Ocean Drilling Program: Leg 200 Scientific Prospectus

DRILLING STRATEGY

The specific location of the H2O junction box is 27°52.916'N, 141°59.504'W. Although it is not within any of the large 20 km by 20 km blocks that we surveyed in 1997, we do have two parallel single-channel lines that run within 1.5 km of the site. The structure is sufficiently smooth that it should be acceptable to assume that it is continuous between the lines. The whole region within 2 km of the H2O junction box is in a "contiguous block" of crust. The SCS data is laterally homogeneous over this block, and the quality of the migrated and unmigrated data is similar. At this stage, we have no new information on sediment thickness. It is no thicker than 100 m, but it is interpreted to be at least 50 m. We will not know for sure until we drill. There may be as little as 8 m of sediment above the first chert layer.

The main objective of the proposal is to drill a borehole that can be used for an OSN permanent seismic observatory. The critical issue is whether there will be enough sediment at the site to set the reentry cone. Four sites have been identified as indicated on track charts and seismic profiles. The locations are shown in Figure 6 and Figure 7 and given in Table 2.

There are three important characteristics for a broadband borehole seismic installation:

(2) The sides and bottom of the hole must be sealed to hydrothermal circulation with the formation. Water circulation around the sonde generates undesirable "installation noise." If the bottom of the hole is composed of unconsolidated and fractured basalt, this means that the hole must be cased to total depth and sealed at the bottom with a cement plug.

(3) To ensure good coupling of the sonde to the Earth, the hole should be in as consolidated a section of basalt as possible and the casing should be cemented to the formation. It is impossible to predict the seismic response of rubble, and it is impossible to couple the sonde and/or casing to rubble. When cementing in poorly consolidated rock, the cement will flow mostly into the formation rather than up the annulus around the casing, so proper coupling of the casing to the formation cannot be obtained.

Our strategy will be to probe the sediment at each site in an attempt to locate the site with the deepest sediment above the chert (Table 2). This will be accomplished by conducting a jet-in test at each site using the jets on the rotary core barrel (RCB) bit to wash down through the upper part of the sedimentary section. We will also attempt to wash through and possibly sample the chert layer to determine how solid it is. In some areas, chert layers are rubble zones that can be washed through when installing the reentry cone. The deepest sediment may be in a fault zone where the basement is highly fractured (e.g., Site H2O-3 in Fig. 9 or Site H2O-1 in Fig. 8), and it may be difficult to drill. The best strategy, if we have the option, may be to select a site high on the block (e.g., Site H2O-4 in Fig. 8) or a midblock site (such as Site H2O-2 in Fig. 9).

Estimating that there may be somewhere between 8 to 25 m of soft sediment above the chert layer, we plan on setting a reentry cone with ~25 m of 20-in casing. We will then RCB core from 25 to ~325 mbsf (meters below seafloor), which will include the deeper sedimentary horizons and ~250 m of basement penetration. The core will be available for a broad spectrum of shipboard and shore-based analyses, and it will give us positive confirmation of the degree of rubble and fracturing in the upper basalts. Though we will likely encounter progressively more consolidated material down to the proposed 250 m of basement penetration, we expect that the bottom of the hole will still be fractured. If the bottom of the hole consists of unconsolidated material, we will plan to core deeper until we reach a more consolidated material or until the allotted time for coring is exhausted (see Table 2 and Table 3 for time estimates). We will then run the triple combination (triple combo) tool and Formation MicroScanner/dipole sonic shear imager (FMS/sonic) in the 9-7/8-in hole to get a continuous record of the well up to 25 mbsf. The hole will then be reamed to ~85 mbsf (~10 m into basement) and 16-in casing will be installed and cemented to this depth. The hole will then be reamed to the total depth (TD) for installation and cementing of 10-3/4-in casing. A borehole compensated sonic log (also called a cement bond log) will be run to check the integrity of the cement behind the casing.

Our original target depth was 400 m of basement penetration, which was a conservative estimate to get into consolidated basalts based on the drilling experience at Hole 504B. The allotted time for Leg 200 will unlikely be sufficient for us to reach this depth. At Hole 504B, sonic logs and resistivity measurements indicate poorly consolidated basalt down to 600 mbsf. If we are fortunate enough to set a reentry cone and casing and reach the current target depth of 250 m of basement penetration ahead of schedule, we will plan to continue drilling until we are in consolidated basalt. A hole that penetrates further into basement will acquire good-quality basalt samples for geochemical studies, will provide adequate penetration into Layer 2 for paleomagnetic analyses, and will provide good hole conditions for in situ experiments.

If the reentry hole is completed before the end of the allotted time on site, there are a number of options, depending on how much time is left (Table 3). If we have less than 24 hr, we suggest repeating the vertical seismic profile (VSP) in the cased hole. If we have a few days left, we plan to core one or more holes at one or more of the H2O sites with the advanced piston corer (APC) and extended core barrel (XCB). The additional holes around the observatory will characterize the lateral heterogeneity of the sediment, confirm the depths to basement, and provide additional observations that can be tied to sites being cored during Leg 199, which is a Paleogene equatorial coring transect aimed at studying the evolution of the equatorial Pacific current and wind system. The information gained would also be useful in interpreting seafloor heat-flow measurements that may be made at the site in the future and provide useful background information for further drilling at the site in the future. If we have roughly a week left, we plan to core other alternate sites proposed for Leg 199 that may not be completed during that leg (e.g., Site PAT-13C, see the Leg 199 Scientific Prospectus at http://www-odp.tamu.edu/publications/prosp/199_prs/199toc.html).

Possible Coring the Nuuanu Landslide
If we complete port call activities in Honolulu at least 24 hrs early, we will double APC/XCB core at Site NU-1A. An alternate operation scenario for coring at this site is given in Table 4 and discussed below in the addendum of this prospectus.

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