Book Description
Research progress on seismic detection and discrimination using ocean-bottom seismographs (OBS's) is outlined. During this first year of DARPA contract work, our investigations have been focused primarily on using existing OBS data to address the problem of sea-floor seismic noise and to constrain models of the near-bottom seismic environment. We have completed noise studies at six of our previously occupied OBS sites. The noise levels of typical amplitude spectra roll off rapidly out to 2-4 Hz and much less rapidly beyond 4 Hz. At frequencies above 4 Hz, the noise levels are low, typically a few nanometers/Hz1/2 or less and comparable to good land-based stations. Several correlations between noise levels and other parameters, such as sediment thickness, distance from the continental margin and sea state, are discussed and used to constrain possible noise mechanisms. We hypothesize that the dominant mechanism of high-frequency (>2 Hz) noise generation at most of the sites examined thus far is of local oceanographic origin, exciting acoustic modes in the water column which couple to Stoneley modes at the sediment-water interface and waveguide modes within the sediment column. This hypothesis is supported by the analysis of noise data from two OBS arrays, which shows that the noise coherence is very low for pairs of sensors separated by as little as 200 m. We also outline some of the conclusions relevant to the seismic noise problem deduced by us from the data collected during the Lopez Island intercalibration experiment.
