Internal gravity waves and horizontal refraction of sound in the ocean

Abstract

Horizontal refraction, i.e., the deviation of acoustic propagation paths from the source–receiver vertical plane, is usually weak in the ocean. In this paper, a perturbation approach [O. A. Godin, J. Comput. Acoust. 10, 123–151 (2002)] to describe horizontal refraction within the ray and the ‘‘vertical modes—horizontal rays’’ theories is extended to media with a combination of stochastic and deterministic inhomogeneities and subsequently utilized to quantify 3-D acoustic effects due to internal gravity waves. Within the perturbation approach, statistical moments of adiabatic mode amplitude and phase and ray travel time are given explicitly by integrals of respective moments of cross-range derivatives of environmental parameters along the trajectory of the sound wave in an unperturbed (range-dependent) medium. Specific scenarios considered simulate conditions of recent propagation experiments. In shallow water, internal wave soliton-induced horizontal refraction can result in travel time biases O(10 ms) at propagation ranges as small as 10 km. In deep water, at ranges about 1000 km horizontal refraction due to internal waves with the Garrett–Munk spectrum leads to acoustic travel times being O(10 ms) less than in 2-D simulations. Without the internal waves, the travel time bias is found to be smaller by an order of magnitude. [Work supported by ONR.]