Propagation and attenuation of acoustic waves in fluid-saturated sediments have been studied theoretically and experimentally. In situ acoustic transmission tests in saturated beach sand show that compressional waves are dispersive within a certain frequency band where the intrinsic attenuation is maximum. This indicates that low-frequency wave velocities in marine sediments are at least 5 % to 10% less than the velocities obtained from high-frequency measurements, and viscous damping, due to the relative motion between solid skeleton and fluid, is the main damping mechanism in the frequency range of 1-30 kHz. The agreement between the experimental results and Biot's theory enables the remote determination of porosity and permeability of marine sediments by using measured compressional and shear wave characteristics. Approximate relations are used to determine the porosity and permeability of the marine sediments using the measured acoustic wave velocities and attenuation.

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According to spectral analyses of many crosswell acoustic tomograms and cores collected from ten seabed locations ͓Yamamoto, J. Acoust. Soc. Am. 98, 2235-2248 ͑1995͔͒, 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 ͑1992͔͒ and Mourad and Jackson ͓J. Acoust. Soc. Am. 94, 344 -358 ͑1993͔͒. 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.