sinchronization with GPS. GPS also provides exact positioning over time. The wireless network Concerning the network and bandwidth, a standard IEEE 802.11g wireless card can provide a peak transfer rate of 54 Mbit/s. The effective transfer rate depends on range, antenna motion, signal-to-noise ratio, and therefore it is uncertain which transfer rate can be achieved in an operational scenario. As an example, at a data

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acquisition rate of 60000 samples per second, with 16 bit discretization, and 8 acoustic channels, 7.7 Mbit of data are generated each second. In a real case, multiple acoustic receiver devices are operated simultaneously, let’s assume 4 or 5. Let’s assume that air operation is carried out by an helicopter that operates a dipping SONAR. This helicopter has onboard a server that performs the task of downloading data from the acoustic receivers. It is likely that transmission conditions will not always be ideal, and therefore such a volume of data can not be retrieved in near real-time. As mentioned above, often it will be necessary to reduce the acquisition rate, or to perform data reduction by a computer procedure. From this point of view it makes sense to include a DSP in the system, in order to perform rapid computations for data reduction operations. The software Several software modules are useful in an acoustic module of this type: • an acoustic acquisition program, that presents the data in files stored on a local

disk; • a module to receive remote commands, for reconfiguration; • a signal pre-processing module; • a module for monitoring the acoustic module. The operating system may be a Linux kernel. The telemetry unit body The telemetry unit is a floating cylindrical container made of anodysed aluminum in 5xxx and 6xxx grades, in order to resist against corrosion caused by salty water. The container must provide a variety of connectors, as for hydrophones, RS-232 port for non-acoustic data, antenna, GPS. Also a mast for wireless antenna, GPS antenna, and RADAR reflectors must be included.

5.3.3 Acoustic emitters

Based on past experiments, here an analysis on acoustic emitters, and possible solutions are provided. Most classical concepts on acoustic tomography and acoustic inversion of seafloor properties were performed in a low frequency band, ranging from 150 Hz to 1600 Hz [19, 24, 25, 41, 42, 57, 68]. In this frequency band, acoustic waveforms propagate across distances larger than 10 km with relatively low transmission loss, allowing reception of signals with acceptable signal-to- noise ratio. For example, during the SWARM Experiment an J-15-3 acoustic source emitting LFM sweeps and CW signals in the band 50-600 Hz with sound pressure level of 160 dB were used. Also a 400-Hz Webb Research Corporation organ pipe tomography source with an amplitude of 180 dB was used to perform transmissions of waveforms with 100 Hz bandwidth across a transect of 42 km. The Yellow Shark

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’95 setup consisted of 5 vertical arrays spanning a transect of 40 km. Acoustic transmissions were performed with two flextensional projectors in the 200-800 and 800-1600 Hz bands. Other acoustic tomography experiments with shorter propagation tracks also used waveforms with frequencies below 1500 Hz (see the INTIMATE, MREA, and RADAR’07 experiments for reference). In order to discuss types of acoustic projector and acoustic emitter systems relevant for permanent and mobile tomographic networks, the equipment used during the RADAR’07 sea trial is taken as for example. Several types of acoustic projector were used:

a general purpose piezoelectric transducer from Lubell Labs Inc., model LL1424. This transducer is a double piston with a useful band of 200 to 9000 Hz, and allows operation with a maximum input power of approximately 800 W, which allows to attain an output of 197 dB at 600 Hz. The beam-pattern is omni-directional. This projector allows a maximum depth of 15 m. It is relatively easy to operate, and from economical point of view presents an interesting solution.

a Netptune T170 ceramic ring. This transducer has a useful band in the range 3.5 to 9.5 kHz, and requires up to 1500 W input power. The horizontal beam-pattern is omni-directional and vertical beam-pattern is toroidal. This is a deep water transducer that can stand virtually any depth.

an ITC 1007 spherical ceramic projector. This transducer has a useful band from

4 to 22 kHz. However the input power attains 10000 W, requiring a high power amplifier. The beam-pattern is omni-directional. This is a deep water transducer that can stand a depth of 1250 m.

A spherical transducer is advantageous over the ring transducer if vertical omnidirectionality is required. The spherical transducer requires significantly more power due to the omnidirectionality. Often acoustic rays with departing angles greater than 45 degrees are not relevant or it is not relevant to data analysis to work with vertical omni-directionality. If emitter-receiver range is large, the energy associated to large departing angles will have negligible amplitude at the receiver. In that case a ring transducer is an option. These transducers consume significantly less power than the spherical transducers. For the sake of illustration: for vertical 45 degrees, the Neptune T170 model presents an amplitude that is 5 dB less than in the horizontal plane. Another ring transducer is the Neptune T161, a transducer with a resonant frequency of 1.8 kHz, which presents for vertical 45 degrees an amplitude that is 8 dB below the amplitude in the horizontal plane. Of course there are many models in the market with different specifications. Some of these devices are relatively bulky. For example, a low frequency towed acoustic source commonly used in tomography, with a band 250-1200 Hz, consists of a ceramic ring with a diameter of approximately 50 cm, high-power electric circuitry, and a frame designed for tow manoeuvres, resulting in a device weighing up to 300 kg. Another important issue is the operation of the acoustic source. In past experiments the acoustic source has been operated in the following configurations:

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• the acoustic source is most commonly towed by the ship. The whole

amplification chain is in the ship and the projector is mechanically attached to a crane or a portic. This requires an assemblage of the transducer in a tow frame, resulting often in bulky devices, weighing easily 300 kg. For example a low-frequency ceramic ring has a diameter of approximately 50 cm. Then high power circuitry and isolation must be included too. In this configuration the source depth is a function of ship speed. The source depth is maximum when the ship is stalled. Examples of sea trials using this configuration are the SWARM Ex, INTIMATE’00, MREA’03, and RADAR’07 sea trials.

• another configuration that uses a ship is where the acoustic source is moored. During the ADVENT’99 sea trial the acoustic source was mounted on a tower moored on the seafloor, and connected to the equipment in the research vessel for power supply and waveform generation.

• the acoustic source can be moored in a site close to shore, and power and

waveforms are supplied from land through a sea cable. This configuration was used during the Yellow Shark 95 experiment.

• the acoustic source is based on a completely autonomous system. This system

consists of a computer that generates waveforms, an amplifier, and the acoustic projector. A battery pack supplies energy. The wave- form generation is scheduled a priori. This system can be moored with an acoustic release for easy recovery. Current standard battery sup- plies and acoustic technology allow an autonomy from hours to days, depending on power required for acoustic transmissions and repetition rate. Devices of this type were used during the High-Frequency Initiative sea trial that took place in Hawaii, in 2005.

In order to minimize transmission loss, whenever possible, experimenters often prefer to deploy acoustic sources below the mixed layer, which often is more than 30 m. In order to accomplish this criterion, acoustic transducers able to stand high depths are required. Ceramic transducers have a survival depth that often attains 500 m to 5000 m depth. As referred above, ceramic spherical and ring transducers require very high power inputs, and can there- fore be used only in situations where a high power amplifiers can be operated (e.g. through a shore connection or a ship). The piezoelectric transducer, like those offered by Lubell Labs is useful when shallow water transmissions are an option, and a very interesting solution in the case of an autonomous emitting device, due to the low input power required.

