Proposed Site OSN-2 is positioned at ODP Leg 138, Site 852, to take advantage of the information previously obtained from geophysical surveys and coring at the site. Four holes were cored at this site with the advanced piston corer (APC), three of which penetrated through roughly the entire sediment column, which was ~116 m thick. Unfortunately, an attempt to sample basement in the last core from Hole 852C with the extended core barrel (XCB) failed to recover any basement material. The redundant coring resulted in recovery of a complete sedimentary section and makes it unnecessary to conduct further sediment coring, except at the sediment/basement contact, if time permits.
Given the tight time constraints, we plan to initiate operations in the hole intended to become the observatory by installing the reentry cone and two of the three casing strings (Table T2). We will first jet-in a reentry cone with 40-60 m of 20-in casing. Next, we plan to drill through the cement plug down to a depth of about 140 mbsf and then install and cement a 16-in casing string to a depth of 130 mbsf. This second casing string will extend through the sediment, the sediment/basement contact, and the upper ~14 m of basement; the latter is often rubbly and can cause hole instability.
Before proceeding with the third casing string, we will use the rotary core barrel (RCB) to drill through the cement plug and then core from 140 down to 216 mbsf or deeper if time permits. The hole will be logged as discussed below. The final step is to ream the hole with a 14.75-in bit down to the total depth (TD) cored and then install and cement 10.75-in casing to ~5 m above TD.
Should the initial port call require less than the planned 4 days and the transit to and operations at proposed Site OSN-2 are completed ahead of schedule, we plan to drill through the upper 80-90 m of sediment in a second hole and then begin RCB coring (Table T3). If we have slightly more than a day available, we should be able to core through the sediment/basement interface and down to ~140 mbsf, thus sampling all the section not previously recovered during Leg 138 or in the cased observatory hole. Again, if sufficient time is available, we plan to log this hole as described below.
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