Acoustic scattering in the ocean from velocity and density fluctuations in the sediments

Abstract

According to spectral analyses of many crosswell acoustic tomograms and cores collected from ten seabed locations [Yamamoto, J. Acoust. Soc. Am. 98, 2235–2248 (1995)], the 3-D power spectra of velocity and density fluctuations in the seabed sediments are strongly anisotropic and dipping in general. Anisotropy, dip, and spectral properties of the fluctuations vary greatly depending on the sediment type and the geographical location. Based on the Born approximation and the Wood sediment model, an analytical solution has been obtained for the acoustic wave field scattered from the velocity and density fluctuations within sediment volume having arbitrary 3-D power spectra. Relative density fluctuations are proportional to relative velocity fluctuations in the sediment. The proportionality constant varies from over ten for soft sediments to one for dense sediments. Thus density fluctuation is the dominant mechanism for scattering from a sediment volume. The grazing angle dependence of acoustic backscattering is strongly affected by the fluctuation anisotropy. Dip in the 3-D power spectra of fluctuations causes azimuthal dependence of acoustic backscattering. The model agrees excellently with the backscattering data reported by Jackson and Briggs [J. Acoust. Soc. Am. 92, 962–977 (1992)] and Mourad and Jackson [J. Acoust. Soc. Am. 94, 344–358 (1993)]. Model-data comparisons confirmed the three theoretical findings: The dominant volume scattering of density fluctuations in soft sediments, the anisotropy-grazing angle relationship and the dip-azimuthal angle relationship of acoustic backscattering. In addition, the model predicts that the moderate frequency dependence of acoustic volume scattering is affected by a power low spectra of velocity and density fluctuations and attenuation within the sediments.