NOVEL ACOUSTICAL INSTRUMENTATION FOR THE STUDY OF OCEAN SURFACE PROCESSES

D. M. Farmer, R.C. Teichrob, C.J. Elder, and D.G. Sieberg

Institute of Ocean Sciences P.O. Box 6000, Sidney, B.C.

V8L 4B2, Canada

Abstract

The ocean surface and upper ocean boundary layer present an especially challenging envi.ronment for scientific measurement. Acoustical techniques are well suited to this task and the field is in a rapid state of development. A novel instrument is described for the measurement of breaking wave distributions, bubble size and spatial patterns, and the structure of organised flow near the ocean surface. It is designed for use as a self-contained and freely drifting device, having both active and passive acoustical components. A 3-dimensional array of broad-band hydrophones allows the passive location of wave-breaking events, multi-frequency sonars permit measurement of bubble size distributions as a function. of depth and time, sidescan sonars reveal the 2-dimensional bubble field structure, and Doppler backscatter is used to derive the velocity field. Both automated and remote control (via a hybrid acoustical/radio packet link) operation are possible. The 3 -dimensional array is achieved with motorised arms that extend when the instrument is at a safe depth and retract for recovery. Acoustical transmission of each frequency and sampling at various bandwidths are controlled from a single clock, allowing coherent detection on all channels. Synchronised multi­channel video cassette recording meets the massive data storage requirements, which in turn require extensive use of high speed digital signal processing for subsequent analysis. The potential of this approach for ocean surface studies is illustrated with examples drawn from recent experiments

Introduction and Overview

Processes at the sea surface control the exchange of heat, gas, momentum and other properties between the atmosphere and ocean, thus playing a critical role in determining ocean surface temperature, currents, wave conditions and the evolution of global climate. As our ability to model the atmosphere and ocean improves, there is increasing need for improved understanding of the physical mechanisms underlying these exchanges. Acoustical methods can contribute by providing remotely sensed measurements from beneath the surface. especially the detection of breaking wave events, bubble clouds and near surface Circulation patterns. We describe an instrument for obtaining

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such measurements and review some of the results that have been obtained.

While the instrument described here has evolved through three generations, the basic concept has remained unchanged: both ambient sound and active sonar data are obtained with a self­contained, battery powered system that can be either subsurface moored or allowed to drift freely, suspended from a small surface float. The decision to develop a battery powered self contained system was necessary because of the need to acquire ambient sound data without contamination by a nearby vessel. Similarly, useful bubble measurements must be acquired well away from a ship because of the persistent inclusion of bubbles in ship wakes. There is an additional advantage of this approach. If the instrument is to be deployed and left alone for 2 -3 days, the ship is free to continue with other work. This makes the measurement program much less demanding of ship-time and thus easier to include in cruises involving several scientific projects. The goal of studying processes on scales of a few centimetres to a few hundred metres dictates a deployment scheme that allows the instrument to be located close to the surface. It is, of course, possible and sometimes desirable to make both active and passive acoustical observations of the ocean surface from the sea-floor, but except in shallow water the resolution permitted by this approach is too poor to allow study of the smaller scale features so important to surface transfer processes.

On the continental shelf it is straight forward to moor the instrument so that it is deployed 20-30m beneath the surface. In the open ocean this would be a major undertaking and we have developed instead a freely drifting deployment in which the instrument is suspended by a rubber cord from a small surface float (Figure 1). The rubber cord largely decouples the instrument, which has a large added mass, from the energetic environment at the ocean surface. At a depth of 25-30m, it closely follows the small residual orbital motion of the swell. The use of a rubber cord is convenient from another point of view: it produces negligible backscatter to vertically oriented sonars. This is important, because precisely calibrated vertical sonars are used to derive bubble size distributions. Concerns about contamination of the vertical sonars

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a) b)

- external bouyancy - acoustic release _ anchor

... :.:. :".:.0:.-: .. ,:

FIG.1 MOORING CONFIGURATIONS

B) suspended from a drifting float in the open ocean b) subsurface moored in the coastal enVironment

must also be considered in the design of a communications link, between the instrument and a radio on the surface float. Since a coiled wire attached at intervals to the rubber cord would contribute unwanted backscatter, communication between the instrument and the radio on the surface float is carried out acoustically during silent periods when the active sonar is switched off.

