SEISMO ACOUSTIC PROPAGATION THROUGH ROUGH SEA-FLOORS

This paper addresses the scattering of seismo/acoustic energy from rough water-solid interfaces. A two-dimensional elastic finite difference method with a velocity-stress formulation is used for all of these models, including all phases, multiples, interface, and interference waves. Initial sinusoidal seafloor models at 15 grid points per wavelength (ppw) show that even medium slopes can greatly increase the amount of energy (especially P to S converted energy) which is transmitted into the bottom. Scattering from microroughness (stair-step definition of a sloping seafloor) is significant for 10-m steps with a pressure pulse source of 10 Hz center frequency. Additional models with much finer grids (30 and 60 ppw) were computed to decrease the microroughness scattering and to check the accuracy of scattering from 10 m steps. Most of the microroughness scattering disappears for the 60 ppw model. Scattering strength is similar for all models with 10 m microroughness. Reciprocal models at 15 and 30 ppw (both with 10-m stair steps) show very good agreement. An important conclusion of this work is that the specification of complex seafloors requires a much finer grid than is necessary for numerical stability and accuracy. This will be true for other numerical methods as well.