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On Leg 78B, the Glomar Challenger returned to Hole 395A on the Mid-Atlantic Ridge and logged the upper 0.5 km of the crust, obtaining excellent resistivity, natural-gamma, and caliper logs throughout the section and reasonable density and velocity logs near the base. The logs show that the crust may be divided into two distinct geophysical units. The upper 400 m of the crust displays high porosities and low velocities, densities and resistivities. Below this, between 400 and 500 m sub-basement, the crust displays a low crack porosity (1-2%), high resistivities (up to 1000 öm), and high velocities (up to 6.0 km/s). For the lower unit, a value of 2.2 was obtained for the exponent m in Archie's Law, which is consistent with low permeabilities measured in the same interval with a packer (2 to 9 μdarcies). Since the boundary between these two units is marked by a sharp increase in alteration products and the lower unit behaves as if it were sealed, we conclude that Hole 395A penetrated a major geophysical discontinuity.

Basement Logging on the Mid-Atlantic Ridge, Deep Sea Drilling Project Hole 395A