Large-scale electrical resistivity and bulk porosity of the oceanic crust, Deep Sea Drilling Project Hole 504B, Costa Rica Rift.

A long-spaced, DC electrical resistivity experiment was run successfully during Leg 83, throughout over 1km of Layer 2 of the oceanic crust at Hole 504B. Apparent crustal resistivities are approx 10ohm-m in approx 600m of pillow lavas, sharply increasing to nearly 500ohm-m in the underlying dikes. Apparent porosities decrease sharply from approx 10% in the pillow lavas to approx 2% in the dikes, and the results indicate 3 porosity layers which seem to correspond to seismic Layers 2A, 2B, and 2C. Comparison of the apparent bulk porosities to shipboard porosity measurements on recovered basalt samples suggests that Layers 2A and 2B have significant open fracture porosity, whereas the dikes of Layer 2C have very little fracture porosity. The sharp increase with depth of apparent resistivity and the order-of-magnitude decrease in apparent bulk porosity are highly consistent with the steep reduction with depth of bulk crustal permeability measured in Hole 504B.-from Author