Acoustic propagation within a surface duct in the western Bay of Bengal

Abstract

Sound speed structure forms a surface duct in the upper 50 m layer in the western Bay of Bengal during late July A range-dependent acoustic ray computation shows that some rays emanating from a source within the upper 30 m, get trapped within this duct while the untrapped rays propagating as refracted bottom-reflected (RBR) rays. The extent of acoustic energy trapped depends on the critical angle of approximately 1 degree. When the source depth coincides with the axis of the surface duct, all the rays within the above critical angle get trapped fully. With further decrease in the source depth, the limit of energy trapping reduces to 0.5 of a degree. The intensity level for a trapped ray is 10-15 dB higher than that of an untrapped RBR ray. The leakage of + 0.6 degree ray out of the surface duct can clearly be seen as change in the intensity level of about 15-20 dB which is attributed to the weakening of the duct at that range.