Reverberation characteristics in shallow water with a strong thermocline

Abstract

The first joint China–U.S. ocean acoustics experiment was conducted in the summer of 1996 in a far offshore area in the Yellow Sea with strong thermoclines. The water depth was about 76 m with a flat seabed. The sediments in the area consisted of silty sand with mean grain diameter of 0.0643 mm. Shallow-water reverberation data were collected by using a 32-element vertical array that spanned the water column. Explosive sources of 1 kg were detonated both above and below the thermocline. The Yellow Sea ’96 reverberation data exhibit strong receiver and source depth dependence. Reverberation intensity level (RL), received in the thermocline, shows a regular interference structure with increasing time (distance). This can be explained by the R-mode theory. In general, seabottom scattering dominated the Yellow Sea ’96 reverberation process. Therefore, the seabottom scattering strength was derived, by inversion, from the reverberation and propagation data. A resonant volume-scattering source in the water column was also observed in the measurements. This demonstrates that water column effects (scattering from internal wave packets, fish groups, etc.) may also be significant for the shallow-water reverberation at some frequencies. [Work supported by ONR.]