Mechanisms for subcritical penetration into a sandy bottom: experimental and modelling results

Abstract

This paper presents the results of a recent study whose overall objectives are to determine the mechanism(s) contributing to subcritical acoustic penetration into ocean sediments, and to quantify the results for use in sonar performance prediction for the detection of buried objects. In situ acoustic measurements were performed on a sandy bottom whose geoacoustic properties were carefully identied. 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 dataset covering grazing angles both above and below the estimated critical angle and frequencies between 2-16 kHz was acquired and processed. The results are compared to models. For the specific bottom type examined, it is shown that for frequencies below 5-7 kHz the sound field into the sediment due to subcritical insonication is dominated by the evanescent field, while scattering due to the surface roughness is the dominant mechanism for higher frequencies. However, it must be mentioned that the contribution of the roughness scattering was always found to be lower than the one obtained at lower frequency with the evanescent wave.