Geophysical methods in which very-low-frequency or direct electrical current is injected into the ground and its potential distribution is measured in order to obtain information about the Earth’s resistivity. Loosely, the term may also include electromagnetic methods, since apparent conductivities (σa) can be used to derive apparent resistivities (ρa) by: σa = 1/ρa. See constant-separation traversing; electrical sounding; electrode configuration; induced polarization.