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TRANSCRIPT
American National Standard on Towed Passive Acoustic Monitoring
and Mitigation Systems
Strawman Draft, July 2015
1. Scope and Organization
This American National Standard (ANSI) concerns the use of passive
acoustic monitoring (PAM) that uses towed arrays from surface vessels for
research or environmental regulatory compliance. A subset of this standard
covers operations that attempt to process data in real-time, either using on-
board or remote operators, with or without the aid of automated detection
software. The standard covers seven aspects of a complete PAM operation:
Initial planning, hardware, software, acoustic localization, operator
qualifications, operations, and performance validation. The planning and
performance validation are described here in some detail; the other aspects
are outlined in the accompanying spreadsheet.
A primary guiding principle of this standard is not to specify how PAM
operations are conducted, but to specify how an operator reports these
activities to contractors, regulators, and outside reviewers. By clarifying and
prescribing how to report PAM operations, communication between these
stakeholders can be enhanced and a virtuous feedback cycle enforced that
may gradually raise the quality and sophistication of this growing activity
without suppressing innovation. Thus the goal is that relatively few actions
in this standard will be required for compliance with this standard; however,
nearly every action discussed here will be required to be documented and
reported in a certain fashion in order to comply with the standard.
From this principle three kinds of actions are specified in this standard:
those that are required, those that must be documented and reported, and
those that are recommended. Required actions are specific actions or
restrictions a PAM operator must undertake (or not undertake) to comply
with this standard, and are indicated in this document by the use of the word
“shall”. Reported actions require an operator to document and communicate
the action in a manner described by this standard in order to achieve
compliance, but the standard does not place restrictions on how the action is
carried out. The phrase “shall report” will indicate this level of compliance.
Recommended actions are statements that reflect an attempt on consensus
opinion of “best practices” by individuals from academia, government, and
industry, and seek to serve as a reference or illustrative benchmark for PAM
activities. The phrase “should” will indicate whether an action is
recommended.
Another guiding principle behind this standard is that it requires a PAM
operator to predict and simulate system performance prior to a survey via a
planning document, and then to report to what degree the predictions and
simulations matched the reality encountered with a follow-on validation
document. The motivation behind this requirement is, once again, to
enhance clarity of communication between operator, contractor, and
regulator by allowing even a non-technical reviewer to understand whether
PAM operations achieved desired goals, and if not, to clearly understand
why. Such a formal procedure helps protect everyone’s interest by clarifying
whether issues with PAM operations arise from the operator, or from factors
outside the operator’s control. One particular scenario this standard seeks to
address is helping contractors and regulators easily understand whether
towing platform noise prevents a towed PAM operation from attaining its
required detection range.
2. Definitions and terms
PAM may be conducted for mitigation or monitoring. Mitigation is
defined here as a project whose only goal is to prevent direct physical injury
to marine life from industrial activities, while monitoring is a project with any
additional goal, including preventing behavioral disruption of marine line,
conducting an acoustic census, or characterizing the acoustic fields
generated by industrial, physical, or biological sources. These definitions are
delineated here because the working group may eventually decide to apply
different criteria to mitigation or monitoring operations. In this document
both operations are treated the same.
A PAM operator is defined as any organization that has responsibility
for the acquisition and interpretation of passive acoustic data from towed
array systems. A contractor is defined as any organization that employs a
PAM operator, and to whom the operator is expected to report. Either the
contractor or operator may be responsible for providing the towed array
hardware. A regulator is any government-based entity that has a legal
responsibility to enforce compliance with relevant laws applicable to the
region where PAM is being conducted.
Acoustical terms used here will generally follow the definitions of ANSI-
ASA_S1.1-2013, “Acoustic Terminology,” and the draft ISO-TC43-SC3-N0037
document on “Underwater Acoustics-Terminology”. In cases where
definitions may conflict between those standards, the definitions listed in this
section will apply:
Impulsive noise:
Signal-to-noise ratio:
Duty cycle:
Sonar equation:
Spherical spreading model:
Species cluster:
Industrial cluster:
3. Initial Planning
3.1 Planning and Validation documents
Before commencement of PAM operations a PAM operator shall
create a report, defined here as a planning document, that shall be provided
to contractors and regulators before operations commence. After a cruise
has been completed, a PAM operator shall create a second, validation,
document that complements the planning document. Only a single pair of
documents needs to be produced for a single operation, even if the operation
utilizes multiple towed arrays. A third document, a briefing document, shall
also be produced and given to PAM operators participating in a give project,
and will be discussed under “Operations”. See Section 9.0 for details on the
validation document.
