A PHYSICAL STUDY OF LOW-FREQUENCY DISPERSION OF ROCK CONDUCTIVITY IN TIME-DOMAIN ELECTROMAGNETICS

Most rocks display conductivity dispersion in the low-frequency range, when the usual displacement currents are neglected. The strong influe nce this low-frequency dispersion (LFD), including the response sign r eversals, was revealed by field experiments with the coincident-loop c onfiguration widely used in transient electromagnetics (EM). Mathemati cal modelling of LFD has been the subject of numerous studies. However , confirmation of the role of LFD or induced polarization (IF) by comp aring mathematical modelling and field data is rather poor, because kn owledge of the properties of rocks in the area of the field measuremen ts is usually insufficient. For this reason physical modelling of LFD has been carried out at Moscow State Geological-prospecting Academy (R ussia) in 1994-95. In order to observe criteria of similarity for both induction and polarization transients, a ring-shaped model was chosen and was represented by an electric circuit, consisting of lumped elem ents (real rock samples included). Qualitatively different transients for dispersive models and their non-dispersive ohmic equivalents were observed.