Ocean bottom seismic noise (1–10 000 s)

Abstract

There is now a good consensus in the geophysical scientific community of the necessity to install broadband seismic stations on the ocean bottom in order to improve our knowledge of the deep Earth dynamics. However, it is necessary to first investigate the level of seismic noise (0.01–10<th>000 s) and to compare it with the expected amplitude of seismic signal due to earthquakes. During the French pilot experiment SISMOBS/OFM (28 April–11 May 1992) in the Atlantic Ocean, two sets of Guralp CMG3 broadband seismometers were successfully installed and recovered. The first set, named OFP, was installed inside the ODP borehole 396B at 292 m below seafloor and the second one, named OFM, was installed on the sea bottom (semi-buried) close to the hole. A comparison of broadband seismic noise (1–3600 s) between OFP, OFM, and a typical continental GEOSCOPE seismic station SSB shows that the noise level of OFM is smaller (resp. larger) at periods longer than 20 s (resp. shorter than 2 s) than the one in the borehole and in SSB. The detection threshold is very low and makes it possible to get good records of teleseismic events as small as 5.4. These results were very encouraging since they demonstrate that in the future ocean bottom seismometers will provide very valuable information on earthquakes and Earth structure.