The scattering of low-grazing angle pulse beams at interfaces between fluid and porous media

Abstract

Many environments in bottom interacting ocean acoustics consist of mud and sands which can be modeled as fluid saturated, porous solids with low shear moduli. To study the physical mechanisms responsible for forward- and backscattering in these media it is useful to have a forward modeling technique which applies to rough and heterogeneous porous bottoms. A numerical scattering chamber using the time-domain finite-difference method applied to the range-dependent Biot equations can be used to study the scattering of low-grazing angle pulse beams from fluid-saturated porous media. For example, for a flat seafloor over a homogeneous porous half-space, converted shear waves and converted compressional ‘‘slow’’ waves are shown to be excited in the subbottom even when the grazing angle is below the critical angle for compressional ‘‘fast’’ waves. Any scattering element near the seafloor will act as a secondary point source by Huygen’s principle and when excited by an acoustic wave will have the potential to generate a family of interface waves and body waves of ‘‘fast’’ compressional, ‘‘slow’’ compressional and shear type.