Diagrams for Magnetotelluric Data: Short Note

where E,, E, and If,, H, represent electric and magnetic components measured along two orthogonal axes (in this paper, for simplicity, to be north and east, respectively). The elements ;,, comprise the magnetotelluric impedance tensor, and they are generally complex due to phase differences between the electric and magnetic fields. All quantities in equation (I ) are frequency dependent. For the special case of “two-dimensional” geology (where structure can he described as having a certain strike direction along which it does not vary), z,, = mmzz2 with z12 i ~mz2,. For the special case of “one-dimerlsionnl” geology (where structure varies with depth only, as if horiTontally layered), z,~ = zzp = 0 and ‘ 12 = -=x1. Magnetotelluric tensor characteristics thus reflect geologic structure, and for this to become fully apparent a tensor is often “rotated”; that is, the values of its elements are computed for different hypothetical orientations of the measuring axes (see, e.g., VozofT, 1972). On a map of several observing sites, the directions of maximum apparent resistivity may then be showsn (Dowling, 1970: Reddy and Rankin, 1971). corresponding to the direction in which the E,-field measurements would be made to obtain a maximum in the absolute value of the element z,~. Sometimes po!ar diagranls are given of the magnitude of one particular tensor element plotted at the angle of rotation of the axes (Berdichevskiy. 1968; Tammemagi and L.illey, 1973). Such polar diagrams for tensor element magnitude are visually not of simple geometric form. as neither would be similar polar diagrams for apparent resistivity. This note describes some simple figures, mainly conic sections. which display certain characteristics of a magnetotelluric impedance tensor. These figures. which may he plotted on a map. may be useful in showing structural trends and classifying geologic structure as one-, two-, 01 three-dimensional. For simplicity. the diagrams to be described are based on the real parts of tensor elements and relate the in-phase part of an electric field to its associated magnetic field. In all cases diagrams based on the imaginary parts of tensor elements could be drawn similarly for the quadrature part of an electric field. Also. in cases 1 and 2. diagrams based WI the complex values of tensor elements could be drawn for the ahsolute magnitude of an electric field. The examples which follow are for an actual lield result,