DRILLING STRATEGY
The proposed drilling program at Site MAF-4B consists of a pilot hole (A) to characterize the composition of the fluids and metamorphosed rock materials of the seamount followed by a second hole (B), which will be equipped with a reentry cone/20-in casing/conductor pipe, 16-in casing string, 10-3/4-in casing string, and the CORK instrumentation (thermistor string and an osmotic sampler). The time estimated to accomplish these objectives is 20.9 days, with coring and logging in Hole A consuming 10.8 days and the establishment of the long-term observatory in Hole B taking 10.1 days.Coring will be conducted with the extended core barrel (XCB) and/or the motor driven core barrel (MDCB) in the first hole in an attempt to optimize core recovery and quality. The hole will be equipped with a free-fall funnel (FFF) to enable XCB bit exchange or reentry with the advanced diamond core barrel (ADCB). Plans call for Hole A to be cored to 400 mbsf, logged with a standard suite of wireline tools (see the "Logging Plan" section), and plugged with cement to prevent communication with the corked reentry hole.
A reentry cone will then be assembled and lowered to the seafloor with ~10 m of 20-in casing (actual length will be determined by a jet-in test after the completion of Hole A). It will be washed to depth and the reentry cone released on the seafloor. The drill string will then be recovered and a drilling bottom-hole assembly (BHA) made up with an 18-1/2-in tricone bit and a 22-in underreamer. Hole B will then be reentered and the hole deepened to ~220 mbsf. The drill string will again be recovered before making up ~200 m of 16-in casing. The 16-in casing string will be deployed and cemented in place before recovering the drill string. Hole B will then be reentered with a 14-3/4-in tricone bit and drilled to ~420 mbsf before installing ~375 m of standard 10-3/4 in casing with ~25 m of screened 10-3/4-in casing on the bottom. After the hole is drilled to depth, the casing will be installed and the drill string recovered. The instrument package will then be made up and attached to the deployment casing (CORK body).
The proposed Site WP-1B drilling program consists of coring two pilot holes (A and B) to characterize the site prior to a third hole (C), in which a reentry cone and casing string will be set and the ION instrument string installed. Time on site estimates to accomplish these objectives are ~26 days. Coring/drilling Holes A and B will consume 10.8 days including wireline logging. Installation and setting up the reentry cone and casing for Hole C is estimated to take 7.5 days, with the remaining time on site devoted to the installation of the instrument and battery package (Fig. 9A, Fig. 9B, Fig. 10). Should time permit, we are planning to core a fourth hole (D) with the APC/XCB as deep as possible.
Oriented piston coring with the APC to refusal (~200 mbsf) will be used in the first hole to ensure the most complete recovery and characterization of the upper sedimentary section. Below that, single XCB coring will be used to core the remaining sedimentary rock above basaltic basement (~370 mbsf) and an additional 10 m into basement, if possible. Hole B will be washed/drilled down with the rotary core barrel (RCB) to ~360 mbsf. Coring will commence and continue ~100 mbsf into the basaltic basement. A full suite of wireline logging will then be initiated (see the "Logging Plan" section).
A reentry cone will then be assembled and lowered to the seafloor with ~60 m of 16-in casing (actual length will be determined by a jet-in test after the completion of Hole A). It will be washed to depth and the reentry cone released on the seafloor. The drill string will then be recovered and a drilling BHA made up with a 14-3/4-in tricone bit. Hole C will then be reentered and the hole deepened to ~425 mbsf. The drill string will again be recovered before making up ~410 m of 10 3/4-in casing. The 10-3/4-in casing string will be cemented in place ~40 m into basement. The drill string will then be recovered and the final BHA to drill a 9-7/8-in hole will be assembled. Because the instrument string should be located in a relatively homogeneous and unfractured zone, the hole is planned to penetrate ~100 m into basalt basement. Actual penetration will be decided upon in the field. This decision will be based on information provided by the cores and the wireline logs from Hole B, drilling data provided by the drillers, and the amount of allowable time remaining in the program to complete the leg objectives.
After the hole is drilled to depth and the drill string recovered, the instrument package will be made up and attached to the 4-1/2-in deployment casing. The instrument package for Leg 195 consists of two seismometers (Fig. 9B, Fig. 10). Centralizers will be equally spaced on the casing string to keep it centered within the borehole. Once lowered into place, the instruments will be cemented in place through the drill string and support tubing. Afterward, the drill string will be disconnected from the hanger/riser, leaving it at a predetermined height above the reentry cone (Fig. 10). A battery package will then be lowered into the throat of the reentry cone (Fig. 10) on the logging line and acoustically released. Finally, the drill string will be released and recovered back aboard the drillship.
At the contingency site in the Southern Okinawa Trough (Kuroshio Current) we are planning to core one oriented hole with the APC to refusal and deepen it with the XCB to 410 m. This hole will be logged with standard logging instruments should time permit. Two additional APC holes will ensure complete stratigraphic recovery for the high-resolution objectives at this site.