Seismic data from a World-Wide Standardized Seismograph Network, established in the early 1960s, accelerated advances in seismology and were a great resource of new discoveries up to the 1970s. During the past 10 yr, 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 by the continuation of long-term observations in geomagnetism (i.e., GEOSCOPE [a project that is run by the Institut de Physique du Globe de Paris], IRIS [Incorporated Research Institutions for Seismology], GeoFon [GEOForschungsNetz; a 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). Although 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 the 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. F1; Site 1107) was chosen because there was no expectation of any technical problems, as previous holes in this area were drilled with a single bit. The site chosen was Site 757 (Fig. F1), which was drilled during Leg 121 in 1988 (Peirce, Weissel, et al., 1989). Installing a reentry cone and casing down to basement was the first step toward the installation of a geophysical ocean-bottom observatory (GOBO). Our intention was to establish a hole that penetrates at least 100-200 m or as deep as possible into the basaltic basement. Although we did not intend to core sedimentary rocks, basement rocks were to 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 was to be much greater than previous drilling at either Sites 756 or 757 along the Ninetyeast Ridge, where only a few tens of meters of penetration were achieved into basaltic basement. In addition, the intention was to obtain a full suite of logs from the borehole to assess its geometry and other geophysical parameters that would allow selection of an optimal position for installing the seismometer. The permanent seismometer instrumentation will be installed after drilling at a later date by a submersible or a surface ship. Simply establishing this cased reentry hole was to require up to a week of ship time. In addition to drilling and casing operations, a series of seismic experiments involving the drillship, as well as the research vessel Sonne, were also planned while on site. These experiments included seismic while drilling (SWD), vertical seismic profile (VSP), and an offset seismic experiment (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.
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 (e.g., 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 of the Earth, 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. It is now possible to consider installing joint observatories to meet the needs of all these programs. During workshops addressing the issue of GOBO (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 a high priority for the next 10 yr.
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; J. Orcutt, pers. comm., 1998) 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 to (1) obtain the seismic noise level in the broadband range 0.5-3600 s, (2) conduct a comparative study of broadband noise on the seafloor, downhole, and on a continent, and (3) 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 F2.
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 an OBS and a borehole seismometer than in a typical broadband continental station such as spinning sidebands (SSBs). The noise is still smaller than the noise at SSBs up to 600 s for OBSs. 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 ocean-bottom and borehole seismometers, 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 ocean-bottom and borehole seismometers compared to a continental station. This might be related to the difference in the crustal structure between oceans and continents and to attenuation of seismic noise caused by ocean surface waves and currents. 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 as well as in a borehole is now possible and can provide the scientific community with high-quality seismic data.
The primary objective of the Ninetyeast Ridge Observatory (NERO) portion of Leg 179 was to prepare a seafloor borehole for future establishment of a GOBO. This objective included drilling a single hole as deep as possible into basement, as well as installing a reentry cone and casing beyond basement to prepare the NERO site as an ocean-bottom observatory. The GOBO will be installed at a later time by Montagner and others 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:
One of the ancillary objectives of Leg 179 was to develop a SWD capability for ODP. The SWD project was funded by the National Science Foundation. SWD uses OBSs to listen to the drillship noise and does not use a VSP tool in the well. SWD has the potential for observing shear waves generated by the drill bit. Of the number of seismic experiments planned for Leg 179, SWD was the only experiment that did not use additional ship time, and it was the only experiment that could be kept in the operational schedule. This was because of contingencies (see below) that resulted in the limited time to achieve the primary objective of drilling the NERO hole.
VSPs 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 air gun shots fired on the surface from a second ship. Typically they take 6-12 hr of drillship time, depending on the depth of the hole, sampling interval, and so forth. In a SWD/VSP experiment, the seismic source is the drill bit, and the sound is received on geophones at the seafloor. SWD technology was developed for land boreholes using surface geophones and has had considerable success.
As a test effort, two OBSs and a drill-pipe pilot sensor were employed during Leg 179. OBSs were deployed and recovered at the NERO site, with initial results and procedures analyzed on board (see "Seismic While Drilling"). The OBSs were deployed, and their locations were surveyed using the ship in dynamic positioning mode during pipe trips to and from the seafloor borehole.
Initial proof-of-concept of SWD consisted of three objectives:
The work necessary to establish a SWD capability fell 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 work that belongs to the third category will mainly be done postcruise because the data from the pilot sensor were not provided in digital form during Leg 179. The United States Geological Survey (USGS) OBSs used during Leg 179 both have three-component inertial sensors and hydrophones and can record autonomously on the seafloor for about 1 week. The pilot sensor data was acquired on the rig floor. Measurement while drilling technology (but not SWD) was tested during Leg 156 (Shipley, Ogawa, Blum, et al., 1995).
