ODP/IODP "CORK" Subseafloor Hydrological Observatories: A Review of Designs and Operations Since 1991

Since 1991, about 25 select holes drilled beneath the seafloor by the Ocean Drilling Program (ODP) or Integrated Ocean Drilling Program (IODP) have been instrumented with long-term sealed-hole hydrological observatories.  We will present a summary of the designs of and experiences with these observatories, term “CORKs” for “Circulation Obviation Retrofit Kit.” 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 an approach to seal select holes with long-term sensor strings and data loggers to monitor both the recovery from drilling disturbances to the in-situ state and for 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 are recovered and/or exchanged at average intervals of ~1-3 years using manned or unmanned research submersibles.  Most installations to date have been in sedimented young ocean crust or in subduction settings, but the approach can certainly be applied to groundwater monitoring in continental margin environments.