Abstract

Long-range, basin-scale acoustic propagation has long been considered deep water and well represented by the two-dimensional numerical solution (range/depth) of wave equation. Ocean acoustic tomography has even recently been demonstrated to be insensitive to the three-dimensional affects of refraction and diffraction (Dushaw, JASA 2014). For frequencies below 50 Hz, where volume attenuation is negligible, the approximation that all propagation of significance is in the plane begins to break down. When examining very long-range propagation in situations where the source/receiver are not specifically selected for open water paths, 3D effects can dominate. Seamounts and bathymetry rises cause both refraction away from the shallowing seafloor and diffraction behind sharp edges. In this paper a set of recent observations, many from the International Monitoring System (IMS) of the United Nations Comprehensive Test Ban Treaty Organization (CTBTO) will be presented, demonstrating observations that are not well explained by Nx2D acoustic propagation. The Peregrine PE model, a recent recoding of RAM in C, has been extended to include 3D split-step Pade propagation and will be used to demonstrate how 3D acoustic propagation affects help explains some of the observations.

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