Abstract
This work reviews high-frequency, low-grazing angle surface and bottom reverberation effects relevant to fish detection in shallow waters. Simple acoustic reverberation models are presented, which include the effects of Bragg scattering from surface wind waves, sediment backscatter, and scattering and absorption within air bubble layers created by breaking waves and boat wakes. A simple example, with a 200 kHz narrow-beam sonar in water depth of 5 m, is used to demonstrate these reverberative interference effects. When present, air bubble layers cause serious interference with fish detection, such that even large fish (TS=−30 dB) could not be detected beyond 50 m range. Under calm conditions without bubbles, reverberation from gravel or rocky sediments would limit detection of small fish (TS=−50 dB) at ranges greater than 50 m. Air bubbles within boat wakes generate strong backscattering and cause shadowing (transmission loss exceeding 20 dB) of targets behind them.
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