TRANSFORMATION OF VLF ANOMALY MAPS INTO APPARENT RESISTIVITY AND PHASE

Abstract

We investigate a transformation of magnetic trans- fer functions into the tangential-electric mode part of the impedance tensor in the scope of the plane-wave electromagnetic tensor-VLF method. The transforma- tion, which is applicable to any 2D data representing the response of arbitrary 3D geoelectric structures, over- comes the difficulties of quantitative interpretation of magnetic transfer functions, which predominantly pro- vide a measure of the lateral changes of the electri- cal conductivity in the earth. We require densely sam- pled magnetic transfer functions of one frequency as input data. These may be decomposed into their nor- mal and anomalous parts (deviation from the response of a layered earth) for a unit external plane-wave source field using the Hilbert transform relationship between the magnetic field components. Faraday's law then directly provides the anomalous toroidal elec- tric field. Unfortunately, there is no chance to esti- mate the normal electric field from magnetic data, since the magnetic field is not sensitive to a layered earth. This constant must be provided as a boundary condi- tion, e.g., from one ground measurement, to derive an impedance tensor and related apparent resistivities and phases.