Abstract:
Scattering from elastic ocean bottoms has been modeled using approximate analytical methods such as perturbation theory and the Kirchhoff and small slope approximations. An alternative approach is to use a numerical technique. This paper describes one such technique, the finite-difference time-domain (FDTD) method, which has been used successively to model scattering from pressure release surfaces [Hastings et al., IEEE Trans. Antennas Propagat. 43, 1183--1191 (1995)]. Monte Carlo scattering strengths are obtained for a tapered incident beam insonifying rough granite and basalt surfaces with either a Gaussian spectrum or a modified power law spectrum. These results are compared with those of formally averaged scattering strengths obtained for several different approximate models. One benefit of using the FDTD approach is that it allows the addition of heterogeneities at little extra computational cost. However, the initial computational cost is great, and the presence of surface and subsurface waves may result in strong scattering at grazing angles despite the use of the tapered incident field. Additionally, grazing incident angles may require surface lengths that are too computationally expensive. These issues and how they affect the applicability of the FDTD method to elastic problems are discussed. [Work supported by ONR.]