Three-dimensional modeling of low-angle seismoacoustic backscatter

Abstract

Predictive modeling codes for seafloor backscatter (e.g., finite difference methods) frequently take advantage of coordinate system symmetries in order to reduce the amount of numerical computation required to obtain a solution. Line sources in models with two-dimensional Cartesian coordinates (x,z) are often computationally fastest but assume symmetry into and out of the sagittal plane. Two-dimensional solutions in cylindrical coordinates (r,z) give the correct spreading and waveforms for point sources but range-dependent features in the sagittal plane represent ‘‘rings’’ about the vertical axis through the source and are somewhat unrealistic. In both of these 2-D cases, propagation and scattering are only considered for P-SV waves. In order to study the limitations of these 2-D approaches and to see if they are excluding any significant scattering mechanisms it is necessary to run some fully 3-D models. In addition to out-of-plane scattering of compressional body waves, vertically polarized shear body waves, and Stoneley (Scholte) waves, 3-D models include the new effects of SH body waves and Love (interface) waves. In this paper backscatter coefficients for single facets on a homogeneous seafloor, for a canonically rough (Goff–Jordan) seafloor, and for volume heterogeneities beneath a flat seafloor are compared. For soft bottoms such as sediment the backscatter coefficients of these models are within 3 dB. [Work supported by Office of Naval Research.]