Geo-electrical methods of prospecting depend on the marked differences in the resistivity which exist between adjacent geological formations, these differences being produced by variations in the porosity of the rocks and in the nature of the saline solutions which they contain. Minerals with a metallic luistre usually have a low resistivity. The variations in resistivity are revealed by the anomalous flow through the ground of an applied current, and the work consists in determining the current flow and interpreting the anomalies in terms of geological structure. The usual methods employed are the surface-potential methods (i.e. the resitivity method, equipotential-line methods, and equiquadrature methods, etc.), and the inductive methods at high or low frequencies, in which the current distribution is examined by means of the magnetic field associatedc with it. The methods may be applied to the location of good conducting ore bodies, faults, and dykes, to the measurement of depths, and also for the plotting of subsurface contours. Under favourable circumstances sulphide bodies may be detected by the natural currents which they produce(spontandeous-polarization method).