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Leg 203 addresses the second of three initiatives outlined in the ODP Long Range Plan—in situ monitoring of geological processes (see p. 49–51; JOIDES Planning Committee, 1996). It also represents an initial step in accomplishing the oceanic crustal component of the third initiative—exploring the deep structure of continental margins and oceanic crust (see p. 52–54; JOIDES Planning Committee, 1996). The drilling is intimately tied to the use of seafloor observatories (see p. 63; JOIDES Planning Committee, 1996) and represents the partnership of ODP with the DEOS multidisciplinary ocean observatory planning effort in the US and the UK and the partnership of ODP with the multinational IASPEI/ION Consortium.

Data obtained from oceanographic expeditions or from the deployment of conventional autonomous recording packages cannot by themselves provide the range or continuity of temporal sampling, or the consistency of spatial sampling, required to address modern observational requirements. There is a pressing need for long-term continuous observations of the present state of the Earth-ocean-atmosphere system as well as the response of the physical, chemical, and biological constituents of that system to natural and anthropogenic change. The DEOS ocean observatory planning initiative was launched to foster a long-term continuous observational presence at the air/sea interface, throughout the water column, at the seafloor, and below. The temporal and spatial scales of such observations must be appropriate to the process under study and range from seconds to decades and from centimeters to millions of meters.

In parallel with DEOS efforts in the US and the UK (planning for which has been supported by the National Science Foundation and Natural Environment Research Council, respectively), the international ION Consortium, representing participants from a number of member states of the European Union, Japan, and the US, has for much of the past decade been implementing plans for a global distribution of deep-Earth seafloor-based seismic observatories and has been planning for establishment of collocated seafloor-based magnetic observatories.

A primary objective of Leg 203 was to establish a deep cased legacy hole in a geographical region identified by a number of studies (e.g., Purdy, 1995) and agreed by ION as essential to the establishment of an unbiased global distribution of broadband digital seismic observatories. The target site was designated OSN-2 after the US-based OSN seismic observatory planning effort was subsequently subsumed into the DEOS planning framework. The site chosen also serves a variety of additional purposes outside of OSN, some of which are detailed below.

The Observatory

Drilling at the proposed OSN-2 site addresses both teleseismic, regional, seismic, and whole-Earth seismic studies. The site is located in a region on the Earth's surface ~2000 km from the nearest continental or island seismic observatory. For uniform coverage of seismic stations on Earth's surface, which is necessary for whole-Earth imaging using modern tomographic inverse methods, a seafloor seismic observatory is required in the eastern equatorial Pacific. This site is one of three high-priority prototype observatories for the OSN (Purdy, 1995).

Global seismic tomography provides three-dimensional images of the lateral heterogeneity in the mantle and is essential in addressing fundamental problems in subdisciplines of geodynamics, such as mantle convection, mineral physics, large-scale geoid anomalies, geochemistry of ridge systems, geomagnetism, and geodesy. Specific problems include the characteristic spectrum of lateral heterogeneity as a function of depth, the anisotropy of the inner core, the structure of the core/mantle boundary, the role of oceanic plates and plumes in deep mantle circulation, and the source rupture processes of Southern Hemisphere earthquakes, which are among the world's largest (Forsyth et al., 1995).

The culturally important earthquakes (those that pose a hazard to structures) in South America are only observed at regional distances on land stations in South and Central America and Global Seismic Network stations on the Galapagos Islands and Easter Island. This restricts the azimuthal information to an arc spanning ~180°. Seafloor stations are required to observe these earthquakes at regional distances to the west and to constrain the earthquake source mechanisms.

It is intended that the infrastructure to be installed at the observatory site will include the facility for real-time data telemetry and for in situ power generation. Because the equatorial observatory data will be available in real time, data will be incorporated into focal mechanism and centroid moment tensor determinations within minutes of Central and South American earthquake events. Other problems that can be addressed with regional data are the structure of the 400-, 525-, and 670-km discontinuities in the northeastern Pacific, the variability of elastic and anelastic structure in the Pacific lithosphere from Pn and Sn, and pure-path oceanic surface wave studies.

