Part II
BACKGROUND

The scientific community has recognized that global seismic observations will remain incomplete until instruments are deployed on the ocean floor. There is asymmetry in station coverage between oceans and continents‹and more particularly between the Southern and Northern Hemispheres. The need for ocean bottom observatories for geodetic, magnetic, and seismic studies is driven by the same factor‹the lack of observations in large tracts of the world ocean where neither continents nor islands are available to place observatories. Some plates, for example the Newsweek and Juan de Fuca Plates and the Easter Microplate, have no islands on which observatories are typically stationed, and, thus, the geodetic measurements needed to evaluate absolute plate motion and plate deformation are not available. The problem of extrapolating the magnetic field to the core-mantle boundary is greatly exacerbated by "holes" in observation sites in the Indian Ocean and eastern Pacific Ocean. Images of the interior velocity heterogeneity, in turn related to thermal and chemical convection, are "aliased" by the lack of control from seismic stations in the Indian and Pacific Oceans. Maps of "holes" from all three disciplines include many common sites. For at least the next five years, it is possible to consider installing joint observatories to meet the needs of all these programs. During the last prospective workshops (IRIS/Hawaii, 1993, ION-ODP, Marseilles, 1995), it was recognized that the installation of a GOBO is now feasible from a technological point of view and represents the first priority for the next ten years.

The installation of ocean bottom seismic stations, their maintenance, and the recovery of data on a timely and long-term basis represent a formidable technical challenge. However, different pilot experiments carried out by Japanese (Kanazawa et al., 1992; Suyehiro et al., 1992), French (Montagner et al., 1994a, 1994b, 1994c), and American groups (OSN1, Dziewonski et al., 1992; Orcutt, pers. com., 1997) demonstrate that there are technical solutions to all the associated problems.

The technical goal of the French Pilot Experiment OFM/SISMOBS (Observatoire Fond de Mer [ocean floor seismometer]) conducted in April and May 1992 was to show the feasibility of installing and recovering two sets of three-component broadband seismometers (one inside an ODP borehole and another inside an ocean-bottom seismometer (OBS) sphere in the vicinity of the hole). Secondary goals were (1) to obtain the seismic noise level in the broadband range 0.5 3600 s, (2) to conduct a comparative study of broadband noise on the seafloor, downhole, and on a continent, and (3) to determine the detection threshold of seismic events. A complete description of the experiment can be found in Montagner et al. (1994a) and a summary drawing is presented in Figure 6.

After the installation of both sets of seismometers, seismic signals were recorded continuously during 10 days. The analysis of these signals shows that the seismic noise is smaller in the period range 4-30 s for both OFM and borehole seismometer (Observitoire Fond de Mer, OFP) than in a typical broadband continental station such as spinning sidebands (SSB). The noise is still smaller than the noise at SSB up to 600 s for OFM. The noise on vertical components is much smaller than on the horizontal ones. The difference might be explained by instrument settling. It was also observed that the noise level tends to decrease as time goes by for both OFM and OFP, which means that the equilibrium stage was not yet attained by the end of the experiment (Beauduin et al., 1996a, 1996b). The patterns of microseismic noise in oceanic and continental areas are completely different. The background microseismic noise is shifted toward shorter periods for OFM and OFP compared to 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 correctly record teleseismic earthquakes of magnitude as small as 5.3 (Montagner et al., 1994b). It was also possible to extract the earth tide oceanic signal. Therefore, the experiment was a technical and scientific success and demonstrated that the installation of a permanent broadband seismic and geophysical observatory at the bottom of the seafloor is now possible and can provide the scientific community with high-quality seismic data.



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