NERO Section Modified by J.-P. Montagner from Proposal 508 Submitted by:
Jean-Paul Montagner, Roland Schlich, Jean-François Karczewski, Kiyoshi Suyehiro, Pascal Tarits, Geneviève Roult, Philippe Patriat, Marc Schamming.
Staff Scientist: TBN Chief Engineer, Part I: Tom Pettigrew
Chief Scientist, Part II: John Casey
Leg 179 is a two-part leg composed of testing the hammer drill-in casing system developed at the Ocean Drilling Program (ODP) and drilling a hole for the Ninetyeast Ridge Observatory (NERO) Project. Testing of the hammer drill will determine the viability of the tool, the complete casing system, and the maximum slope that can be spudded by the tool. Fifteen days have been allocated for these tests. The NERO project will enable a major gap in the global coverage of seismic, magnetic, and general geophysical data to be filled. Currently, geophysical observatories are only present on continents and islands, thus data collection is incomplete. Establishing a cased reentry hole will be the first step toward the installation of a Geophysical Ocean Bottom Observatory. This observatory will be part of the future network of seafloor observatories proposed in the International Ocean Network (ION) program. From a scientific point of view, the geophysical applications can be considered at two different scales. At a global scale, NERO meets the scientific objectives of ION regarding global geodynamics and earthquake studies and is in good agreement with ODP's Long Range Plan. At a regional scale, NERO enables a more specific investigation of the dynamics of the Indian Plate, which has had a complex geologic history characterized by high intraplate seismicity and may contain a diffuse plate boundary between the Central Indian Ridge and the Indonesian Arc. From a technological point of view, previous experiments (Japan, Leg 128, 1991; France, OFM/SISMOBS; Hole 396B, 1992) demonstrated that the installation of broadband seismometers in boreholes is now feasible. The selected site on the Ninetyeast Ridge should not produce any technical problems, since previous single bit holes were drilled in this area (ODP Sites 756 and 757 during Leg 121 in June 1988). To install a seismometer in the hole at a later date, it is necessary to first install a reentry cone and case down to basement. Seven days have been allocated for this operation. After the leg, the same kind of instrumentation used during the OFM/SISMOBS experiment will be positioned in the hole.