Sediment acoustic models and Biot's theory

Abstract

The development of geo-physical descriptions of the sediment and models to predict the acoustic properties are critical to the application of sediment acoustics. The former follow standard geo-physical methods of sediment classification, based on grain size, density, and other physical descriptors. The latter started as a fluid approximation, followed by a visco-elastic approximation with five frequency-independent parameters, consistent with sediment acoustic data up to the 1980s. Recent experimental data have revealed deficiencies in this approach, particularly in the case of sandy sediments, which cover a large fraction of the continental shelves. The measurements are more consistent with a poro-elastic model, consisting of Biot's theory with extensions to account for the particular physics of granular media. There are currently two approaches to the remedy: (a) a visco-elastic model with frequency dependent parameters that mimic the experimental data and (b) a poro-elastic model with the necessary attributes. It is shown that (a) would be a significant improvement over existing models, but (b) is the preferred solution. A recent discovery concerning the viscosity of nano-meter water films has resolved a problem with the dimensions of the grain contact gap. Future plans will center on further rationalization and reduction of input parameters to develop a practical poro-elastic model. [Work supported by the Office of Naval Research, Ocean Acoustics Program.]