The 497-nmi voyage to proposed Site PAT-8C was accomplished in 47.0 hr at an average speed of 10.6 kt. At 0030 hr on 14 November, the vessel slowed to 5 kt, and a short 3.5-kHz survey was made from north to south across the site. Upon conclusion of the 45 min survey, the vessel came about and returned to site as the thrusters were lowered and the DPS was activated. The GPS receiver interface to the dynamic-positioning system was employed, and the vessel was over the coordinates of the new location by 0115 hr. A beacon was deployed at 0241 hr. The corrected PDR depth was 4848.0 mbrf.Hole 1218A
After the drill string was deployed to a depth of 4811 mbrf, the driller then lowered the bit until the heave compensator appeared to activate, suggesting contact with a firm sea bottom at a bit depth of ~4840 mbrf. Hole 1218A was spudded with the APC at 1000 hr on 14 November. The seafloor depth indicated by the recovery of the initial core was 4837.3 mbrf, 10.7 m shallower than the PDR depth.
Hole 1218A was deepened to 188.7 mbsf (base of Core 20H) before it became necessary to switch to XCB operations to accomplish the deeper objectives of the hole. Cores 3H through 20H were oriented. All 20 piston cores fully stroked, but the core barrels for Cores 19H and 20H could not be pulled free with 80,000 pounds of overpull, and thus both were drilled over to release them from the sediment. The average recovery for the APC section was 102.8% (Table T1).
Ten XCB cores were taken over the interval from 188.7 to 276.8 mbsf (Cores 21X through 30X). A small sample of basalt was obtained in the last core, thus confirming basement contact. XCB coring deepened the hole 88.1 m and recovered 72.4 m (82.2% recovery). The total recovery of Hole 1218A was 266.4 m, representing 96.3% of the cored interval (Table T1).Downhole Logging Operations (Hole 1218A)
To prepare Hole 1218A for logging, the drill string was pulled up to 78 mbsf and then lowered back to 277 mbsf. The hole was then displaced with 110 bbl of 8.9-lb/gal sepiolite mud, and the bit was placed at the logging depth of 85 mbsf. Logging rig-up operations began at 1130 hr on 16 November and, after some minor technical problems, were completed by 1645 hr. The triple combination (triple combo) tool was then run to the bottom of the hole (276.8 mbsf), and logging began at 2210 hr on 16 November.
Two passes were made with the triple combo tool in Schlumberger mode and a third pass in Lamont mode for multisensor gamma ray tool (MGT) data acquisition. Two passes were also made with the Formation MicroScanner-Sonic Velocity (FMS-Sonic) toolstring tool string. At the end of the second FMS-Sonic pass, the drill pipe was gradually withdrawn to a depth of 65 mbsf thus providing an extra 20 m of logged formation. In total, five logging runs were made with no bridges encountered, and the bottom of the hole was reached on all attempts. After logging operations, the bit was pulled free of the seafloor at 1917 hr on 17 November and the vessel offset 20 m north of Hole 1218A.Hole 1218B
In order to obtain a good stratigraphic overlap with cores from the first hole, the spud-in position for Hole 1218B was 3 m shallower than for Hole 1218A. Hole 1218B was spudded with the APC at 2050 hr on 17 November. Eighteen piston cores were obtained over the interval from 0 to 165.40 mbsf with orientation beginning on Core 3H. The average APC recovery was 102.1%. Heat flow measurements were obtained at 32.4 mbsf (Core 4H), 60.9 mbsf (Core 7H), 89.4 mbsf (Core 10H), and 108.4 mbsf (Core 12H). Following APC operations, 11 XCB cores were collected over the interval from 165.40 to 263.50 mbsf with an average recovery of 92.2%. Core 25X was drilled only 5 m instead of the usual 9.6 m in order to maintain a good stratigraphic overlap with cores from Hole 1218A. The total recovery of the hole was 259.25 m representing 98.4% of the cored interval.Hole 1218C
Once the bit cleared the seafloor at 1654 hr on 19 November, the vessel was moved another 20 m north, and Hole 1218C was spudded at 1735 hr. The coring objective for the third hole was to fill the gaps in the compiled stratigraphic record of the first two holes. The hole was washed ahead to 55 mbsf and then piston cored to 163.5 mbsf. The average recovery from the 11 piston cores was 103.4%. All the cores were oriented. It was necessary to drill from 140.5 to 144.5 mbsf to maintain the required overlap with cores from Holes 1218A and 1218B. Heat flow measurements were made at 83.5 mbsf (Core 3H), 102.5 mbsf (Core 5H), and 121.5 mbsf (Core 7H).
After APC coring operations ceased, the hole was deepened with 10 XCB cores to a total depth of 256.8 mbsf (Table T1). The interval from 220.0 to 223.0 was drilled to maintain stratigraphic overlap with the cores from Holes 1218A and 1218B. XCB operations resulted in 90.3 m cored with 99.7% recovery. The total for the hole was 194.8 m cored and 198.05 m recovered (101.7%).
After drilling operations ceased, drilling equipment was secured, and the beacon was successfully recovered. The transit to the next site began at 1300 hr on 21 November.
Next Section | Table of Contents