Sensitivity of acoustic propagation modeling to variations in ocean environmental conditions

Abstract

A parabolic equation acoustic model has been used to estimate transmission loss for a mid-frequency sound source to assist in marine mammal behavioral response studies. The sound propagation is modeled for a range-dependent environment using the Navy Digital Bathymetric Data Base and Bottom Sediment Type database, NOAA Global Ocean Sediment Thickness Dataset, and the High-Resolution Global Sea Surface Wind Speed monthly climatology. The sound speed along the propagation path is modeled using two ocean temperature/salinity fields of different spatial and temporal resolution: monthly 0.25 deg outputs of the Generalized Digital Environmental Model (GDEM), and the daily 1/12 deg outputs of the regional Hybrid Coordinate Ocean Model (HYCOM). The modeling was done for two operational areas with strikingly different ocean environments, the Southern California Bight and the Gulf Stream region off Cape Hatteras. The latter represents a much more dynamic ocean environment with profound temperature fronts, highly variable meso- and submesoscale eddy activity, and sharp bathymetry gradients along the shelf break. Received sound levels measured on tagged marine mammals, and synchronized sound source/sound receiver tracklines are used to estimate uncertainty of the acoustic modeling results, and the non-linear sensitivity of the model to wide-range variations in the ocean environmental conditions.