Acoustic backscattering by the anisotropic and dipped structure of velocity fluctuations within the bottom sediments

Abstract

The three-dimensional power spectra of the velocity fluctuations within the seabed measured by high-frequency (1–50 kHz) crosswell acoustic tomography experiments indicate that the velocity fluctuation spectra are anisotropic in general and are often dipped; i.e., the major and minor axes of anisotropy are tilted from the vertical and the horizontal direction (Yamamoto, this meeting). The aspect ratio of the horizontal scale to the vertical scale ranges from 4 to 10 in the shallow-water sediments. The angle of tilt, called dip, is found as large as 30 deg. The intensity of the fluctuation spectrum depends on the sediment type. These parameters of the three-dimensional power spectrum affect the scattering of acoustic waves. An analytical solution to acoustic wave scattering by a dipped anisotropic 3-D velocity fluctuations in the sediments has been obtained. The strong dependence of acoustic backscattering on the grazing and azimuthal angle observed by Jackson and Briggs (1993) is excellently predicted when the realistic anisotropy and dip structures of the velocity fluctuations are incorporated in this analytical model of scattering by sediment volume fluctuation. [Work supported by ONR.]