SCIENTIFIC OBJECTIVES AND METHODS
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:
1.Logs: After initial reentry with a logging bottom hole assembly (BHA), a temperature log
will be run first, followed by three logs to delineate the fine-scale permeability structure of the
section penetrated by Hole 395A. These three logs include a flowmeter log, the Schlumberger
combination Formation MicroScanner/Array Sonic string, and the Schlumberger Triple
Combo geophysical string, to be deployed in an order to be determined at sea for greatest
operational efficiency. The upper 300 m of basement is known to be quite permeable on
average from the downhole flow, packer measurements, and an incomplete flowmeter
experiment during the DIANAUT program (Becker et al., 1984; Hickman et al., 1984;
Becker, 1990; Kopietz et al., 1990; Gable et al., 1992; Morin et al., 1992). Detailed
permeability information and structural control will be required to allow interpretation of the
data to be collected from the CORK experiment long after Leg 174B in terms of active
hydrogeological processes in discrete zones of the formation. This sequence of logs will
require a pipe trip, plus about 36 hours logging time, for a total of 2-2.5 days on site.
2. CORK: Deployment of a fully configured CORK to seal the hole, instrumented with a 600
m-long, 10-thermistor cable, a pressure sensor in the sealed section, and a reference pressure
sensor at seafloor depth. This installation will provide a long-term record of (a) the rebound
of temperatures and pressures toward formation conditions after the emplacement of the seal,
(b) possible temporal variations in temperatures due to lateral flow in discrete zones, and (c)
pressure variations, which in a sealed hole would be the primary manifestation of changes in
the forces that drive the natural circulation system. Approximately 1.5-2 days will be required
to deploy the CORK experiment (Table 1).
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.
1. If the model of active lateral circulation is basically incorrect, and downhole flow is indeed
simply an artifact of drilling, then sealing the hole should remove the driving force for the
downhole flow, and temperatures and pressures will slowly and smoothly trend toward
values consistent with conductive, hydrostatic processes.
2. If there is some element of truth to the model of active lateral circulation in basement, with
this circulation providing the driving pressure differential for the downhole flow, then sealing
the hole will not change the driving force, and lateral circulation should continue even though
the seal has stopped the downhole flow. Pressures in the sealed hole should approach a
nonhydrostatic value in an irregular fashion that reflects variability in the natural
hydrogeologic processes. Similarly, temperatures will rebound towards values consistent with
the circulation system, also in an irregular fashion that reflects natural hydrogeologic
variability. In addition, differences in the behavior of the temperature sensors should reflect
vertical variations in the lateral flow regime due to fine-scale permeability variations. We
understand so little about crustal hydrogeology that simply defining the natural time- and
space-scales of such variability will be a very important result.
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