The data recording demands of our measurement system are substantial. A primary concern throughout its evolution has been the need to record high quality broad band (-20kHz) ambient sound, in addition to active sonar signals. The recording

FIG.2 INSTRUMENT WITH ARMS EXTENDED

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system must encompass a large dynamic range and permit the equivalent of continuous battery operation for periods of 32 -SOh. Fortunately the combination of the commercially available Pulse Code Modulation (PCM) digital audio processor, coupled with a series of video cassette recorders meets the requirements. Use of standard VHS recorders with S hour tapes allows 5 gigabytes of digital data to be stored on each tape, with additional digital storage on the analog channels. We have used up to 10 VCRs in various modes that have allowed up to 80h of operation on a deployment, although greater bandwidth requirements and other considerations may limit the total duration. In the most recent version discussed here, we have used 8 VCRs, operated in pairs to provide a total of 32h of operation.

It is central to the design philosophy of the instrument that it be flexible in allowing use of a variety of different sensors. An earlier version included a vertically oriented video camera to provide simultaneous video and acoustical observations. Junction boxes on the instrument frame permit different transducers or sensors to be designed for simple 'plug-in' connection, and the data logging system has a corresponding flexibility. This flexibility is also inherent in the degree of software control available through the controlling computer.

A significant development in the latest version of the instrument has been the inclusion of motor driven arms that can extend when the instrument is at a safe depth and retract for recovery (Figure 2) . These have a span of 8. 5m and provide a base for a 3-dimensional hydrophone array and bistatic sonar systems.

FIG.3 MAIN ELECTRONICS PACKAGE

Component Descriptions

System Outline

The heart of the instrument resides in a 1.5m by 0.30m diameter pressure housing. Figure 3 shows the acoustical electronics, the recording system and control electronics.

In its present configuration there are 7 vertically oriented sonars operated at 28, 50, 88, 100, 120, 200 and 400kHz. The transducers for each frequency are mounted on the instrument top plate, with electrical connection via a junction box to the electronics pressure housing. Four 100kHz sidescan sonar transducers are mounted transversely at the ends of the extendable arms and directed 20° above the horizontal so as to view the ocean surface. A broad band hydrophone is mounted at the end of each arm, on the top plate, and suspended a few metres beneath the instrument housing.

SURFACE� �----� COMMUNICA�

ENVIRONMENTAL

I SENSORS

FIG.4 INSTRUMENT ELECTRONICS BLOCK DIAGRAM

r-------------------

I

MIX FREQUENCY PHASE LOCK LOOP

�-------------------� r-------------------l I TRANSMIT FREQUENCY I I PHASE LOCK LOOP

�------------------- REF"'ERENCE FREQUENCY 139B.125 Hz

OUTPUT SIGNAL

2756.25 Hz

FIG.5 SONAR/SIDESCAN TRANSCEIVER BLOCK DIAGRAM

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Control and recording of these various acoustical systems is managed by a Tandy Model 102 lap-top computer, with appropriate external interface cards connected to the computer bus. The computer is also connected to a Packet Terminal Node Controller (TNC) for communication to the ship.

The interface cards include circuitry for switching on and off various parts of the instrument under computer control, for actuating the sonars, controlling the motor driven arms, and for interrogating and digitising various environmental sensors.

A block diagram illustrating the various primary electronic components is shown in Figure 4. These components will be discussed in sequence below.

The sonar tLansceivers are nearly identical except for frequency dependent components such as filters and crystals. In order to retain coherent operation of sonars, all signal generation is derived from a single digital sampling clock. This is a 44. lkHz signal on one of the two pulse code modulators (PCM) . The 44.lkHz signal is divided down to 1378 . 125Hz , which is used as a reference frequency from which each sonar source signal is generated by multiplication. This procedure ensures that the sonar transmissions always start with the same phase.

