Multipath propagation in a deep ocean including modified rays near grazing incidence at the bottom

Abstract

A ray description of acoustic propagation between communicators submerged in a deep ocean may include rays that almost graze the interface between water and sea‐floor sediment. For example, this can occur in range‐independent environments where the squared refractive indices in water and in sediment decrease linearly with depth. Four pairs of such rays connect communicators near the surface and separated in range by at most 4(H/γ)1/2, where H is the ocean depth and γ is the gradient of square refractive index in water. One ray in each pair is entirely waterborne; the other intercepts the sediment and grazes a caustic. We present a uniform asymptotic analysis of these ray pairs that incorporates diffractive finite frequency effects as in the modified ray theory of Murphy and Davis. These effects include the appearance of a frequency‐dependent caustic and a modification of the usual π/2 phase shift experienced by the classical ray that intercepts the sediment. Comparison of these results with normal mode calculations is excellent. Vector diagrams (after Bartberger) also identify mode groups corresponding to the modified rays.