Scattering of low-frequency sound by shallow underwater targets

Abstract

Rayleigh scattering of sound by a target can be described as a wave radiated by virtual point sources inside the target. When a target is located close to the ocean surface or another reflecting boundary, reflections of the incident and single-scattered waves from the boundary lead to multiple scattering from the target, with the target being insonified by nearby virtual sources. At low frequencies and for shallow targets, the distance from a virtual source to the target is not necessarily large compared to acoustic wavelength or the target’s dimensions. This paper takes advantage of the virtual source concept and recently derived explicit analytic representations of 2-D and 3-D acoustic Green’s functions in unbounded fluids with inclusions of a circular cross-section, to develop a simple and numerically efficient model of multiple scattering. Scattering from soft, hard, fluid, and solid objects is considered. The model is used to study the acoustic field in the vicinity of shallow targets, calculate normal-mode compositions of the multiple-scattered field, examine conditions of applicability of the single-scattering approximation, and investigate implications of multiple scattering for target detection and classification.