Abstract:

Small, controlled land detonations on and near the beach recorded by land geophones and by offshore seismoacoustic sensors are used to characterize the coupling between land seismic and underwater acoustic fields. These data were collected in the Marine Physical Laboratory's Adaptive Beach Monitoring program. The predominant arrival on land is an elastic surface wave. Although an ocean bottom interface wave can be seen clearly in seismoacoustic recordings just outside the surf zone, it attenuates to nearly background noise levels by the time it reaches a bottom hydrophone array 1.5 km offshore in 12-m water. Travel-time and frequency/wave-number analyses indicate that the hydrophone array's received energy is composed mostly of dispersive body waves propagating as the lowest water-borne mode, with frequency-dependent phase and group velocities around 2.0 and 1.8 km/s, respectively. The frequency at which this lowest mode cuts off is the corner frequency of the high-pass filter representing the propagation from land to the underwater acoustic field. These results and numerical modeling, guided by insights obtained from classic work on acoustic propagation in a fluid wedge, are used to explain offshore underwater recordings of land vehicle activity. [Work supported by ONR, Code 32.]