M INERALS Article Effective-Medium Inversion of Induced Polarization Data for Mineral Exploration and Mineral Discrimination: Case Study for the Copper Deposit in Mongolia Michael Zhdanov 1,2 , Masashi Endo 1, *, Leif Cox 1 and David Sunwall 1 1 TechnoImaging, Salt Lake City, UT 84107, USA; mzhdanov@technoimaging.com (M.Z.); leif@technoimaging.com (L.C.); david@technoimaging.com (D.S.) 2 Department of Geology and Geophysics, The University of Utah, Salt Lake City, UT 84112, USA * Correspondence: masashi@technoimaging.com; Tel.: +1-801-264-6700 Received: 30 November 2017; Accepted: 7 February 2018; Published: 14 February 2018 Abstract: This paper develops a novel method of 3D inversion of induced polarization (IP) survey data, based on a generalized effective-medium model of the IP effect (GEMTIP). The electrical parameters of the effective-conductivity model are determined by the intrinsic petrophysical and geometrical characteristics of composite media, such as the mineralization and/or fluid content of rocks and the matrix composition, porosity, anisotropy, and polarizability of formations. The GEMTIP model of multiphase conductive media provides a quantitative tool for evaluation of the type of mineralization, and the volume content of different minerals using electromagnetic (EM) data. The developed method takes into account the nonlinear nature of both electromagnetic induction and IP phenomena and inverts the EM data in the parameters of the GEMTIP model. The goal of the inversion is to determine the electrical conductivity and the intrinsic chargeability distributions, as well as the other parameters of the relaxation model simultaneously. The recovered parameters of the relaxation model can be used for the discrimination of different rocks, and in this way may provide an ability to distinguish between uneconomic mineral deposits and zones of economic mineralization using geophysical remote sensing technology. Keywords: effective-medium; induced polarization; 3D inversion 1. Introduction The induced polarization (IP) effect is caused by the complex electrochemical reactions that accompany current flow in the earth. These reactions take place in a heterogeneous medium representing the rock formations in areas of mineralization. It was demonstrated almost forty years ago in the pioneering papers [1–4] that the IP effect may be used to separate the responses of economic polarized targets from other anomalies. However, until recently, this idea had very limited practical application because of the difficulties in recovering the induced polarization parameters from the observed electromagnetic (EM) data, especially in the case of the 3D interpretation required for efficient exploration of the mining targets. The quantitative interpretation of IP data in a complex 3D environment is a very challenging problem. The most widely used approach to solving this problem, which is considered the industry standard, was developed by the University of British Columbia’s Geophysical Inversion Facility (UBC-GIF). This approach is based on an assumption that the chargeability is relatively small and the IP data can be expressed as a linear functional of the intrinsic chargeability [5,6]. The corresponding linear inverse problem is then solved to obtain the chargeability model under an assumption that the data are not affected by EM coupling. The main limitation of this linearized approach is that it ignores Minerals 2018, 8, 68; doi:10.3390/min8020068 www.mdpi.com/journal/minerals