Figure 5 shows the signal generation scheme. The reference frequency is used to control a crystal voltage controlled oscillator (VCO) the frequency of which is divided down in two stages. The first stage is used to drive the sonar via a transmit gate. It is divided again in a second stage to generate the reference frequency, which is then phase compared with the original reference frequency to maintain phase lock of the VCo.

The reference frequency is also used in the same way to generate a mixing frequency that, for those channels having a s. 5kHz bandwidth, differs from the sonar transmission signal by 2756.25Hz. The received signal is then mixed with this frequency to yield a signal with 2756.25Hz carrier which is sampled at exactly four times this frequency (1l. 025kHz) for recording. In this way the signals are quadrature sampled by the PCM analog to digital converter under control of the same clock that generates the source signal. This scheme can be adapted for different bandwidth requirements. For example, if an ll.0kHz bandwidth is required, a m�x�ng signal of 5512. 5kHz is used to generate a signal that is sampled at 22.05kHz.

Each sonar transceiver is mounted on a single board. The output stage consists of a 24 Volt DC power supply fed through a switching power amplifier into a transformer and inductor to drive the transducer at approximately 70 Watts. Transformers and inductors are individually wound to match the impedence of each transducer.

The receiver contains four stages amplification, each with a programmable gain.

of

Hydrophone Array

We have used Met Ocean NH4l23 hydrophones and preamplifiers. These have a low frequency cut-off of 32Hz and a sensitivity of -187dB re:l�Pa-l. The instrument is not designed for detecting lower frequencies, which would require different mounting procedures. A hydrophone is mounted at the end of each of the four arms, providing a span across opposite arms of 8. Sm. A fifth hydrophone is on the same plane close to the centre of the top plate; the sixth is attached to the anchor cable 10m beneath the array.

Control Software

Control software is based on the four different modes of operation during deployment. After the instrument is deployed there is a preset time delay. This ensures that if communication cannot be established, within a certain period, the arms will be extended and recording will begin using default parameters. Normally, however, recording will begin on command from the ship, following a check of the instrument status and extension of the arms.

The second operational mode covers normal operation with alternate sonar operation and hydrophone listening. Various parameters can be set to control specific sonar operation and to adjust the relative proportion of time in hydrophone and sonar mode. Since the sonars interfere with the acoustical communication link, a predetermined period, such as the first IS minutes of each hour, is set aside for hydrophone operation. This provides a window during which instrument status can be checked and recording parameters can be modified. The software ensures that all environmental sensors are monitored, including the recording status of the VCRs, and also checks the communications link for new commands.

The third mode is for standby, in which the instrument neither records, nor counts out a delay, but simply waits for further commands. Finally, the fourth mode occurs when the full complement of VCRs have run to completion and the instrument enters a 'sleep' mode. The arms retract and the electronics are switched off to conserve power. The communications hardware is switched on at predetermined intervals to allow determination of its status prior to recovery.

Recording System

The recording system includes the multiplexer and digitiser/encoder mentioned above, and the video recorders. While these can be used in various configurations, the present discussion relates to the synchronous use of two video recorders in parallel. One records output from the six hydrophones and seven vertical sonar signals; the other records a hydrophone and four sidescan signals.

Each digitiser/encoder is based on two printed circuit boards used in a Sony digital audio processor (PCM-SOl/60l), with two analogue input

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channels of 44. 1kHz sampling rate at 16 bits resolution, and an output encoded in NTSC video format suitable for recording. These have been modified to operate on the voltages available in the instrument, to reduce their size and power consumption, to accept multiplexed inputs, and to add digital synchronising bits and words. As discussed above, the master clock for all the active sonars is derived from one of the digitisers, resulting in a synchronous signal from each sonar which is sampled at its quadrature points in order to record amplitude and phase on a single channel.

