Magnetotellurics (MT) is a geophysical method to study the distribution of electrical conductivity in the earth. It uses natural electromagnetic field variations (caused by solar radiation/particles or atmospheric sources, e.g., lightning discharges), which induce electric currents in the subsurface. The depth of penetration of the field variations depends on period length and the conductivity of the subsurface (skin effect).
By measurement of magnetic and electrical fields at the earth's surface one obtains estimated values of impedance (the relation of electrical and magnetic field strength, normally represented as apparent resistivity and phase shift between both fields) as function of period. By modelling and inversion an image of the distribution of electrical conductivity is obtained. Highly conductive zones hint, e.g., at fluids and partial melts in crust and upper mantle or graphites in shear zones.
Alternating currents of varying periods flow in the electrically conductive ionosphere in approximately 100 km height and in the magnetosphere, which are caused by the influence of the solar radiation (primary winding). These currents are connected with a magnetic field, which induces electric currents in the conductive earth layers (secondary winding, Fig. 1), whose effects can be observed at the earth's surface (note that ionospheric currents produce a near-field effect and are not used for MT soundings). A further cause of these currents in the earth stems from the radiation of electromagnetic energy of thunderstorm lightnings and its propagation in the space between earth and ionosphere (atmosferics or sferics). Thereby currents are induced, too.
Fig. 1: Electric currents in the interior of the earth are caused by current systems in the ionosphere/magnetosphere and by radiation of lightnings.
Fig. 2: Recording of short-period electromagnetic signals (atmosferics) in the magnetic and electrical horizontal field components at an observation site in the Lybian desert (left). Top right: The spectrum of geomagnetic and geoelectrical variations. Lower right: This part of this spectrum shows the Schumann resonances, standing waves in the waveguide, which is formed by the electrically conductive earth and ionosphere. The different characteristic with frequency of the magnetic field (here the north component Bx) and the electrical field (east component Ey) reflects a layering of the conductivity of the underground.
Fig. 3: At high frequencies a characteristic signal shape of atmosferics (the so-called slow tail) is often observed. It results from frequency-dependent attentuation of the electromagnetic waves along the propagation path in the waveguide earth-ionosphere (dispersion).
The occurrence of short and long periods enables a depth sounding of the underground. Similar to atmospheric temperature (daily and yearly variations) short-period waves penetrate only into shallow ground while long-period signals reach depths to over 100 km (skin effect, see Fig.4).
Fig. 4: Long-period electromagnetic waves penetrate deeper into the underground than short periods. So long-term measurements must be accomplished in order to detect a good conductor at greater depths of the crust or upper mantle.
Fig. 5: Electrical resistivities occurring in nature are very rock dependent and cover order of magnitudes.
Fig. 6: Apparent elektrical resistivity and phase of an one-dimensional subsurface model with two layers: an overburden (10 Ohm m) with a thickness of 200m above the basement (1000 Ohm m).