American NationalStandard on Towed PassiveAcousticMonitoring 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 coverssevenaspects of a complete PAM operation: Initial planning, hardware, software, acousticlocalization, 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.
Aprimary guiding principle of this standard is not tospecify how PAM operations are conducted, but to specify how an operator reportsthese 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 arerequired, those that must be documented andreported, and those that are recommended. Requiredactionsare 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. Recommendedactions 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 becausethe 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, abriefing 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 shalldescribe the time period and geographical region to be covered by the PAM effort.
3.1.2:The planning document shall list marine mammal speciesexpected 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 documentshall explicitly state which species shall not be covered by operations.
3.1.3: The planning documentshall 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 activitiesexpected 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 clusterdefined 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, combinedwith 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 documentshall 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 notrecommended 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 documentshallcitedatabases or other resources where theseinputshave 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 levelson 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 3.1.5at 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.
3.4Timing 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 clusters[1]. 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 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 seconds[2].