One multiplexer samples the four sidescan signals at l1.02SkHz into one channel of its PCM, with the second channel retaining the full 44. lkHz sampling (22kHz bandwidth) for one hydrophone signal. The other multiplexer inputs sixteen signals in two sets of eight, each at 11. 02SkHz. The hydrophones are sampled by default, whenever the sonars are not switched in (six inputs are used) . Whenever a sonar is triggered, an alternate set of eight inputs is switched in to sample the seven sonars for a period long enough to ensure data capture up to the surface. Each signal sampled at 11kHz is bandpass limited to SkHz using a switched capacitor filter with a low pass Cauer function. The multiplexers substitute the two least significant bits of each sample to encode synchronising information, and retain the upper fourteen bits of data. Each sonar trigger is marked in the data stream with a unique sequence of eight words to allow simple detection for analysis. A modification to this scheme allows annotation data to be added to these marks so as to allow accurate synchronisation of the data in time, especially when multiple tapes are recorded simultaneously.

The video recorders are portable VHS units, each providing 8 hours of recording time. Power on and record status are available to the instrument computer, and control is achieved using the function generator chip from the remote control unit that comes with each VCR. The video track records the pulse code modulated digital data, while the audio track records FSK serial annotation data supplied by the instrument computer. In the present configuration, 8 VCRs are used in pairs, allowing an equivalent continuous operation period of 32 hours.

Environmental Sensors

Various environmental sensors are used to detect the orientation and operation of the instrument. These include a ParoScientific pressure sensor, a magnetic compass, two tilt indicators, an accelerometer aligned with the instrument's vertical axis, an additional tilt indicator mounted on one of the arms for detecting their deployment statusJ and battery voltage sensors. The control computer and associated interface circuitry is used to interrogate these sensors which are then recorded via a 1200 baud modem on the audio channels of the video tape.

Communications Link

If the instrument is deployed from a ship it is useful to be able to interrogate its various

functions and have the option of controlling its operation remotely. This is achieved with a hybrid radio and acoustical link.

The system consists of a radio link from the ship to the surface float, and an acoustical link from the float to the instrument. Pack(lt radio AX.25 protocol is used at 1200 baud on the radio link and at 300 baud on the acoustical link.

The equipment on the ship consists of a computer used as a terminal, a packet Terminal Node Controller (TNC) and a UHF radio. The buoy contains a UHF radio, a dual port TNC and an underwater transceiver built by Orcatron Manufacturing Ltd. The instrument contains a matching underwater transceiver and a TNC which is interfaced to the controlling computer. The packet radio protocol permits data to be transfered from one station to another through a repeater station. In our configuration the buoy acts as the digital r-epeater. Error correction and packet retransmission take place between the ship and the buoy or between the buoy and the instrument, so that the two links are independent and errors detected at the buoy are corrected prior to forward transmission. This is much more efficient than the conventional method of digital repeating where retransmissions must pass through the entire link.

The use of a hybrid acoustical/radio link can also be used in the moored configuration (Figure Ib) . In this case the surface float would be replaced by a nearby bottom anchored surface buoy.

Batteries and Power Supplies

Batteries are housed in a separate 1. 1m by O.3m diameter pressure housing. The main pack consists of seven l2 Volt 32 Amp Hour Gel Cell rechargable batteries. Six of these are wired in series and parallel to provide 24 volts at: 96 amp hours. The seventh battery provides 12 volt power for the control computer and the acoustical link.

The main battery pack provides power directly to the sonars. It is also used to drive four DC to DC converters to provide the instrument electronics and video recorders with a regulated ±12 volts and ±5 volts. The 24 volt battery pack is designed to provide sufficient power to operate the 8 VCRs and associated electonics for the 32 hours of equivalent continuous operation that is possible with 8 hour extended play tapes. The 12 volt battery is able to run the computer and acoustical communication system in standby mode for a minimum of 72 hours.

