Acoustic propagation in the upper sound channel

Abstract

This paper deals with acoustic propagation in sound channels occurring naturally in the ocean, allowing for the presence of random scattering irregularities. These irregularities are generally very weak and are often large compared to the acoustic wavelength. The parabolic equations for propagation of the moments of the wave field can be used in these circumstances and an approximate solution has been obtained for the second moment which is valid even in the case of multiple scatter. Although the physical meaning of the solution is clear, the expressions can be difficult to evaluate for most sound channels, and special care must be exercised in the neighborhood of caustics and foci. A detailed description is given of the methods used to evaluate the solution of the second moment equation with a particular discussion of the numerical techniques used. Results are presented for a sound-speed profile modeled by a cubic polynomial corresponding to channeling conditions observed in the North Atlantic. It is found that the presence of scattering irregularities has only a very small effect on the mean intensity in and near the channel and that the present methods are of only limited value at caustics. The spatial coherence of the acoustic field is, however, strongly affected by the irregular structure.