An assessment of near surface biological volume scattering off the continental shelf of Virginia

Acoustic models were used to estimate volume scattering from fish and marine mammals in the vicinity of the Surface WAves Dynamics Experiments (SWADE) at 37.5 N and 74 W as part of planning for the Office of Naval Research Acoustic Reverberation Special Research Project (ARSRP) sea surface scattering experiment. Animal densities were derived from fisheries assessments and airborne surveys of marine mammals, Animal target strengths were based on resonant models of fish with swimbladders, bent cylinder models of fish without swimbladders, and in situ measurements of mammals. Estimates of scattering were made for a broad frequency range of 10 Hz to 10 kHz with emphasis placed on the planned experimental frequencies of 100 Hz to 800 Hz. Average layer scattering strengths calculated for average densities of animals at two potential experimental sites ranged from -74 to -53 dB between 100 and 800 Hz. Since most fish and marine mammals will be aggregated to some degree, volume scattering can be expected to vary within the area insonified during ARSRP measurements. A model of fish school encounter suggests that only aggregations of porpoise, mesopelagic fish and nonswimbladder bearing fish are widespread enough to produce consistent reverberation, producing layer strengths slightly above -80 dB. Given the variations with frequency, location, time of day, and uncertainties of animal distribution, layer strengths at the experimental sites are expected to be between -80 and -50 dB. The