OPERATIONS STRATEGYPrevious drilling in the Japan Trench region was in the DSDP era before the advent of advanced hydraulic piston corer/extended core barrel (APC/XCB) technology (Legs 56 and 57 in 1977 and Leg 87 in 1982). Previous ODP drilling at plate subduction zones were at Izu-Marianas, Barbados, Costa Rica, Cascadia, Nankai, and Peru margins, which span quite different subduction types. Both Sites 1150 and 1151 are located at the deep-sea terrace at 2681- and 2182-m water depths, respectively.
A unique aspect of Leg 186 was the installation of two permanent borehole geophysical observatories. The installation required a complex operation that has never before been done. At this environment, a clear deep-seismic reflector is the Cretaceous unconformity, below which the physical properties such as P-wave velocities and densities become more favorable for the sensors. However, it did not seem realistic to drill to depths of more than 1.5 s in two-way traveltime at the two sites to penetrate the Cretaceous unconformity. Such an operation would have required an interim supply of casing joints during the leg and considerably more time than available on a standard leg. Therefore, a provisional depth of 1000 m was targeted for RCB coring and to check if the physical properties values were acceptable for the instruments. Logging data and observations from cores allowed us to monitor physical properties and decide on a suitable depth.
For initial coring objectives, one site with a fast sedimentation rate was to be double APC/XCB cored to ensure complete recovery in the Pliocene and younger section. Because of large uncertainties in installation time requirements, double APC coring was postponed until Site 1151.
Each site was equipped with a reentry cone and was cased through unstable sections leaving a 50- to 100-m open-hole section at the bottom. Because the sensor package diameter cannot run through the drill string as do logging tools, it had to be connected at the bottom of the drill string. To cement in the sensors, which is essential for the strain measurements, the drillship was required.
At Site 1150, the original plan for XCB coring to 450 mbsf level was deepened to 723 mbsf. Hole 1150B was thus drilled to 703 mbsf and cored to 1182 mbsf. Below ~1100 mbsf, the physical properties seemed to be adequate for the instrument. Logging data further confirmed and pinpointed where the instrument depth should be. The unfortunate abandonment of reentry Hole 1150C because of probable defects in the reentry cone caused a major change of planning. We had to go back to the port of Yokohama to obtain another reentry cone, at which time we also loaded the remaining 4 1/2-in casing pipes purchased for hanging the instrument package in the drilled and cased holes. At this point, we focused the use of time on accomplishing the primary objectives for the leg, which meant that many other objectives might have to be temporarily abandoned.
The Site 1150 installation was successfully completed at 1900 hr on July 28, with only 18 days left for operations at the second site. It was then that we decided to skip APC/XCB coring and logging and instead aim for the shortest pathway to complete the observatory installation. Thus, Hole 1151A was RCB cored all the way to 1114 mbsf. The operations at Hole 1151B proceeded rapidly, and we successfully completed the instrument installation on August 9, just 12 days after arriving on site. After the installation, we proceeded to complete the remaining objectives for the hole, which included double APC coring this site to ~100 mbsf, and then logging to 870 mbsf.
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