BOUNDARY SCATTERING LIMITATIONS TO FISH DETECTION IN SHALLOW WATERS

This work reviews high-frequency, low-grazing angle surface and bottom reverberation effects relevant to fish detection in shallow waters. S imple acoustic reverberation models are presented, which include the e ffects of Bragg scattering from surface wind waves, sediment backscatt er, 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 reverb erative interference effects. When present, air bubble layers cause se rious interference with fish detection, such that even large fish (TS = -30 dB) could not be detected beyond 50 m range. Under calm conditio ns 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 c ause shadowing (transmission loss exceeding 20 dB) of targets behind t hem. (C) 1998 Elsevier Science B.V.