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Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration

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A special issue of Minerals (ISSN 2075-163X). This special issue belongs to the section "Mineral Exploration Methods and Applications".

Deadline for manuscript submissions: closed (22 December 2023) | Viewed by 25062

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Special Issue Editors

Dr. André Revil

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Université Grenoble Alpes, Université Savoie Mont-Blanc, CNRS, UMR CNRS 5204, EDYTEM, 73370 Le Bourget du Lac, France
Interests: critical zone; hydrogeophysics; volcanology; near-surface geophysics; biogeophysics

Dr. Damien Jougnot

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Sorbonne Université, CNRS, EPHE, METIS, F-75005 Paris, France
Interests: hydrogeology; hydrogeophysics; near surface geophysics; environmental geophysics; petrophysics
Special Issues, Collections and Topics in MDPI journals

Dr. Jacques Deparis

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BRGM, 45060 Orléans, France
Interests: environmental geophysics; hydrogeophysics; near surface geophysics

Special Issue Information

Dear Colleagues,

The goal of mineral exploration is the discovery of new deposits of economic value with the purpose of extracting them to feed the needs of industry. Geophysics describes a set of non-intrusive techniques useable to image the subsurface and extract their physical properties. It is used in mineral exploration to reduce risk associated with geochemical and geological data and drilling. In the last several decades, a great deal of progress has been made in the development and use of geoelectrical and electromagnetic geophysical methods in mineral exploration—especially regarding the exploration of targets located the first kilometer below the ground surface. In addition to electrical conductivity, induced polarization is a very sensitive technique that can be used to localize and decipher minerals in the subsurface of the Earth. This Special Issue targets contributions from laboratory to field measurements, numerical forward and inverse modeling, as well as petrophysical models able to connect the properties of minerals to the geophysical properties of interest. Our goal is to provide an updated view of the state-of-the-art in terms of geoelectrical and electromagnetic methods applied to mineral exploration. This Special Issue is also open to a variety of methods, including magneto-resistivity, magneto-induced polarization as well as seismoelectric and electroseismic effects, just to cite a few.

Dr. André Revil
Dr. Damien Jougnot
Dr. Jacques Deparis
Guest Editors

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Keywords

induced polarization
mineral exploration
electrical conductivity
petrophysics
inverse modeling

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Published Papers (11 papers)

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Research

22 pages, 6963 KiB

Open AccessArticle

Combining Electrical Resistivity, Induced Polarization, and Self-Potential for a Better Detection of Ore Bodies

by Zhaoyang Su, André Revil, Ahmad Ghorbani, Xin Zhang, Xiang Zhao and Jessy Richard

Minerals 2024, 14(1), 12; https://doi.org/10.3390/min14010012 - 20 Dec 2023

Cited by 5 | Viewed by 3006

Abstract

Electrical resistivity (ER), induced polarization (IP), and self-potential (SP) are three geophysical methods that have been broadly used in the realm of mineral exploration. These geophysical methods provide complementary information, each exhibiting a distinct sensitivity to various types of mineral deposits. Considering the relationship among these three methods, we propose an integrated approach that merges their respective information to offer an improved localization technique for ore bodies. First, we invert the electrical conductivity distribution through electrical resistance tomography (ERT). Then, we use the inverted conductivity distribution to invert the IP and SP data in terms of chargeability and source current density distributions. Then, we normalize the inverted chargeability and source current density distributions and we combine them to obtain an ore body index (ORI)

χ

used to delineate the potential locations of ore deposits. We design this index to be sensitive to the presence of ore bodies, which are reflected by either strong and localized source current density (SP) and/or strong chargeability values (IP). The proposed method is first validated using a synthetic model with two distinct anomalies characterized by different properties. The results show the limitation of individual inversion, as each method exclusively detects one of these anomalies. The combined approach allows a better characterization of the target. Then, the approach is applied to a sandbox experiment in which two metallic bodies are buried in water-saturated sand used as the background. Again, the proposed methodology is successfully applied to the detection of the metallic targets, improving their localization compared with individual methods. Full article

(This article belongs to the Special Issue Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration)

