Seismic data from a World-Wide Standardized Seismograph Network (WWSSN) established in the early 1960s accelerated advances in seismology and were a great resource of new discoveries up to the 1970s. During the past ten years, our knowledge of the processes of the deep Earth has been greatly improved by the development of new generations of global monitoring networks in seismology and geodesy and the continuation of long-term observations in geomagnetism (GEOSCOPE [project name that is run by the Institut de Physique du Globe de Paris], IRIS [Incorporated Research Institutions for Seismology], GeoFon [GEOForschungsNetz; geophysical research network] on a global scale; and MedNet [MEDiterranean NETwork], Poseidon, CDSN [China Digital Seismic Network], GRSN [German Regional Seismic Network] on a regional scale). While the quantity and quality of data have increased, this new information has revealed that there are large departures from lateral homogeneity at every level from the Earth's surface to its center. The intensive use of broadband data has provided remarkable seismic tomographic images of Earth's interior. These models are now routinely used in geodynamics for earthquake studies and to obtain the complex time histories of the inhomogeneous earthquake faulting related to tectonics. Improvements in the observatory locations for seismology, geodesy, and geomagnetics, particularly in the oceans, can greatly enhance our understanding of the Earth's interior.

The observatory planned for the Ninetyeast Ridge will be part of the future network of seafloor observatories proposed in the International Ocean Network (ION) program. The selected site on the Ninetyeast Ridge (Fig. 5) should not produce any technical problems, as previous holes in this area were drilled with a single bit. The primary site is ODP Site 757 and the alternate site is Site 756; both were drilled during ODP Leg 121 in 1988 (Peirce, Weissel, et al., 1989). Installing a reentry cone and casing down to basement is the first step toward the installation of a Geophysical Ocean Bottom Observatory (GOBO). A hole will then be established that penetrates at least 100 200 m into the basaltic basement. Although the sedimentary rocks will not be cored, basement rocks will be cored to allow a wide range of petrological, geochemical, and geophysical studies on the rock samples recovered. The extent of the coring and penetration into basement will be much greater than previous drilling at either Sites 757 or 756 along the Ninetyeast Ridge, where only a few tens of meters of penetration were achieved into basaltic basement. The permanent seismometer instrumentation will be installed after drilling at a later date. Establishing this cased reentry hole will require up to a week of ship time. In addition to drilling and casing operations, a series of seismic experiments involving the drill ship, as well as the research vessel Sonne, are also planned while on site. These experiments include seismic-while-drilling (SWD), vertical seismic profile (VSP), and offset seismic experiments (OSE), as well as the possible deployment of a broadband wide dynamic range seismometer in the borehole to test the deployment procedure and shock resistance of the instrument, as well as the characteristic of seismic noises under the seafloor. These seismic experiments will require four additional days of ship time.

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