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Drilling Strategy and Operations

After arriving on site, we planned to drill and core two pilot holes to determine the geology of the formation and to establish the casing requirements for the reentry hole that was to house the borehole seismic observatory. The first was to be APC cored to refusal (estimated at 200 mbsf) with the Tensor core orientation tool then XCB cored to basement, estimated at ~370 mbsf. After a jet-in test, a second pilot hole was to be drilled to basement and then cored 100 m into basement using the RCB to determine the nature of the basement. This hole would then be logged to identify a suitable interval for setting the seismometer package.

Once the pilot holes had been completed, we planned to offset and jet in a reentry cone and ~60 m of 16-in casing. The hole would then be reentered and deepened to ~425 mbsf with a 14.75-in tricone bit to lower a 410-m string of 10.75-in casing ~40 m into basement and cement it in place. The hole would then be reentered again and deepened to ~100 m in basement for the seismometers. After the seismometer package had been made up, lowered into the hole, and cemented in place, a battery package would be lowered into the throat of the reentry cone and acoustically released. The pipe would then be tripped to the surface, bringing the deployment to completion so that it could be activated by an ROV at a later time. As at Site 1200, however, the actual operations at Site 1201 departed significantly from those that had been planned.

The JOIDES Resolution arrived on site at 1600 hr on 31 March 2001, following a 2-day transit from Guam. After the pipe was lowered to the seafloor, Hole 1201A was spudded with the APC/XCB to study the sediment section but the hole was abandoned after one core because of a premature APC shear pin failure. APC coring was then initiated in Hole 1201B at 1905 hr on 1 April and continued to a depth of 46.7 mbsf, after which the hole was deepened with the XCB to 90.3 mbsf (Table T2). The vessel was then offset 15 m to the west and a third APC hole, 1201C, was spudded and cored to refusal at 46.7 mbsf to provide a repeat section through the soft sediments. The Tensor core orientation tool was used on the third, fourth, and fifth cores in both Holes 1201B and 1201C, and a temperature measurement was taken with the Adara shoe at a depth of 44.6 mbsf in Hole 1201C.

A problem with the spooling of the coaxial cable used for the undersea television camera became apparent during the deep-water operations at the site. Since the undersea camera was required for reentry and the deployment of the seismic observatory, we decided to search for a deep-water pocket where we could fully extend and retension the cable in an attempt to correct the problem. After steaming 204 nmi to the northwest to a small basin indicated on Japanese hydrographic charts, we deployed the coaxial cable to a depth of 6183 m and fixed the spooling problems.

At 0718 hr on 5 April, the vessel was back at Site 1201 and the main pilot hole, Hole 1201D, was spudded 120 m south of Hole 1201C. The hole was drilled with a center bit to a depth of 80.4 mbsf, where RCB coring was initiated. Coring proceeded without incident to basement at 510 mbsf, which was considerably deeper than initially predicted, and then continued another 90 m into the basement to a total depth of 600 mbsf. After releasing the bit, Hole 1201D was logged with the triple combination (triple combo) tool from 80 mbsf to total depth. A second logging run with the FMS-sonic tool could not pass an obstruction at 366 mbsf because of deteriorating hole conditions. After completing the run in what was left of the open hole, the pipe was lowered again in an attempt to reopen the hole for logging but an impassable bridge was reached at a depth of 90 mbsf. At that point, a 50-m plug of cement was set to prevent future fluid communication with the cased reentry hole and seismometer.

The hole for the seismometer was initiated at 1600 hr on April 14 when Hole 1201E was spudded with a reentry cone and 16-in casing and jetted in to a depth of 39.1 m. The hole was then drilled to 543.0 mbsf and cased with 10.75-in casing to a depth of 527.0 mbsf, or 15 m into basement. After cementing the casing in basement, the 9.875-in ION installation hole was drilled to a total depth of 580 mbsf. By 1230 hr on 23 April, the seismometer instrument string was assembled and final electrical integrity checks were completed (Fig. F17). Hole 1201E was then reentered and the instrument package was lowered into the hole without incident. The seismometer package was cemented in place with the end of the stinger located at a depth of 568.4 mbsf (Fig. F18). The top of the uppermost seismometer was placed ~558.4 mbsf, or ~46.4 m below the basement contact. At 0830 hr on 24 April, the battery platform was lowered through the moonpool and landed in the reentry cone at 1400 hr (Fig. F19). A handheld acoustic command unit was used to release the platform. Proper platform installation was confirmed with the subsea television camera, and by 1100 hr on 25 April, the ship was secured for transit and under way to Site 1202 (alternate Site KS-1).

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