The penetration of highly directional acoustic beams into sediments

A planar array of seven hydrophones buried in a sand sediment was used to study the propagation of 20 kHz sound waves through a water/sediment interface. Two different sources were used: one a parametric source with a beamwidth near 2°, and the other a linear source with a beamwidth near 10°. Measurements were made with each source at four grazing angles between 14° and 78°. It was found that at low grazing angles the behavior of the wave produced by the parametric source departed significantly from the prediction of plane wave theory for a lossy sediment. When the parametric source was used, the wave fronts penetrated more steeply into the sediment and the attenuation with depth was less than predicted by plane wave theory. It was observed experimentally that near the critical grazing angle the narrow acoustic beam is displaced along the interface before entering the sediment.