Propagation of acoustic waves in marine sediments, a review

Abstract

The acoustic properties of natural unconsolidated marine sediments are essentially those of a suspension of solid particles in a fluid, with usually a small rigidity or shear modulus evident. Because of this rigidity the system may be modelled as a fluid saturated porous elastic solid. The theory of acoustic propagation in such a medium was first presented in the early work of M.A. Biot, which maintains a fundamental approach aimed at including all relevant physical mechanisms in a quantitative manner. Application of Biot theory to saturated porous solids yields 3 wave solutions ? 2 dilatational waves and a shear wave ? but in the case of unconsolidated marine sediments only one dilatational wave is observable. The properties of these wave solutions ? their velocities, attenuations and characteristic impedances ? are related to the many physical properties of the sediments invoked by Biot theory. The main problems of applying Biot theory lie in the large range of input parameters required to be specified. Some of these parameters are well defined but others are difficult if not impractical to specify. These limitations and the range of applicability of the theory to the study of marine sediments are further discussed. Each sediment can be shown to be characterised by two frequency regimes separated by a transition zone, the frequency of which depends primarily on the permeability and pore size of the sediment. In the "low" frequency regime there is complete coupling between the motion of the fluid and solid particles. The attenuation of a sound wave is then solely due to the visco elasticity of the sediment frame structure and the sound speed approaches that of a non interacting mixture of fluid and solid particles, the modulus of which is increased slightly by some structural stiffness. At "high" frequencies there is a partial decoupling of the motions of fluid and solid resulting in an increase in total wave attenuation due to viscous damping. There is also an increase in sound speed. Other properties of interest to underwater propagation studies, such as the reflectivities of interfaces, have been examined in terms of Biot theory. These analyses show some significant differences from the results of treating the sediment as a lossy fluid and have importance in bottom loss studies.