Scientific Objectives | Table of Contents
Borehole Seismic Observatories
The scientific importance of establishing long-term geophysical stations at deep ocean sites has been acknowledged by the earth science and ODP communities and is expressed in various reports (JOI-ESF, 1987; Purdy and Dziewonski, 1988; JOI/USSAC, 1994; Montagner and Lancelot, 1995; Ocean Drilling Program Long Range Plan, 1996). The objective is to understand the processes driving Earth's dynamical systems from a global to a regional scale by imaging the Earth's interior with seismic waves. Unfortunately, few seismometers are located on the 71% of the Earth's surface covered by oceans; this makes high-resolution imaging of some parts of the mantle impossible. Many new ocean-bottom sensors, whose locations will be carefully selected to maximize results (Fig. 1), are needed to accomplish the goals of the international geoscience programs that rely on earthquake data. Aside from Site WP-2, which will be drilled and instrumented during Leg 191, several other western Pacific sites have been selected for instrumentation. Observatories at Sites 1150 and 1151, on the inner wall of the Japan Trench, were installed during Leg 186 (Suyehiro, Sacks, Acton, et al., in press). In addition, Site WP-1, located in the Philippine Sea, is scheduled to be drilled and instrumented during Leg 195. The instrumentation for these enumerated long-term borehole observatories has been developed by an ongoing national program of Ocean Hemisphere Network Project (OHP) within Japan. The data from these observatories will eventually become accessible worldwide through the OHP Data Center.
Aside from plugging an important gap in the global seismic array, the Site WP-2 observatory will produce high-quality digital seismic data. Tests with other borehole seismometers show that the noise level for oceanic borehole instruments is much less than most land counterparts (e g., Stephen et al., 1999; Fig. 5). Recent studies that exploit high-quality digital seismic data obtained on land have shown exciting new phenomena on mantle flows. In the western Pacific, for example, Tanimoto (1988) showed that there exists a strong l = 2 (angular order) pattern of deep (>550 km) high-velocity anomalies from waveform inversions of R2, G1, G2, X1, and X2 surface waves. This suggests a complex interaction of subducting slabs with the surrounding mantle, including the 670-km discontinuity in the region (Tanimoto, 1988). However, because of sparse global coverage by existing seismic stations, current seismic wave resolution is insufficient to image the actual interaction of the plates with the mantle. More recent studies show the potential of new mantle-imaging techniques, with finer scale images having been obtained in certain locations where high-quality data are dense. Two examples are the deep extension of velocity anomaly beneath ridges (Zhang and Tanimoto, 1992; Su et al., 1992), and the fate of subducted plates at 670-km discontinuity (van der Hilst et al., 1991; Fukao et al., 1992). These detailed conclusions result from the extraction of more information from existing seismograms. Such studies are limited by sparse data coverage, a barrier that new ocean-bottom stations can help break.
Seismic Observatory Design
The WP-2 observatory is to be equipped with two broadband seismometers (Guralp CMG-1) attached to a pipe hung from the reentry cone (Figs. 6, 7), situating the seismometers near the bottom of the drilled hole. Installation of two identical seismometers is a step designed to add redundancy to the observatory. Digital seismic signals from the seismometers are passed uphole by wires using RS-422 Serial Interface Protocol to be recorded in a data control box of multiple access expandable gateway (MEG) data. The observatory will be continuously powered for about three years by four units of six-Watt batteries (SWB1200, Kornsburg Simrad) attached to a frame that sits on the reentry cone (Figs. 6, 7).
In September 1989, a feedback-type accelerometer capsule was installed in Hole 794D in the Japan Sea during Leg 128 (Ingle et al., 1990; Suyehiro et al., 1992, 1995). The instrument recorded a teleseismic event (body-wave magnitude [Mb] = 5.4 at ~4000-km epicentral distance) that clearly showed a surface wave dispersion train (Kanazawa et al., 1992). In May 1992, a comparison of seafloor and borehole (Hole 396B) sensors was made using a deep-sea submersible for installation and recovery (Montagner et al., 1994). Although at this stage there is no consensus as to how we should establish seafloor seismic observatories, it is becoming clearer that oceans can provide low noise environments. In August 1999, a seismometer and a strainmeter were cemented at Sites 1150 and 1151 in the deep-sea terrace of the Japan Trench during Leg 186 (Suyehiro, Sacks, Acton et al., in press). The cementing was done to stop fluid motion around the sensors to make the noise level lower and to achieve highly sensitive broadband seismic observations. Because it is imperative that no fluid motion occurs around the broadband seismometers at proposed Site WP 2, the sensors will be cemented during Leg 191 as well. Once instruments are installed in the hole, an ROV will activate the observatory by handling underwater mateable connectors (UMC). In November 2000, Kaiko, an ROV (Fig. 3) designed to operate in water depths up to 10,000 m by the Japanese Agency of Marine Science and Technology Center (JAMSTEC), is scheduled to visit Site WP-2 to begin observations.
