Scattering from a partially buried target: A modified acoustic ray model

Abstract

An acoustic ray model, recently presented by Kargl et al. (IEEE J. Ocean. Eng., DOI: 10.1109/JOE.2014.2356934), describes the scattering of sound from a target within a homogeneous waveguide. For a proud target, four ray paths account for the interaction of incident sound with the target and its local environment. In marine environments, a target at the water-sediment boundary can be partially or completely buried. Ray paths that may reach a target then depend on the burial state. Modifications to the ray model are presented. The simplest approach introduces a Heaviside step function into the model, which abruptly turns a ray contribution off as a ray's point of impact on a target transitions from above the boundary to below. In geometric acoustics, the Heaviside step function is a zeroth order approximation to the Fresnel integral for the diffraction of sound at a shadow boundary into a shadow region. The Fresnel integral provides a smooth transition in the intensity across the boundary. Similarly, a smooth transition should occur within a modified ray model for partially buried targets. The modified ray model will be compared to finite-element model results for a solid aluminum cylinder and an aluminum replica of a underwater unexploded ordnance. [Research supported by SERDP and ONR.]