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Seafloor Pressure Array Studies at Ultra-Low Frequencies
CALIFORNIA UNIV SAN DIEGO LA JOLLA
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Broadband inertial measurements of environmental noise on the seafloor have always been difficult technically and few examples of successful measurements are available. A knowledge of environmental noise processes, however, is essential for future broadband instrument design and deployment. In order to measure broadband noise routinely, a low frequency pressure gauge designed for deep ocean recording was adapted for use with the Scripps OBSs and calibrated in the laboratory. The instruments were deployed twice in array configurations along with other pressure sensors in 4.1-km deep ocean southwest of San Diego, California. Spectra from the hydrophones showed a stable 0. 1 5 Hz microseism peak throughout the deployment period with some evidence of dispersion. Smaller peaks at 0.3-1 Hz appeared when the local wind became stronger. Highly coherent peaks at 0.085 Hz, originally interpreted as single frequency microseisms, were found to be Rayleigh wavetrains from large earthquakes. Reverberations in the water column caused by P-wave arrivals from the earthquakes were also observed. Coherence in the microseism band was significant only for instruments separated by less than 6 km, lending support to the hypothesis that the source is at least in part isotropic and random. Wavenumber analysis was successful only for the well-defined earthquake signals while for the microseism band, low coherence and the location of the sensors reduced the arrays useful azimuthal resolution.
APPROVED FOR PUBLIC RELEASE