An overview and general results of the Lopez island OBS experiment |
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| Authors: | G H Sutton F K Duennebier B Iwatake J D Tuthill B T R Lewis J Ewing |
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| Institution: | (1) Hawaii Institute of Geophysics, University of Hawaii, 96822 Honolulu, Hawaii, USA;(2) University of Washington, 98195 Seattle, Washington, USA;(3) Woods Hole Oceanographic Institution, 02543 Woods Hole, Massachusetts, USA |
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| Abstract: | The purpose of the experiment was to determine the effects of coupling and bottom currents on ocean bottom seismometers. Twelve operational OBSs, three specially designed three-component systems, and a hydrophone were compared with each other. Unlike seismometers placed on hard rock at land stations, ocean bottom seismometers can be affected by soft sediments (which act as lossy mechanical springs) and by buoyancy. Coupling through soft sediments can modify the response to ground motion much as a low pass filter does, and high buoyancy tends to counteract this effect. These effects are observed in the Lopez data, which consist of signals from mechanical transient tests, cap shots, airgun pulses, and general background noise. The modification of response is pronounced for some instruments and barely noticeable in others. Instruments that stand high in the water relative to their base width tend to be susceptible to rocking motion that shows up as a mechanical cross coupling between horizontal and vertical motion. Correlation of Lopez results with coupling theory suggests that it is possible to design ocean bottom seismometers that will couple well to any sediment. Current levels at the Lopez site (<5 cm s-1) were too small to produce noticeable effect on any of the instruments; however, the same design criteria that will minimize coupling problems will also lessen problems caused by ocean currents.Hawaii Institute of Geophysics Contribution No. 1171. |
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