5.3.4 An emitter system for a permanent observatory

A permanent observatory would be installed in a site with a watercolumn that is at least 100 m. The configuration would be one of two possibilities: either the acoustic source is installed on a tower, to be deployed on the seafloor; or the acoustic source is suspended in the middle of the watercolumn. The first possibility may offer an increased protection against maritime agitation and currents. It consists of a 5 meter tall metallic structure holding the acoustic source. This must be attached to ballasts heavy enough to prevent the tower to be moved or thrown over in the

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case of severe sea conditions. The option where the acoustic source is suspended consists of a subsurface float attached to the acoustic source. This float must have a buoyancy that is sufficient to create a tension on the mechanical cable in order to suspend the acoustic source in the water column. The acoustic system consists of a signal generator, a high power amplifier, and a transducer: • the signal generator is a device based on a computer with digital- to-

analog (DAC) converter. This computer has network capabilities for communications over a TCP/IP protocol, in order to allow re- configuration, monitoring of the acoustic transmission, and upload of waveforms.

• a computer program controls the generation of electric waveforms. Waveforms can be uploaded or generated by the software.

• the power amplifier is driven by the DAC output. Depending on the acoustic

transducer, the power amplifier should be able to deliver up to 10000 W. • the acoustic source is based on a spherical transducer or a ring trans- ducer.

If one assumes that acoustic paths with ranges attaining up to 20 km, than it is acceptable that waveform in the range 400 to 2000 Hz are going to be used to carry out acoustic tomography.

Optimal sound pressure level requirements are difficult to define in a generic study, since this depends on several factors, such as emitter-receiver range, frequency band, water depth, noise level. This is part of the design of an acoustic observatory, and can be adjust by means of acoustic propagation models by taking into account these factors. Nonetheless, based on past experience, it is expected, that in many cases the required sound pressure level, enabling sufficient signal-to-noise ratio, range in the interval 180 to 200 dB.

5.3.5 Mobile emitter systems

Mobile emitters are employed in mobile tomographic networks. These net- works are usually deployed with ships or helicopters, for short duration ac- tivities (several days). Often a ship remains in the area under operation until the observation is over.

One can assume at least two scenarios: • an acoustic source is operated by an helicopter or a ship. • autonomous acoustic sources are deployed. An acoustic source can be tailored to be towed by a research vessel. This consists of a transducer mounted in a tow frame comprising a rudder and horizontal stabilisers. It required an electromechanical cable for deployment manoeuvres. For example, in the case of a low-frequency high power tranducer based on a ring transducer capable

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to attain to 210 dB in the frequency range 250 to 2000 Hz, the following baseline specification may be taken: • tow frame with rudder and horizontal stabilisers in order to allow the structure

to perform a stable movement during tows. The frame is made of marine quality stainless steel. The typical weight of the whole structure is approximately 300 kg.

• The electromechanical cable is connected to the tow frame. It should meet

specifications to stand tow speeds up to 5 kn. A working load of at least 5000 N is required, provided that the tow frame has a good hydrodynamic design.

• The electric cable has a an isolation voltage of 1000 V. • The power amplifier should be rated to meed the transducer’s maximum input

power. It must be able to drive the acoustic source at maximum power respecting the maximum voltage allowed for the tow cable and matching the source impedance along the frequency band of interest. The power amplifier should be protected against all leaks, including a full short circuit, either on the cable or at the transducer end.

This type of device can be operated deeper than 30 m. The piezoelectric double piston transducer requires a significantly smaller tow frame, since the transducer itself is smaller than the low-frequency ring transducer. Taking the LL1424 model as an example, the design of this transducer has already hydrodynamic characteristics. Concerning the electric specifications, in the case of the LL1424 model, a maximum tension of only 80 V rms is allowed. A professional audio amplifier with a 2000 W output has been used with this transducer model. Impedance adapters are required to match the power amplifier with the transducer input circuitry. This device can be operated at maximum depths of 15 m. Another type of design that may become relevant in tomographic observatories is an autonomous acoustic source. Existing lithium battery technology allows assembling of battery packs able to supply power for an acoustic transducer. However, in practice, the feasibility of such a system is conditioned to transducers with relatively low power input. For example Neptune Sonar and ITC Transducers offer transducers that require a maximum input in the range 350 to 750 W, which is significantly less than the input required by the transducers referred above. However, these transducer operate in higher frequency ranges, starting at 2 kHz. Model T335 by Neptune Sonar requires a maximum input power of 750 W and operates in the band 2-8 kHz. On the other hand the Model T313 requires a maximum input power of 380 W and operates in the band 7-16 kHz. These frequency ranges are clear away from frequency ranges used in classical tomography. For distances to be covered in a tomographic network, 10 to 20 km, at such frequency ranges acoustic modelling is difficult task, and also transmission loss mechanisms will cause the acoustic signals to be significantly attenuated. These frequencies can be considered only in tomographic applications where emitter-receiver distances of up 2 or 3 km are to be considered.

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Piezoelectric double piston transducers may play a role in autonomous emitting devices, since they are significantly more efficient in transducing electric energy into acoustic energy, specially at low frequencies. Model LL916 by Lubell Labs can achieve sound pressure level of 180 dB at 1 kHz with only 38 W of input power. Low energy consumption is a very important issue when it comes to autonomous devices. This transducer operates in the frequency range 0.5 kHz to 20 kHz, with a peak response at 1 kHz. The disadvantage of this transducer is that maximum depth is limited to 15 m. An autonomous acoustic source would consist of the following components: • an acoustic transucer responding to specific needs in terms of frequency range,

operating depth, and horizontal range; • an amplifier matching the transducer’s specifications; • for signal generation, a computer with a sound card or dedicated digital- to-analog

converter. A computer program would be used to control signal generation (manage audio files, emission schedule, waveform am- plitude, etc.);

• a container with a battery pack matching electric specifications of the transducer,

and autonomy requisits. This device can be deployed in a moored configuration using a ballast, an acoustic release, and a subsurface float. Also a free-drifting device can be considered. This will, however, require other components such as GPS receiver and wireless communication system in order to allow for monitoring of position and battery level for timely recovery, before the system is shut off. 5.4 Summary This chapter is an attempt to understand which technical solutions will be employed in future tomographic networks. First literature on past experimental studies was reviewed with a particular focus on the acoustic in- strumentation used, with the objective understanding which technical spec- ifications are relevant. Then acoustic receiving and emitting equipment is discussed, in order to provide possible solutions of instruments adapted to permanent or mobile tomographic networks. Technical solutions for acoustic instruments and detailed specifications are presented. This was based on systems successfully implemented and applied in experimental studies. First acoustic acquisition systems are described, one for a cabled network and an autonomous system. These systems are relatively complex, since they consist of a significant number of systems and accessories, both hardware and software. Then several transducer types were reviewed and then considered for solutions tailored to different scenarios and applications. For permanent observatories and towed acoustic sources, classical solutions can be considered. When it comes to mobile solutions that require multiple acoustic emitters, then autonomous systems will be a solution to be taken into account. Although

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autonomous acoustic source have not been yet employed so often, the existence of low-consumption transducers, and recent advances in lithiumbattery technology and small electronic devices for con- trolling signal generation, allow for the design of small and versatile emitting devices, able to cover a broad range of solutions. One important remark is that past collaborations in scientific projects allowed to earn knowledge allowing for the development of relatively complex data acquisition systems, able to meet the requirements of Acoustic Tomography Observatories being discussed under the ESONet programme. 5.5 References 5.1 W. Munk, P. F. Worcester, and C. Wunsch. Ocean Acoustic Tomography.

Cambridge Monographs on Mechanics. Cambridge University Press, Cambridge, 1995.

