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The induced polarization influence in transient geoelectromagnetics


  • James R. Wait


In inductive methods of electromagnetic prospecting the transient magnetic field response, for a step function current in the source loop, is measured. It has been observed that the waveform of the received signal may change sign if the medium is polarizable in the sense that the complex conductivity is a function of frequency. We present a simple spherical shell model which incorporates such IP (induced polarization) in the formulation of the time dependent secondary field. It is shown that the electromagnetic and the induced polarization effects are additive if the target is weakly polarizable. In general, however, the EM and the IP influences are intertwined in a nonsimple manner. We also approach the problem using a general functional formulation which leads to a relatively simple procedure to deduce the form of the negative slow tail of the waveform which is the portion resulting from frequency dispersion. We conclude that such dispersive effects may have a profound influence even when the medium is weakly polarizable.

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