Examination of three-dimensional effects using a propagation model with azimuth-coupling capability (FOR3D)

A three-dimensional wave propagation model of parabolic approximation type (FOR3D) is used to examine 3-D ocean environmental variations. The background theory and characteristics of the model are reviewed briefly. Propagation situations that are classified as 3-D, N×2-D, and 2-D are described in connection with FOR3D and are interpreted in several ways. An analytic exact solution is used to demonstrate the model’s accuracy and its capability for treating fully 3-D propagation, when coupling exists between solutions in adjacent vertical planes of constant azimuth. It is also employed to illustrate a procedure for using approximate conditions at vertical side boundaries in a 3-D calculation. An application is made to an Atlantic Ocean shelf-slope environment with realistic bottom topographic variations and sound-speed profiles. The occurrence of significant azimuthal coupling is demonstrated in propagation loss versus range curves. It follows that, while the N×2-D approximation of no azimuthal coupling is useful in many situations, not all 3-D ocean acoustics problems can be adequately solved without a fully 3-D propagation model.