A method in which orthogonal components of the horizontal electric field associated with currents induced in the earth by natural sources of energy are measured simultaneously at two or more stations. The measurements from one station, which serves as a base, are used to normalize the measurements from other stations to compensate for variation of the source with time. The normalized measurements, plotted as vectors at each station, may outline an ellipse if signals from several different azimuths are recorded as the source changes. The relative area of the ellipse at each station is (ideally) inversely proportional to the conductance of the sedimentary section above the basement. The orientation of the ellipse yields information about the direction of current flow.