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doi:10.2204/iodp.proc.301.104.2005

Introduction

This paper aims to provide a concise but comprehensive review of 1989–2003 designs and operations in support of long-term hydrogeological monitoring with 20 Circulation Obviation Retrofit Kit (CORK) subseafloor sealed-hole observatories installed in 18 holes (Fig. F1) during the Ocean Drilling Program (ODP). It aims partly to provide some technical and historical background for the deployment of three more CORKs of a refined design during the very first expedition of the Integrated Ocean Drilling Program (IODP) (Fisher et al., this volume). More importantly, it attempts to collect sufficient detail about the designs and operations of ODP CORKs to provide a reference for deployment of CORKs of established design or further development of CORKs of new designs during IODP cruises. We also provide a complete history of submersible operations and a bibliography of publications to date related to ODP-era CORKs (see the "Appendix"), although we do not attempt to review the scientific contributions of the overall CORK effort. Finally, because of their relevance to IODP Expedition 301 CORK-II installations, we provide digital files (see "Supplementary Material") of most of the data files recovered to date from the four ODP Leg 168 CORK installations on the eastern flank of the Endeavour segment and the two nearby installations from ODP Legs 139 and 169 at Middle Valley.

The origins of the CORK experiment can be traced back to discussions during a 1987 workshop on wireline reentry of deep-sea boreholes (Langseth and Spiess, 1987). By that time, considerable experience had been gained in interpreting thermal observations of vertical flow in Deep Sea Drilling Project (DSDP) and ODP holes left open between permeable formation and ocean bottom water (e.g., Hyndman et al., 1976; Becker et al., 1983). However, we recognized that such borehole flow, which seemed to be fairly common in holes penetrating through sediments into oceanic basement, represented serious perturbations to the hydrological systems we were attempting to study via scientific ocean drilling; thus, some sort of sealed-hole experiment was necessary to allow reestablishment of equilibrium in in situ conditions to understand hydrogeologic state and processes. In 1989, we began to pursue the concept of an "instrumented borehole seal" (Fig. F2) with additional scientific collaborators, the science advisory structure, ODP engineers (principally T. Pettigrew), and funding agencies. The first two installations, planned for the Middle Valley sedimented spreading center, were added to the ODP schedule as of December 1989, based on a drilling proposal originally submitted in early 1986. They were successfully completed during Leg 139 in the summer of 1991 (Davis et al., 1992).

It was as we sailed for Middle Valley that the acronym "CORK," or "Circulation Obviation Retrofit Kit," was coined by ODP Operations Superintendent Glen Foss. The configuration of the original design bears an obvious resemblance to a cork in a bottle; "CO" referred to stopping the fluid exchange between formation and ocean bottom water that was to be expected if holes into hydrologically active formations were left unsealed; "RK" referred to the fact that the experiment could be installed in any reentry hole, whether drilled the day before or 20 y earlier. The CORK effort grew in prominence within ODP to a greater degree than we ever envisioned, and it became one of two primary threads (the other being the borehole broadband seismology effort, e.g., Purdy and Orcutt, 1995) that led to identification of "in-situ monitoring of geological processes" as one of three featured initiatives in the final ODP long-range plan (JOI, 1996).

Figure F3 illustrates a generic CORK configuration and summarizes the range of primary scientific objectives of these hydrogeological observatories. During the course of the ODP CORK effort, 14 installations were made during 1991–2001 with an original single-seal CORK design (Davis et al., 1992), and 6 more installations were made during 2001–2002 with three different models incorporating capabilities to isolate multiple zones in a single hole (Table T1).

The term "CORK" is sometimes used generically to represent any long-term sealed-hole experiment, but it is also used to refer specifically to the original single-seal CORK design. Following a two-part (December 1997/February 1998) CORK science and engineering workshop (Becker and Davis, 1998), the term "advanced CORK" was used generically to represent a sealed-hole experiment with a multizone isolation capability (Fig. F4). However, the proper noun "Advanced CORK" (acronym ACORK) is also used specifically as the name of the first of the three multizone models actually developed (Shipboard Scientific Party, 2002). The other two multizone models developed during the ODP period were (1) the "CORK-II" (Jannasch et al., 2003), based on a borehole instrument hanger design originally developed to support deployment of broadband seismometer/strainmeter packages in deep holes (Shipboard Scientific Party, 2000), and (2) a wireline instrumented multipacker system or "wireline CORK" (Becker et al., 2004), deployed from an oceanographic research ship using a wireline reentry "Control Vehicle" (Spiess et al., 1992). Design summaries for the original CORK and the three multizone models are provided below, as well as primary references for further details.

Obviously, the ODP CORK effort succeeded far beyond the vision we originally sketched out on a dinner napkin in 1989! Nevertheless, despite the welcome programmatic embrace of the concept and the substantial technological evolution that has taken place since then, some key proponent-driven aspects continued throughout ODP very much in the mode that led to the original deployments. These include

• The critical need for close collaboration among program engineers and scientific proponents to refine the measurement program incorporated in the installations;

• The sharing of costs between program funds for the seafloor and subseafloor infrastructure and additional funds for the "third-party" scientific instrumentation, raised by proponents' proposal submissions to national agencies; and

• The support of postinstallation submersible operations by national funding agencies outside of ODP programmatic oversight or commingled funds.

As IODP begins, these models appear to be continuing, so these aspects are explored in greater detail after the technical summaries of CORK designs and operations.

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