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.

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.

I. Seismic-While-Drilling Vertical Seismic Profile
One objective of Leg 179 is to develop a seismic-while-drilling capability for the Ocean Drilling Program. The SWD project was funded by the National Science Foundation. SWD uses OBSs to listen to the drill ship noise and does not use a VSP tool in the well. However, to evaluate the performance of the SWD system, a conventional VSP, with which to compare results, is critical. The conventional VSP could be carried out with the vertical component instrument already on board, but it would be better to run it with a three component VSP tool. SWD has the potential for observing shear waves generated by the bit and it would be useful to compare this with any shear waves in the VSP converted by scattering.

The SWD experiment will be conducted at the NERO site to develop seismic-while-drilling capability for the Ocean Drilling Program (ODP). Vertical seismic profiles have proven extremely useful over the history of ODP in correlating borehole properties with regional seismic properties. Normally they are carried out with a borehole seismometer and airgun shots fired on the surface from a second ship. Typically they take 6-12 hr of drill ship time depending on the depth of the hole, sampling interval, etc. In an SWD/VSP, the seismic source is the drill bit and the sound is received on geophones at the seafloor. No additional drill ship time is necessary to acquire an SWD/VSP. The SWD technology was developed for land boreholes using surface geophones and has had considerable success. We propose here to extend the SWD capability to deep-ocean boreholes. For the NERO experiment SWD/VSPs and traditional VSPs will be compared for data quality and utility. If successful, the technology will be transitioned to ODP for routine use.

As a test effort, two OBSs and a drill-pipe pilot sensor on Leg 179 will be utilized. OBSs will be deployed, recovered, and redeployed at the NERO site, with initial results and procedures analyzed on board. The OBSs can be deployed and recovered using the ship's workboat. Five additional GEOMAR Ocean Bottom Hydrophones (OBHs; Flueh and Bialas, 1996) will also be deployed around the drill site and used during the SWD experiment.

Initial proof-of-concept of SWD will consist of three objectives:

1.A demonstration of the generation and recording of drill bit signal on the pilot sensors at the rig floor. Analysis will consist of producing filtered autocorrelation functions at depth intervals of less than 5 m over a range of bit depths sufficient to see pipe multiple arrivals and their characteristic moveout. Spectral and temporal characteristics of drill bit signal will be documented.

2.A demonstration of the recording of drill bit direct arrivals (P- and S-waves) in the OBS data. Analysis will consist of producing filtered cross-correlation functions (between the OBS and pilot sensor data) at depth intervals of less than 5 m over a range of bit depths sufficient to observe P and S-wave moveout. Filtering would include polarization filtering, bandpass filtering, and multichannel spatial filtering so that direct arrival signals can be distinguished from other interference.

3.A demonstration of the recording of P and S reflections. Analysis will consist of wavefield separation of direct and converted energy and isolation of primary bit-generated reflections.

The work necessary to establish a SWD capability falls into three categories: (1) acquisition of the OBS data during drilling; (2) acquisition of the pilot sensor data on the rig floor during the drilling operations; and (3) reduction of the OBS and pilot sensor data to a VSP format for seismic analysis.

The USGS-OBSs both have three-component inertial sensors and hydrophones and can record autonomously on the seafloor for about one week. The operations necessary to do the processing are computing autocorrelations and cross-correlations between selected channels and bandpass and notch filtering. The pilot sensor data will be acquired on the rig floor. Measurement-while-drilling technology (but not SWD) was tested on Leg 156 (Shipley, Ogawa, Blum, et al., 1995).

II. Conventional Vertical Seismic Profiling
About 12 hr will be allotted for the conventional VSP during the logging program following completion of drilling. The experiment will be conducted in a similar manner to other VSPs on ODP Legs 118, 123, 148, and 164 (Swift et al., 1991, Bolmer et al., 1992; Swift et al., 1996; Holbrook et al., 1996). A water gun and an airgun will be floated from the aft port crane and the Schlumberger three-component tool will be used as the borehole receiver. The tool will be clamped at 10-m intervals within basement and through the cased sediment section. These data will define the vertical seismic velocity and attenuation properties within a few tens of meters of the borehole. The availability of the VSP tool for the leg is critical in evaluating the SWD experiment and to carry out the oblique seismic experiment (described below).

