Mid-frequency sound propagation and attenuation through a biological deep scattering layer

An acoustic autonomous underwater vehicle (AUV) deployed during the New England Shelf Break Acoustics experiment in May 2021 investigated the effect of environmental variability on acoustic propagation and scattering. Shipboard 38 kHz echosounder data revealed the presence of a biological deep scattering layer (DSL) approximately 100 m below the AUV mission depth of 300 m. A prolate spheroid gas-filled swimbladder scattering model was used to estimate the DSL acoustic attenuation resulting from resonant scattering by fish. These range-dependent attenuation coefficients were applied to a comparison of 2.5-4.5 kHz AUV source signals received on mooring hydrophones following propagation paths through and above the DSL. A parabolic equation model was used to simulate transmission loss through the scattering layer along the selected propagation paths, as well as account for physical oceanographic variability. The DSL in this study was estimated to account for roughly 1-1.5 dB/km of attenuation through the layer, which improves the final propagation path comparison results and emphasizes the importance of accounting for both biological scattering and physical oceanographic variability in attenuation studies.