Prospecting that makes use of three fundamental properties of rocks. One is the resistivity, or inverse conductivity. This governs the amount of current that passes through the rock when a specified potential difference is applied. Another is the electrochemical activity with respect to electrolytes in the ground. This is the basis of the self-potential method. The third is the dielectric constant. This gives information on the capacity of a rock material to store electric charge, and it must be taken into consideration when high-frequency alternating currents are introduced into the earth, as in inductive prospecting techniques. Electrical methods are more frequently used in searching for metals and minerals than in exploring for petroleum, mainly because most of them have proved effective only for shallow explorations.