TY - JOUR

T1 - Spectral induced polarization in anisotropic rocks with electrically conductive inclusions

T2 - Synthetic model study

AU - Gurin, Grigory

AU - Titov, Konstantin

AU - Ilyin, Yuri

AU - Fomina, Ekaterina

N1 - Funding Information: This work was supported by the Russian Science Foundation, grant no. 20-47-04402 'Development of the induced polarization theory with application to exploration of strategic raw materials'. Equipment of RC 'Geomodel' Core Facility of St. Petersburg State University was used. We thank the Associated Editor Fern Storey, Matthias Bucker and anonymous reviewer for very constructive comments. The respective data are available upon request to authors. Publisher Copyright: © 2020 The Author(s) 2020. Published by Oxford University Press on behalf of The Royal Astronomical Society. Copyright: Copyright 2020 Elsevier B.V., All rights reserved.

PY - 2021/2

Y1 - 2021/2

N2 - Electrically conductive minerals (e.g. graphite, pyrite, chalcopyrite and magnetite) occur in the various geological contexts. They might represent economic resources or serve as indicators of such resources. In addition, they can be sources of contamination of soil and groundwater. Therefore, characterization of rocks containing electrically conductive inclusions is an important task in many sectors of science and economy. We conducted laboratory measurements to study the impact of the shape, composition, size and passivation character of electrically conductive inclusions on the induced polarization (IP) parameters. This paper presents results of time-domain IP measurements performed on 22 synthetic models, which were made of sieved sand mixed with electrically conductive particles. We carried out the IP measurements while varying orientation of the electrical field relative to the long axis of the inclusions. We found that the total chargeability of the models (M) strongly depended on the volumetric content (ζ), shape and characteristic size (l) of the inclusions. It also depended on the angle between the electrical field direction and the orientation of the long axis of the inclusions (α), which made the models anisotropic. Experimental relationships between M, ζ, l and α were found consistent with predictions of the generalized Maxwell-Garnett mixing equation in the tensor form. In contrast to M, the relaxation time (τ) of the studied models was almost independent of $\alpha $. Exceptions were the models with cylindrical electrically conductive particles, which showed a strong relationship between τ and $\ \alpha $. Despite the previous assumptions, no unique relationship between τ and the characteristic length of electrically conductive inclusions was observed. However, for all particle shapes, $\tau $ was proportional to the surface area of the particles. We also studied how passivated areas on the surface of the inclusions modified the spectral IP parameters. We found that passivation of electrically conductive inclusions led to anisotropy of M, while τ remained almost unaffected by the orientation of the polarizing field. Based on the experimental data, we show that the polarization magnitude of electrically conductive inclusions is proportional to the normal component of the electrical current density on their surface. We also show that the relaxation time is proportional to the area of the active surface of the inclusions. The obtained relationships highlight the importance of the interfacial polarization mechanism of the electrically conductive inclusions.

AB - Electrically conductive minerals (e.g. graphite, pyrite, chalcopyrite and magnetite) occur in the various geological contexts. They might represent economic resources or serve as indicators of such resources. In addition, they can be sources of contamination of soil and groundwater. Therefore, characterization of rocks containing electrically conductive inclusions is an important task in many sectors of science and economy. We conducted laboratory measurements to study the impact of the shape, composition, size and passivation character of electrically conductive inclusions on the induced polarization (IP) parameters. This paper presents results of time-domain IP measurements performed on 22 synthetic models, which were made of sieved sand mixed with electrically conductive particles. We carried out the IP measurements while varying orientation of the electrical field relative to the long axis of the inclusions. We found that the total chargeability of the models (M) strongly depended on the volumetric content (ζ), shape and characteristic size (l) of the inclusions. It also depended on the angle between the electrical field direction and the orientation of the long axis of the inclusions (α), which made the models anisotropic. Experimental relationships between M, ζ, l and α were found consistent with predictions of the generalized Maxwell-Garnett mixing equation in the tensor form. In contrast to M, the relaxation time (τ) of the studied models was almost independent of $\alpha $. Exceptions were the models with cylindrical electrically conductive particles, which showed a strong relationship between τ and $\ \alpha $. Despite the previous assumptions, no unique relationship between τ and the characteristic length of electrically conductive inclusions was observed. However, for all particle shapes, $\tau $ was proportional to the surface area of the particles. We also studied how passivated areas on the surface of the inclusions modified the spectral IP parameters. We found that passivation of electrically conductive inclusions led to anisotropy of M, while τ remained almost unaffected by the orientation of the polarizing field. Based on the experimental data, we show that the polarization magnitude of electrically conductive inclusions is proportional to the normal component of the electrical current density on their surface. We also show that the relaxation time is proportional to the area of the active surface of the inclusions. The obtained relationships highlight the importance of the interfacial polarization mechanism of the electrically conductive inclusions.

KW - Electrical anisotropy

KW - Electrical properties

KW - Electromagnetic theory

KW - Hydrogeophysics

UR - http://www.scopus.com/inward/record.url?scp=85097505138&partnerID=8YFLogxK

U2 - 10.1093/gji/ggaa480

DO - 10.1093/gji/ggaa480

M3 - Article

AN - SCOPUS:85097505138

VL - 224

SP - 871

EP - 895

JO - Geophysical Journal International

JF - Geophysical Journal International

SN - 0956-540X

IS - 2

ER -