Temperature dependence of electrical resistivity of water-saturated rocks

The electrical resistivity of several rocks saturated with an aqueous solution has been determined experimentally in the range from room temperature to 250 °C. Of particular interest are the data for temperatures above 200 °C, scarcely documented in the literature. All the rock samples used in the present study experience a quasiexponential decay of resistivity with temperature up to 200 °C. The same temperature dependence is found for the resistivity of the saturating solution, thus confirming that conduction in watersaturated rocks is essentially electrolytic. At temperatures above 200°C, some specimens of highly porous dacitic tuff still follow closely the saturating solution in the pattern of resistivity variation with temperature, exhibiting a minimum around 220 °C. However, the behaviour of other types of rock, notably low-porosity crystalline rocks, where a steady decay of resistivity is maintained up to 250 °C, departs from that of the saturating solution. Resistivity hysteresis phenomena are observed in most rock samples. This experimental evidence points out that mechanisms different from water characteristics, such as growth of microcracks or chemical reactions, contribute to electrical conduction at high temperature. The electrical signature of the thermally induced growth of microcracks (thermal cracking) is made clear for welded tuff and granite, and the possibility of using electrical measurements to monitor hydraulic fractures in Hot Dry Rock is suggested.