The planning document shall contain the information listed below.
3.1.1: The planning document shall describe the time period and
geographical region to be covered by the PAM effort.
3.1.2: The planning document shall list marine mammal species
expected to be present over this time period and region, with citations of
specific databases or literature used to create this list. (An Appendix of
database resources or literature reviews can be appended to this standard,
e.g. Richardson and Greene, “Marine mammals and Noise,” or “Marine
Mammal Observer & Passive Acoustic Monitoring Handbook”). If an
operation does not plan to monitor or mitigate for certain species, the
planning document shall explicitly state which species shall not be covered
by operations.
3.1.3: The planning document shall list the acoustic bandwidth that will
need to be monitored for each species listed in 3.1.2, with appropriate
references. If a set of species is expected to share similar acoustic
characteristics, the report may then define “clusters” of species that will
streamline the rest of the planning document. For example, some clusters
might include “low-frequency baleen whales,” “beaked whales,” or “high-
frequency echolocators”. It is recommended that sperm whales and right
whales always be treated as their own species clusters.
3.1.4: The planning document shall list the industrial activities
expected during operations. This list may include not only different acoustic
sources, but also different modes of operation for each source. For example,
airgun array sources may have a “mitigation” mode and a “full power”
mode. Self-noise from the towed platform shall be listed as one such
activity. Groups of similar industrial activities may be combined into
industrial “clusters,” to simply discussion in the rest of the planning
document. It is recommended that clusters consist of activities that share
similar duty cycles, bandwidths, and 1-minute averaged received levels of 6
dB or less.
3.1.5: The planning document shall list the minimum detection ranges
to be achieved by the system. Each combination of species and industrial
cluster defined in 3.1.3 and 3.1.4 shall have a separate detection range
listed, and the manner by which this range has been assigned shall be
reported. For example, the detection range may be obtained by (i) a preset
requirement provided by a regulatory agency or contractor; (ii) derived by
the operator, contractor, or regulator, based on propagation simulations of
various industrial sources, combined with estimates of PTS and TTS
thresholds of specific species; or (iii) estimates based simply on past
empirical experience.
3.1.6: The planning document shall list key features of the hardware
system, including the hydrophones, array cable, signal conditioning, and
recording system. See “Hardware” section below and on spreadsheet for
specific reporting requirements and recommendations.
3.1.7: The planning document shall describe the software used by
human operators to monitor the acoustic data, as well as software intended
for use for automated detection and classification. See “Software” section
below and on spreadsheet for specific reporting requirements and
recommendations.
3.1.8: The planning document shall describe by what means the
bearing or range of bioacoustic signals from the towed array will be
estimated. See “Localization” section below and on spreadsheet for details.
3.1.9: The planning document shall list the expected monitoring
schedule of PAM operations (e.g. day only vs. day/night), the number of
human observers to be used during operations, the qualifications of the
observers, shift schedules, shift procedures, and data integrity protocols.
See “Operations” section below and on spreadsheet for specific reporting
requirements and recommendations.
3.2 Planning document simulations
In addition to the topics listed above, the planning document shall present
the results of simulations that illustrate the expected detection performance
of the system using the best prior information available. A simulation shall
be required for each combination of species and industrial cluster defined in
3.1.3 and 3.1.4. The simulation may be as simple as the sonar equation, or
as complex as a numerical acoustic propagation model combined with
empirical noise samples, but every simulation in the planning document shall
describe the following factors:
3.2.1: Each simulation shall list the source levels and directivity
assumed for each species cluster, and time series for typical calls for each
species cluster shall be generated, using digital sampling rates used by the
recording system. Amplitude-scaled recordings of the species in question
are recommended for producing these time series, but calls may also be
simulated by synthetic FM sweeps or impulses, provided that the algorithm
for generating the artificial signals is specified.
