Large-scale electrical resistivity and bulk porosity of the upper oceanic crust at Hole 395A

A long-spaced, DC electrical resistivity experiment was run during Leg 109, through over 400m of the upper oceanic crust in Hole 395A. The same experiment had been run during Leg 83 in Hole 504B, and the results from the two holes are compared. Apparent crustal resistivities are about 30ohm-m through most of the pillow lavas cored in Hoel 395A, and increase to about 300ohm-m at the base of a thick unit of sealed pillows near the bottom of the hole. A cyclic downhole variation of resistivity with a wavelength of 20-100m correlates with the main lithologic units; within each unit, resistivity increases and porosity decreases with depth. Bulk porosities in Hole 395A were estimated by applying Archie's Law, and range from 10%-15% in the uppermost pillows to about 4% in the resistive unit near the bottom of the hole. -from Author