1. In magnetotelluric exploration wherein orthogonal measurements E.sub.x and E.sub.y of the earth's telluric field and orthogonal measurements H.sub.x and H.sub.y of the earth's magnetic field are made along a line of exploration, said magnetotelluric measurements being related to the impedance of the earth by the determinant of the matrix of impedance tensor components: ##EQU5## where ##EQU6## is the impedance tensor relating the earth's electric fields E.sub.x, E.sub.y to the earth's magnetic fields H.sub.x, H.sub.y, the method of plotting said magnetotelluric measurements to distinguish the structure of resistivity anomalies and separate near surface and depth anomalies comprising: filtering magnetotelluric measurements of E.sub.y, E.sub.y H.sub.x and H.sub.y taken at sites along lines of exploration x and y to produce measurements of E.sub.x, E.sub.y, H.sub.x and H.sub.y for a plurality of successively low frequencies, each successively lower frequency representing a successively lower depth; determining the phase (.phi..sub.DET) of the determinant of the matrix of impedance tensor components for said filtered measurements; plotting contours of equal values of the phase of said determinant as a function of the points on said line of exploration at which said measurements were made, for each of said successively lower frequencies.