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Measurements of the bottom-reflection losses for a low-frequency sonar signal (0.7–3.0 kc/sec) are reported for two areas off the California coast and for a third area in the Bering Sea. At all three areas, the bottom losses were large at small grazing angles. Comparisons are made between the measured values of bottom loss and those calculated for a plane sound wave reflecting from a layered model of the ocean sediments. The sediment model consists of a number of absorbing solid layers. Reasonable agreement is shown between the experimental and calculated values of bottom loss.

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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.

Scattering from the sea surface was studied using a displacement hydrophone in order to observe the effect of surface-wave shadowing. The displacement hydrophone, able to receive the acoustic field at or near the sea surface, is particularly suitable for observing the effect of shadowing. A stable platform at sea was provided by the use of the NUWC Oceanographic Tower off San Diego. Comparison of preliminary results with Wagner's geometrical shadowing function indicates that the observed shadowing is less than would be expected theoretically. For example, with a wind speed of 20 kt the surface appears acoustically to be only 20% shadowed at 4° grazing angle, whereas geometrically 40% would be in shadow. This suggests that corrections to the scattering function using the geometrical approximation to shadowing may not be applicable. [The author received support under the Advanced Study Program from Naval Undersea Warfare Centers, San Diego, California.]