Leg 174B Scientific Report


Hole 395A
Leg 174B represents the fourth time Hole 395A has been reentered since it was drilled on Leg 45 (1975-1976). It was reentered for logging and downhole experiments during Legs 78B (1981) and 109 (1986) and using the French submersible Nautile during the DIANAUT reentry expedition (1989). Past observations from Hole 395A indicate a continuous downhole flow of ocean bottom water and generally support a model of lateral flow of seawater in the upper basement beneath the sediment pond in which the site is located. The logs and CORK experiment deployed during Leg 174B will provide essential information about the formation pressure and permeability structure, which are keys to understanding the crustal hydrogeology at the site.

Initial shipboard interpretation of the DVTP and Schlumberger logs supports the following preliminary results: like all past temperature logs in Hole 395A, the DVTP (Fig. 4) and temperature logging tool (TLT) logs show virtually isothermal borehole temperatures from the seafloor down to 350 mbsf. From 350 to 450 mbsf, there is a slight increase in temperature; below 450 mbsf there is a much stronger increase in temperature. This reconfirms prior indications of a strong flow of ocean bottom water down the hole, at a rate of 1000-2000 l/hr, exiting into the formation between casing and 450 mbsf. Shipboard analyses of the Leg 174B Schlumberger logs (Figs. 5, 6) and comparisons to Leg 45 core description and logs from Legs 78B and 109 clearly show that Hole 395A consists of definable layers of pillow basalts, massive flows, and fluid aquifers that correlate to changes in the resistivity, velocity, and bulk density logs. Distinct changes in the high-resolution temperature gradient log and anomalies in the SP log indicate that at least two major aquifers are active in the hole at approximately 310 and 420 mbsf. Zones of high resistivity and high sonic velocity distinguish massive lava flows, and both resistivity and velocity generally increase toward the bottom of the hole. High-resolution borehole images, cement bond quality, formation strength, and elastic properties can be extracted from the Formation MicroScanner (FMS) and DSI logs. The ARI data (Fig. 7) produced images that show the character and orientation of individual pillow basalts and the heterogeneity of crustal structures at a vertical scale of approximately 1 m. From the comparison of FMS and ARI images, the extent of pillows and flows near the borehole may also be distinguished.

No CORK data will be available until the first submersible revisit to the site, tentatively scheduled for February of 1998, utilizing Nautile. The features of the log data described above are particularly relevent to the hydrogeologic structure in Hole 395A and illustrate the physical state of the ocean crust in unprecedented detail. Overall, the Leg 174B logging program has solidified the position of Hole 395A as the most important reference hole for young oceanic crust formed at a slow spreading rate.

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