Because of the delays caused in port at Cape Town, South Africa, delays in waiting for a resupply ship to arrive at Site 1105 on the Southwest Indian Ridge, and longer than expected transits, the operational schedule for NERO was severely impacted and had to be curtailed. The total number of days spent on site was reduced from 11.5 to 5.5, which resulted in barely enough time to meet the primary objective of drilling and casing the NERO hole for installation of the borehole seismometer. This was the highest remaining priority of Leg 179 and superseded all others. We were able to conduct the SWD experiment simply because it involved no additional ship time. In addition, components of the primary objective including coring basement, opening the borehole to depths of 200 m, and logging the hole had to be eliminated from the operational schedule. Ancillary objectives to the NERO hole also were not possible. These included the OSE experiment, the VSP experiment, and test deployment of the broadband seismometer.
Virtually all of the current "global" seismic network resides on land, creating large gaps in coverage for the ~71% of the Earth's surface below sea level. These gaps produce both bias and incomplete images of the Earth's interior. The ION project hopes to fill some of these major gaps in the Indian and Pacific Oceans by installing borehole seismometers in places that are at least 2000 km from any continent or ocean island where a possible seismic station observatory could be established. The site chosen for NERO fits this criteria.
Hole 1107A drilled during Leg 179 lies midway between Holes 757B and 757C, which are 200 m apart. These holes were drilled during Leg 121. Hole 1107A is located at 17º07.4180'S, 88º10.8497'E along the crestal portion of the Ninetyeast Ridge (Fig. F1). The hole was positioned to drill into the sedimentary cover and the underlying basaltic basement sampled during Leg 121 (Shipboard Scientific Party, 1989). The Ninetyeast Ridge is the product of a long-lived hot spot located near the Kerguelen archipelago, which is thought to have also formed the Broken Ridge, the Kerguelen-Heard Plateau, and the Rajmahal Traps in eastern India (Luyendyck and Rennick, 1977; Duncan, 1981, 1991; Saunders et al., 1991; see Fig. F5). 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., Duncan, 1991; Saunders et al., 1991).
Previous drilling (Shipboard Scientific Party, 1989) at Holes 757B and 757C penetrated 369-m-thick sedimentary sequence that consisted from top to bottom of Eocene-Pleistocene nannofossil ooze, lower Eocene calcareous ooze and chalk, followed by ash, tuff, lapilli, and pebbly material. Below the sediment cover, Hole 757C penetrated 42 m of basaltic basement dated at 58 Ma by Duncan (1991). Site 1107 was selected in close proximity because of the generally good drilling conditions and penetration rates experienced during Leg 121. Holes 757B and 757C were drilled at water depths of 1652.1 and 1643.6 m, respectively. Hole 1107A was drilled at a water depth of 1648 m. Drilling was expected to encounter roughly 370 m of sedimentary cover above basement, based on Leg 121 results.
One inherent problem in ocean borehole seismic observatories is the noise levels created at the sea surface, which is derived from wind and waves through direct forcing at long periods and by nonlinear coupling of elastic waves at short periods (Webb, 1998). Based on experience in Hole OSN-1, in which a borehole seismometer has already been deployed south of Hawaii, fairly high seismic noise levels were problematic (Dziewonski et al., 1992). The problem has been attributed to two factors. One is that the borehole casing is thought to have penetrated only 9 m into basement, and the seismic sonde was clamped to the casing. It is thought that the sonde was clamped above the basement/sediment interface (J. Orcutt, pers. comm., 1998). This problem was accommodated in the planning of Hole 1107A by casing to at least 40 m into basement. In a related problem, others have suggested that the noise level is high because the noise is coupled into the top of the borehole at the massive exposed reentry cone and propagates down a poorly cemented casing (Webb, 1998). In this case, proximity of the seismic sonde to the casing may also be problematic, so assuring a good cemented bond for the casing and/or deepening the hole significant distances below the casing should improve the noise level. In this way, the seismic sonde can be either clamped to the borehole wall or grouted into the borehole at a significant distance away from the casing (Webb, 1998; J. Orcutt, pers. comm., 1998). Others have suggested filling the borehole with sand. In the case of Hole 1107A, our drilling strategy included deepening the borehole beyond the casing as much as time allowed. At least 10 m was required for the installation of the seismometer. Our drilling plan involved washing through the sedimentary cover to 370 m and then drilling 40 m into basement. The borehole would then be cased and cemented, and, finally, the borehole would be cored and drilled to a maximum depth of 200 m below the sediment/basement interface. Because of the time problem described above, we developed a no-coring strategy to maximize penetration through the cement and into the borehole with the small time available. The strategy fell short of saving enough time for the OSE experiment, but it allowed us to drill through the casing shoe, cement, and rathole fast enough to allow establishment and further deepening of the borehole.
1Examples of how to reference the
whole or part of this volume can be found under "Citations" in the preliminary
pages of the volume.
2Shipboard
Scientific Party addresses can be found under "Leg 179 Participants" in the
preliminary pages of the volume.
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