Before a ship even leaves a dock, before any items are shipped, and before any operator is hired , any organization hired to conduct PAM needs to determine whether PAM is even appropriate for the task at hand, and if so, what the fundamental performance requirements of the system must be. Often, the biggest mistakes in using PAM start at the planning stage. By explicitly defining procedures for initial planning, the standard recognizes that no two PAM surveys can or need be exactly alike, and thus avoids the single biggest concern about standardization: that it forces a “one size fits all” mentality.
The working group will define a checklist of minimum information that needs to be researched by any organization attempting a PAM operation. Whenever feasible the standard would identify databases or other resources that will allow this information to be obtained. Examples of checklist items that the working group may consider include the following:
(1) Determining what time period and what region needs to be covered by the towed PAM system.
(2) Identifying what marine mammal species would be reasonably be expected to be present during this time in this region. Flagging databases and/or literature reviews that condense this information.
(3) Obtaining best-informed estimates of source levels and frequency bandwidth of expected call types from the species identified in (2). For many years, the book “Marine Mammals and Noise,” by Richardson et al, provided a useful reference, but more recent sources may be available.
(4) Extracting information about the acoustic propagation environment in the expected operating area. This information may include bottom bathymetry, bottom composition, and sound speed profiles. The standard will list resources where this information can be obtained, or describe how to make best-practice estimates of parameters when no such information is available.
(5) Establishing estimates of the expected “self” noise spectra of the towing vessel and other expected contributing sources (e.g., other vessels or sources which may be acting in concert with the PAM vessel), which may vary with time or operational state of the tow or support vessels.
This step needs to recognize a variety of potential situations facing a PAM organization. For example, the organization may be required to deploy off a certain platform, but in other cases the organization can select its own platform. An unresolved question is what to do if the characteristics of the towing and/or other vessels are unknown during the planning stage. Would it be sufficient to use reference measurements for certain size and classes of vessels, if no direct measurements of the specific vessel in question are available? Can a PAM organization comply with an ANSI standard if the towing vessel remains unknown until shortly before the deployment?
(6)Defining and setting signal-to-noise ratios (SNR) or other detection threshold criteria for various categories of sounds. For example, one set of SNR values might be provided for situations where a human would be reviewing a scrolling spectrogram, while another SNR value might be provided for the case where an automated detector is monitoring the system. The definition of SNR for impulsive and repetitive transient sounds may also need clarification, perhaps by exploiting related ANSI standards on noise measurement.
(7) In addition to these six items, the standard will assume that a minimum required detection range for PAM has been independently specified. There are three reasons why the current standard would not tackle the topic of how to determine a minimum detection range.:
- First, setting a minimum detection range would require knowledge about what sound levels at which marine mammals would be expected to be affected (either physiologically or behaviorally) by an activity, which in turn would require knowledge or estimates about the hearing sensitivities of marine mammals, a subject where a community census may not yet be possible.
- Second, the steps required in determining a minimum detection range (i.e. knowledge of physiological and behavioral thresholds, environmental propagation conditions, and anthropogenic noise sources) are so involved that they deserve an entire standard to themselves.
- Finally, for better or worse, federal guidelines already exist for minimum detection ranges for a variety of environments, and to encourage rapid adoption of the standard by industry and regulators, it is best to use those ranges.
Once this research phase has been completed, the standard will then describe procedures and models (e.g. the sonar equation, standard acoustic propagation models) for determining whether the required detection range can be reasonably obtained for the species required and under the conditions expected. If a towed PAM system cannot obtain the required detection range, given the knowledge available, then the standard will recommend that PAM should not be used in that situation. The working group might also include criteria for determining to what degree localization capability is required of the system. It is anticipated that this subject may take a substantial amount of the working group’s time to achieve consensus, as there are several tradeoffs involved. For example, the ability to localize sounds to some degree would provide independent confirmation that the system is attaining its design goals; however, any localization capability will undoubtedly increase the complexity and cost of the subsequent system.
The first result of complying with the Initial Planning section of the standard would be a go/no-go decision on using PAM. If a “go” decision is obtained, then the reward resulting from all the time spent by the standard on the planning phase will be greater flexibility and simplicity in defining requirements for hardware, software, and operations.