In 1998, in the pilot experiment at the OSN-1 site established by ODP (Site 843) in seafloor west of Hawaii, three broadband seismometers were deployed (one on the seafloor, one buried in the sediment, and one in the borehole) to compare the performance of different styles of installation. Figures F9 and F10 summarize for vertical and horizontal component data, respectively, the improvement that we expect to see in ambient seismic noise by placing a sensor in basement rather than on or in the sediment. Above 0.3 Hz, the seafloor, buried, and borehole spectra at the OSN-1 site show the borehole installation to be 10 dB quieter on vertical components and 30 dB quieter on horizontal components (Stephen et al., 1999; Collins et al., 2001). Shear wave resonances within the thin sediments are the physical mechanism responsible for the higher noise levels in or on the sediment.

The site of the future seismic observatory established during Leg 203 will also play a role in completing the global distribution of permanent seafloor magnetic observatories. Long-period data from magnetic observatories are essential for studies of the geodynamo convection of the outer core, rotation of the inner core, the structure of the mantle near the D discontinuity and the core-mantle boundary, studies of variations in the length of day, and investigations of the temperature, composition, and state of the upper and midmantle as revealed by the three-dimensional variations in electrical conductivity structure. The spatial sampling requirements for such observations are similar to those of global seismology, with a particularly severe bias introduced by the absence of seafloor stations.

The remote location of the site and the infrastructure to be installed in support of the intended geophysical observatories provides an opportunity for multidisciplinary observations of the air/sea interface, the water column, the seafloor, and below. The cased legacy hole established during Leg 203 may, thereby, serve as the first component of a future multidisciplinary marine laboratory.

Basement Drilling on the Pacific Plate

As noted in the Leg 200 Scientific Prospectus, there are no deep boreholes (>100 m) in the Pacific plate, the largest modern tectonic plate. Table T1 summarizes the boreholes drilled on "normal" crust on the Pacific plate that have >10 m of basement penetration and crustal ages <100 Ma. ODP/DSDP holes in seamounts, plateaus, aseismic ridges, and fracture zones are not included. Holes with crustal ages >100 Ma are not included because they would be affected by the mid-Cretaceous super plume (Pringle et al., 1993). In 30 yr of deep ocean drilling and >1000 ODP/DSDP holes worldwide, there have been only 17 holes with >10 m penetration into the normal igneous Pacific plate, only five holes during ODP and three holes with >100 m penetration. Furthermore, there are no boreholes off axis in "very fast" spreading crust. Thus, Leg 203 provides a reference station in normal 10- to 12-Ma ocean crust that will constrain geochemical and hydrothermal models of crustal evolution.

Although fast-spreading ridges represent only ~20% of the global ridge system, they produce more than one-half of the ocean crust on the surface of the planet, almost all of it along the East Pacific Rise. Most ocean crust currently being recycled back into the mantle at subduction zones was produced at a fast-spreading ridge. If we wish to understand the Wilson cycle in its most typical and geodynamically significant form, we must examine ocean crust produced at fast-spreading ridges. We have also known for longer than 40 yr that crust created by fast seafloor spreading is both simple and uniform, certainly so in terms of seismic structure (Raitt, 1963; Menard, 1964). Successful deep drilling of such crust during Leg 203 is likely to provide fundamental information that can be extrapolated to a significant fraction of the Earth's surface (Dick et al., 1996).

Drilling Strategy

Leg 203 was governed by two primary scientific objectives, in order of priority:

  1. To drill, case, and cement a legacy hole with sufficient penetration depth into basement (~100 m) for successful coupling in a low-noise environment of a long-term DEOS/OSN observatory broadband seismic package; and
  2. To sample and log the sediment–basement transition in young fast-spreading Pacific crust and the basement to a depth of 100 m or more.

The Leg 203 operational strategy stressed the importance of preserving the integrity of the cased legacy hole (Objective 1) while attempting to achieve the goals of Objective 2. The first objective was accomplished, although problems with inserting 16-in casing into the 18 1/2-in well bore forced us to complete Hole 1243A without coring or scientific logging. The second objective was largely satisfied by the decision to jet in to near the sediment/basement interface and then to core with the rotary core barrel (RCB) nearby Hole 1243B, although we did not achieve crustal penetration to the desired 100 m but managed to penetrate to 85 m in basement before drilling became too difficult to continue.