5.2 W. H. Munk and C. Wunsch. Ocean acoustic tomography: A scheme for

large scale monitoring. Deep-Sea Research, 26 A:123–161, 1979. 5.3 P. F. Worcester, W. H. Munk, and R. C. Spindel. Acoustic remote sensing

of ocean gyres. Acoustics Today, 1(1), 2005. 5.4 P. F. Worcester and R. C. Spindel. North pacific acoustic laboratory. J. Acoust.

Soc. Am., 117(3):1499–1510, 2005. 5.5 F. Jensen, W. Kuperman, M. Porter, and H. Schmidt. Computational Ocean

Acoustics. AIP Series in Modern Acoustics and Signal Processing, New York, 1994.

5.6 M. J. Buckingham. Ocean-acoustic propagation models. Technical Re- port

Publication no. EUR 13810 of the Commission of the European Communities, 1992.

5.7 M. J. Hinich. Maximum-likelihood signal processing for a vertical array.J.

Acoust. Soc. Am., 54:499–503, 1973. 5.8 H. P. Bucker. Use of calculated sound fields and matched-detection to locate

sound source in shallow water. J. Acoust. Soc. Am., 59:368–373, 1976. 5.9 M. D. Collins and W. A. Kuperman. Focalization: Environmental fo- cusing

and source localization. J. Acoust. Soc. America, 90:1410–1422, 1991. 5.10 A. M. Richardson and L. W. Nolte. A posteriori probability source

localization in an uncertain sound speed, deep ocean environment. J. Acoust. Soc. America, 89(5):2280–2284, 1991.

5.11 M. J. Hinich and E.J. Sullivan. Maximum-likelihood passive localization using

mode filtering. J. Acoust. Soc. America, 85(1):214–219, 1989. 5.12 J. M. Ozard. Matched field processing in shallow water for range, depth and

bearing determination: Results of experiment and simulation. J. Acoust.

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Soc. America, 86:744–753, 1989. 5.13 R.M. Hamson and R.M. Heitmeyer. Environmental and system effects on

source localization in shallow water by the matched-field processing of a vertical array. J. Acoust. Soc. America, 86(5):1950–1959, 1989.

5.14 C. Feuillade, W. A. Kinney, and D. R. DelBalzo. Shallow water matched- field

localization off panama city, florida. J. Acoust. Soc. America, 88:423–433, 1990.

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CHAPTER 6

Stationary versus Mobile Observatories

CRISTIANO SOARES and PAULO FELISBERTO

Institute for Systems and Robotics University of Algarve Campus de Gambelas

8005-139 Faro Portugal

e-mail: {csoares, pfelis}@ualg.pt

http://www.siplab.fct.ualg.pt/

6.1 Introduction The present chapter will discuss possible architectures for tomographic net- works designed for the remote observation of three-dimensional temperature fields in coastal shallow-water regions. This will mainly consist in proposing basic experimental setups, and analyse their capabilities in achieving a degree of observability of the environment spanned by that acoustic network. First a review of relevant studies will be carried out as an introductory section. Then, building on those studies, both permanent and mobile observatories are discussed. A tomography network system should ideally be able to significantly aid in the observation of the three-dimensional spatial structure of oceanographic features. A few studies attempted the development of methods for retrieval of the spatial distribution of water temperature or currents in shallow-water areas. So far, these methods consist of two stages: the first stage is an acoustic inversion that allows for obtaining range-independent integral estimates of sound-speed, or other physical properties, over multiple ocean transects containing an acoustic source and a receiver array; a second stage is to meld those integral sound-speeds with direct measurements of temperature [1], or to meld the set of integral sound-speed estimates in order to produce an estimate of the spatial structure [2,3]. Elisseeff et al. propose a two stage acoustic data assimilation scheme that combines acoustic tomographic estimates and local direct measurements of non-acoustic data. In a first stage integral sound speed profiles are estimated along multiple acoustic tracks by acoustic inversion, from data collected by multiple hydrophone arrays, and most efficiently with a moving emitter. Note that the resulting estimates are range-averaged temperature estimates. In a second stage the tomographic estimates are interpolated at the nodes of a specified grid, and melded with direct temperature measurements to provide a spatial distribution of the spatial field. The acoustic inversion is carried out according to a model that relates travel time perturbations to

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sound speed perturbations, in the form of a system of equations. Another set of linear equations is used to meld these temperature perturbations estimates with the direct measurements.

Figure 6.1: Spatial coverage achieved with a moving source during the Haro Strait experiment (1996). The straight lines connect transmission positions with receiver positions. Circles: acoustic moorings with emitters and receivers. Stars: moving source consisting of light-bulb deployments. Squares: environmental moorings [1]. Although each individual acoustic inversion has no resolution over range, the acoustic data provides good resolution in depth and across transmission tracks. As an example, Figure 6.1 shows the Haro Strait sampling geometry with the transmission positions connected to receiving positions with light gray lines, in order to emphasize the spatial coverage obtained by using multiple receivers. With an adequate distribution of a few receiver arrays and adequate spatial spacing of the acoustic transmissions, a large spatial coverage can be provided within a short period of time, with high azimuthal resolution with respect to each receiver array. Another two stage method for retrieving the three-dimensional spatial distribution was proposed by Felisberto et al. [3]. In Ref. [3] the temperature perturbation was represented by a single set of EOFs dependent only on depth. Figure 6.2 shows an example of an experimental setup using only two receiver arrays and a moving emitter. This measurement procedure assumes that the oceanographic features under observation have a very slow variability, since it takes at least several hours until a vessel has performed such an emission path. The first stage is carried out as a Matched-Field Tomography (MFT) procedure with the primary objective of obtain integral temperature perturbations over different vertical cross-sections containing emitter and receivers, and the secondary objective to account for uncertainties of geometric and seafloor parameters.

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Figure 6.2: Distribution of moving emitters and receiver arrays over an area of interest. The division of the area into cells allows for retrieving the spatial distribution of temperature in a two stage inversion method. The output is a set of range-independent EOF- coefficients for each propagation track. In the second stage the area under observation is divided into horizontal cells crossed by the propagations paths of the first stage. Each cell is characterized by a set of EOF-coefficients. The second stage is to estimate the EOF coefficients associated to each cell by means of an equation system that relates these with the range-independent EOF coefficients estimated in the first stage through an observation matrix. The elements of the observation matrix are calculated as an integral of the EOFs over the eigenrays’ paths across each cell. Therefore a ray-tracing model for forward model computations is used in the second stage, requiring only one forward model for each propagation path. From the computational point of view, this very interesting, enabling this method to be applied for quasi real-time applications. The largest computational burden is presented by stage one, which, nonetheless, with today’s available computational resources does not present a constraint, as the computed forward models are range-independent. The results presented in Ref. [3] were obtained with simulated data based on real oceanographic data. The spatial temperature structure was modelled by two EOFs. The first EOF, the most important component could be well resolved. The method proposed by Felisberto goes one step further, in comparison to that proposed by Elisseeff, since it attempts to retrieve the spatial distribution of an oceanographic feature by relying directly on the acoustics. Concerning the formulation based on travel-time perturbations, this is advantageous, in the sense that inverse problem can be literalized, contributing for fast computations.

