By leaving Hole 395A open for over 20 years, with revisits for discrete data sampling roughly every five years, we have only learned that the downhole flow has apparently continued at a significant rate. We have no resolution as to possible variations in downhole flow rates with time (as has been documented in Hole 504B), let alone the constancy or variability of (a) the driving forces responsible for the downhole flow or (b) the formation hydrologic properties that may limit it. Furthermore, we still do not understand exactly where the downhole flow is directed in the formation, other than the general statement that it is directed into the upper 300 m or so of basement.

The Leg 174B program is intended to address these important issues by providing essential information about the permeability structure and formation pressure, which are keys to understanding the crustal hydrogeology at Site 395. Approximately five days will be spent at Hole 395A during Leg 174B. The program will begin with four logs designed to provide an estimate of the downhole flow rate in 1997 and to assess the fine-scale distribution of permeability in the hole. The hole will then be CORKed with pressure sensors and a thermistor cable, for a long-term record of the pressure and temperature variations in the sealed hole as the natural hydrologic system re-establishes itself. In more detail, the following sequence of logs and experiments will be deployed during trips of the drill string:

Data from the CORK experiment will be collected for a still-unspecified time after Leg 174B, utilizing a submersible or remotely operated vehicle (ROV) to be supported by the National Science Foundation (NSF). The primary purpose of the CORK experiment is not necessarily to assess the equilibrium pre-drilling thermal regime (which we can estimate from detailed heat-flow surveys as in Fig. 2), but instead to monitor how the hydrologic system varies with time as natural hydrogeological conditions are re-established. Full thermal re-equilibration could require many tens or hundreds of years if it occurs only by conductive processes, but could also occur in much less time if the Langseth et al. (1984, 1992) model of active lateral circulation is correct. We are interested primarily in exploring the causes of the hydrogeological state and any possible temporal variations, with the simplest goal to determine how these are associated with and controlled by formation pressure and/or permeability structure. It is impossible to model or predict all of the possible outcomes of the experiment, but considering two possible end-member results might be instructive.

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