Acoustic Wave Backscattering in a Random Inhomogeneous Ocean Sediment

The backscattering of sound by inhomogeneities of the ocean sediment may provide a remarkable effect on underwater acoustic wave propagation. It may also be used as a means of remotely estimating complicated sediment properties. In this paper, a theoretical model of acoustic waves backscattered from an inhomogeneous sediment is formulated based on the Born approximation. The model not only contains the formal homogeneous bottom case but is also extended to the more realistic stratified bottom case. A complex wavenumber, in which an attenuation coefficient is introduced, reveals significant changes of the penetration depth within the sediment. The model predicts that for the stratified bottom, the backscattering strength is rapidly oscillating and decreases sharply at small grazing angles owing to the refraction of the waves caused by the sound velocity gradient. In order to reduce the number of independent variables, Biot's theory is applied to relate three-dimensional density fluctuations to sound speed fluctuations through porosity. A transverse-isotropic model is also developed to access the three-dimensional sound speed fluctuation spectrum. Geoacoustic surface and cross-hole tomographic data acquired from different sites characterizing sandy and silty bottoms are used to obtain three-dimensional sediment volume inhomogeneities. Backscattering strengths are evaluated for those bottom cases. The results agree with intuition and other published data. INTRODUCTION The investigation of acoustic backscattering from the ocean bottom began during World War II and has been carried out for a wide band of frequencies in many ocean regions of 9-1