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A long-spaced, DC electrical resistivity experiment was run successfully during Leg 83, throughout over 1 km of Layer 2 of the oceanic crust at Hole 504B. This experiment responds to in situ crustal resistivities over vertical and radial averaging scales of about 5 to 50 m. Apparent crustal resistivities are about 10 ohm-m in about 600 m of pillow lavas, sharply increasing to nearly 500 ohm-m in the underlying dikes. Bulk crustal porosities were approximated by applying Archie's Law to the apparent resistivities. Apparent porosities decrease sharply from about 10% in the pillow lavas to about 2% in the dikes, and the results indicate three 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.

Large-Scale Electrical Resistivity and Bulk Porosity of the Oceanic Crust, Deep Sea Drilling Project Hole 504B, Costa Rica Rift