Effects of atmospheric moisture on rock resistivity

The role of water in the electrical resistivity of rocks has long been recognized as an important one. This study examines the changes in resistivity of rock samples as induced by atmospheric moisture. An analysis of the term dry rock leads to the conclusion that oven drying and evacuation are methods that yield only relative degrees of desiccation. To obtain absolute values of the moisture present in a sample, one should probably use nuclear magnetic resonance and mass spectrometry. Experiments were performed on samples of hematitic sandstone, pyrite, and galena. The sandstone underwent a change in resistivity of 4 orders of magnitude when it was measured in a vacuum of 5 × 10−7 torr and in air of 37% relative humidity. Pyrite and galena showed no variations in resistivity when they were measured under the same conditions. These results, plus others obtained elsewhere, indicate that rocks of the resistive type are affected in their electrical properties by atmospheric moisture, whereas rocks of the conductive type are not. The experimental evidence obtained is difficult to reconcile with a model of aqueous electrolytic conduction on the sample surface; it is instead suggested that adsorbed water molecules alter the surface resistivity in a manner similar to that observed in semiconductors and insulators.