Computation of apparent resistivity profiles from VLF‐EM data using linear filtering1

A simple filter is developed which transforms VLF‐EM real magnetic field 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‐dimensional earth model. The performance of this simple autoregressive filter is tested for modelled and real survey data. The technique yields profiles of apparent resistivity very similar, both in magnitude and in wavelength, to those which would have been obtained using VLF‐EM resistivity measurements or d.c. resistivity profiling. This low‐pass filter has the advantage of reducing high‐wavenumber noise in the data; therefore only the major features of the VLF‐EM profile are displayed.