The effect of pressure on the electrical resistivity of water-saturated crystalline rocks

Electrical resistivity of eight igneous rocks and two crystalline limestones was measured at pressures to 10 kb. The rocks were saturated with tap water or salt solution, and the pore pressure was maintained near zero. The dependence of resistivity on temperature, porosity, and pore fluid salinity suggested that conduction was primarily electrolytic throughout the entire pressure range, even though the porosity of some rocks was less than 0.001. Resistivity increased with increasing pressure. The average increase over the 10-kb range amounted to a factor of 250. The changes of resistivity with pressure parallel changes of compressibility with pressure, being rapid over the first 2 kb and tapering off more gradually at higher pressures. The data suggest that the electrical conduction of these rocks consists of (1) conduction along cracks, below a few kilobars pressures, and (2) volume and surface conduction along a network of pores which persist throughout the entire pressure range. Surface conduction of the rocks saturated with tap water was 10 to 20 times greater than the volume conduction of the pores. The dependence of conductivity on porosity for all the samples saturated with saline solution followed the same empirical law that is observed for porous sedimentary rocks, σ(rock) = σ(solution) × η2.