COMPUTATION OF APPARENT RESISTIVITY PROFILES FROM VLF-EM DATA USING LINEAR-FILTERING

A simple filter is developed which transforms VLF-EM real magnetic fie ld transfer functions into apparent resistivities. It is based on the relationship between the horizontal derivative of the surface electric field and the vertical magnetic field at the surface of a two-dimensi onal earth model. The performance of this simple autoregressive filter is tested for modelled and real survey data. The technique yields pro files of apparent resistivity very similar, both in magnitude and in w avelength, to those which would have been obtained using VLF-EM resist ivity measurements or d.c. resistivity profiling. This low-pass filter has the advantage of reducing highwave-number noise in the data; ther efore only the major features of the VLF-EM profile are displayed.