The effect of an ocean front on sound propagation in shallow water

Abstract

From a perspective of sound propagation, a sloping bottom found in typical shallow-water environments can cause propagating sound to horizontally refract offshore. Additionally, an offshore ocean front can cause horizontal reflection/refraction shoreward. Combining these two effects, sound tends to propagate along the front, and ‘‘whispering gallery’’ modes can be observed. To model this effect, we first derived an analytic solution for the sound field using a point source in a rigid wedge environment that has a front placed offshore. The analytic solution gives us clear physical insight into this problem. For a more complicated environment, we exploited a 3-D acoustic propagation program in the Acoustics Toolbox, which uses a vertical mode and horizontal Gaussian beam method. This model shows sound energy trapped ahead of the front with observable frequency and modal dependence. Also, when including the foot of the front, which is commonly seen on the continental shelf and extends inshore along the bottom, the model shows less modal attenuation, which raises the level of the trapped energy.