ANALYSES OF HIGH-FREQUENCY BOTTOM AND SUBBOTTOM BACKSCATTERING FOR 2 DISTINCT SHALLOW-WATER ENVIRONMENTS

High-frequency (40-kHz) bottom backscattering data obtained by the Ben thic Acoustic Measurement System from two different shallow water loca tions were analyzed. One of the sites has a hard sandy bottom, and the other a soft silty bottom. Since the data were recorded on two slight ly separated arrays, the differential phase technique was used to loca te the scatterers. It is found that scattering at the sandy site origi nated from the seafloor interface, while the scattering at the silty b ottom was caused by a scattering layer about 1 m beneath the seafloor (believed to be methane gas voids). The backscattering strengths at th e silty bottom were about 3 dB higher than those at the sandy bottom. A simple sub-bottom scattering model for the silty bottom is assumed a nd compared with the data. When the sound refraction and attenuation a re taken into account, the model-data fit yields reasonable results.