Mechanical Design

The instrument in its freely drifting mode is supported by a 37" diameter steel buoy which also contains 2 VHF direction finding beacons, 2 strobe lights, a UHF transceiver, an acoustical transceiver and packet communications system. The buoy is connected to the instrument with a natural rubber 'bungy' cord which is 17m unstretched, but typically 25m when stretched with a 72 kg tension. The cord effectively decouples the instrument from the rapid motion of the surface float. Special care is taken

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to ensure that there are no metal on metal moving parts in the connections, so as to avoid acoustical contamination. For this reason also the surface buoy is covered with I" thick plastic foam strips to reduce the sound of wave impact.

The instrument itself has 12 syntactic foam floatation blocks at its top end, each posessing 20 kg buoyancy. Most of the instrument's total weight in air of 692 kg is located close to the bottom of the instrument frame so as to maximise the righting moment. In water the instrument is 45 kg buoyant. Care has been taken to balance the buoyancy and weight distribution so that the instrument is typically within 1 or 2D of vertical.

Additional buoyancy is attached to the anchor cable beneath the instrument, sufficient to bring the instrument and acoustic release to the surface in the unlikely event that all pressure housings flood. (No such problems have been encountered thus far!) Beneath this buoyancy is an acoustic release and the anchor, which is a barrel of concrete weighing 270 kg in water. As a safety precaution a pressure activitated release is mounted in parallel with the acoustic release, that would release the anchor weight if the rubber cord was severed and the instrument sank below 100m.

The instrument's height is 3m, its diameter 1. 25m and the extended arms have a total span of 8.5m. The hinged arms are actuated by a DC electric motor that drives a threaded rod running vertically up the centre of the frame. A threaded nut running along the rotating rod drives the arms in or out. A sensor detects when the movement is complete and switches off the motor.

Data Recovery

Two types of VCR data storage are used: the primary recording of PCM encoded data on the video channel, and the storage of 'annotation' and environmental data on the audio tracks. The data storage is substantial. The video channel alone handles 5.05 gigabytes on a single 8 hour video tape. Moreover the high data rates dictate use of real time data processing on playback where possible. A schematic diagram of the components used on playback is shown in Figure 6.

On playback, a Sony PCM-501/601 digital auoLo processor receives the video data from a VCR and decodes the video signal into a serial digital data stream and synchronising clocks. These signals are extracted from the PCM and interfaced to a digital signal processor for analysis.

Subsequent processing depends upon the nature of the data acquired. We have developed two systems. In the case of data acquired from the hydrophone array, we convert the serial data from the hydrophones to parallel 16 bit words and store them in a hardware FIFO buffer. A Motorola 56001 digital signal processing board (manufactured by Spectrum Signal Processing Inc.) performs the first stage of processing and transfers the intermediate

data via the serial ports to a second 56001 DSP board for the next stage of processing. The output

FIG.S PLAYBACK PROCESSING SYSTEMS

from this second processor is then fed to a Hewlett Packard Vectra 386 computer for final processing, disk storage and generation of appropriate displays.

Sidescan and vertical sonar data is processed by a different system. Sonar data for a given transmission has a finite length of useful information, for example the time taken for a pulse to return from the ocean surface. Digital synchronisation marks identifying the time of the pulse transmission are embedded in the data stream. Once a mark is detected in the data stream, the desired numbers of words are then transferred to the serial port of an AT&T DSP32C digital signal processing board (also from Spectrum Signal Processing Inc.). A host Hewlett Packard 386 Vectra computer takes processed data for storage on disk and appropriate display.

In both systems, annotation data stored on the VCR analog channels is demodulated by a phase lock loop into a serial digital data stream, converted to parallel by a UART, and stored in a hardware FIFO buffer. The host computer can take the data as required for adding time and date stamps to the data as well as applying corrections for tilt and direction,

Results

The instrument has been used in one form or another in the FASINEX, OCEAN STORMS and SWAPP projects. In FASINEX (1986) (Frontal Air Sea Interaction Experiment) a much smaller version was employed, without computer control or digital recording of sonar data. A distinguishing feature of that project was the use of a vertically oriented video camera that was used to identify breaking waves from beneath. By the OCEAN STORMS cruise (1988) digital recording of sonars was achieved allowing Doppler processing of the vertical sonars. In the SWAPP (Surface Wave Processes Project) in February/March 1990, the instrument had evolved to the extent described above. The number of sonars increased from 4 to 7, all channels were digitally

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recorded, the motorised arms were added and a 3-dimensional array of hydrophones used.

