Abstract:

Land-based vehicle activity can be tracked by underwater acoustic sensors located at least as far as 3.4 km offshore. This capability is demonstrated with data from two near-surf-zone experiments as part of the Marine Physical Laboratory's Adaptive Beach Monitoring program. In one case, four tracked vehicles spaced every 150 m traveled down the beach. Spectra estimated from a single hydrophone of a bottom line array 1.5 km offshore in 12-m water show a high-pass character compared to land geophone recordings because of the coupling characteristics between the land seismic and underwater acoustic fields. Adaptive plane-wave beamforming over the 30- to 70-Hz band shows, at times, beam-peak-to-background levels exceeding 35 dB. The acoustic tracks agree with those from visual logs and suggest that the four vehicles can be enumerated acoustically, particularly when combined with results from the 120- to 130-Hz band. In a second case, plane-wave beamforming results for two tracked vehicles traveling on the beach and recorded by a bottom array 3.4 km offshore in 20-m water show a beam-peak-to-background level approaching 20 dB. A videotape shows the evolution of 2-D wave-number spatial spectra during the vehicle transit. [Work supported by ONR, Code 32.]