DRILLING STRATEGYThe complete Leg 205 sequence of operations and time estimate is shown in Table 1 and Table 2.
The first site occupied during Leg 205 will be Site 1039R. 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 ~340 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 hydrologic borehole observatory is shown in Figure 8. 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.
We are reviewing the existing drilling and seismic data to determine if we will request permission to relocate Site 1039R closer (0.5 km) to the trench axis. This will have the advantage of minimizing the amount of gabbro sill that will need to be penetrated to reach oceanic crust. However, we will be farther from the known section cored at Leg 170's Site 1039; this may require having to core the entire section if Pollution Prevention Safety Panel (PPSP) permission to drill without coring is not obtained.
At Site 1040R, we will drill three holes. Each of these holes will be completed with a modified CORK identical to that described for Site 1039R above. Each installation will be designed to monitor a single zone. The first hole will monitor the uppermost underthrust sediments and the second two holes will both monitor the décollement. After installing the reentry cone and 16-1/2-in casing, the first hole at Site 1040R will be washed from 0 to ~180 mbsf and then cored from ~180 to 410 mbsf, penetrating the upper fluid conduit, décollement, and terminating in the upper underthrust sediments. This will help us identify the best place to seat the packer and 10-3/4-in casing shoe and locate the décollement accurately for the next two holes planned at this site. It will also save time as no further cores will need to be taken at this site. During the coring, we anticipate conducting about five temperature measurements to complement existing data. After the 10-3/4-in casing is cemented in place (with the casing shoe below the décollement), the osmo-sampler will be emplaced with the packer seated below the 10-3/4-in casing shoe in the uppermost underthrust sequence.
The second hole at Site 1040R will be identical, with the exception that the casing shoe and packer will be seated just above the décollement and the osmo-sampler emplaced into the décollement to monitor it. Although no cores need to be taken in this and the third hole, we must use the RCB bit to drill out the cement shoe in the bottom of the 10-3/4-in casing so we can core specific horizons in the latter two holes at Site 1040R if required. The third hole will also target the décollement but will be offset by ~100 m.
No logging is planned for any of these holes based on the experience with LWD and drilling at Site
1040 during Leg 190.
Alternate Site 1043R
Site 1043R is an alternate site, at which installing long-term monitoring of fluids in the décollement could be attempted if drilling at Site 1040R fails for any reason. The strategy there would be basically the same as for Site 1040R, with one hole aiming to monitor the underthrust sequence and at least one hole sampling fluid flow within the décollement.
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