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Both of the concepts presented above show that it is conceptually possible to remotely observe the distribution of temperature in a given oceanographic volume, if multiple acoustic arrays are employed and if acoustic signals are transmitted from multiple positions, in order to generate multiple acoustic transects. The transmission of acoustic signals from multiple emitter positions to multiple receiver positions is the key that opens this possibility: generating acoustic data from one emitting position to multiple receiver positions naturally conducts to a gridding of the environment into cells, in order to produce local environmental estimates, rather than a single integral estimate over a given propagation track. In this way, one can expect to be able to observe the interior of a volume bounded by a set of acoustic emitters an receivers by crossing that volume with multiple acoustic paths. There is one step further to go: each acoustic inversion may be carried out with acoustic data collected simultaneously at multiple hydrophone arrays. Yet, it is uncertain which performance in accuracy and resolution over range can be attained. In the following sections architectures for stationary and mobile architecture will be provided. 6.2 Permanent acoustic tomographic networks This section aims at providing possible solutions for permanent acoustic to- mography networks. The idea is to design a network that progressively tends to enable the coverage of a relatively large area, following a gridding scheme, in order, to enable the retrieval of meaningful environmental estimates over that gridding space. It is assumed that the main infrastructure will follow a similar architecture as existing Ocean Observatories, e.g., Neptune Canada [4, 5]. This infra-structure has a backbone cable that carries both power and fiber-optic bi-directional communications across the network with repeaters that amplify the optical signal carrying information. The backbone also contains branching units to distribute power and communications to spur cables connecting network nodes. The nodes work as an interface between the backbone and the junction boxes of local networks, providing communications to those net- works with dedicated wavelengths, and converting power-supply from high- voltage into medium-voltage. After a node come the junction boxes, which provide power-supply and communications to the instruments. The junction boxes convert medium-voltage into low-voltage to supply the instruments with energy and take care of the data communication by means of ethernet switches. These junction boxes may appear in paralell or in cascade. Here it is assumed that a tomographic network, or a branch of a network comes after a junction box, depending on the physical distances of between the instruments, and the capacity of a junction box handle a number of instruments. In this study it will be assumed that the available power-supply and communication bandwidth is sufficient to serve the tomographic network, and therefore no considerations on this

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issue will be included. In terms of implementation, it is acceptable to imagine that a tomograhic network would be built in several phases, where at each phase a number of instruments or a branch with instruments is added. Thus, it is likely that the network will start with a minimal number of instruments, large enough to allow for a systematic observation of oceanographic quantities.

Before starting to set up possible configurations of tomographic networks, it is necessary to make assumptions on several aspects, in order to create a frame. The objective is to set up tomographic networks that can cover an area of 100 km2 - let’s say a 10 km x 10 km square - in a shallow water region. In the present framework, it is assumed that a tomographic network is a set of instruments connected to a junction box, or, at an advanced stage of development, several sets of instruments, connected to one or multiple junction boxes, i.e., multiple junction boxes branch from a node. As instruments that are truly part of the network the following types will be considered: • acoustic sources; • acoustic acquisition systems with vertical arrays of hydrophones; • environmental acquisition systems with vertical arrays of temperature and

conductivity sensors; These instruments are moored over the area according to a pre-defined architecture, in order to allow for the assessment of environmental information on the spatial distribution of oceanographic features. Mainly temperature fields. The proposed architectures will build on the concepts presented by Elisseeff et al. and Felisberto et al., which are based on the existence of multiple receivers and emitters, or multiple emitting positions. Also moving emitters, that may be used in the framework of a permanent tomographic network (provided that the environmental distribution remains stationary during the observation window considered). The generic idea is to set up the acoustic equipment such that it can cover a given area with a minimum number of instruments and cable length, in order to acoustically observe the interior of a given ocean volume. This is conceptually possible by setting up architectures that take advantage of the inclusion of multiple acoustic emitters and receiver arrays, such that multiple propagation paths cross at positions contained in that ocean volume. In terms of signal processing, e.g., let’s consider an approach such as a Matched- Field Processing scheme, setting up multiple emitters and receiver arrays will take the experimenter to implement inversion algorithms with acoustic data collected at multiple acoustic arrays, whose acoustic waves have origin at multiple acoustic sources. It is known that the capability of an acoustic system to provide discrimination of the solution of an acoustic inversion problem is highly dependent on the uniqueness of the acoustic response of the medium between emitter and receivers. Observing an environmental portion from multiple emitter positions to multiple receiver positions can significantly increase the overall acoustic field uniqueness, which may provide the key for solving the spatial distribution of an oceanographic feature, as readily demonstrated in past studies. While in the case of an emitter-receiver pair local values of a given environmental parameter or local features can not be observed, i.e., only an integral

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or average value of that parameter within the propagation track can be assessed, merging acoustic data involving multiple sources and receivers present a way to enable the observability of parameters governing the acoustic propagation.

6.2.1 A simple geometry: a line network Acoustic instruments moored across a line allow for a range-dependent observation of the environment. Siderius et al. [6] have reported successful geoacoustic inversion results with data collected during the Yellow Shark 95 sea trial from 5 hydrophone arrays deployed across a 40 km transect at ranges 8, 16, 24, 32, and 40 km from the acoustic source. The observed transmission loss was used to determine range-dependent bottom properties with a marching search to fit the TL measured at the five ranges using a parabolic equation propagation model. This marching technique was used to deter- mine the best fit at each array location before proceeding to the next range. Those experimental results illustrates the viability of this type of acoustic configuration for remote sensing of environmental properties across a given ocean transect. A cabled implementation of such configuration would conceptually al- low for a long-term observation of range-dependent ocenographic quantities across an ocean transect. This would consist in deploying hydrophone arrays over a given transect and one acoustic source at each extremity, as shown in figure 6.3. Each pair of hydrophone arrays sets the boundaries of an environmental sector. In principle, the unknown environmental parameter to be acoustically observed in each environmental sector is modelled as range-independent. Merging estimates across multiple environmental sectors will result in an overall range-dependent environmental picture. The deployment of an acoustic source at each extremity of the transect is of crucial importance for the success of this architecture, more specifically, in coping with the difficulties that would arise if only one extremity would receive an acoustic source:

Figure 6.3: Line acoustic tomography network. Acoustic source are set up at the network extremities, and multiple hydrophone arrays and environmental sensors are set in between.

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as mentioned above, the key for parameter observability is to enable reduced ambiguity in the solution space of the inverse problem. It has been demonstrated in many occasions that the number of array receivers plays a major role in ameliorating parameter discrimination. One can expect that merging acoustic data collected with 2 or more acoustic arrays will contribute even more for reducing the solution ambiguity of the inverse problem.

if hydrophone arrays are to be deployed across tens of kilometres, it is expected

that the data collected by the furthest arrays will suffer a significant degradation in signal-to-noise ratio (SNR). Bi-directional acoustic transmission will contribute for an improved balance in SNR, if an estimator with data collected at two arrays or more is considered.

If a marching technique across the environmental sectors is used for proceeding

with parameter estimation, it is expected that estimation errors will be accumulated as the marching proceeds from sector to sec- tor. These errors have origin in multitude of sources, especially, model mismatch, search algorithm, and acoustic noise. Setting up an acoustic source at the opposite end allows to create an additional anchor that allows for the marching procedure to follow in the reversed direction of the ocean transect, and therefore iteratively correct previous estimation results.

