Wedge Diffraction Based Scattering Model for Acoustic Scattering from Rough Littoral Seafloors

摘要

Models for acoustic scattering from rough surfaces based on Biot and Tolstoy's (BT) exact wedge diffraction theory have proven accurate and useful in a number of experimental and numerical studies (1). Because the BT solution is restricted to impenetrable wedges (acoustically hard or soft boundary conditions), scattering models based on the BT solution have thus far been limited to the rough air/sea interface where the actual boundary conditions are very nearly pressure-release (soft). Recently, important theoretical work (2,3) has extended the exact BT theory to density-contrast but isospeed wedges. This new development makes possible the application of wedge diffraction based scattering models to the roughness at the sea floor where the change in the acoustic impedance at the boundary is dominated by changes in density and only weakly affected by changes in sound speed. However, it is important to confirm that small amounts of sound speed contrast do not perturb the diffraction too much. To contribute to the understanding of how the diffracted wave is affected by sound speed contrast and get some idea as to the practical limitations of wedge-diffraction based scattering models for littoral seafloors, a simple numerical experiment involving a highly accurate Finite-Difference Time-Domain (FDTD) solution to the acoustic wave equation and a wedge-shaped boundary has been explored. This paper presents the results of FDTD experiments designed to quantify any changes in the diffracted field brought about by sound speed contrast. An ad hoc treatment of sound speed contrast is developed based on the requirement that the diffracted wave must smooth out the reflection discontinuity and preserve the continuity of the total field.