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Abstract

The method presented here is a direct transformation of magnetic transfer functions into apparent resistivities and phases . The derivation is an extension of the two-dimensional procedure developed by Gharibi and Pedersen (2000) to arbitrary conductivity structures in the subsurface. The VLF technique (very-low-frequency) is a well established electromagnetic tool for mapping near surface structures of geological targets. Here, artificial source fields generated by powerful VLF transmitters situated at several locations all over the world and radio transmitters are used in induction studies for shallow applications. By combining two or more transmitters located in different directions, one can calculate a set of transmitterindependent induction parameters for each point of measurement (Pedersen [1989), Pedersen et al. (1994], Gharibi & Pedersen [2000]), which are the same as the magnetic transfer function in geomagnetic depth sounding applications (GDS). The Geological Survey of Sweden (SGU) operates a 'tensor'-VLF device as an add-on tool in airborne surveys and acquired maps of magnetic transfer functions (tipper) for frequencies around 16 kHz. We show, that the magnetic transfer function is related to the full TE-mode impedance tensor besides a constant in the electric field, which has to be estimated by other means. Use must be made of the potential field character of the magnetic field in the air half-space, yielding an successive approximation of the anomalous magnetic fields for an arbitrary primary magnetic field. When the anomalous vertical component is explicitly known, solving Faraday's equation in the wavenumber domain by using the divergence theorem leads to an estimation of the anomalous electric field of TE-mode. The normal electric field is provided by an impedance measurement at one point in the area of investigation. We demonstrate , that structural information in terms of apparent resistivity and phase reflecting different kinds of lithologies can be extracted from magnetic measurements and related to geological investigations. In the following sections, the decomposition of electric and magnetic fields and transfer functions is introduced and the procedure of the transformation is developed.