Sound propagation in regions overlaying gaseous range-dependent sublayers

Abstract

Some regions of the ocean consist of bottom layers, which have imbedded bubbles. The bubbles may consist of methane gas. The consequence is that the particular layer has a sound speed, which is lower than in the overlying layer. Depending on the wavelength of the sound source relative to the layer thickness, sound may either reflect off the layer, or it may partially tunnel through the layer to the next set of layers. This effect is frequency dependent, and at some point, when the wavelength in the layer is small relative to the layer thickness, no sound will traverse the layer. The effect of this is that the modal eigenspectrum will be altered and will be largely characterized by the gaseous layer structure in horizontal and vertical directions. The influence of range-dependent gaseous regions on modal spectrum and measured signal structure in different underwater waveguides, which can be considered as strongly range dependent for spatially localized gaseous regions, is modeled by the normal mode code SWAMP for different frequency ranges. [Work sponsored by ONR, the Naval Research Laboratory, and the University of New Orleans.]