ODP Technical Note 10


B. Leg 146 CORK Operations

Two new CORK systems were deployed during Leg 146, one in Hole 889C on the Vancouver margin, and one in Hole 892B on the Oregon margin. Site 889 is located on a 15-20-km-wide zone of low-gradient slope off Vancouver Island. The site was positioned to allow penetration and sampling of a gas hydrate zone and bottom-simulating reflector (BSR); CORK emplacement at this site is hoped to elucidate the geothermal, geochemical, and hydrogeological conditions conducive to BSR and hydrate formation. Hole 889C was drilled to 384.5 mbsf, with 259.5 m of casing, and a perforated, closed-end 5-in. liner extending to 322.9 mbsf. Deployment of the CORK data logger and thermistor/sampling string was hampered by poor weather and obstructions within the liner, apparently due to infilling of sediment through the 3/8-in. liner perforations. The fluid-sampling tube was damaged during initial deployment attempts and was eliminated from the experiment, and a shortened 240-m thermistor string was installed.

Site 892 is located on the seaward flank of the second landward ridge at the toe of the Oregon accretionary margin. The site was positioned to intersect both a BSR and an apparently hydraulically active, landward-dipping fault at 68 and 105 mbsf, respectively. Hole 892B was drilled to a total depth of 178.5 mbsf, with casing to 93.6 mbsf, and a perforated, closed-end 5-in. liner extending to 146.1 mbsf. Once again, sediment appears to have flowed in through perforations in the liner, reducing the amount of available open hole. Because this hole was significantly shallower than originally intended for CORK emplacement at the Oregon margin, a long thermistor string had to be doubled over upon itself several times to shorten it to 127 m. This overlap in the thermistor string significantly thickened the string, requiring that the fluid-sampling tubing be omitted from the second Leg 146 CORK deployment.

During Leg 146 it was also intended that the thermistor string hanging beneath the CORK emplaced in Hole 857D (on Leg 139) would be replaced. The original thermistor string was too short because Hole 857D ended up being deeper, and the cased section longer, than originally planned. Poor weather during Leg 146, in combination with the delicate nature of CORK recovery operations, resulted in the Hole 857D CORK being severely damaged. A skirted running tool was lowered to the seafloor and latched on to the upper end of the CORK, which projected above the reentry cone. A series of large heaves while the heave compensator was being adjusted resulted in application of excessive weight to the CORK, driving the data-logger down through the CORK mandrel. The CORK running tool then disengaged from the CORK and dragged across the ROV platform and off the reentry cone to the seafloor. The exposed data logger and failed mandrel remain in the reentry cone, and were observed leaning at an angle against the ROV platform opening. The data-logger case may still be intact, as may be the primary CORK seal. An attempt to extract data from the logger at Hole 857D will be made from a submersible in August 1993.

The Leg 146 experience illustrates that the successful deployment of CORK seafloor observatories is weather and formation dependent. Scientists considering emplacement of CORKs on upcoming legs should alert ODP as early as possible in the planning process so that these and other factors can be considered. One significant modification which is now being planned by CORK PI's is the addition of an acoustic link to allow logger integration and reprogramming from a conventional surface vessel.

Section VI-C is a reprint from Leg 139 and has been ommitted here.

To Pressure Core Sampler, Section A.
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