Techniques for Investigating Scattering from the Subbottom in Shallow Water

This report reviews work carried out to identify promising techniques for determining the major acoustic mechanisms governing scattering from the subbottom in shallow water, Two approaches were investigated: (1) the application of T-matrix scattering theory to scattering from geological features in the subbottom, and (2) the coupled differential equation theory of gradient driven coupling of shear and compressional waves. T-matrix theory is well developed and has most of the characteristics needed for application to modeling scattering from the subbottom. T-matrix theory is a full wave approach, has few fundamental restrictions, includes both compressional and shear waves, applies to multiple structured scatterers of quite general shape, and can be formulated for a waveguide geometry. The basic theory of gradient driven coupling has been developed, but still has some features that are not well understood. The theory predicts "resonance frequencies" at which coupling between shear and compressional waves may be greatly enhanced. The resonance frequencies can be fairly high (>1 kHz) for gradients estimated for volume inhomogeneities in shallow water sediments. The effects of the resonance have not been studied nor has a stochastic theory of gradient driven coupling been developed. This report concludes with some recommendations for future research to extend these approaches and to apply them to understanding scattering from the subbottom.