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doi:10.2204/iodp.proc.301.107.2005

Hole completion

Hole 1026B was drilled and cored during Leg 168 to a maximum depth of 295.2 mbsf, but hole conditions in basement were poor (Shipboard Scientific Party, 1997). The exact depth of top of basement is not known, but hard formation was encountered during drilling at 247.1 mbsf, suggesting that the hole penetrated 48.1 m into basement. The 10¾ inch casing shoe was placed at 248.5 mbsf. At the end of coring operations, the top of fill was tagged at 269.9 mbsf and a liner was subsequently fabricated from 5 inch drill pipe, extending access to the upper 31.8 m of basement (Shipboard Scientific Party, 1997). There was no open hole remaining in Hole 1026B; all formation access was past the liner and 10¾ inch casing that stabilized the basement interval.

The CORK set in Hole 1026B during Leg 168 included a thermistor string, an OsmoSampler, pressure gauges, and a data logger. These instruments were recovered in 1999, except for the OsmoSampler and a sinker bar, which were stuck in the hole either inside the liner or between the liner and the casing. When we arrived on site during Expedition 301, there was no instrumentation installed in the CORK but all mechanical and hydraulic systems remained in place. We needed to recover the CORK before we could install a replacement observatory. Once the old CORK was recovered and the cone was cleared of a BioColumn instrument (as described in "Operations"), we prepared the new observatory for deployment.

The new Hole 1026B CORK system was the simplest of the systems deployed during Expedition 301 (see "Operations" and Fisher et al., this volume). It comprised a CORK-II body with 4½ inch casing extending to 201.5 mbsf and a single packer element set in casing near the bottom of the 4½ inch casing (Fig. F8). No 4½ inch casing was installed below the packer element sub because the hole was completely cased to a depth below the liner and there was no need to protect the instruments hanging below the bottom plug (Fig. F8).

We used surplus umbilical from Leg 205, comprising a single ½ inch packer inflation line and three ¼ inch pressure-monitoring and fluid-sampling lines. The three ¼ inch lines were run through the single packer and ended in small wire-wrapped screens that were attached just below the inflation element. All of the CORK systems deployed during Expedition 301 included nine pass-throughs within the packers and across the upper 10¾ inch casing seal so that we could use a single design for these systems and achieve sampling and monitoring goals within multilevel CORKs. Most of the extra pass-throughs in the Hole 1026B CORK were capped, but one line through the 10¾ inch casing seal was plumbed into a two-way valve in the CORK head. This valve was topped with an Aeroquip connector so that during a future submersible or ROV dive expedition it will be possible to check pressure below the casing seal but above the packer element. If the pressure monitored below the packer is different from that above the packer, this will give a positive indication that the CORK system is properly sealed. As with all other valves in the CORK head, this one was left open during deployment to prevent air from being trapped in the sampling and monitoring lines.

After we deployed the CORK system from the ship and reentered Hole 1026B, the CORK body was held a few meters above the cone so that we could deploy the instrument string. The Hole 1026B instrument string was also relatively simple, including three OsmoSampler packages (Fig. F8). The uppermost OsmoSampler contains copper coils for gas sampling, the middle one has PTFE tubing for fluid sampling and tracer injection, and the lower one contains microbiological incubation substrate and an acid-addition OsmoSampler for metals analyses. There is a single, self-contained temperature logger in each of the lower two OsmoSamplers. We decided to not deploy additional temperature loggers in Hole 1026B because earlier studies had already determined the upper basement temperature at this site and because this sampler and measurement string were too short to extend into basement. We elected to keep this string short to make sure that the samplers and data loggers can be recovered during a future drilling or dive expedition and will not be caught in or alongside the liner that fills much of the lower part of the hole.

Instrument string deployment went smoothly, and the CORK was set in the cone and the packer was inflated in casing. The submersible/ROV platform was assembled in the moonpool and lowered onto the CORK by wireline. We went back down with the camera to inspect the landing platform prior to releasing the CORK running tool and found that the platform was cocked and held above the top of the valve and sampling bays. We released from the CORK head and used the running tool to "nudge" the high side of the landing platform. It uncocked and slid down the CORK head, landing on the rim of the cone, ending seafloor operations in Hole 1026B.

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