3.2.2: Each simulation will describe the acoustic propagation model
used for each species cluster. The model may be theoretical or derived from
empirical measurements. It is not recommended to use a simple spherical
spreading model for situations where the ocean depth is less than the
detection range, where the acoustic wavelength of the lowest frequency
component is 5% or more of the ocean depth, or where a significant
thermocline is expected in the area.
3.2.3: Inputs to any numerical propagation model shall be listed,
including bottom bathymetry, bottom composition, and sound speed profiles.
The document shall cite databases or other resources where these inputs
have been obtained.
3.2.4: The simulation shall list expected received levels of noise
generated by each industrial cluster on individual hydrophones for the towed
array system, for each bandwidth defined by each species cluster. These
received levels may be empirical measurements from previous operations, or
theoretical estimates using estimated source levels and appropriate
propagation modeling. It is strongly recommended that actual empirically-
measured time series from similar industrial clusters be used to estimate
received levels, in order to capture the non-stationary characteristics of each
noise source. If no empirically-based measurements are available, then the
planning document shall use published source levels for a given industrial
cluster, converting into estimated received levels using a propagation loss
model.
3.2.5: The simulation shall generate a time series representing the
noise generated by the various industrial clusters. If empirical data samples
are available, then the time series shall be filtered over an appropriate
species cluster bandwidth, and then the amplitude of the time series shall be
adjusted to convert received levels of the empirical measurements to
estimated received levels on the upcoming operation. The propagation
model used to make these adjustments shall be specified.
If no empirical measurements are available, then synthetically-
generated noise may be used to model the time series generated by the
activity, using filtered white noise, FM sweeps, or gated pulses, depending on
the time-characteristics of the noise source.
The noise time series may incorporate directional characteristics of the
industrial noise, if available.
3.2.6: If no significant source of industrial noise is expected for a given
species cluster and industrial cluster, then the so-called “Wenz” curve may
be used to generate “colored” noise time series with appropriate power
spectral densities listed in the curves. Industrial noise shall not be
considered a significant source of background noise if the power spectral
density of the noise lies 10 dB below the Wenz prediction of the species
cluster bandwidth-limited noise at sea state 1.
3.2.7: Time series of flow noise self-generated by the towed array
shall be estimated, using either empirical data measurements at the planned
tow speed (recommended) or by generating appropriately band limited white
noise, scaled to an appropriate amplitude.
3.3 Simulation outputs
For every simulation discussed in the previous section, a synthetic
spectrogram shall be generated, and a performance curve shall be
generated for each automated detector to be applied to the species cluster
used in the simulation.
Spectrogram: The time series generated for each species cluster
(3.2.1) shall be scaled in amplitude to match the expected received level of
the sounds when generated at the appropriate detection range specified in
3.1.5 at a representative calling depth for each species cluster. These call
time samples shall be combined with the industrial, ambient, and flow noise
time series discussed in 3.2.5, 3.2.6, and 3.2.7 respectively, to generate a
final simulated time series for each combination of species and industrial
noise cluster. If array processing is to be used in the PAM operation, then
array gain and other directionality enhancements may be applied to
generate the final “simulated” time series.
A spectrogram shall be generated from each simulated time series and
presented in the planning document. The spectrogram should cover a time
period over which a human observer would be expected to monitor real-time
data during the actual operation. The purpose of the spectrogram is to
provide a visual indication that a human operator would be able to detect a
representative signal generated by a species cluster at the specified
minimum detection range.
Performance curve:
If an automated detector and classifier (ADC) is to be used in any
capacity in the PAM operation, then a performance curve shall be plotted in
the planning document, for every combination of species cluster and
industrial cluster. The performance curve shall be generated by generating
1000 one-minute simulated time series realizations, that combine the
biological, industrial, ambient, and flow noise time series discussed in 3.2.1,
3.2.5, 3.2.6, and 3.2.7. The ADC will be applied to each realization, and a
Detection Error Tradeoff Graph (false negative rate vs. false positive rate)
shall be generated for the ADC. ADC settings that will be “default” values
during the upcoming operation shall be marked on the DET graph. The
purpose of the graph is to provide insight into what tradeoffs are expected
between the false negative rate (“miss rate”) and the false positive rate
(“false alarm”). This standard shall not specify a particular false negative
rate or false positive rate; rather, these performance metrics shall be
specified in the planning and follow-on validation documents.
