Sediment geoacoustic properties determined from local and spatially integrated measurements of interface waves

Abstract

In Aug./Sept. 1998, as part of a sediment classification exercise at a site near Panama City, Florida and coordinated by NRL, Stennis, a vertical hydrophone array was deployed at mid-water depth to measure the velocity and directionality of interface waves. An airgun, 20-m deep on the seabed and at a range of approximately 1 km, excited head waves, and the received signal was used to invert for the sound speed using two methods: first, by comparing the arrival times of the water wave with those of the interface wave, and second, by considering the critical angle computed from the vertical directionality of the received signal as found from a cross correlation of signals from the hydrophones at known separations. The first method gives an average of the sound speed integrated along the path length, while the second method establishes the sediment properties near the array. The array was allowed to drift, giving measurements along various paths and at different local positions. By comparing the local and averaged properties of the sediment, the heterogeneity was established and the spatial variation determined more completely than could be achieved by either purely averaged or local measurements alone. [Work supported by ONR.]