Range-dependent normal modes in underwater sound propagation

Abstract

Normal mode theory is best suited for the case of stratified media. Range dependence of the medium properties and of its boundary may nevertheless be taken into account in the framework of an adiabatic approximation provided the changes with range are sufficiently gradual. We have extended this approach and have included possible range variations of the boundaries. To lowest order, the solution furnished by this method consists of the depth functions of a locally stratified medium; in higher order, the range functions satisfy a system of coupled equations, with the coupling terms causing an exchange of energy between modes. As an application, we have evaluated acoustic fields in an isovelocity wedge-shaped ocean (continental shelf) using the normal-mode method with adiabatic range variation, obtaining good agreement with the exact solution due ~ to Bradley- and Hudimac.