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Magnetotelluric sounding infers subsurface electrical resistivity from naturally varying electric fields and magnetic fields. Surface electrodes measure potential differences while magnetic sensors record the same fluctuations. In the frequency domain, the relation between horizontal field components defines an electrical impedance that depends on the conducting structure below the station.

Low frequencies generally penetrate farther than high frequencies, as expected from the skin depth of a conductor. A range of frequencies therefore constrains a range of depths. The inferred electrical resistivity depends on connected fluids, composition and temperature; a conductive zone does not uniquely identify molten rock.^[ref]