Numerical modeling of reverberation from facets in an elastic seabed

Abstract

The field of ocean bottom reverberation has been lacking efficient numerical models for simulation of elastic scattering from subbottom facets. The only available models have been of finite-difference and finite-element types, both of which are extremely computationally intensive due to the spatial discretization requirements, restricting the applicability to very-short-range reverberation problems. An alternative hybrid boundary element-wave number integration approach has recently been developed [P. Gerstoft and H. Schmidt, J. Acoust. Soc. Am. Suppl. 1 87, S83 (1990)] limiting the discretization requirements to the boundary of the facets and thus allowing for both short- and long-range reverberation simulations in a stratified seismoacoustic ocean environment. For a facet inhomogeneity in an otherwise horizontally stratified environment, the total field is expressed in terms of Green's theorem. By choosing a Green's function satisfying the boundary conditions at all horizontal interfaces, only the interface between the irregularity and the layered medium contributes to the surface integral in Green's theorem. A modified version of SAFARI [H. Schmidt and F. B. Jensen, J. Acoust. Soc. Am. 77, 813–825 (1985)] is applied to compute all Green's functions involved in the boundary integral in both the exterior and interior domains. The boundary element equations, expressing the discretized boundary conditions between the two domains, are then solved, and the total reverberant field is again computed by means of SAFARI. Simulations of both forward and backward reverberation from facets such as salt diapirs and Arctic ice keels will be presented, in part in the form of a computer graphics animation of the time development of the scattered seismoacoustic field. [Work produced at the Naval Res. Lab., Code 5160 Super mini-computer Center and supported by ONR.]