As an example, to estimate environmental parameters in the first section requires merging the acoustic data collected at array number 2 during a transmission from acoustic source number 1, with acoustic data collected at array number 1 during a transmission from acoustic source number 2. Basically, one takes intersection of the bi-directional acoustic propagation within the environmental boundary defined by two contiguous hydrophone arrays. At the present time, research on acoustic inversion with multiple acoustic emitters and receiver arrays is being undertaken, in order to establish minimal hardware requirements and to obtain an idea on the achievable performance for a such an tomographic network. There are also issues such as the maximum range that can be covered by a line network and the range between instruments that allow for meaningful acoustic inversion results. The limiting factors are transmission loss and environmental modelling. While one wishes to maximize the spatial covery, one must be aware that increase ranges will require increasing sound pressure levels, and that modelling errors cummulate with increasing range. The spatial coverage offered by a line network can be significantly increased if a moving acoustic source is employed. Figure 6.4 shows an example of the line network with a moving acoustic source. The dashed lines connecting the source positions to the receiver positions indicate the propagation paths generated at each case. Given the number of receiver arrays and the number of emitting positions, the area can be densely covered by many crossing propagation paths. Assuming that the oceanographic quantities vary slowly, a number of acoustic transmissions may be performed to obtain a spatial picture of those oceanographic quantities.

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6.2.2 Volume observation with a two branch network The past section has analysed a line network that allows for covering a multi- environmental ocean transect. However, with a configuration of this type only the environmental plane containing the array and the acoustic sources can be acoustically observed. One can go further, by setting up configurations of instruments to attempt the observation of an ocean volume. Of course the complexity of the network has to evolve, namely, by setting up more than one instrumentation branch. For example, considering an initial single branch network, one can add one more branch departing from the same position as the existing one, to shape a triangle. The minimal network would consist of one hydrophone array and an acoustic source at each vertex, as illustrated in figure 6.5(a). Altough acoustic instruments are set only at the vertices of the triangle, this configuration allows for observing environmental sectors or areas away from the acoustic instruments. The area can be divided in environmental cells reflecting the network configuration. The large cells contain an emitter/array pair. The small cells in between have no instruments but they may be large enough to significantly influence the acoustic propagation across each propagation path. The large cells are crossed by two propagation paths, each connecting a vertex to the other two, which, in terms of inversion problem, provides increased solution constraint, since acoustic data collected at two acoustic arrays may enter the inversion, and may, therefore, be seen as anchors. A multi-iterative inversion scheme would be set up such that it initially esti- mates parameters in the large cells.

Figure 6.4: Line acoustic tomography network with a moving source. The blue crosses away from the transect indicate acoustic transmission positions

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Figure 6.5: Acoustic tomography network in triangular configuration: (a) acoustic instruments placed only on the triangle’s vertices; (b) acoustic in- struments placed on trangle’s vertices and edges. Once these were considered reasonable estimates of the physical quantities of interest, they will be maintained fixed and then the environment in the small cells is estimated. This process can be repeated until an overall convergence of the global environmental parameterization is achieved. The rationale behind this algorithm is based on the equivalent model concept: the initial iterations of the inversion process may yield estimates with a relatively large error, although the underlying acoustic model produces replica fields compatible with the observed field. Provided that the replica fields yielded by the initial model estimates are on a trajectory allowing for model convergence, as soon as each transect is broken into environmental sectors, on can expect that convergence of across inversion iteration takes place. An important remark concerning the capability of this configuration is an increase in spatial resolution, in comparison with traditional MFP approaches, where usually only an integral estimate for a transect containing the acoustic instruments is obtained. The triangle configuration in Fig. 6.5(a) has still the liability of being unable to cover the interior of the triangle. This limitation is overcome naturally by adding hydrophone arrays halfway of each edge (see Fig.6.5(b)). For cost reasons, this change may not be implemented on every edge of the triangle, since it will imply the extension of the sea-cable to connect the receiver system on the third edge. Theoretically one can add as many receiver systems as wanted, in order to increase the resolution or area covered. One must be aware that the computational resources must be powerful enough to handle all the collected data, and the inversion algorithm must be capable to deal with the increasing complexity of the instruments’ geometry.

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Also the instruments must be sparse enough in order to allow each environmental section or cell to be representative enough to allow an unambiguous solution to be attained. This is a trade-off in terms of spatial resolution such that the spatial variability of oceanographic quantities can be observed. A slight variation of the two branch configuration can be implemented deploying two parallel sea cables beyond the node, i.e., to obtain a square rather than a triangle. This allows for significantly increase of the area covered by the tomographic network, with the cost of a slight increase in the number of instruments and sea cable length. Figure 6.6(a) shows an example of that configuration. An emitter-receiver pair is deployed at each vertex of the square. This allows each acoustic transmission to be received by three acoustic arrays. This results in the following acoustic cross-sections:

• The environmental cells in the corners are crossed by 3 transmission paths. • The inner cell is crossed by 2 transmission paths. • The cells on the edge are crossed by 1 transmission path.

Note that this configuration allows for considering that each transmission path has two ways, which is significant for promoting solution constraint, if necessary. The intensity of the red colour in the scheme depicted in Figure 6.6(a) reflects the number of acoustic path crossing each environmental cell. The observability of the parameters characterizing an environmental cell should be increased by taking into account an increasing number of acoustic paths departing from and arriving to that cell.

Figure 6.6: Acoustic tomography network in square configuration: (a) acous- tic instruments placed only on the triangle’s vertices; (b) acoustic instruments placed on vertices and edges.

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Other variations of this type of network can be considered for increased area covery. Figure 6.6(b) shows an example. The number of instruments can be increased, without necessarily including emitters at every new position. The change presented in Fig. 6.6(b) may produce the following effects:

• increase the number of acoustic paths crossing the respective environ- mental cells. In terms of acoustic inversion, it is expected that this brings an improvement in the observability of the respective environ- mental cells.

• increase the number of environmental cells, due acoustic path crossings resulting

by the inclusion of the new acoustic instruments.

• increase the number of acoustic paths crossing other cells, eventually al- lowing for increasing the observability of those cells too. In other words the new acoustic arrays allow for collecting additional information on other environmental cells.

• an additional benefit is the inclusion of acoustic data with improved signal-to-

noise ratio. This issue may become significant when emitters and receivers are well apart, such that the received sound pressure level is low.

This change can be introduced in order to increase the number of environ- mental cells, the observability of cells crossed by few acoustic paths, or for improving the overall signal-to-noise ratio. In the example of Figure 6.6(b), if the area of the network was maintained, the spatial resolution of the tomo- graphic network had been increased, since the inclusion of 2 receiver arrays induced the 6 additional acoustic path crossings, that may originate 6 additional environmental cells. These examples show that there are several open issues with serious implications in the design of a tomographic network, that deserve to be specifically addressed: • for a given configuration, which environmental cells are generated, and which is

the size of each? A complementary issue is: which are the minimal distances that enable the observability of an environmental cell?

• how many acoustic paths should be considered for the estimation of an

environmental cells? Note that although taking multiple acoustic paths in an acoustic inversion, one must consider the risk that this may become over constraint.

• as mentioned above, an iterative inversion scheme to be adopted for such a

tomographic network will be one that estimates one environmental cell at a time, where besides acoustic and environmental data, also provisory estimated parameters of neighbour cells must be included. Thus, an estimation hierarchy must be followed. For networks as complex as the example of Fig. 6.6(b) establishing that hierarchy may be cumbersome. Ideally, an algorithm that for a given architecture automatically establishes that hierarchy and mutual parameter dependencies would provide an important aid for the iterative inversion problem.

