Data processing and interpretation in geomagnetic deep sounding

This paper is a review of techniques of processing magnetic variation data from the point of view of their effectiveness in determining the parameters that describe a particular electromagnetic induction problem. Among the factors that influence the choice of data-processing technique are: (1) the relative importance of induction by vertical and horizontal magnetic field variations, and (2) the relative importance of local induction in the conductivity anomaly as compared with its influence on the flow of worldwide current systems.The response of an anomaly can be calculated by transfer-function methods, and presented in the form of frequency-dependent induction vectors or ellipses. The usefulness of internal/external field separation procedures is limited by the problems involved in estimating the spatial behaviour of the normal variation fields.