ASA 126th Meeting Denver 1993 October 4-8

2aUW7. Quantitative backscattering coefficients from the numerical scattering chamber.

R. A. Stephen S. A. Swift

Dept. of Geol. and Geophys., Woods Hole Oceanogr. Inst., Woods Hole, MA 02543

Models of geoacoustic interaction with the seafloor are essential to link the backscattered acoustic field with geological and geophysical descriptions of the seafloor. Beamforming has been implemented into numerical scattering chamber formulation so that backscatter coefficients and functions can be obtained. This process is discussed for a flat, homogeneous seafloor; a single facet on a flat, homogeneous seafloor; and a canonically rough, homogeneous seafloor. This study suggests that representing the backscattered field by a single, angle-dependent coefficient is an oversimplification. Coherence of the scattered field across the beam before stacking and the time spread of the stacked field are significant issues. The numerical stacking chamber computes solutions to the elastic (or anelastic) wave equation by the finite difference method in a two-dimensional Cartesian geometry. The finite difference approach has the following advantages: (a) it includes all shear wave (rigidity) effects in the bottom; (b) it can be applied to pulse beams at low grazing angles; (c) both forward-scatter and backscatter are included; (d) multiple interactions between scatterers are included; (e) arbitrary, range-dependent topography and volume heterogeneity can be treated simultaneously; (f) problems are scaled to wavelengths and periods; and (g) the method considers scattering from structures with length scales on the order of acoustic wavelengths.