Electromagnetic conductivities of rock cores: Theory and analog results

Abstract

Measurement theory developed for a laboratory investigation of electromagnetic characteristics of conductive and magnetic rocks agreed with experiments performed on synthetic conductive samples using a multifrequency coil‐bridge system in the 10 kHz to 4 MHz range. The theoretical analysis also provides a basic and useful method for the study of inductive [electromagnetic (EM)] conductivity and magnetic susceptibility spectra from conductive and/or magnetic samples. The skin effect, an important consequence of eddy current behavior, affects EM measurements and can be recognized by varying the energizing frequency. Investigations of the EM respses of 104 synthetic cores, of five types, revealed that textural effects, i.e., the structure, arrangement, and particle size of the conducting elements exerted a large influence on results. These analog cores contained a range of conductors, including copper wires, graphite powder, aluminum particles, and magnetite‐ilmenite grains, set in a matrix of insulating plaster or wax, representing models of layered, stringer, disseminated, network, massive, and magnetite‐rich mineralizations. For a heterogeneous sample, the laboratory‐measured EM conductivity depends strongly on the structure and orientation of the conducting materials and usually differs from the galvanic conductivity. For a sample with insulated conducting particles, the measured EM conductivity decreases with the increasing square of the sample diameter. This indicates that EM field methods may not respond well to disseminated targets with dispersed conducting minerals even at high concentrations. A mechanism attributed to magnetic loss produced unrealistically high apparent conductivities for magnetite‐bearing samples, especially at low frequencies (f < 100 kHz). For a conductive magnetite sample (σ >1 S/m), the effect of magnetic loss can be eliminated at high frequencies (f > 400 kHz ), and the ohmic conductivity of the magnetite sample can then be estimated.