Abstract:
During a recent near-surf-zone experiment, the underwater acoustic noise field was dominated by biological sounds over the 50-Hz to 10-kHz band. At night, members of the croaker (Sciaenidae) family migrated from the surf zone out to the 20-m water, where two bottom hydrophone arrays were located. Often during the night, individual vocalizing fish came within several meters of an array element so that the fish's sequence of 7--13 knocks, each with peak-to-peak source levels of about 160 dB re: 1 (mu)Pa @ 1 m, was clearly discernible above the background din. The arrivals from some of these knocks display low-frequency (around 60 Hz) oscillatory tails due to excitation of interface waves at the ocean bottom. These Scholte waves have phase velocities around 110 m/s and a group velocity near 30 m/s. Using adjacent array elements to locate the fish at each knock, its movement over the sequence creates a synthetic aperture that is used in inversion for the shear wave velocity in the uppermost part of the ocean bottom. [Work supported by ONR, Code 32.]