A finite element model for acoustic scattering from objects near the ocean bottom

We present a finite element model (FEM) for the numerical simulation of time-harmonic waves scattering from objects located near the interface between two distinct fluid regions. One practical application is to model the interaction of sound waves with naval mines, whether close-tethered, proud, partially buried, or completely buried in ocean sediment. A better understanding of the physics of this fluid-structure interaction is needed to advance current mine detection and identification capabilities. The model assumes that the sediment is a dissipative fluid, and that the ocean and sediment are of infinite extent. For numerical tractability, both fluid domains are truncated and a perfectly matched layer (PML) is introduced near the truncation boundary. The accuracy of the FEM/PML model is demonstrated for the case of scattering in a homogeneous acoustic medium, where analytic solutions are available. For the case of scattering from a buried spherical shell, the model is compared lo results from a T-matrix model and to experimental measurements taken at the Naval Research Laboratory. The model is relatively easy to implement, readily handles undulating fluid-fluid interfaces and complex elastic structures, and offers a very high level of accuracy.