THE FRENCH PILOT EXPERIMENT OFM-SISMOBS - 1ST SCIENTIFIC RESULTS ON NOISE-LEVEL AND EVENT DETECTION

The seismic data recorded during the French Pilot Experiment OFM-SISMOBS (Observatoire Fond de Mer) were carefully analyzed. This experiment was successfully conducted between 28 April and 11 May 1992. Notwithstanding the technical goal of the experiment, which was to show the feasibility of installing and recovering two sets of three-component broadband seismometers (one inside an Ocean Drilling Program (ODP) borehole and another inside an ocean-bottom seismograph (OBS) sphere in the vicinity of the hole), the second goal of the experiment was to obtain for the first time the seismic noise level in the broadband range 0.5-3600 s, to conduct a comparative study of broadband noise on the sea-floor, downhole and on a continent, and to determine the detection threshold of seismic events. After the installation of both sets of seismometers, seismic signals were recorded continuously for 10 days. The analysis of these signals shows that the seismic noise is smaller in the period range 4-30 s for both sea-floor seismometers (OFM; Observatoire Fond de Mer) and downhole seismometers (OFP; Observatoire Fond de Puits) than at a typical broadband continental station such as GEOSCOPE Station SSB. The noise is smaller than that at SSB up to 600 s for OFM. The noise on vertical components is much smaller than on the horizontal ones. This difference might be explained by instrument settling. It was also observed that the noise level tends to decrease with time for both OFM and OFP, which means that the equilibrium stage was not attained by the end of the experiment. The patterns of microseismic noise in oceanic and continental areas are completely different. The background microseismic noise is shifted towards shorter periods for OFM and OFP compared with a continental station. This might be related to the difference in the crustal structure between oceans and continents. The low level of seismic noise implies that the detection threshold of earthquakes is very low, and it has been possible to record correctly telescismic earthquakes of magnitude as small as 5.3. It was also possible to extract the Earth tide oceanic signal. Therefore, the experiment was a complete technical and scientific success, and demonstrated that the installation of a permanent broadband seismic and geophysical observatory on the sea-floor is now possible and can provide the scientific community with high-quality seismic data.