Acoustic Modeling Using a Three-Dimensional Coupled-Mode Model

Abstract

The overall goal of this research is the development of an accurate and reliable propagation model applicable to environments which exhibit strong range dependence in all three spatial dimensions. OBJECTIVES The objective of this work is to gain an understanding the physics of propagation in continental shelf areas, specifically horizontal refraction and mode coupling induced by three-dimensional (3D) inhomogeneities in the waveguide. A coupled-mode approach has been applied for this purpose. The coupled-mode approach is attractive for solving problems involving 3D propagation for several reasons. First, this technique provides intuitive results for understanding the features responsible for observed propagation effects in range-dependent environments. For example, upslope propagation is characterized by acoustic energy radiated into the bottom at discrete depths associated with mode cutoff. The modal decomposition of the acoustic field has also been used to describe horizontal refraction in a wedge-shaped ocean, for which the single-mode interference pattern associated with rays launched up and across the shelf has been well documented [Weinberg and Burridge (1974)]. Furthermore, coupled-mode solutions can be highly accurate and have been used for benchmarking solutions to range-dependent problems [Jensen and Ferla (1990)]. In order to appreciate the limitations of existing 3D models, it is necessary to have methods which can provide reference solutions for comparison. APPROACH A 3D acoustic propagation model based on the stepwise coupled-mode approach [Evans (1983)] implemented as a single-scatter solution has been developed. This technique is based on a hybrid modeling approach for which normal modes supply the vertical dependence and a parabolic equation (PE) solution provides the horizontal dependence. The inhomogeneous Helmholtz equation for pressure P(r, θ , z) at range r, azimuth θ , and depth z from a point continuous wave source of amplitude S(ω), 1 DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.