3.4 Timing of planning document
The planning document shall be completed and delivered to the
contractor with enough lead time to allow the contractor (and potentially
regulators) to make go/no-go decisions on using PAM.
4.0 Hardware
This section lists required and recommended factors regarding towed array
system hardware, and lists specifications that shall be described in the
planning and/or validation documents. (See also attached spreadsheet for
division of key points into required, required to report, s recommended
columns).
4.1 Hydrophones
At least one hydrophone in each tow cable shall have a sensitivity that
spans the vocalization bandwidth of each species cluster defined in 3.1.2.
[The minimum sensitivity within a given bandwidth will be 200 uPa/V?] At
least one hydrophone in each tow cable shall be calibrated over a bandwidth
sufficient to characterize all noise sources that overlap the combined
bandwidths of all species clusters1. Every towed array cable in the
operation shall have one extra, or backup hydrophone.
The PAM operator shall report the following information about
hydrophones in the planning document, as a function of frequency: noise
floor, calibration curve with respect to frequency, directivity, mechanical
depth limit, and flow noise compensation characteristics. If multiple
hydrophones in a towed array cable share the same specifications, then the
number of hydrophones that share a given set of specifications shall be
reported as well.
4.2 Array geometry
For each towed cable in the water, the planning document shall report
the number of hydrophones in the cable, the spacing between hydrophones,
and the distance between the stern of the vessel and the hydrophone closest
to the vessel. If multiple cables are deployed from a single platform, or if
cables are deployed from multiple platforms, then the relative positions of all
1 While in principle a system with localization capability may be able to verify
detection ranges with uncalibrated hydrophones, a calibrated system is
needed to verify that the various noise source levels estimated in the
planning document are consistent with actual measurements.
hydrophones with respect to each other should be specified, whenever
information from the cables will be combined for localization purposes.
The planning document shall list the expected tow depths of the
hydrophones during expected tow speed conditions. We recommend that
the tow depths be listed as a function of several tow speeds.
4.3 Auxiliary sensors
The planning document shall list the existence and location of auxiliary
sensors in each tow cable, including depth, inclination and/or acceleration
sensors. The resolution and sampling rate of each sensor shall be reported.
We recommend that a tow cable have at least a depth sensor embedded
within the cable that can be periodically sampled every 10 seconds2.
4.4 Cable properties
The tow cable-operating load shall be twice the load expected during
maximum towing speeds.
The planning document shall list the following mechanical properties of
the towing cable: Maximum operating load, cable diameter, linear density,
and maximum radial curvature permitted. The use of weights, anchors, or
other submerging technology shall be described. The model and make of
the deployment and retrieval winch shall be specified, including the
2 As discussed below, the primary motivation for measuring towed array depth is to determine whether the system is above or below a strong thermocline.
maximum load permitted. The planning document shall specify whether the
electrical connection needs to be broken when operating the winch.
The planning document shall list the following electrical properties of
the towing cable: number and gauge of conducting wires, arrangement (e.g.
twisted single-pair), whether a grounding shield exists, and whether data are
transmitted by analog or digital means. If an analog-to-digital convertor is
employed at the hydrophones, the type of converter, the number of bits per
sample, and the sampling rate shall be specified. See Section *** for
requirements on analog/digital conversion. We recommend that cables
transmitting analog data use twisted single pairs that are in turn surrounded
by a grounding sheath.
The planning document shall describe the make and model of any
connectors used to connect tow cables together, or to connect the tow cable
to any deck cable. All connectors (including deck connector) shall be able to
survive full immersion in water without disrupting or shorting the signal. The
length of deck cable needed to connect the tow cable to the recording
electronics shall also be specified.
4.5 Signal conditioning and recording equipment
Acoustic data shall be recorded with a minimum of preprocessing or
filtering before being recorded by a dedicated digital recording system (not
used for multiple purposes like manual monitoring or manual detection). The
acoustic data shall pass through the recording system before being fed
through any computer running automated detection algorithms or subjected
to further filtering and processing for manual review. Signals shall be passed
between equipment using BNC cables (and RCA cables shall never be used).
