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doi:10.2204/iodp.sp.350.2013

Drilling and coring strategy

Expedition 350 will begin with a 2 day effort at proposed Site IBM-4GT, coring a 150 m deep geotechnical test hole for potential future deep drilling (5500 mbsf) at proposed Site IBM-4 (Izu fore arc) using the riser D/V Chikyu. The Center for Deep Earth Exploration (CDEX) requires sediment property information at a site considered for a potential future deep drilling program. In particular, shear strength data are needed to design the top hole for riser installation. Preliminary data from ODP Site 792 (0–64 mbsf) indicate a formation strength that is weaker than those for other Chikyu riser expeditions (Nankai and Shimokita). Based on IBM proposal proponent and Expedition 350 co-chief support, IODP-Texas A&M University (TAMU) agreed to honor a request by CDEX to provide the geotechnical test cores at proposed Site IBM-4GT.

Following initiation of operations at proposed Site IBM-4GT, the operations strategy for Expedition 350 is to core and log through the Neogene to Oligocene–Eocene volcaniclastic sediment and rock sequences and, if possible, into the crystalline basement at one site in the IBM-3 rear arc area in the eastern half of the Izu-Bonin rear-arc seamount chains, ~90 km west of the Myojin-sho arc volcano (Figs. F4, F5, F8, F9). The IBM-3 sites are based on six intersecting low-fold MCS profiles obtained during JAMSTEC Cruises KY06-14 (December 2006) and KR07-09 (June and July 2007) (Figs. F9, F10, F11, F12). Proposed Site IBM-3C is the primary site and is located at 31°47.38′N, 139°01.58′E in 2114 m water depth. Three alternate proposed Sites IBM-3D, IBM-3E, and IBM-3F are specified a few kilometers northwest and west of primary proposed Site IBM-3C in case the latter proves unsuitable (mainly because of the proximity of a submarine cable). Objectives for the alternate proposed sites would be the same as those for the primary proposed site, and the stratigraphy is essentially the same.

Although many of the scientific objectives can be met by penetrating the Neogene seismic Layers L1–L4 (1200 mbsf; Table T1), we hope to reach the igneous basement. The transition of seismic velocity to >5 km/s is generally thought to represent the transition to igneous crystalline rocks. As shown in Table T1 and Figure F14, the 5 km/s iso-velocity contour is estimated at 2080 mbsf at proposed Site IBM-3C based on the analysis of ocean bottom seismic (OBS) refraction data.

Expedition 350 will begin on 30 March 2014 with a 5 day port call in Keelung, Taiwan. The transit to the IBM-4 area will take 4.2 days. After completion of the geotechnical test hole at proposed Site IBM-4GT (1.4 days), the transit to the IBM-3 area will take 0.3 days. At the end of operations, the JOIDES Resolution will sail to Yokohama, Japan, which will take 0.9 days. This leaves ~50 days for onsite operations at Site IBM-3 (Table T2).

Our drilling and coring plan at proposed Site IBM-3C includes two scenarios: one without casing and one with casing. Both scenarios may reach a total depth of ~2100 mbsf if good to excellent hole conditions prevail; however, hole problems may limit the actual total depth reached. In both scenarios, we begin with a jet-in test to determine the length of 20 inch casing that can be deployed as a stinger to the reentry cone, taking <1 day (Hole A). Next, we use the advanced piston corer (APC) and extended core barrel (XCB) systems to core the soft to firm sediment at the top of the sequence. Based on ODP Leg 126 results, the expected APC/XCB penetration is ~200 m and will take ~1 day (Hole B). At this point, the drill pipe is recovered and the rotary core barrel (RCB) assembly is made up and deployed (Hole C).

In Scenario 1, RCB coring in Hole C is assumed to proceed without hole problems and reach total depth of 2300 mbsf in ~41 days. Coring will be followed by ~4 days of wireline logging.

In Scenario 2, coring in Hole C is terminated at 600–1200 mbsf after ~7 days because of hole conditions that warrant casing of the upper section. Wireline logging is conducted for ~2 days. Next, a reentry cone with a 20 inch casing stinger is deployed and jetted-in to begin Hole D. Hole D is then drilled (without coring) to somewhat less than the total depth of Hole C. Casing strings, 16 and 10¾ inch, are made up and deployed to depths determined by the logs of Hole C. RCB coring is then resumed to a total depth of 2100 mbsf, followed by ~3 days of wireline logging, for a total operational time in Hole D of ~34 days.

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