A full description of procedures developed to analyse resulting data lies beyond the scope of this report. Reference is made to Farmer and Vagle1 for an example of the techniques used and the kind of scientific results that can be achieved, Here we summarise just a few examples of typical experimental output.

Figure 7 shows backscatter from bubble clouds. By using different frequencies such images can also illustrate the variability in bubble size distribution, which is a key element in understanding bubble cloud dynamics and air-sea gas transfer.

Figure 8 shows a sidescan image of bubble clouds. Bubble cloud organisation into long rows just beneath the ocean surface is a consequence of Langmuir circulation,

In Figure 9 a correlation plot is shown of the ambient sound signal from two hydrophones in the 3-dimensional hydrophone array. The vertical axis corresponds to time delay between the hydrophones, the horizontal axis is time (in second;;) and the

m " '-+' " .5 I f-a. UJ 0

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100

80

60

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FIG,7 VERTICAL SONAR IMAGE SHOWING BACKSCATTER FROM BUBBLE CLOUDS

10 Pings averaged SIDESCAN DISPLAY

Sidescan 1

FIG,B SIDESCAN SUNAR IMAGE OF BUBBLE CLOUDS SHOWING ORGANISATION INTO ROWS BY LANGMUIR CIRCULATION

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DATA31.SCT Time(sec) Chan3-J.

FIG.9 CORRELATION PLOT OF DATA FROM TWO SPACED HYDROPHONES SHOWING. SOUND OF BREAKING WAVES MOVING ABOVE THE INSTRUMENT

grey scale denotes signal correlation above some threshold level. The dark streaks represent correlation peaks associated with the passage of breaking waves. The breaking waves travel in nearly straight lines; they are curved on the display because of the geometrical distortion of the time delay calculation. By using additional hydrophone pairs it is possible to generate 2-dimensional images of the moving breaking events. Such data provide a basis for testing models of the wave­breaking and of the processes of momentum transfer with which it is associated.

Concluding Comments

The basic concept of signal generation and digital recording is easily adapted to different experimental configurations. In one of these we used a three dimensional array of hydrophones suspended beneath ice in the Arctic2• Digital broad band recording allowed detailed analysis of sounds radiated by individual cracking events.

In general we have adopted the approach of broad-band recording rather than preprocessing, because this technique imposes no assumptions on the nature of the raw signal. As our understanding of the underlying physical mechanisms develops, we can gradually implement preprocessing of specific portions of the data. In developing such processing approaches, VCR recordings provide a very convenient representation of the data; in effect the real time processing developed for use with VCR output in the laboratory can be directly implemented without modification for preprocessing on the instrument, since the data rates and formats are identical.

In concluding we comment that acoustical methods have great potential for the study of ocean surface processes. The availability of inexpensive digital processors and broad band digital recorders combined with novel deployment schemes as described here, opens up the possibility of a wide range of

ocean measurement approaches that can contribute to our under�tanding of the upper ocean boundary layer.

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Acknowledgement: Development and use of this instrument has been made possible through financial support of the Canadian Panel on Energy Research and Development, the U.S. Office of Naval Research and the Institute of Ocean Sciences. Paul Johnston developed the earlier versions of the instrument . Svein Vag1e, Len Zedel and Li Ding played a key role in developing the analytical procedures for using the data.

References

Farmer, David M. and Svein Vagle, 1989. Waveguide propagation of ambient sound in the ocean­surface bubble layer, J. Acoust. Soc. Am. 86 (5), 1897-1908.

Farmer, David M. and Yunbo Xie, 1989. The sound generated by propagating cracks in sea ice, J. Acoust. Soc. Am. 85(4) , 1489-1500.