According to the Archie equation
, the electric resistivity of rocks with relatively small amount of clay is directly proportional to the electrical resistivity of the pore fluid. The latter, in its turn, monotonically decreases with temperature (right figure) with the beta
coefficient being close to 0.02 (e.g., for NaCl solutions beta
= 0.026). Thus, for example, increasing the temperature to 200°С will decrease rock resistivity in about 6 times
, which makes the EM geophysical methods especially useful for geothermal exploration.