The subject of this note is a method of acquisition and reduction of data in connection with what is commonly known as the “telluric method,” which primarily deals with the parameter known as “relative ellipse area.” Let us suppose that at an instant t the orthogonal components of the telluric (electric) field at a point B, called Base Station, are Ex and Ey. At the same instant, the components at another point F, called Field Station, measured in another orthogonal coordinate system, are Eu and Ev. It is assumed that these components are related to each other as follows:
where a, b, c, and d are real numbers, called the “correlation coefficients,” which depend only on the directions of the measuring axes and on the electrical properties of the subsurface. It follows that the Jacobian of the transformation from the x-y system to the u-v system,
depends only on the electrical properties of the subsurface; it is called the relative ellipse area at F with respect to B. For information concerning the validity of equation (1), and the geologic meaning of J, the reader may refer to a book by Berdichevskii (1960).