Modeling techniques for marine-mammal risk assessment

Propagation modeling in the ocean may be said to be a fairly mature subject, with a number of reliable and efficient acoustic models freely distributed. However, acoustic modeling to predict effects of sound on marine mammals presents some particular challenges. Standard sonar models predict the mean power levels for static receivers. However, marine-mammal researchers have shown a strong interest in being able to predict the actual time series that a moving mammal would experience as it swims through an ensonified ocean. The time series can then be used to directly model auditory models of the mammalian ear. To do this properly requires attention to subtle Doppler effects. The authors present a Gaussian-beam-tracing method that handles all these issues. Another key element needed for such models is the ability to rapidly predict three-dimensional (3-D) acoustic fields for lots of source/receiver combinations. This problem arises in trying to choose optimal locations for navy exercises, considering also a variety of hypothesized mammal-migration patterns. The authors discuss a precomputation approach to solve this problem. Finally, they examine a technique to reduce the computation needed for the one-third octave transmission loss (TL) averages. The one-third octave average is often used as a metric for the assessment of risk to mammals. The brute-force solution to this problem requires propagation modeling at many frequencies in the band. Here, the authors develop a general relationship to replace those frequency averages with much more easily computed range averages. The novelty of this approach relative to the previous range-averaging techniques is that it extends those methods to the range-dependent conditions.