LEG 132

Western and Central Pacific

During Leg 132, tests were conducted at two sites (Sites 809 and 810) to refine the hard rock guide base (HRB), drill-in bottom hole assembly (DI-BHA), and diamond coring system (DCS), hardware required to meet future endeavors at difficult-to-drill sites and the long-standing objective of core recovery from axial regions of active spreading ridges. The HRB is a quickly deployable seafloor structure that stabilizes a drill bit/bottom hole assembly while initiating a hole in hard bare rock. It can accept a seafloor slope of up to 25 deg, allow for the support of one or more casing strings, and accommodate moving or recovery options. The DI-BHA is intended to isolate unstable formations, allow separation from the drilling assembly without retrieving the bit, provide adequate bit life, allow nested deployment capability in multiple unstable zones, provide drill collars/connections capable of surviving rotation above the seafloor without lateral support, and provide a through-bore for the deployment of wireline coring systems through the isolation strings. The DCS is a wireline coring system capable of drilling and coring through highly-fractured, poorly cemented, crystalline rock.

Until Leg 132, ocean ridge formations were cored with conventional roller-cone bits about 10 in. in diameter, identical to bits often used in the oil industry. When rotary core bits are used in recently erupted volcanic rocks, the walls of the drill holes tend to collapse, binding the drill string and destroying the bit. The DCS system is similar to that employed in drilling brittle and fractured formations on land and consists of smaller-diameter diamond bits rotating at high speed with a computer-controlled low-level drill-string weight on the bit. While the Leg 132 DCS Phase II had been modified from a prototype tested during Leg 124E, the HRB and DI-BHA were newly developed systems.

Operations were conducted at Sumisu Rift, an incipient backarc basin west of the Izu-Bonin Arc system in the western Pacific and at Shatsky Rise, a thickly sedimented plateau of Mesozoic age on the Pacific Plate east of Japan. Full testing of the DCS was conducted at the Sumisu Rift site where almost all components of the system, including the computer-controlled secondary heave compensator, were thoroughly evaluated and, in several cases, onboard modifications were fabricated. This was the first successful coring of "zero-age" basalt in volcanic terrain on the seafloor. A hole 79 m deep was cored with significant recovery and no problems with side-wall stability. Additional vital information was obtained regarding HRB placement procedures on bare volcanic rock and for improvements to the core-retention capability of the system.