Acoustic backscattering from a rough surface at extremely low grazing angles

Abstract

This study is concerned with the wide discrepancy between published experimental data and existing theories for acoustic backscattering at low grazing angles. A new theoretical development based on the Helmholtz theorem is presented which is valid at low grazing angles and which includes the effect of shadowing. At low grazing angles a scattering strength nearly independent of grazing angles and an inverse relationship between scattering strength and wind speed is predicted. A low-grazing-angle experiment was performed at sea on a stable platform where conditions approximated those found in the open ocean. Observations were obtained on the acoustic backscattering of the sea surface as a function of grazing angle down to 1.5° and wind speed from 5 to 20 knots. The effect of shadowing was observed with acoustic radiation. The sea surface was measured with a wave staff to obtain the rms slope of the surface and the wave-height spectrum. Previous theories are shown to give incorrect results because of improper use of rough-surface boundary conditions. It is also shown that the effect of shadowing needs to be included, but a relatively simple correction is adequate.