Establishment of Geophysical Ocean Bottom Observatory (GOBO)
The primary objective of the NERO portion of Leg 179 is to drill a single hole 200 m into basement and install a reentry cone and casing to prepare Site 757 (or 756) along the Ninteyeast Ridge as an ocean bottom observatory. The GOBO will be installed at a later time and will be part of the future network of seafloor observatories proposed in the ION program. The scientific objectives that can be addressed with geophysical data from long-term ocean bottom observatories include two broad subject areas: Earth structures and natural hazards. These two areas can each be divided into subareas according to the scale under investigation: global, regional, and local.

1. Global scale: mantle dynamics, core studies, moment tensor inversion. The ION report emphasizes that "oceans are seismic deserts!" Except for a few stations on oceanic islands, very large zones are unmonitored, particularly in the Pacific, South Atlantic, and East Indian Oceans. With the present station coverage (FDSN [Federation of Digital Seismic Networks], Fig. 7), the best expected lateral resolution is larger than 1000 km. The same problem arises for geomagnetic observatories. There are many shadows or poorly illuminated zones in the Earth. Due to the nonuniformity of earthquake and seismic station distribution, seismic waves recorded in stations do not illuminate the whole Earth. For example, the transition zone (in a broad sense: 400-1000 km of depth) is poorly covered by surface waves and body waves below oceanic areas.

2. Regional scale (wavelengths between 500 and 5000 km): oceanic upper mantle dynamics, lithosphere evolution, and tsunami warning and monitoring. In terms of oceanic upper mantle seismic investigations, only very long wavelengths have been investigated. In addition, surface waves are the only waves sampling the oceanic upper mantle, and there are no direct measurements of body waves. To understand the lithosphere's evolution, it is necessary to improve the lateral resolution of tomographic seismic studies.

The Indian Ocean crust is considered to be the most complex in any ocean basin. Since the 1970s, magnetic anomalies, fracture zone information, and other geophysical information (McKenzie and Sclater, 1971; Norton and Sclater, 1979; Schlich, 1982; Royer and Sandwell, 1989) have been used to understand the tectonic history of the Indian Ocean, which is characterized by irregularities in kinematic behavior (e.g., ridge jumps, reorganization of the ridge system, asymmetric spreading, spreading velocity changes, and finally collision between India and Asia). Few tomographic investigations have been performed so far in the Indian Ocean (Montagner, 1986; Montagner and Jobert, 1988; Debayle and Lévêque, in press). These studies display a good correlation between surface tectonics and seismic velocities down to 100 km (Fig. 8), but there seems to be some offset at deeper depths for the Central Indian Ridge, as a consequence of the decoupling between the lithosphere and the underlying mantle. This complexity at deeper depths is also present in global tomographic models. However, the lateral resolution is still quite poor and it makes it necessary to increase the station coverage of oceanic areas. The next step in tomographic techniques regards the simultaneous use of surface waves and body waves. By installing only one station in the Central Indian Ocean, it will be possible to obtain direct measurements of delay times and, therefore, unique and fundamental information on the local anisotropy (from SKS splitting), particularly for the 410 km and 660 km discontinuities (from converted seismic waves) and for pure oceanic paths. As shown in Figure 9, the future observatory is surrounded by seismically active areas. This ensures there will be a reasonable amount of data within one or two years after borehole instrumentation.

3. Local scale (wavelengths <500 km): oceanic crustal structure, sources of noise, and detailed earthquake source studies (tomography of the source, temporal variations).

Supplementary Objectives:
1. Sample Characterization
In addition to the objectives related to the emplacement of a GOBO at the previously drilled site, at least 100-200 m of the basaltic basement will be cored and a significant basaltic sample set is likely to be recovered. These recovery depths into basement are significantly deeper than previous coring into basement at Sites 757 and 756. The basaltic basement at the proposed site along the Ninetyeast Ridge includes eruptive units thought to have formed above a mantle plume in the Southern Indian Ocean (e.g., Saunders et al., 1991). The coring provides the opportunity to conduct an in-depth study of a volcanic section formed over an oceanic mantle plume. Detailed descriptions, as well as geochemical, petrologic, and geophysical studies of these basalts will help to further characterize the origin of these basalts, as well as the volcanic stratigraphy of the Ninteyeast Ridge. Petrophysical studies including measurements of P and S-wave seismic velocities of the samples recovered should help to characterize the site and local velocity structure.

2. Geophysical Site Characterization
An extensive suite of seismic experiments will be conducted in conjunction with drilling activities at the site chosen for the installation of GOBO. These experiments include seismic while drilling, vertical seismic profile, and oblique seismic experiments, as well as the possible temporary 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 characteristics of seismic noise levels under the seafloor. These seismic experiments will require four additional days of ship time and will provide one of the most complete borehole seismic datasets available. We briefly review these studies and objectives below.

To 179 Part II: Seismic Experiments

To 179 Table of Contents

ODP Publications

ODP Home