Telluric Current

(also earth current), an electric current flowing in the earth’s crust; its existence is associated mainly with variations of the earth’s magnetic field (which induce currents in accordance with the law of electromagnetic induction) and with the electric field of the atmosphere (seeATMOSPHERIC ELECTRICITY) and electrochemical and thermoelectric processes occurring in rocks.

Telluric currents of inductive origin may be either regional or global; currents caused by the atmosphere’s electric field or by electrochemical and thermoelectric processes in rocks are more local. The intensity and direction of telluric currents vary in time with periods ranging from several years (cyclical, annual variations) to several minutes and seconds (short-period variations). Fluctuations of the electric-field intensity E of telluric currents are studied on the basis of the potential difference between electrodes inserted into the ground or dropped into the sea for distances ranging from several hundred meters to several kilometers. The amplitude of these variations fluctuates from fractions of a millivolt to hundreds of millivolts per kilometer, depending on the composition of the basement rocks, the geographic position of the measuring point on the earth’s surface, and the state of disturbance of the geomagnetic field. The maximum values of E occur in outcrops of the crystalline basement of the earth’s crust, in the auroral oval, and during magnetic storms.

Telluric currents in the sea are characterized by smaller values of E, which, however, increase near coasts (the coast effect). The measurement of currents induced in seawater as a result of the water’s motion in a constant geomagnetic field makes it possible to determine the velocity of sea currents. Telluric currents also make it possible to obtain valuable information on short-period oscillations of the geomagnetic field. Observations of telluric currents are widely used in mineral prospecting and in deep subsurface studies of the upper mantle.