III. Oblique Seismic Experiment (OSE)
An OSE, using the same single-node three-component borehole seismic tool as the conventional VSP, will be conducted at the NERO hole on the Ninetyeast Ridge. Goals are to: (1) determine interval velocities over the depth of the hole for comparison with well logging and core sample measurements; (2) map lateral heterogeneity at the site with a resolution of ~100 m over ranges up to 12 km; (3) check for anisotropy within the sediments and volcanic sections; and (4) obtain in situ measurements of attenuation in the sediments and volcanic section at very-low frequencies (VLF). These measurements will be necessary to determine the effects of local structure on the ultra-low frequency (ULF; 0.001-5.0 Hz) observations of ambient noise and teleseismic waves (earthquakes) to be made at the site as part of ION, and to place the site in a geological and geophysical context for extrapolation of the ULF results to other regions of the seafloor. Even though the compressional and shear wavelengths in the ULF band are long with respect to the heterogeneities and geological structure at the hole, seismometer coupling and ambient noise are sensitive to sub-wavelength scale features. During ODP Leg 179 on the Ninetyeast Ridge, the JOIDES Resolution will drill a hole at least 100-200 m into basaltic basement near Site 757 (17°S). The project is a joint effort between U.S. (Project NOSE) and German (Project SINUS) scientists. Scientists from GEOMAR will conduct a refraction experiment from the Sonne using seafloor receivers. The drill ship will coordinate with the Sonne and record their shots using the Schlumberger three-component tool clamped near the bottom of the borehole. The OSE results will be integrated with an experimental seismic-while-drilling VSP experiment, a conventional VSP, Schlumberger logs, and physical properties measurements of cores. In contrast to most 'normal' ocean crust, the igneous section on the Ninetyeast Ridge was created at very high magma extrusion rates that resulted in large, horizontal sheet flows in the upper igneous section. A detailed study of the seismic response of these sheet flows (converted shear waves, anisotropy, interference effects, etc.) will constrain models for inferring the rate of magma injection from single-channel and multichannel seismic reflection surveys elsewhere.

The low cost of this study is made possible by cooperation with a geophysics survey on the Ninetyeast Ridge lead by Dr. Ernst Flueh at GEOMAR, FDR. The JOIDES Resolution and the Sonne will be at the NERO site at the same time in late May, 1998. Both ships are currently scheduled to arrive at the NERO site on May 16th. Dr. Flueh will obtain the bathymetry and sediment thickness data needed to reduce the OSE traveltime data. Dr. Flueh will also deploy 20 30 OBHs and OBSs on the seafloor around the site and will shoot with a tuned airgun array in a pattern of circles and radial lines around the borehole. The data from these instruments will define the seismic structure on a range of scales from a few hundred meters up to a few tens of kilometers. This is essential to characterization of the site because of the strong lateral gradients on these scales inherent in the construction of the volcanic Ninetyeast Ridge. Other seismic studies proposed will complement Dr. Flueh's by providing much greater detail about the basement and sediment structure out to ranges of a few hundred meters.

The two OBSs deployed close to the drill ship will stay on the seafloor and will be used for recording during the OSE experiment. The type of OBSs used is well suited for recording converted S-waves as demonstrated during a similar two-ship experiment in conjunction with Leg 164 (Pecher et al., 1997). The three-component Schlumberger seismic tool will be clamped at a single depth near the bottom of the borehole at about 100 m in basement. GEOMAR scientists aboard the Sonne will shoot a series of concentric circles around the borehole at ranges of 2, 4, 6, and 8 km using radar and dithered global positioning system (GPS) navigation to steer. A series of four straight lines will be shot across the borehole at 45° angles. The Sonne will coordinate shooting with the JOIDES Resolution. Schlumberger will provide seismic recordings for each shot using timing synchronized to a GPS clock. GEOMAR will also survey the bathymetry of the survey region using a multibeam system aboard the Sonne and will collect multichannel reflection profiles to determine the thickness of sediment above basement and provide control on compressional velocities. The advantages of this approach are (1) determination of velocity on vertical scales finer than a conventional VSP and, (2) in the future, the ability to obtain crustal velocity information without using drill ship time for a conventional VSP.

IV. Pilot Deployment of a Broadband Seismometer
To test a Japanese borehole seismometer installed via the drill ship, temporary deployment of a broadband wide dynamic range seismometer in the borehole at the NERO site will be conducted. This will allow testing of the deployment procedures and shock resistance of the instrument. The characteristics of seismic noises and their level in the borehole will also be examined. This test will address questions of future installations of borehole seismographs using the drill ship. A minimum of 12 hr will be allotted for the test. The instrument will be retrieved at the end of the test.

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