A numerical scattering chamber for studying low-angle seafloor backscatter.

Abstract

A numerical scattering chamber, based on the finite-difference solution to the two-way elastic (or anelastic) wave equation in the time domain, is a powerful and convenient approach to studying the physics of strong seafloor scattering. Scattering from both surface roughness and volume heterogeneities at scale lengths comparable to wavelengths can be treated. The method includes all shear wave and interface wave effects and all multiple interactions between scatterers. Bottom parameters varying from soft sediments (with shear wave velocities much less than water velocity) to hard basalts (with shear velocities higher than water velocity) are studied. For low-angle (less than 20-deg grazing angle) backscatter problems a Gaussian pulse beam is used as the incident field and the resultant scattered field is computed on arrays of receivers surrounding the scattering region. However, initially narrow beams (about six wavelengths across) diffuse significantly as they propagate across the chamber (which for low-angle problems can be as long as 60 wavelengths). This is a consequence of the physics of the problem and is not a numerical issue. Broader beams diffuse less but have a larger ‘‘footprint’’ on the seafloor. There can be many tens of discrete, deterministic scatterers within the footprint of a single beam. Broader beams also require larger computational domains forcing a trade-off between computational effort and beamwidth. Further thought must also be given to expressing the actual incident field in the ocean in terms of Gaussian beams. [Work supported by Office of Naval Research.]