Observations of Seafloor Ambient Noise with an Ocean Bottom Seismometer Array.

摘要

In 1987 an array of ocean bottom seismometers (OBS) was deployed in the deep ocean off southern California for the purpose of recording ocean bottom ambient noise. By lowering the instruments with a wire, it was possible for the first time to create an array which allowed coherent processing of short wavelength energy in the frequency range 0.05 to 30 Hz. The array consisted of 9 working instruments in place for 30 days, allowing for the recording of ambient noise during several changes in the surface environment. Sea surface conditions were determined from the U.S. Navy's Global Spectral Ocean Wave Model (GSOWM). Noise levels at the ocean floor have been correlated with the swell height and wind speed predicted by the GSOWM. However, the theoretical transfer function between surface wave height and pressure fluctuations on the bottom did not predict the recorded pressure signal to a high degree of accuracy. The array achieved a uniform sampling of distances between sensors, and a good estimate of spatial coherence has been made up to 156 m. A characteristic pattern of coherence length dependence on frequency is observed throughout the deployment. Through modeling of seismic energy it has been possible to estimate the range to the source of the noise energy. The wavenumber decomposition or beamforming of the ambient noise presented some difficulties. Theses. (aw)