While multi-emitter multi-receiver acoustic tomography is a significant step forward in transforming this remote sensing concept in an effective measurement tool, with

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significant advantages in terms space coverage and resolution in comparison to traditional approaches, several important accessory issues need to be investigated for seamlessly handle the acoustic observations generated with such a network in the acoustic inversion problem. 6.3 Mobile acoustic tomographic networks There are several situations where observation of oceanographic phenomena are observed for a limited period, and where mobile network concepts naturally apply: the transitory nature of some oceanographic phenomena (e.g. up- wellings of

cold water), with a duration of days. the phenomenon is in an area that is not accessible for deployment of sea-cables. in the case of natural or man-made hazard, moving instruments can be employed

with short notice in Rapid Environmental Assessment (REA) operations. In any case, a mobile network will use instruments adapted to deployment operations to be completed within hours, which is a characteristic inherent to immediate use or rapid response applications. A mobile network is not cabled, and therefore it uses autonomous instruments, i.e., it is powered up with batteries, and the equipment includes wireless communications, local storage and computation capabilities. Also a GPS system with readily avail- able data at any time should be on board, in order to enable tracking of instruments. The instruments, both acoustic emitter and receivers, or environmental data acquisition systems are tailored for deployment in free drifting configu- ration or moored configuration. One should be aware that deployment and recovery operations of moored instruments are significantly more complex than free-drifiting instruments. Deciding on the deployment may depend on serveral factors, such as: • autonomy of each instrument. For example: if an instrument has a long

autonomy (several days) the tendency is to decide for a mooring in order to prevent the instrument to drift to far away from the initial position; on the other hand, if an instrument must be recovered on a daily basis for battery replacement or other maintenance operations, then the tendency is to decide for a free-drifting configuration in order to avoid the repetition of complex and lengthy deployment and recovery operation.

• this choice is also related to the permanent presence of a platform (e.g. vessel)

able to monitor the position of the network instruments at all times. At sea, often the maximum range enabling two node to perform wireless communications is restricted to 10 km.

• If currents are low and predictable then it is not mandatory to continuously

monitor the position of an instrument. Under that situation free-drifting configuration is an option, even if the instrument can not be monitored at all times.

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A free-drifiting instrument has also the advantage of being less susceptible to maritime agitation. For example, currents cause a vertical acoustic array to suffer a tilt, which must be taken into account in acoustic modelling. Cur- rent acoustic inversion techniques are robust to small position uncertainties. Whenever an instrument is deployed, in either mode, one must necessarily take the risk of loosing it. The geometric setups discussed in section 6.2 may well be considered for mobile tomographic networks. However, in some aspects the mobile network presents some issues that in practice may be relevant: • in free-drifting mode the instruments may significantly drift away from initial

positions. In order to keep the impact of this factor reduced, free-drifitng instruments should not drift too far apart.

• the network is not cabled, there is the possibility that not all nodes are in

reach for data access at all times if the processing platform and acoustic instruments are not favourably located. This may hinder real- time acoustic inversion, if the objective is to obtain quasi real-time oceanographic observations.

• in general, at least a towed acoustic source may be considered, since in general

vessel time will be available. Since oceanographic features vary slowly, multiple acoustic transmissions received at different times can be treated as simultaneous receptions, for obtaining of a static picture of the observed volume. It is important to define a meaningful tow geometry integrable with receiving nodes and other transmitting nodes (if existing). The integration of a moving node will significantly in- crease the number of transmitting positions. In other words, if a towed acoustic source is used in tandem with fixed emitters and receivers, the volume coverage can be significantly increased.

Concerning the uncertainty in position associated to drifting instruments, it has been demonstrated that this has little impact in the acoustic data inversion [7], provided that GPS data is available for each instrument. Matched- Field Processing methods are sufficiently robust to small uncertainties in latitude and longitude, since these errors will be compensated with estimation errors in environmental parameters. 6.4 Summary This chapter has clearly suggested a trend towards three-dimensional acoustic tomography in shallow water areas. This is motivated by earlier work within this scientific objective [1, 2, 3], and the recognition that rapid advances in electronics, computer technology, and battery technology will soon allow for a team of experimenters to handle simultaneously an increasing number of instruments. Also the increase of available computational resources is playing its role in contributing for the maturing of the three-dimensional tomography concepts, which will significantly increase the complexity of the inherent inversion problem. The proposed architectures use multiple emitters and receivers. The joint inversion of acoustic data received simultaneously at multiple receiver arrays is the key to enable

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an acoustic network to model and observe spatially varying temperature fields, in opposition to more traditional approaches where only one acoustic arrays is considered at a time. Future work will develop inversion algorithms for multiple acoustic array inversion, and demonstrate the feasibility of that concept.

6.5 References 6.1 P. Elisseeff, H. Schmidt, M. Johnson, D. Herold, N. R. Chapman, and M. M.

McDonald. Acoustic tomography of a coastal front in Haro strait, British Columbia. The Journal of the Acoustical Society of America,106(1):169–184, 1999.

6.2 P. Felisberto. Data Assimilation Applied to Ocean Acoustic Tomography. PhD

thesis, University of Algarve, Faro, Portugal, 2005. 6.3 P. Felisberto and S. M. Jesus. An hybrid acoustic-oceanographic method for

estimating the spatial distribution of sound-speed. In S. M. Jesus and O. C. Rodriguez, editors, Proc. European Conf. on Underwater Acoustics (ECUA) 2006, pages 663–668, June 2006.

6.4 S.T. Lentz. The Neptune Canada communications network. In OCEANS

2007, pages 1 –5, October 2007.

6.5 P. Phibbs and S. Lentz. The implementation of the neptune canada backbone network. In OCEANS 2007, pages 1 –5, October 2007.

6.6 M. Siderius and J.-P. Hermand. Yellow shark spring 1995: inversion re- sults

from sparse broadband acoustic measurements over a highly range- dependent soft clay layer. J. Acoust. Soc. America, 106(2):637–651, Au- gust 1999.

6.7 Cristiano Soares, Sergio M. Jesus, and Emanuel Coelho. Environmental

inversion using high-resolution matched-field processing. The Journal of the Acoustical Society of America, 122(6):3391–3404, 2007.

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CHAPTER 7

Concluding Remarks

MICHAEL TAROUDAKIS

and

THE ESONET OCEAN ACOUSTIC TOMOGRAPHY GROUP

7.1 General This report presented the concept of ocean acoustic observatories and details on the associated technology and on the acoustic data processing and analysis methods. A large part of the report is devoted to the Ocean Acoustic Tomography, which is the main framework supporting the function of these observatories, and we showed that this concept has reached an advanced stage of maturity, such that it can be immediately implemented as a fully operative system. Overall, we hope that this report demonstrates the capabilities of the ocean acoustic observatories, and thereby encourage incorporation of those into future multi-disciplinary ocean observatories. This chapter presents an overview of the issues that should be taken into account in the design and implementation of an acoustic observatory for ocean monitoring. In order that an ocean acoustic observatory is fully exploited by the scientific community, it has to fulfill the basic ”4M’s” specifications : Mobility Modularity Multipurpose Multidisciplinary

The reasoning for adopting the above specifications for ocean acoustic observatories has become clear by the preceding chapters. The additional characteristics of the ocean acoustic observatories should be related to the actual applications they are expected to deal with, and the end-users who will undertake the task to operate the observatories and exploit their outcome. Besides, the cost of the instrumentation should be such that the development and deployment of an acoustic observatory is reasonable. Therefore, the design of an acoustic observatory should be based on user-driven technology – easy to handle – easy to operate. It should be taken into account that the ultimate goal of the ocean acoustic observatory is to provide information in real or almost real time. The end user who may not be experienced should be able to “observe” the processes in the water as they are happening. All these are challenging

