Bulletin of the Seismological Society of America
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Bulletin of the Seismological Society of America; April 2006; v. 96; no. 2; p. 647-664; DOI: 10.1785/0120040064
© 2006 Seismological Society of America
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Article

Seismic Broadband Ocean-Bottom Data and Noise Observed with Free-Fall Stations: Experiences from Long-Term Deployments in the North Atlantic and the Tyrrhenian Sea

T. Dahm1, F. Tilmann2 and J. P. Morgan3

1 Institut für Geophysik, Universität Hamburg
20146 Hamburg, Germany
 (T.D.)
2 Bullard Laboratories, Department of Earth Sciences
University of Cambridge
Cambridge, United Kingdom CB3 OEZ
 (F.T.)
3 IfM-GEOMAR
24148 Kiel, Germany
 (J.P.M.)

In a comparative study of two long-term deployments we characterize the seismic noise on the seafloor in the North Atlantic south of Iceland and in the Tyrrhenian Sea north of Sicily. We estimate the teleseismic body-wave detection threshold to be approximately magnitude 6.0 at frequencies below the microseismic band (f < 0.1 Hz) on vertical components at the quietest sites in both regions. At the microseismic peak (~0.25 Hz) in the North Atlantic, the minimum magnitudes for events to be recorded most of the time are Mw 7.0 for the Tyrrhenian Sea deployment and above 8 for the North Atlantic deployment. By correlating seismic noise and oceanic waveheight amplitudes we are able to find the major generation areas of microseismic noise in the North Atlantic. Although the high noise of secondary microseisms at 0.24 Hz is generated far away from the ocean-bottom stations at three near-coastal regions, the microseismic noise at about 1 Hz is generated directly at the stations. We present a technique to estimate the noise generation areas prior to future deployment by using noise at nearby land stations.

The ambient low-frequency noise below 0.1 Hz occurs mainly on horizontal components and is probably induced by seafloor-current-induced tilt. The power spectral density of this noise varies by a factor of up to 10,000 between different stations and deployment sites, indicating in some cases wobbling deployments, possible problems of frame weakness, and a possible higher noise sensitivity of external packs to seafloor currents. Cross-coupling between horizontal and vertical channel noise is strong at some of our stations, demonstrating that the leveling mechanics can be further improved to reduce vertical channel noise.




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W. C. Crawford, R. A. Stephen, and S. T. Bolmer
A Second Look at Low-Frequency Marine Vertical Seismometer Data Quality at the OSN-1 Site off Hawaii for Seafloor, Buried, and Borehole Emplacements
Bulletin of the Seismological Society of America, October 1, 2006; 96(5): 1952 - 1960.
[Abstract] [Full Text] [PDF]


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