The minimum bits per sample for any ADC system shall be 16, and the
sampling rate of any recording system shall exceed twice the maximum
frequency expected from all the species clusters. The frequency-dependent
gain and phase characteristics (transfer function) of the total conditioning
and recording system shall be measured and shall be reported in the
planning document, to permit raw numbers of digital time series to be
calibrated in terms of Pascal. **Recording on tape or other analog recorders
shall/shall not be permitted(?)**.
The recording system shall have the ability to record data
continuously. If data are not being recorded continuously, then the
recording system shall be able to buffer 30 seconds of data, so that if the
recording system is activated the previous 30 seconds shall also be stored.
The planning document shall provide a flow chart illustrating any signal
conditioning that takes place prior to data recording. The make and model
of the recorder, its ADC type, number of bits per sample, and sampling rate
shall be reported, along with the make and model of each piece of
equipment that the signal passes through before being recorded.
If any auxiliary sensors are being recorded, including GPS, the
existence of the data streams shall be listed in the planning document, as
well as the data format and sampling rate. Data shall be sampled at least
once a minute, and must be recorded continuously while tow cable is in
water. The recording system need not be the same system recording
acoustic data. Files storing data shall have a time stamp. We recommend
that at a minimum array depth and ship GPS be logged concurrently with any
acoustic data sampling.
5.0 Software
This section addresses software for both real-time and post-cruise
acoustic data processing. It is divided into software for manual review and
software for automated detection and classification.
5.1 Software for real-time human monitoring
Software intended for real-time monitoring by manual (human)
operators shall have the following features:
-The ability to generate scrolling spectrograms in a “ring buffer”: that
is, the past N seconds of acoustic data are always visible on screen.
-The ability to plot two spectrograms covering two different bandwidths
simultaneously.
-The ability to adjust spectrogram time, frequency, and intensity
scaling on the fly, the ability to adjust the Fourier transform and data
overlap, and an ability to plot either a linear or logarithmic frequency axis.
-The ability to draw bounding boxes around signals of interest, and
permanently store the associated calibrated time series from all
hydrophones in the system.
This section will probably define what non-acoustic information
(metadata) needs to be logged by software.
5.1 Automated detection and classification software
Please see spreadsheet for examples of topics for automated signal
processing.
6.0 Localization
See spreadsheet for some possible examples. In general, the required topics
will be presented first, following by the required to report topics, and then
recommended topics.
7.0 Operator Qualifications
See spreadsheet for some possible examples. In general, the required topics
will be presented first, following by the required to report topics, and then
recommended topics.
8.0 Operations
See spreadsheet for some possible examples. In general, the required topics
will be presented first, following by the required to report topics, and then
recommended topics.
9.0 Performance Validation
The PAM operator shall produce a validation document after cruise
completion that serves as a bookend to the planning document. The
following topics shall be contained in this document:
(A) Location of acoustic data streams, associated auxiliary data
streams, and metadata, along with contact information for obtaining this
data.
(B) Plot of self-noise power spectral density of conditioning and
recording electronics, as well as calibration curve.
(C) Plots of cumulative distribution functions of background noise
levels throughout the entire cruise across entire operational bandwidth, in
terms of power spectral densities, as a function of frequency.
(D) For each species cluster, the validation report shall plot
spectrograms of background noise levels, using times representing 10th,
50th, and 90th percentiles of noise levels computed across each cluster
bandwidth.
(E) Spectrogram examples showing species cluster calls at minimum
required range or greater. If no calls from a particular species cluster were
detected during cruise, then simulated species signals (same as used in
planning document) shall be inserted into spectrograms generated from time
series noise data from 10th, 50th, and 90th percentiles. The purpose of this
requirement is to provide a "common sense check" that platform noise levels
are consistent with predicted detection range, and the simulated
spectrograms generated in the planning document.
(F) "Duty cycle," or fraction of time that PAM system was running and
monitored, shall be reported. Times where mitigation/monitoring not
required may be excluded. The duty cycle includes times when system is
running, and operator effort is on.
(G) Plot of CTD/XBT cast and resulting sound speed profile, for
situations where the cast was required.
(H) Metadata for cruise shall be reported using convention:
http://abirger.github.io/animal-telemetry/passive-acoustic/#toc_5
(I) Validation of automated detectors: how?
Noise profile data shall be made available to regulator upon request, in
order to improve databases of expected platform noise levels.