Three-Dimensional Shallow Water Acoustics

Abstract

LONG-TERM GOALSBoth physical oceanographic processes and marine geological features in the continental shelf can cause the medium properties to have lateral heterogeneity, so horizontal refraction of sound can occur and produce significant three-dimensional (3-D) sound propagation effects. The long-term goals of this project are targeted on understanding the 3-D acoustic effects caused by the environmental factors existing commonly in the continental shelf and shelfbreak areas. OBJECTIVESOne of the research objectives of this project is to develop efficient and accurate 3D models (both theoretical and numerical models) for studying underwater sound propagation in 3-D ocean environments. The ultimate scientific objective of the proposed work is to study the underlying physics of the 3-D sound propagation effects caused jointly by physical oceanographic processes and geological features. To achieve this goal, individual environmental factor will be first studied and then considered jointly with a unified ocean, seabed and acoustic model. Another major objective is to develop a tangent linear model to predict acoustic fluctuations due to 3-D sound speed perturbation in the water column. This model will be useful for the sensitivity analysis to assess the joint ocean and seabed effects. APPROACHThe technical approaches employed in the 3D sound propagation study include theoretical analysis, numerical computation and real data analysis. A 3-D normal mode method has been used to study canonical environmental models of shelfbreak front systems [1] and nonlinear internal wave ducts [2][3]. 3-D parabolic-equation (PE) wave propagation models with improved split-step marching algorithms [4][5][6] are used to study sound propagation in realistic environments. When the acoustic mode coupling can be neglected, a vertical-mode horizontal-PE model is used. These PE models employ the following higher order operator splitting to increase the PE approximation accuracy.(1)