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Shear wave resonances in sediments on the deep sea floor [poster]
Zeldenrust
Irene
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Working Paper
Data from seafloor observatories are generally of poorer quality then continental site data
because the sea surface is an important and local source of broadband noise. This noise is the
result of wind and wave action through direct forcing at long periods and by non-linear
coupling to elastic waves at short periods. By analysing the noise and looking for patterns we
sometimes can constrain the noise source and subtract it from the spectrum and thereby
improve the quality of the data. In another case we can use patterns in the spectrum to derive
information on the structure of the ocean bottom.
This report will present an analysis of broadband noise at two locations in the Pacific.
The first location is ODP Site 843B, and is situated about 225km south-west of Oahu at a
waterdepth of 4407m. Between February and May 1998, the Ocean Seismic Network Pilot
Experiment (OSNPE) acquired over 115 days of broadband borehole seismic data at a
sampling rate of 20 samples/second. The other location is situated near 28N latitude, 142W
longitude (about halfway between Hawaii and California) at a waterdepth of 5000m. In
September 1998, a permanent deep ocean scientific research facility – the Hawaii-2
Observatory, or H2O – was installed on a retired AT&T submarine telephone cable recording
broadband seafloor seismic data at 160 samples/second.
Presentation
Shear wave resonances at frequencies between about 0.1 and 10Hz are a ubiquitous feature of ambient noise and controlled source seismic data acquired on sedimented sea floors. They are a major factor in the ambient noise field and mask many useful seismic arrivals. For controlled source experiments shear wave resonances are a major source of incoherent, signal generated noise and coda. The peaks of the ambient noise spectra associated with the resonances, however, can be used to infer the sediment rigidity and thickness. The theory of Godin and Chapman (1999) has been used to infer shear velocity and sediment thickness from the resonance peaks in horizontal component power spectra for two sites in the Pacific. At ODP Site 843B (OSN-1), about 225km southwest of Oahu, the sediment thickness is known from drilling and we can infer from the resonances that the uppermost shear velocity is 76m/s. At the Hawaii-2 Observatory (H2O) Site, in 5000m water depth half-way between Hawaii and California we predict a sediment thickness of about 50m by assuming the same uppermost shear velocity as at OSN-1.