Leg 174B Scientific Report


The main objective of Leg 174B was to reenter Hole 395A to run a selected suite of downhole logs followed by installation of an instrumented borehole seal or CORK (Circulation Obviation Retrofit Kit). The purposes of the logs and CORK experiment were (1) to document the in-situ physical properties and hydrogeology at this young crustal reference site and (2) to test a hydrological model developed from observations obtained during three earlier reentries since the hole was drilled over 21 years ago in 1975-1976. Leg 174B operations at Hole 395A were very successful in achieving the following objectives: a temperature log and three advanced Schlumberger logs were run and the hole was smoothly CORKed. Sufficient time was left to collect advanced hydraulic piston cores through the full sedimentary section (predominantly nannofossil ooze) and an extended core barrel core in basaltic basement at Site 1074, 4 km northwest of Hole 395A.

Leg 174B represents the fourth time Hole 395A has been reentered since it was drilled on Deep Sea Drilling Project Leg 45 (1975-1976). 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. This model was generally confirmed by Leg 174B logs from Hole 395A and cores from Site 1074. Like all past temperature logs in Hole 395A, the Leg 174B data show virtually isothermal borehole temperatures from the seafloor down to 350 mbsf. 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. The Leg 174B Schlumberger logs and comparisons to past core descriptions and logs 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 spontaneous (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 were obtained with the formation microscanner and two advanced tools new to the Ocean Drilling Program: the digital shear imager and azimuthal resistivity imager. The Leg 174B log data 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. No CORK data will be available until the first submersible revisit to Hole 395A during the winter of 1998.

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