The close proximity of Site 1243 to Site 852 permits us to take advantage of information previously obtained from geophysical surveys and coring at Site 852. Evidence from the seismic reflection survey suggests that Holes 1243A and 1243B are sufficiently close in character to Site 852 such that data obtained from that site will still be broadly representative of the same formations and conditions. During Leg 138, four holes were cored at Site 852 with the advanced hydraulic piston corer (APC), three of which penetrated through roughly the entire sediment column, which was ~116 m thick. The redundant coring resulted in recovery of a complete sedimentary section.

With the exception of some refinements in paleomagnetic interpretation that newly cored sediments might make possible, the existence of a complete sedimentary section made it unnecessary to conduct further sediment coring during Leg 203. A shipboard proposal to jet in and use APC coring at a proposed hole (Hole 1243C) was approved by the scientific party. However, the decision by ODP to change the end-of-leg port to Victoria, British Columbia, Canada, rather than the originally scheduled port of San Francisco, California, necessitated cutting operations by 4 days. This exacerbated pressure on operating time that was related to difficulties in drilling Hole 1243B and curtailed plans to attempt Hole 1243C.

Logging Plan

In order to integrate properties across the areas sampled by coring (Hole 1243B), we planned a full suite of logs. However, because of the aforementioned problems with casing Hole 1243A, we were unable to run the usual suite of logs in that hole because it was fully cased. We did, however, run both an inclinometer log and a cement bond log.

In Hole 1243B we deployed, as planned, the following:

  1. The standard logging triple combination (triple combo) tool string, including tools for measurements of gamma ray activity, density, porosity, resistivity, and temperature;
  2. The Formation MicroScanner (FMS)-sonic tool string, including tools for measurement of elastic properties and high-resolution resistivity images of the borehole wall; and
  3. The Well Seismic Tool (WST) for check shot and vertical seismic profile (VSP) seismic survey.

The triple combo tool string is a combination of five tools, beginning with the Hostile Environment Gamma Ray Sonde (HNGS) on top. This tool measures the natural radioactivity of a formation, including the measurement for K, Th, and U contents. It is applicable for determining the formation's mineralogy and geochemistry, especially for the detection of ash layers and clay intervals, as well as for different lithostratigraphic units and their boundaries. The Accelerator Porosity Sonde (APS) measures the total rock porosity of a formation and is able to define differences in the crustal structure. In combination with the Hostile Environment Litho-Density Sonde (HLDS), which measures the formation's density, this tool yielded information about the drilled lithology, especially where core information is missing. This is particularly germane to Hole 1243B because the core recovery was almost entirely basement material with a 25% total recovery rate. The HLDS also measures the photoelectric effect, which gives additional information about the matrix composition. Either the Dual Induction Tool (DIT) or the Dual Laterolog (DLL) tool can be used to measure rock resistivity. The DIT provides an indirect measurement of the resistivity and the spontaneous rock potential as well as the conductivity of the formation at three invasion depths, whereas the DLL measures the direct resistivity at two invasion depths. The last tool of the triple combo tool string was the Lamont-Doherty Earth Observatory Temperature/Acceleration/Pressure (TAP) tool. Unfortunately, the TAP tool did not return any data.

The main components of the second tool string (FMS-sonic) are the FMS and the Dipole Sonic Imager (DSI). Applications are mainly identification of structural characteristics, estimation of fracture porosity, and the creation of a seismic impedance log. The FMS tool obtains a high-resolution microresistivity picture of the borehole wall, mainly leading to the identification of lithologic units and tectonic features (e.g., presence of fractures and faults, their orientations, and their degree of alteration). The FMS tool also incorporates a caliper log, which is used for hole size estimation and to infer the degree of mechanical competence of the hole walls.

The WST was also used in Hole 1243B. The WST provides a complete check shot survey, a depth-traveltime plot, and a rudimentary VSP survey. A set of seismic interval velocities were obtained that showed a velocity structure within the basement that was in broad agreement with the sonic logs.

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