Observations of anomalous acoustic penetration into sediment at shallow grazing angles.

Abstract

Buried mines pose a difficult problem for mine countermeasures operations. Mines may naturally bury in coastal waters where wave-induced effects are significant near the bottom. Current techniques for acoustic detection and classification of a buried mine require a high-resolution imaging sonar with bottom penetrating capability. Recently, anomalous acoustic penetration into sediment at shallow grazing angles was observed during a series of experiments which acoustically characterized the shallow water environment. These experiments employed a stationary sonar tower complete with pan and tilt motors. In the initial experiments, backscattered returns from objects of opportunity were recorded; these objects were either partially or completely buried near the water–sediment interface. In a later experiment, buried calibrated retroreflectors and a buried hydrophone were also utilized; the hydrophone was used to measure in-sediment sound-pressure levels. Results of the backscattered returns from the objects of opportunity and calibrated retroreflectors will be presented. Also, the measured in-sediment sound-pressure levels will be compared to the SAFARI (seismic and acoustic fast field algorithm for range-independent environments) code and will be further compared to results reported in the literature. [Work supported by the Office of Naval Research.]