Leg 174B will run selected logs and then install a Circulation Obviation Retrofit Kit (CORK) in Hole 395A, which penetrates over 500 m into 7-m.y.-old crust on the western flank of the Mid Atlantic Ridge. The overall purpose of the experiments in Hole 395A is to understand hydrogeologic processes at this important reference site for young crust formed at a slow spreading rate. The logging program will include four tool runs‹temperature log, flowmeter log, and two Schlumberger tool strings‹focused on documenting the permeability structure in the hole and understanding the downhole flow of ocean bottom waters which has been observed since the hole was drilled in 1975. After the logging program is completed, the hole will be instrumented and sealed with a long-term CORK observatory, shutting off the long-lived downhole flow. The purpose of this CORK experiment is to allow natural hydrogeological conditions to re-establish after sealing the hole and then to monitor how the hydrologic system varies with time. Monitoring the return to the natural thermal and hydrological regime will allow us to determine if the pressure differentials which drive the downhole flow observed to date at this site are dynamically maintained due to active circulation occurring in the basement or simply an artifact of drilling.

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