Electrical resistivity changes in saturated rocks during fracture and frictional sliding

Changes in electrical resistivity were observed as a function of compressive stress in a variety of crystalline rocks that were subjected to confining pressure of up to 5 kb and to pore pressure of water of 500 bars. In the majority of the rocks, resistivity increased slightly up to about half the fracture stress; just the reverse effect has been noted elsewhere for rocks that were apparently partially saturated. Beyond half and particularly within about 20 per cent of the fracture stress, resistivity dropped typically by an order of magnitude. This sharp decrease corresponded closely to an increase in porosity, or dilatancy, which took place under compressive stress. Detailed study of one rock, Westerly granite, showed that changes in resistivity and, hence, porosity with stress were insensitive to effective pressure, when stress was normalized with respect to fracture stress. This suggests that fracture occurred at a critical crack porosity that was pressure independent. The changes in resistivity with stress that accompany frictional sliding on a fault are insignificant when the measurement volume contains the fault, even though faulted rock under pressure can support high stress.