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issues. In the following the basic features of the ocean acoustic observatories are summarized. 7.2 Location There is no a-priori restriction related to the location of an ocean acoustic observatory. The location should be decided on the basis of scientific issues related to the expected outcome of the observatory. For instance, acoustic observatories could be created in shallow (coastal) areas aiming at the monitoring of the sediment processes, or the quality of the marine environment which is vulnerable to land activities. On the other hand, they can be built in deep water areas if the oceanographic processes there, are relevant to the climate change, just to mention two of the many applications of ocean acoustic observatories. Ocean acoustic tomography initially was intended for long range applications, necessarily involving deep areas. Therefore the ocean bottom played minor role or was neglected. Acoustic ray paths encountering the ocean bottom were excluded from the analysis of the observables. Later on, applications of ocean acoustic tomography in shallow water areas, made it necessary the inclusion of the bottom properties in the modeling of the associated forward and inverse problems, Now there is a trend to include bottom classification (geoacoustic inversions) as an additional modulus of the ocean acoustic observatories (see Chapter 3). 7.2 Instrumentation The basic elements of the acoustic observatory are of course its receivers. The reason that the receivers are mentioned first is that the receivers (hydrophones) are the only elements of an ocean acoustic observatory used in all the applications and functioning of the observatory. For instance acoustic sources are necessary only if the observatory has an active function, whereas when the observatory operates in passive mode, the sound signals come from natural biological sources (e.g marine mammals) or other random sources in the marine environment (ambient noise). The receivers should be chosen on the basis of the general operational characteristics of the observatory. The effective frequency response range of the receivers range should be chosen on the basis of the acoustic signals to be exploited. Receivers optimized to work in very low frequencies (below 100 Ηz) are intended for seismic applications only. Hydrophones specially intended for active applications (ocean acoustic tomography) should have a frequency response compatible with that of the sources. Hydrophones intended for the exploitation of the ambient noise should have a wide frequency response while those intended for biological purposes should have additional response range to accommodate possible ultrasound emitted by the marine mammals. For detailed analysis of the specifications of receivers used in past tomography experiments the reader is referred to Chapter 5. When ocean acoustic tomography is considered, the long range propagation requirements dictate the use of relatively low frequency acoustic sources. Of course these sources have the disadvantage of being expensive and relatively big and heavy. Therefore a compromise should always be done using preliminary theoretical studies

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of acoustic propagation properties in the area that the acoustic observatory will be developed. For detailed analysis of the specifications of sources used in past tomography experiments the reader is referred to Chapter 5. One should take into account that the power emitted in water should be low enough so that good performance of the observatory is ensured without disturbing the living creatures of the marine environment. With respect to the use of arrays of sources/hydrophones instead of single elements one has to take into account the data inversion methods to be applied and of course the available funding. The optimum exploitation of the sound for ocean observation is achieved by arrays of sources and receivers, placed vertically in the water column for practical reasons. When an array of sources is deployed, many additional source handling features (for instance beamforming not mentioned in previous Chapters) may be exploited. However, for practical reasons and as a compromise with respect to cost of the installation and operation, a single source and an array of hydrophones in each mooring would be enough for an observatory for ocean acoustic tomography. Figure 7.1 presents a typical configuration of a pair of transceivers intended for such an observatory.

Figure 7.1 A typical pair of transceivers in an ocean acoustic observatory The measurements should ideally be transmitted to a shore station for immediate processing. The transmission of data could be done by radio link or by cable. The decision on cabled or non cabled ocean acoustic observatory is of course based on many issues the principle of which is of course the location of the installation close or away to the shore and the cost. There is an on-going dispute however on which one is the best practice. Many scientists are in favor of the cabled observatories, stating that whenever possible the cabled observatory is the optimal solution to the problem of data transmission. On the other hand there are scientists believing that alternative methods of data transmission including purely radio link with antennas mounted at the buoys of the moorings, or acoustic transmission in water to other moorings having cabled capabilities should be preferred.

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Figure 7.2 Basic integration of data at an acoustic observatory It is not our intention to participate in this dispute. In any case appropriate modems should be developed or purchased and be placed at the transmitting and receiving locations. In Chapter 5 the reader will find useful information and details on the data transmission protocols and requirements. In addition data storage is necessary especially in cases that real time transmission of data is not possible. In this case the main issue is the capacity and required power for the data keeping. 7.3 Integration Integration and processing of the data is another important issue of the ocean acoustic observatory. The basic integration of all the information gathered at the observatory is described in Figure 7.2. Taking into account the multidisciplinary character of the observatory it is assumed that additional data could be collected by appropriate sensors. Therefore it is expected that the data collection will include acoustic signals, oceanographic and biochemical parameters as well as positioning information. This set of data should be appropriately handled by some kind of preprocessing at the platform (e.g data compression) prior to their transmission at regular intervals to the base station which could be at the shore (in case of a permanent observatory) or at a supporting ship or other marine vehicle in the case of mobile observatories. The final processing will necessary take place at the base station. In what concerns acoustic signals, the final process involves first of all the decoding and decompression of the signals to appear in its initial form, followed by the identification of the specific observables to be applied in the inversion processes (see details in Chapter 3). The

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inversion procedure follows, based on the most appropriate method for ocean acoustic tomography or geoacoustic inversion related to the technological issues chosen for the observatory. The theoretical methods have been briefly described in Chapter 3 while an example of integrated computer software for the processing of the data was presented in Chapter 4 with additional information presented in Chapter 5. The overall task of data processing is schematically presented in Figure 7.3. This sketch describes the process of retrieving oceanographic information for the water column and the sea-bed from acoustic data. It does not include biological applications of tracking and monitoring marine mammals. A slight modification of the sketch to take into account this application is not however difficult. After the initial signal processing, data inversion is performed to provide the user with estimations of the geoacoustic parameters of the water column and the sea bed. These parameters could feed back the inversion tools in case of iterative inversion processes to ensure maximum reliability of the results. The geoacoustic parameters are the results of the ocean acoustic observatory at the initial (first) stage. In most cases however this is not the ultimate goal which involves the use of oceanographic models to predict critical states in the ocean. To this end, the inverted data, along with initial acoustic data feed appropriate data assimilation models which will give the final answer to the scientists involved in the problems of marine (ocean) monitoring. However we have not reached the final stage yet. It remains to include the task of visualization, which is necessary for the end-users who are not necessarily the scientists to have a comprehensive and easy way of understanding the processes in the marine environment, their impact in the society and be able to take the necessary decisions on a specific problem related to the sensitive marine environment.

Figure 7.3 Data integration in an ocean acoustic observatory

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To this end, a Geographical Information System (GIS) can play an important role. It is the most appropriate tool for projecting data (initial acoustic and inversions) in a map of the environment and by means of additional mathematical tools to present statistical and probabilistic spatial analysis of the processes expected in some area. Therefore the GIS is not only a way to see maps with useful parameters at it is the usual cases in commercial representations of the data obtained by ocean observatories, but at the same time a dynamical tool for decision making. The analysis of the potential and the specifications of GIS for ocean acoustic observatories is however not a subject of the current report. As a final conclusion we believe that it has become clear that the acoustical oceanography committee of Europe is ready to contribute with modern tools to the observation of the marine environment. Of course synergy with other scientific areas is required to maximize the quality of the end product.

Figure 7.4 The elements of the Geographic Information System

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We need a better environment

____

We must joint efforts to get it.

