New Geophysical Technique for Mineral Exploration and Mineral Discrimination Based on Electromagnetic Methods

Abstract

The research during the first two years of the project was focused on developing the foundations of a new geophysical technique for mineral exploration and mineral discrimination, based on electromagnetic (EM) methods. The developed new technique is based on examining the spectral induced polarization effects in electromagnetic data using effective-medium theory and advanced methods of 3-D modeling and inversion.The analysis of IP phenomena is usually based on models with frequency dependent complex conductivity distribution. In this project, we have developed a rigorous physical/mathematical model of heterogeneous conductive media based on the effective-medium approach. The new generalized effective-medium theory of IP effect (GEMTIP) provides a unified mathematical method to study heterogeneity, multi-phase structure, and polarizability of rocks. The geoelectrical parameters of a new composite conductivity model are determined by the intrinsic petrophysical and geometrical characteristics of composite media: mineralization and/or fluid content of rocks, matrix composition, porosity, anisotropy, and polarizability of formations. The new GEMTIP model of multi-phase conductive media provides a quantitative tool for evaluation of the type of mineralization, and the volume content of different minerals using electromagnetic data.We have developed a 3-D EM-IP modeling algorithm using the integral equation (IE) method. Our IE forward modeling software is based on the contraction IE method, which improves the convergence rate of the iterative solvers. This code can handle various types of sources and receivers to compute the effect of a complex resistivity model. We have demonstrated that the generalized effective-medium theory of induced polarization (GEMTIP) in combination with the IE forward modeling method can be used for rock-scale forward modeling from grain-scale parameters. The numerical modeling study clearly demonstrates how the various complex resistivity models manifest differently in the observed EM data. These modeling studies lay a background for future development of the IP inversion method, directed at determining the electrical conductivity and the intrinsic chargeability distributions, as well as the other parameters of the relaxation model simultaneously. The new technology introduced in this project can be used for the discrimination between uneconomic mineral deposits and the location of zones of economic mineralization and geothermal resources.