Site 1027

Abstract

During Ocean Drilling Program (ODP) Leg 168, Hole 1027C was drilled to 632 meters below seafloor (mbsf), 2.2 km east of Hole 1026C, where sediment thickness is 575 m above a buried basement low (Figs. F1, F2). The upper part of the hole was cased through sediments and uppermost basement, with 54 m of open hole. The open interval near the base of Hole 1027C comprises a diabase sill, intercalated sediments, and basalt breccia overlying 26 m of extrusive volcanic rocks (Shipboard Scientific Party, 1997). A subseafloor borehole observatory ("CORK") that included a data logger, pressure sensors, thermistors at multiple depths, and a fluid sampler was installed in Hole 1027C during Leg 168. Observatory instruments were retrieved in 1999, and the pressure logging system was replaced. The CORK in Hole 1027C was scheduled to be replaced during Integrated Ocean Drilling Program (IODP) Expedition 301, but problems setting the CORK in Hole U1301B and a lack of time and materials prevented the completion of any Hole 1027C operations during that expedition. Hole 1027C was fully sealed and continued to record formation pressure before, during, and after Expedition 301. The pressure record from this hole provided evidence of a long-term, cross-hole response to drilling and other operations in Hole U1301B as well as a subsequent period of fluid flow down into Hole U1301B, which was not completely sealed (Fisher et al., 2008;Davis et al., 2010).Researchers visited Hole 1027C with DSRV Alvin during summer 2009 to prepare the CORK for replacement by retrieving a battery pack and communications package from the remotely operated vehicle (ROV) platform and removing an underwater electrical connector from the top of the wellhead. Unfortunately, the connector was corroded in place and could not be removed, but the ROV platform was cleared for CORK retrieval operations.The primary scientific objectives at Site 1027 during Expedition 327 were to recover the existing CORK in Hole 1027C, core and deepen the hole by ~40-50 m, run hydrologic tests of the open hole, and deploy a new multilevel CORK for monitoring, sampling, and associated experiments. These operations could not be completed because the CORK in Hole 1027C could not be unlatched from the reentry cone and casing system (see "Operations"). Thus, the old observatory remains in place, and additional servicing will be attempted during future dive operations to