Shallow-Water Broadband OBS Seismology

The recent development of broadband ocean-bottom seismometers that can be deployed for more than a year has led to the construction of large ocean-bottom seismometer (OBS) fleets and to many successful experiments studying Earth structure and tectonics beneath the oceans. However, ocean surface waves raise noise levels at deep ocean-floor sites far above those at continental sites in the microseism band between 0.2 and 10 sec period, and currents and ocean waves raise noise levels at longer periods. Broadband OBSs are rarely deployed in shallow water because of a fear of loss due to bottom trawling and an expectation of very high noise levels from strong currents and the nearby ocean surface. However, these noise sources can be overcome such that shallow OBS deployments may provide noise levels that are comparable to deep-water sites at periods > 10 sec and lower than deep-water sites at shorter periods. Burial of the instrument into the sediments can shield the seismometer from current noise, while the noise from deformation under wave loading can be removed using pressure gauge data. We predict the noise levels can be reduced to allow the detection of Rayleigh waves from 20 to 200 sec period with good signal-to-noise ratio (SNR) from teleseismic earthquakes as small as M(w) 5. Short-period (< 2 sec) noise levels will be 20-30 dB lower in shallow water than in deep water because short-period microseisms are greatly attenuated during propagation from deep to shallow water. Short-period (0.5-2 sec) teleseismic body waves should be detected with good SNR from events as small as M(w) 4.5.