University of California Santa Cruz, United States

Since 1991, the Ocean Drilling Program (ODP) and Integrated Ocean Drilling Program (IODP) have instrumented 24 holes drilled beneath the seafloor with long-term sealed-hole hydrological observatories called "CORKs." We present a historical summary of the designs of and experiences with the CORK observatories, starting from the 1989 concept sketch on a dinner napkin and continuing through three new installations planned in young Atlantic crust during IODP Expedition 336 during autumn of 2011. Understanding subseafloor hydrology in a variety of type environments has been a prime objective of scientific ocean drilling since the late 1970's. However, early experience indicated that holes that penetrated through marine sediments into underlying oceanic basement often allowed open exchange between formation fluids and ocean water, perturbing if not totally disturbing the in-situ hydrogeological state. This motivated the CORK approach to seal select holes with long-term sensor strings and data loggers, to record the recovery from drilling disturbances to the in-situ state and monitor natural hydrological signals. The original design included a single seal at the seafloor, and later designs have allowed for monitoring multiple zones in a single hole sealed by packers. The sensor strings have always included pressure and temperature monitoring, and many have included self-contained fluid samplers driven by osmotic pumps ("OsmoSamplers") that can be tuned for a variety of geochemical and microbiological sampling objectives. Typically, data and samplers have been recovered and/or exchanged at average intervals of approximately 1-3 years using manned or unmanned research submersibles. One CORK is now connected to the NEPTUNE Canada cable network; this allows 1 Hz sampling frequency and eliminates demand on battery power and the need for submersible visits to collect data. Cable connections to other holes are now planned in both the Juan de Fuca and Nankai regions. Nearly all installations to date have been in sedimented young ocean crust or in subduction settings, but the approach can certainly be applied to hydrogeological, geophysical, microbiological, and other monitoring throughout subseafloor hydrological environments explored by scientific ocean drilling.