D. B. Creamer
B. J. Orchard
U.S. Naval Res. Lab., Code 7127, Washington, DC 20375
An analysis of acoustic wave propagation in a random shallow water waveguide is presented, in which deviations of the index of refraction are a stochastic process. The specific model studied is motivated by the oceanic waveguide in shallow waters, in which the subbottom sediment leads to energy loss from the acoustic field, and the stochastic process results from internal (i.e., density) waves. In terms of the normal modes of the waveguide, the randomness leads to mode coupling while the energy loss results from different attenuation rates for the various modes (i.e., mode stripping). Monte Carlo simulations of stochastic coupled-mode equations are presented [similar to L. B. Dozier and F. D. Tappert, J. Acoust. Soc. Am. 63, 353--364 (1978)], from which the average modal intensity (second moments of the field) and the scintillation index for this model are derived. It is shown that the usual concept of equilibrium or saturated statistics must be modified. In the context of an unrealistically simplified model with only two modes, an interpretion is provided whereby this result is due to the competition between the mode coupling terms, which redistribute the modal energies, and mode stripping, which results in an irreversible loss of energy.