Leg 198 officially began at 1100 hr on 27 August 2001 with passage of the first line ashore in Yokohama, Japan, ending Leg 197 ~19 hr early. At 0930 hr on 31 August, after 4 days of port call activities, the last line was released and the ship maneuvered into the harbor.
The transit to the first site was initially slowed by the presence of Typhoon Wutip, which was forecast to cross over Site 1207 about the same time as our full-speed arrival projection. To maintain a safe distance from the storm, we kept the correct course but at reduced speed for the first several days of the transit. As the typhoon increased speed and began moving to the northeast of the site, we increased our speed accordingly. Overall, the 1158-nmi, 5.2-day transit was accomplished at an average speed of 9.2 knots (kt). Upon arriving at the coordinates for the site, the thrusters and hydrophones were lowered at 1730 hr on 5 September, followed by deployment of a positioning beacon.
Once the vessel settled on location, an APC/XCB/motor-driven core barrel (MDCB) outer barrel assembly was assembled and the MDCB system was deck tested prior to deployment of the APC. After two unsuccessful attempts at obtaining a mudline core, a 4.82-m core was retrieved at 1225 hr on 6 September, initiating Hole 1207A. With the bit positioned at a depth of 3107 meters below rig floor (mbrf), the mudline core indicated a seafloor depth of 3111.7 mbrf, or 3000.7 meters below sea level (mbsl).
Piston coring advanced to 181.8 mbsf, with recovery averaging 103% (Table T2). Cores 3H through 18H were oriented. The advance-by-recovery technique was used for Cores 19H and 20H. Piston coring was terminated when these two successive cores failed to fully stroke, impeded by chert layers. We then switched to the XCB system.
The first XCB core (Core 21X) was advanced 6.7 m, with good recovery (101%) until encountering a chert layer, which destroyed a soft formation XCB cutting shoe. A second XCB core barrel was deployed, equipped with a hard formation cutting shoe; however, only minimal advancement (0.1 m) was achieved after 1 hr of rotating time, no recovery, and the destruction of the hard formation shoe. An XCB center bit was then deployed to drill through the chert, advancing to a depth of 197.9 mbsf. Three successive cores were then cut (Cores 23X through 25X); the first two recovered 100% and 99%, respectively. Core 25X advanced only 2.0 m in 75 min rotating time with no recovery. The XCB center bit was deployed for a second time to drill through the chert. Another XCB core was attempted; however, Core 26X advanced only 2.0 m, recovering only a few chert fragments (5% recovery). The center bit was deployed for a third time to drill through a chert layer to a depth of 237.3 mbsf. Core 27X advanced 6.4 m recovering a few pieces of chert (1% recovery). The center bit was deployed for a fourth time to clean up the hole and hopefully clear any remnant carbide cutters and debris from the previously used XCB cutting shoe. Prior to recovering the center bit, a 30-barrel (bbl) mud sweep was pumped to aid in flushing any debris from the hole.
We then switched to the MDCB system in an attempt to increase recovery. Upon retrieval of MDCB Core 28N (5% recovery), drilling personnel observed that the core barrel had been heavily scoured. In addition to the scouring on the core barrel, the crown of the diamond bit was severely damaged. These observations suggested that either the MDCB bit had been in contact with a primary bit cone or that there was debris in the hole. Because of the doubt about the condition of the primary bit and hole, additional MDCB coring was suspended. A final XCB core was attempted with only 0.2 m advanced in 35 min. With no recovery in this core and concerns about bit and hole conditions, we decided to terminate coring in Hole 1207A at 256.6 mbsf in favor of RCB coring in Hole 1207B. Overall, the XCB was deployed for eight cores, with an average recovery of 71.1%. The drill string was retrieved, clearing the rotary table at 0015 hr on 9 September, ending Hole 1207A.
After retrieving the drill string, an RCB bottom-hole assembly (BHA) with a center bit assembly was deployed, and Hole 1207B was spudded at 1115 hr on 9 September. Approximately 4.25 hr was spent drilling the hole down to a depth of 157.0 mbsf and recovering the RCB center bit assembly. An RCB core barrel was deployed, and RCB coring advanced to a depth of 622.8 mbsf, taking 49 cores. As anticipated, recovery was poor through an interval of closely spaced chert layers interbedded with ooze. Coring parameters were varied continuously in an attempt to boost recovery. In addition, several core catcher configurations were also used, including flapper core catchers. Beginning with Core 40R, a combination of flapper core catcher and no core liner was used to minimize or eliminate jamming. Although somewhat unorthodox for coring in sedimentary strata, recovery did improve over the last 96.3 m. Improved recovery also coincided with increased lithification of the interbedded carbonate layers, so it is difficult to attribute improved recovery solely to coring without liners. Overall, recovery in Hole 1207B averaged 12.9%.
Coring was terminated when the depth objective was attained, and the hole was swept with 40 bbl of drilling mud, followed by a wiper trip to 159.9 mbsf. No overpull or drag was noted. During the return trip to bottom, 40 m of soft fill was noted and circulated out of the hole. Another 40-bbl mud sweep was conducted, and the bit was released. The hole was then displaced with 217 bbl of logging mud, and the pipe was tripped back and positioned at 126.4 mbsf in preparation for logging.
The first logging run was made with the triple combination (triple combo) tool suite (see "Downhole Measurements," in the "Explanatory Notes" chapter for details on the logging tools and see "Downhole Measurements" for further details on the logging operations), which was deployed to a depth of ~621 mbsf, or within 1.5 m of total depth. A second pass with this tool suite failed to pass ~379 mbsf.
The second logging run included two passes with the FMS-sonic tool string. Both passes with this tool suite were also halted at a depth of ~379 mbsf. The third and final logging run was made with the geologic high-resolution magnetic tool (GHMT) tool suite and reached a depth of ~367 mbsf.
After completion of the logging operations, the drill string was retrieved, clearing the seafloor at 1715 hr on 13 September and ending Site 1207. Upon recovery of the beacon and retraction of the thrusters and hydrophones, the ship began the transit to Site 1208.