Hard-Rock Reentry System
The HRRS is being developed to provide ODP with the ability to establish a reentry casing on sloped and fractured hard-rock outcrops on the seafloor. The system uses a Model 260 downhole fluid hammer developed by SDS Digger Corporation of Canning Vale, Western Australia, along with a bit to advance the hole while casing is installed simultaneously. Presently, 13-3/8-in casing is being used in the prototype development program. The rough sea states encountered during Leg 179 tests demonstrated the need for more robust bits that could withstand the torque, lateral pivoting (i.e., rocking) movements, and weight on bit fluctuations experienced during this first offshore trial. All three of these parameters contributed to the premature failure of the bits tested on that leg.
The next generation of bits developed for the HRRS testing program during Leg 191 were all tested onshore. Corrections and improvements to the bits were made based on the observations of these land tests. Despite the limited onshore testing, the next generation of bits appear much superior to those used during Leg 179.
There are five primary objectives for testing the HRRS during Leg 191. These goals include:
1. Characterization of the Model 260 fluid hammer operating parameters (i.e., flow rates, pump pressures, and weight on bits).
2. Characterization of the hammer-drill and bit-spudding capabilities without casing.
3. Testing of the entire HRRS system by drilling in 20+ m of 13-3/8-in casing in a fractured hard rock environment with little or no overlying sediment or talus and with little or no slope.
4. Testing of the entire HRRS system by drilling in 20+ m of 13-3/8-in casing in a sloped fractured hard-rock environment with little or no overlying sediment or talus.
5. Testing of the entire HRRS system by drilling in 20+ m of 13-3/8-in casing in a sloped fractured hard-rock environment with overlying sediment or talus.
Two new bit types have been developed for testing on Leg 191; these include an underreamer and ring-type bit. There are two different versions of the underreamer bits that will be tested as well as two versions of the ring-type bits. Underreamer bits have retractable arms to open a larger hole than the pilot bit they are mated onto. Ring bits are composed of two major parts that include a casing shoe and pilot bit. The casing shoe has a ring of tungsten carbide buttons that works in tandem with the pilot bit. However, unlike the underreamer bits, which are totally recovered at the completion of the installation process, the casing shoe is left in the hole on the bottom of the casing after the pilot bit and hammer are withdrawn.
The Leg 191 sites are located in the northwest Pacific Ocean east of Japan. The Mesozoic M-series magnetic lineations in the region (Fig. 4) show that the lithosphere in this area was formed in Late Jurassic to Early Cretaceous time (Larson and Chase, 1972; Sager et al., 1988; Nakanishi et al., 1989). Paleomagnetic studies indicate that this part of the Pacific plate formed ~30° south of its present position, near or slightly north of the equator (Larson and Lowrie, 1975; Larson et al., 1992). The magnetic bight created by the intersection of "Japanese" and "Hawaiian" lineations implies that the spreading ridges that formed the lithosphere met at a triple junction that defined the northwest corner of the growing Pacific plate (Larson and Chase, 1972; Sager et al., 1988). Shatsky Rise, an oceanic plateau with an area about the same as California, began to form in latest Jurassic time coincident with a major reorganization of the spreading ridges and triple junction (Sager et al., 1988; Nakanishi et al., 1989). Evidently the plateau formed rapidly at first, perhaps from a nascent mantle "plume head" (Sager and Han, 1993; Sager et al., 1999). The plume seems to have captured the triple junction and kept it at the plume location until the plume waned just before the Cretaceous Quiet Period (Nakanishi et al., 1999).
The history of the northwest Pacific plate since the formation of the lithosphere and Shatsky Rise seems to be one of northward drift and low sedimentation. Sediments atop Shatsky Rise are as thick as 1.2 km in thickness, because the rise top remained above the carbonate compensation depth and thick pelagic carbonate sediments accumulated (Sliter and Brown, 1993). Sediments in the adjacent abyssal basins are thin, typically 300-500 m thick (Ludwig and Houtz, 1979), owing to seafloor depth and distance from major sediment sources.
Scientific Objectives | Table of Contents