Ray-based description of mode coupling by sound speed fluctuations in the ocean.

A traditional approach to the analysis of mode coupling in a fluctuating underwater waveguide is based on solving the system of coupled equations for the second statistical moments of mode amplitudes derived in the Markov approximation [D. B. Creamer, J. Acoust. Soc. Am. 99, 2825-2838 (1996)]. In the present work, an alternative approach is considered. It is based on an analytic solution of the mode coupling equation derived in the high frequency approximation [A. L. Virovlyanskii and A. G. Kosterin, Sov. Phys. Acoust. 35, 138-142 (1987)]. This solution, representing the mode amplitude as a sum of contributions from two geometrical rays, is convenient for statistical averaging. It allows one to easily derive analytical expressions for any statistical moments of mode amplitudes. The applicability of this approach is demonstrated by comparing its predictions for a deep water acoustic waveguide with results of a full wave numerical simulation carried out using the method of wide angle parabolic equation.