Telluric current sounding near Kilbourne and Hunts Holes, New Mexico
— J. E. O'Donnell, R. Martinez, and J. Williams

The telluric content method utilizes natural electromagnetic phenomena known as geomagnetic micropulsations which are almost continuously present. The measurements consist of simultaneous recording of the horizontal component of the electric field at two sites, such as base and rover sites. If we know the subsurface configuration at the base site and the behavior of the electric fields at both the base and rover sites, we may deduce the subsurface configuration at the rover site. Telluric soundings can delineate anomalous low-resistivity areas, at depths greater than a few kilometres, which complements the shallower sounding methods such as AMT and D.C. soundings. The assumptions on which the telluric method is based and the properties of the relative ellipse area J, its geologic significance, and its dependence on frequency are stated by Berdichevskii (1960). For example, the assumptions usually made are that the earth consists of conductive layers over an insulating basement and that the thickness of the layers is much less than a wavelength in the earth. The J value is then equal to the ratio squared of the conductance at the base site to conductance at the rover site. Conductance, S = SUM(h_i,e_i ,i)where h_i and e_i are the thickness and resistivity of the ith layer.