Abstract:

During a recent seismoacoustics experiment, a moored, near-bottom, source just outside the surf zone (approximately 500-m seaward) in 9-m deep water, transmitted 8 tones from 70 to 700 Hz. Recordings were made by several fixed sensor systems, including: 1) a bottom hydrophone/3-component geophone package 15-m distant; 2) two horizontal bottom hydrophone arrays in a nearly perpendicular configuration in 12-m water 1.23 km away and 1.06 km farther offshore; and 3) five 3-component geophones buried at approximate 14-m spacing across the beach at 480--530 m range. These data were used to create 5-min time series of the spectral levels and directionality of the tones every 1/2 h over an 8-h period. Results indicate that within a 5-min period, fluctuations of 10--15 dB occur in the bottom hydrophone array signal levels which, typically, are not correlated from frequency to frequency. Fluctuations in the land geophone data are somewhat smaller, and only the 2 or 3 highest-frequency tones are detectable. Variations in water depth, measured by a package also located just outside the surf zone, are used with an adiabatic normal mode model to predict the fluctuation effects of ocean surface wave activity alone. [Work supported by ONR, Code 32].