Finite-difference methods for low-angle seafloor scatter

Abstract

Finite-difference methods applied to the two-way elastic wave equation are the most promising approach to understanding scattering from both surface irregularities and volume heterogeneities. A finite-difference code appropriate for low-angle scatter (less than 20° grazing angle) has been developed. The incident wave field is a Gaussian pulse beam. It has a Gaussian amplitude profile normal to the propagation direction with a time dependence (in pressure) that is the third derivative of a Gaussian. The peak frequency of the pulse is 50 Hz. The pulse beam can be “shone” on a 2-D representation of the seafloor including volume heterogeneities and rough water-sediment and sediment-basement interfaces. Time series of the combined incident and scattered fields are obtained on vertical and horizontal profiles around the scattering region. The incident field can be computed separately and removed from the observed field to give just the scattered field. The rms energy in the scattered field is plotted as a function of angle and includes forward and backscattered energy and energy scattered into the bottom as well as into the water. Time spread of the scattered energy can also be investigated. These concepts will be demonstrated using three models: a flat, laterally homogeneous reference structure, a similar model with surface irregularities, and a similar model with volume heterogeneities.