Coherent leakage of sound from ocean surface ducts of nonlinear sound speed profile

Abstract

The sound speed variation with depth that defines a surface acoustic duct does not necessarily adhere to the uniform gradient of perfectly isothermal water. Prior consideration of the coherent leakage of sound from a surface duct has almost always been made for an isothermal layer, and not for a realistic duct with a nonlinear sound speed profile. This paper presents the results from a brief study in which the rate of leakage has been obtained by using a normal mode model, for an ocean with various surface duct types for which the sound speed varies nonlinearly with depth. Initial results indicate that the rate of variation of leakage with acoustic frequency is strongly linked to the duct trapping frequency, but that the absolute level of leakage is related to the depth of the duct. The formation of these conclusions is illustrated by results from numerical modelling. Reference is also made to theoretical expectations for ducts of uniform gradients weaker than for isothermal water.