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DRILLING STRATEGY

The complete Leg 205 sequence of operations and time estimate is shown in Table 1 and Table 2.

Site 1039R-A
The first site occupied during Leg 205 will be Site 1039R-A. This site has been located 1.3 km closer to the trench axis than Site 1039 drilled during Leg 170 to minimize the amount of gabbro sill that will need to be penetrated to reach oceanic crust. One hole is planned, which will be partially cored and into which we will install the long-term hydrologic borehole observatory. Our first step will be to wash a reentry cone and 16-1/2-in casing into the seafloor. We will then reenter this hole with the RCB and drill without coring to ~370 mbsf. RCB coring will commence from there through the sill, basal sediments, and ~100 m into basement. This basement coring is intended to obtain the samples required to characterize basement but also to open up the circulating system within the upper basement for the borehole observatory. While drilling/coring, a number of Davis Villinger temperature probe (DVTP) temperature measurements will be made, as formation conditions permit, to complement existing data. If hole conditions permit, we will log the entire hole with the triple combination tool and the Formation MicroScanner (FMS)/sonic velocity tools. The next step will be to reenter the hole with a larger bit and open the hole up to 14-3/4-in in preparation to install the 10-3/4-in casing. The 10-3/4-in casing will then be emplaced into the hole (with a mud motor and underreamer if necessary) to just above the zone that the observertory will monitor. The casing will be cemented in place to inhibit communciation between the casing and the formation. Next, we will drill out the cement shoe and clean the rat hole with the RCB bit. Then, we will assemble a 4-1/2-in casing screen, casing packer, and casing to the instrument hanger (Fig. 8). Then the entire assembly will be lowered into the hole and latched in to seal the borehole outside of the 4 1/2-in casing. The casing packer will then be inflated to seal off the zone at the bottom of the hole that is targeted for monitoring. Finally, the osmo-sampler with integral temperature sensors will be lowered through the center of, and latched into the bottom of, the 4-1/2-in casing; this will completely seal the zone to be monitored. The final installed configuration for this modified CORK geochemical and hydrologic borehole observatory is shown in Figure 8A. Two absolute pressure gauges including data loggers will be installed within the instrument hanger head: one sensor will sense pressure within the sealed-off fluid sampling zone at the bottom of the hole, the other one will record pressure variations present within the borehole above the sealed-off section. One additional

sampling line, running from the instrument hanger head all the way down to the sealed-off zone, will be installed for future pressure/fluid sampling purposes.

Sites 1040R-A, 1040R-B, and 1040R-C
Each of these sites will be completed with a modified CORK identical to that described for Site 1039R-A above. Each installation will be designed to monitor a single zone. The first two, 1040R-B and 1040R-C, will both monitor the décollement (Figure 8B). The ~500 m separation between these two sites minimizes the chances that seal failure or other difficulties at one site will affect the other installation. They are close enough, however, that any seismically-induced pressure transients may be expected to propagate from the down-dip to the up dip location. At each of these sites, we will install a reentry cone with 16-1/2-in casing and then drill to total depth. Although the uppermost section will be drilled without coring, the rest of the section will continuously cored with the RCB (300-365 mbsf at Site 1040R-B; 220-279 mbsf at Site 1040R-C) to determine the exact location of the décollement as well as the best location to set the 10-3/4-in casing shoe and the 4-1/2-in casing packer. While drilling/coring at Site 1040R-B only, a number of Davis-Villinger temperature probe (DVTP) temperature measurements will be made, as formation conditions permit, to complement existing data. Following the coring, the next step will be to reenter the hole with a larger bit and open the hole up to 14-3/4-in in preparation to install the 10-3/4-in casing. Finally, the 10-3/4-in casing will be cemented in place (with the casing shoe just above the décollement) and the osmo sampler will be emplaced with the packer seated below the 10-3/4-in casing shoe in the uppermost décollement sequence.

Site 1040R-A, the last site planned for Leg 205, will monitor the uppermost underthrust sediments (Figure 8C). After installing the reentry cone and 16-1/2-in casing, the hole will be opened to 14 3/4-in and the 10-3/4-in casing installed with the casing shoe below the decollement. The osmo sampler will be emplaced with the packer seated below the 10-3/4-in casing shoe in the uppermost underthrust sequence. Currently there is no time available to core any of the section penetrated at Site 1040R-A. If time permits, limited coring would include the decollement and the upper underthrust sequence (~320-410 mbsf).

No logging is planned for any of these sites (Sites 1040R-B, 1040R-C, or 1040R-A) based on the experience with LWD and drilling at Site 1040 during Leg 170.

Alternate Sites
Alternate sites for each of the primary sites are presented in Table 2 and in the "Site Summaries" section.

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