COMPLEX DIELECTRIC PERMITTIVITY IN ROCKS: A METHOD FOR ITS MEASUREMENT AND ANALYSIS

Abstract

The interfacial effects arising in electrical measurements of rocks, when a two‐electrode system is used, are analyzed via the Maxwell‐Wagner effect. The similarity in electrical behavior between rock samples and heterogeneous dielectrics with nonnegligible ohmic conductivities leads to a general analysis of the Maxwell‐Wagner effect. We conclude that, in general, it is necessary to have volume charge accumulations at interfaces within the sample and at the electrode‐sample interface in order to have electrical steady‐state conditions in rock samples. The expression for the charge accumulation at the interface as a function of frequency is obtained as well as expressions for the effective dielectric permittivity in a composite material; the relaxation time associated with the charge accumulation and that of the effective dielectric permittivity are the same. Given a sample consisting of two different materials, it is shown that whenever both components have nonnegligible ohmic conductivities, the imaginary component of the effective dielectric permittivity tends to infinity when the frequency tends to zero; such behavior is not observed when one of the two conductivities is negligible. This fact constitutes the basis of the method of measurement for heterogeneous dielectrics with nonnegligible ohmic conductivities proposed herein. The method consists essentially of utilizing a dielectric with negligible ohmic and dielectric conductivities as one of the two components of the sample, which isolates the other one from the electrodes. Theoretical examples of the behavior of such a composite material for various cases are analyzed and three experimental examples are presented. The method presents the advantage of permitting application of theory developed for dielectrics in general.