Comparison between subcritical penetration models and in situ data

This paper presents the recent results of a study whose overall objectives are to determine the mechanism(s) contributing to anomalous high-frequency sound penetration into sediments, and to quantify the results for use in sonar performance prediction for the detection of buried objects. To date, several mechanisms have been hypothesized in order to explain this phenomenon, the most frequently mentioned being: (a) the existence of a Biot slow wave in the sediment, (b) surface roughness, and (c) scattering of the evanescent wave by volume inhomogeneities within the sediment. In situ acoustic measurements were performed on a sandy bottom whose geoacoustic properties were carefully identified. A parametric array mounted on a tower moving on a rail was used to insonify hydrophones located above and below the sediment interface. An extensive data set covering a wide range of grazing angles (both above and below the estimated critical angle) and frequencies (2–50 kHz) was acquired and processed. The results are compared to models for the three mechanisms stated in an attempt to explain the anomalously high sound-pressure levels measured. Dependence on frequency, depth into the sediment, and grazing angle is explored.