Modeling of seafloor wave propagation and acoustic scattering in 3-D heterogeneous media

Abstract

Results from a 3-D finite difference code for the elastic wave equation are used to compare scattering from volume and surface heterogeneities at the seafloor. The 2-D and 3-D models for flat ocean crust containing volume heterogeneities, single facets, and statistically rough seafloors are compared. For both hard bottom (shear velocity greater than water velocity) and soft bottom (shear velocity less than water velocity) environments, heterogeneities scatter energy both forward and backward into the water column via three mechanisms: primary scattering from the direct compressional wave reflecting off the seafloor heterogeneities, secondary scattering from transmitted compressional and shear waves interacting with volume heterogeneities and from interface waves scattering from near surface heterogeneities, and tertiary scattering from interface waves spawning additional interface waves. There is significant secondary and tertiary scattering from rough, hard bottoms representative of basaltic seafloors. The results indicate that two-dimensional modeling techniques may be appropriate for modeling sub-bottom volume heterogeneities with velocity contrast less than 10% but may underestimate scattering from rough seafloors. It is also found that subsurface conversion to horizontally polarized shear waves is significant for both volume and surface heterogeneous crust in the three-dimensional model.