Ray Theory versus the Parabolic Equation in a Long Range Ducted Environment

An important type of underwater acoustic environment involves a deep water SOFAR channel with a superimposed (winter) surface duct capable of trapping acoustic energy. In this report we examine the effects predicted by a computer model of three ducted sound speed profiles and two analytic profiles on acoustic propagation at 300 Hz. The purpose of this study was to better understand the capabilities and limitations (both theoretical and numerical) of a ray theoretic model for predicting mid and long range signal transmission loss (TL) in the winter North Atlantic. Several source and receiver configurations (combinations with one or both in and out of the duct) have been studied. To test validity, the ray theoretic calculations are compared to previously accepted results predicted by a propagaton model based upon the parabolic equation. Both model predictions agree qualitatively (TL measurements and acoustic field patterns are comparable), while any quantitative discrepancies are explained by the theoretical or numerical limitations of the different approaches. We conclude that when both the source and receiver are at least several wavelengths from the sea surface, ray theory can be quite accurate in its predictions for these ducted environments.