Person: Earl E. Davis

Abstract

An essential aspect of the forty years of deep-sea scientific drilling has been to maximize the scientific return during each expedition while preserving samples for future investigations. This philosophy also extends to borehole design, providing the community with tens of cased legacy boreholes that penetrate into the basaltic crust, each ripe for future investigations of crustal properties and experiments to determine crustal processes (Edwards et al., 2012a). During Integrated Ocean Drilling Program (IODP) Expedition 336 to North Pond on the western flank of the Mid-Atlantic Ridge at 22N, Hole U1383B (Fig. 1) was planned to be a deep hole, but was abandoned when a 14.75-inch tri-cone bit catastrophically failed at 89.9 meters below the seafloor (mbsf) (Expedition 336 Scientists, 2012). Thisresulted in about 36 meters of open hole below casing, similar to conditions within tens of legacy boreholes. Because the overall experiment required a return to the “natural” hydrologic state in basaltic basement, it was critical to seal the hole to prevent a hydrologic “short circuit”. Thus, a plan emerged at sea to seal Hole U1383B with a simplified Circulation Obviation Retrofit Kit (CORK) termed “CORK-Lite” that could be deployed by a remotely operated vehicle (ROV) on a planned dive series five months later. To prepare for this deployment, a standard ROV platform that is used with CORKs was modified to be self-guiding in the re-entry cone and deployed. The next step was to design a CORK system that could seal the borehole, yet be physically manageable with an ROV, and be ready for shipping and deployment within three months. Several key functional aspects dictated the design of the new CORK-Lite (Table 1).

Abstract

Integrated Ocean Drilling Program (IODP) Expedition 327 installed two new subseafloor borehole observatory systems (“CORKs”) in 3.5 m.y. old upper ocean crust on the eastern flank of Juan de Fuca Ridge in Holes U1362A and U1362B. Expedition 327 participants also recovered part of an instrument string previously deployed in a CORK in Hole U1301B and deployed a short replacement string. These observatories are part of a network of six CORKs that was designed to monitor, sample, and complete multidisciplinary cross-hole experiments. We present an overview of project goals and describe the design, construction, and deployment of new CORK systems. We also provide an update on the status of preexisting CORK systems as of the start of Expedition 327. Additional CORK servicing and sampling are scheduled for summer 2011 and 2012, including a long-term free-flow perturbation experiment that will test the large-scale directional properties of the upper ocean crust around the observatories.