ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

1pAO1. Three-dimensional velocity fluctuation structure of the seabed imaged by high-frequency crosswell tomography.

Tokuo Yamamoto

Appl. Marine Phys. Div., RSMAS, Univ. of Miami, Miami, FL 33149

High-resolution images of the compressional wave velocity fluctuation structure of the seabed are inverted from the travel times measured by high-frequency (1--50 kHz) crosswell acoustic tomography experiments at three different sites in shallow water. The three-dimensional power spectra of the velocity fluctuations are determined from the velocity images. The velocity fluctuation spectra are anisotropic in general, i.e., the fluctuation frequency in the vertical direction is much higher than in the horizontal direction. The aspect ratio of the two ranges from 4 to 10. In addition, the major and minor axes of anisotropy are often tilted from the vertical and the horizontal direction. The angle of tilt, called dip, is found as large as 30(degrees). 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. The strong dependence of acoustic backscattering on the grazing and azimuthal angle observed by Jackson and Briggs (1993) is excellently predicted when the anisotropy and the dip structure of the velocity fluctuations are incorporated in an analytical model of scattering by sediment volume fluctuation (Yamamoto, this meeting). [Work supported by ONR.]