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doi:10.2204/iodp.proc.301.206.2009

Summary of hydrogeologic findings and plans for future work

Expedition 301 hydrogeologic test results stand out in comparison with the rest of the global data set of results from similar experiments (Fig. F6). The global data set tends to show a decrease in bulk permeability with depth into basement. Values higher than 10^–13 m² are restricted to the upper 300 m of basement, although data from deeper crustal levels are limited. Tests of smaller depth intervals in upper basement tend to yield higher permeabilities, as often seen in field studies and data compilations from aquifers and crustal systems on land. Packer test results from Hole U1301B appear to be anomalously high in comparison to the global permeability versus depth trend based on packer and thermal flow meter experiments (Fig. F6) and permeability versus age trends for the upper basaltic crust (Becker and Davis, 2003; Fisher and Becker, 2000). Curiously, bulk permeabilities inferred from packer tests are at the limit of those inferred from regional hydrogeologic models (Hutnak et al., 2006). In other locations, packer experiments tend to yield bulk permeability values that are 0.5–1.0 orders of magnitude lower than those determined using a thermal flow meter in the same holes, and both of these experimental methods tend to yield bulk permeability values 2–4 orders of magnitude lower than those inferred from numerical modeling and analysis of responses to tidal pressure variations and tectonic events. This difference in calculated properties is generally attributed to the heterogeneous nature of permeability within the oceanic crust and the characteristic scales and assumptions inherent in the different methods.

It is curious that basement permeability estimates made from the crosshole response between Sites U1301 and 1027 are lower than estimates based on packer experiments in Hole U1301B, even through the latter experiment tested a much smaller volume of crustal rock. This could result from azimuthal anisotropy in basement permeability, as described earlier, or perhaps basement around Site U1301 is unusually permeable because it only recently ended a long phase of vigorous, low-temperature hydrothermal circulation that kept the crust cool and limited the extent of alteration (Hutnak and Fisher, 2007). Completion of the next drilling expedition and the subsequent crosshole testing program should help to resolve the extent of crustal heterogeneity and anisotropy in this area.

As of summer 2008, Holes U1301A and U1301B remain unsealed. Attempts to seal Hole U1301B using a cement delivery system with the submersible Alvin in summers 2006 and 2007 were unsuccessful. The U.S. IODP R/V JOIDES Resolution will return to Site U1301 in summer 2009 to conduct additional cementing operations, pumping much larger volumes than is possible using a submersible.

Shimmering fluid was observed discharging from Hole U1301A during and after summer 2007 dive operations. No such evidence for upflow from the borehole was observed during earlier visits, suggesting that Hole U1301A must have "turned around" sometime between 2006 and 2007 servicing operations. In fact, downhole temperature loggers recovered from Hole U1301A in summer 2008 provide a detailed record of this flow reversal, which appears to have occurred spontaneously when the formation overpressure recovered sufficiently so as to overcome the excess pressure generated by cold, downflowing fluids. Why this required 3 y following Expedition 301 is not understood. Remarkably, Hole U1301B continues to draw fluid rapidly into basement, even though it is located just 36 m from Hole U1301A, which is vigorously discharging warm formation fluid to the ocean. The turnaround of flow in Hole U1301A in 2007 has no influence on the interpretation of crosshole response between Holes U1310B and 1027C, based on data collected in 2004–2005 (Fisher et al., 2008). But understanding of the pressure and thermal interactions between Holes U1301A and U1301B and implications for local and regional crustal hydrogeology will require additional investigation.

The next drilling expedition to the Expedition 301 area appeared on the IODP 2008 schedule briefly in 2007, but then the expedition was postponed when the JOIDES Resolution went into dry-dock for refit. Expedition 301 proponents have an additional observatory servicing expedition with the Alvin scheduled for summer 2009, after the drillship cements the unsealed CORKs at Site U1301, but little more can be done to advance long-term experimental objectives until the next full drilling expedition.

The next drilling expedition will begin with operations in Hole 1027C, recovering the existing CORK and deepening the hole by 30–40 m. This will make room to hang drill collars (making CORK installation safer), provide upper-crustal samples for microbiological and other analyses, and open up the formation for large-scale testing. Emplacement of a two-level CORK system will optimize the configuration for crosshole tests and allow acquisition of long-term geochemical and microbiological samples.

Hole SR-2A (Fig. F1C) will be the deeper of two new basement holes, located between Sites 1026 and U1301. The operational approach in Hole SR-2A will be similar to that used for Hole U1301B on Expedition 301, with drilling, casing, coring, wireline logs, single-hole packer work, and emplacement of a multilevel CORK. Nearby Hole SR-2B will penetrate the upper, most permeable crustal layer(s) and will be the main perturbation well for long-term experiments. Once this hole is drilled and (partially) cased, researchers will initiate a 24 h pumping test with seawater and tracers, then set a multilevel CORK observatory. Multiyear crosshole tests will be run by submersible or ROV 1–2 y after drilling operations are complete, using the naturally overpressured formation and an autonomous flow meter to test properties within an enormous crustal volume surrounding the boreholes.

Work completed thus far shows that crosshole testing can be done at a crustal scale using CORK observatories. The development of tools and methods that allow this kind of testing comprises a significant accomplishment for the scientific ocean drilling community. Instruments and methods developed for Expedition 301 are now being adapted for use in other settings, and researchers are likely to see similarly ambitious experimental programs undertaken around the world in coming decades. Long-term borehole observatories provide opportunities to explore the subseafloor realm in new ways, across a broader range of processes and scales, and with greater accuracy and resolution than was possible in the past. It is challenging to install, maintain, and use these systems, but persistence and patience is providing a new and more nuanced understanding of marine hydrogeologic systems.

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