The ESONET

Ocean Acoustic Tomography Group

February 2011

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8. Annex II – Sea Water Electrodes

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1. - Sea Water Electrodes: Observaories in a Corosive Enviroment

Klaus Schleisiek

SEND Off-Shore Electronics GmbH - Rostocker Str. 20 - D-20099 Hamburg Fon +49 40 18036234-1 Fax +49 40 18036234-9 - URL www.send.de Managing Director: Elke Andresen - HR B106038 beim Amtsgericht Hamburg

1.1 Industrial use Commercial fibre optic underwater cables for the telecommunication industry usually only have one single conductor for the power supply of repeaters. Therefore, the "current return" has to go through the (salt) water, which happens to be an excellent conductor, because of its virtually infinite conductor width. Appropriately designed electrodes need to be used for coupling.

A telecommunication cable always extends from a shore station through the water to another shore station. Therefore, the electrodes on both ends are servicable. Usually, graphite electrodes are used. Most of the systems use direct constant current sources, some use alternating constant current sources. A problem of the DC stations is chlorine gas pollution on the anode. Because the energy source feeds a constant current, fluctuations in resistance of the conductors have no influence on the energy that can be extracted by a repeater.

In the utility industry there exist a number of power lines (HVDC power transmission), which go through water, e.g. between Sweden and Danmark. Very often a single conductor cable and a current return through the water is used both for cost and efficiency reasons. Arrays of massive graphite electrodes are buried close to the shore for coupling.

1.2 Ocean bottom observatories In ocean bottom observatory installations there is one big difference to the "classical" sea water current return scheme: An observatory will be located somewhere in the deep sea, a considerable distance away from shore. Therefore, the electrode of the observatory will be located in the sea, not on shore. As a consequence, it can not easily be serviced. In addition, deep sea observatories connected to underwater cables are a new phenomenon and therefore, we have to look elsewhere for guidance on which electrode material to use.

Two industries have solved the problems we are faced with long ago:

1.3 Electro plating

Electro plating of precious metals uses the structure to be plated as cathode, a solution of precious metal salts and a mesh electrode as anode. The mesh electrode consists of a titanium structure, which is platinum plated. This anode is able to carry 600 A/m2 with low degradation, as described below. ‘Little chlorine gas‘ is produced by this type of electrode, but no substantial quantitative results are available. Apparently it does not constitute a notable health risk at the work place.

1.4 Cathodic protection Cathodic protection is used e.g. to save steel pilings from corrosion in every harbour. A comprehensive overview on these techniques is presented (in German) in: Gerd Zimmermann,

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"Technische und wirtschaftliche Aspekte aus Planung, Bauausführung und Betriebsüberwachung von kathodischen Korrosionsschutzanlagen für Meeresbauwerke" (c) Germanischer Lloyd, Hamburg, in "Tagungsband der 1. Tagung Korrosionsschutz in der maritimen Technik", 4. und 5. 12. 2001 at the University of the German Armed Forces, Hamburg.

Cathodic protection works in such a way that e.g. a steel structure becomes the cathode of a galvanic system and the current density is controlled in such a way that always "some" metallic material deposits on the steel structure, which consequently keeps it from corroding.

Several materials have been used successfully:

Iron-Silicon-Chromium

Magnetit Platinized Titanium / Niobium / Tantalum

Metal-Oxide covered Titanium

Wastage rate [g/A*a] 500 20 0,08 0,04 max. Current density [A/m2] 50 70 600 600

1.5 Platinized titanium anode Why is a platinized titanium anode so rugged?

It has to do with the physical properties of titanium. Titanium is extremely ignoble - even more so than aluminium. But titanium oxide has exactly the same density as metallic titanium. Therefore, whenever titanium is scratched in air or in water it will immediately impregnate itself with a thin layer of titanium oxide, which is an insulator. This insulation layer has a breakdown voltage of about 40 V.

This is why titanium is a popular corpus material for precious metal electrodes. The current flow will always go through the precious metal. Areas, which are not covered by the precious metal, will cover themselves with an insulating layer of titanium oxide. The degradation mechanism is as follows: Because of sand particle impact, small holes in the platinum plating will be produced over time. Then, the underlying titanium will oxidize and the oxidization zone will slowly migrate under the platinum plating. After a while, small areas of the 5 μm thick platinum will break away, widening the small hole in the plating.

Therefore, a titanium electrode electro plated with a precious metal will gracefully degrade: As more and more electro plating is abrased, the current density in the remaining plating increases. But only when all the plated precious metal is gone, the titanium corpus itself will start to carry current and disintegrate into titanium oxide rapidly.

When platinum is used as plating material of an anode in salt water, it doubles as a catalyst, which predominantly produces oxygen and only small amounts of chlorine. The latter is temperature dependent. It is unclear how much of this chlorine in statu nascendi will actually be set free as chlorine gas.

A quantitative study should be made measuring the chlorine gas production under varying temperatures and current densities.

It is worth noting that mixed-oxide plated titanium is used as anode in sanitizing water pools, because the mixed-oxide plating predominantly produces chlorine even under light salt conditions.

A market leader in platinizing is Umicore Galvanotechnik GmbH, located in Schwaebisch Gmuend, Germany.

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1.6 A real example In November 2006 a cable based Tsunami warning station was deployed in the East-Sea for the South-Korean Meteorological Administration (KMA). It extends 15 km into the sea South of Ulleong-Do island. The observatory is at 2200 m depth.

The station is powered by a 460 mA direct constant current source. The picture below shows the voltage drops produced by different current paths:

The current source itself is situated 5 km away from the beach. Therefore, a 2 wires cable had to be used from the source itself to the man hole next to the beach. The negative side of the current source is connected to the copper tube inside the fibre optic underwater cable. The cable is fed into the Ocean-Bottom-Unit (OBU), which extracts 12.7 V @ 460 mA electrical power. On its other end a brass rod is used as an electrode serving as cathode. From there the current returns an electrode on the beach through the sea water serving as anode. The anode is a platinum plated rod of titanium with a diameter of 20 mm, 250 mm long. An insulated wire connects the anode to the beach hole and the land cable.

In December 2010 the station had to be repaired, because of cable damage. This gave an opportunity to look at the pressure cylinder and the deep sea electrode. The system had been operating for 3 years continuously. The titanium pressure cylinder looked like new, the bronze cathode was not corroded at all, screws were lightly covered with salt - mainly calcium, I suppose.

Unfortunately, the anode could not be inspected. It is mounted about 1m below sea surface at low tides at the shore. But judging from the fact that the loop voltage at a constant current of 460mA did not change at all during the past 3 years (and after the cable repair), I conclude that the sea electrode is in good shape as well.

1.7 Conclusion When commercially available fibre optic underwater cables are used to connect to an ocean bottom observatory, only one conductor will be available as power feed. Therefore, the sea water itself has to be used as "return conductor".

Fig. 1: Voltage drops along the Korean tsunami warning station

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For the sake of servicability, the anode, which will degrade over time, should be located on the shore side. A platinum plated rod of titanium or niobium is an optimal choice for the anode. It should be sheltered by a plastic container from sand abrasion and direct rock impact. Current densities up to 600 A/m2 can be safely used.

The choice of the cathode material at the observatory end is less critical. A brass rod serves this purpose well. The choice of material should be made according to the material(s) of the observatory, which are in direct contact with the water. Ideally, no electrochemical potential differences should be present in the metallic structure or else corrosion problems might occur.

Klaus Schleisiek, SEND Off-Shore Electronics GmbH, kschleisiek@send.de

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