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13 pages, 14370 KiB

Open AccessArticle

Investigating the Formation of Hot-Dry Rock in Gonghe Basin, Qinghai, China

by Yang Yang, Fangbo Chen, Siliu Yu, Yubin Zheng, Sujie He, Yan Zeng, Xiaoli Xie, Jie Zhu and Nan Luo

Minerals 2023, 13(8), 1103; https://doi.org/10.3390/min13081103 - 19 Aug 2023

Cited by 2 | Viewed by 1766

Abstract

The Gonghe Basin, Qinghai Province, China, has rich geothermal and hot-dry rock resources. Through a magnetotelluric survey line with 400 points, combined with regional geology data, the deep geoelectrical structural background and thermal source mechanisms of the Gonghe Basin were explored. The results showed that (1) a deep structure with high conductivity may exist at a depth of 15 km in the basin, and could be compared to the layer-shaped, low-velocity, high-conductivity structure in the eastern part of the Qinghai–Tibet Plateau; (2) the rushing reverse fault played a crucial role in heat control and conduction from the hot field; and (3) high-temperature heat storage existed, including four layers of geothermal resources. This study proposed a triple-polymorphism model of hot-dry rock in the area; that is, the high-conductivity layer in the Middle–Late Cenozoic crust was the principal heat source; the Middle–Late Triassic granite was the essential heat-storing body, as well as a parent rock to the hot-dry rock; and the Cenozoic sedimentary rock was the effective caprock. This model is critical to understanding geothermal causes, predicting geothermal resources, and planning, on the Qinghai–Tibet Plateau. Full article

(This article belongs to the Special Issue Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration)

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15 pages, 3588 KiB

Open AccessArticle

3D Multicomponent Self-Potential Inversion: Theory and Application to the Exploration of Seafloor Massive Sulfide Deposits on Mid-Ocean Ridges

by Zhongmin Zhu, Chunhui Tao, Zhigang Shan, André Revil, Zhaoyang Su, Zuofu Nie, Jinsong Shen, Xianming Deng and Jianping Zhou

Minerals 2023, 13(8), 1098; https://doi.org/10.3390/min13081098 - 17 Aug 2023

Cited by 3 | Viewed by 1858

Abstract

The marine self-potential (SP) method is currently playing an increasing role in the exploration and resource evaluation of seafloor massive sulfide (SMS) deposits. SP surveys are conducted using autonomous underwater vehicles (AUV), which yield multicomponent electric field datasets. By comparing with the single-component electrical field data used to date, the inversion of these multicomponent data is expected to provide a more accurate description of the 3D structure of SMS deposits beneath the seafloor (like gradiometry in gravity surveys). We introduce an inversion algorithm specifically adapted to multicomponent SP data. A synthetic model demonstrates that the inversion of multicomponent datasets allows us to better recover the amplitude of the current density and the morphology of the ore bodies compared to using a single component of the electrical field. Next, we apply our approach to a multicomponent SP dataset collected during the DY58 oceanic cruise at the Yuhuang hydrothermal field on the Southwest Indian Ridge. Subsequently, we reconstruct the three-dimensional (3D) geometry of the SMS deposits beneath the seafloor. The AUV-based SP system with the collection of multicomponent SP data inversion appears to be a powerful tool in the exploration and evaluation of seafloor sulfide resource and, in the future, could be used in concert with induced polarization data. Full article

(This article belongs to the Special Issue Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration)

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18 pages, 30930 KiB

Open AccessArticle

Simulation and Observations of Audio Magnetotelluric Measurements over Water-Covered Areas

by Qiong Wu, Yong-Bo Li, Hong-Ze Mi, Gang Wang and Zhen-Yu Zhang

Minerals 2023, 13(8), 990; https://doi.org/10.3390/min13080990 - 25 Jul 2023

Viewed by 1392

Abstract

Electromagnetic (EM) surveys play a significant role in mineral exploration. However, the EM method often faces limitations when investigating minerals in areas covered by rivers, lakes, or other water bodies. This paper introduces audio magnetotelluric (AMT) observation technology that utilizes separated electric and magnetic channels to deal with this challenge over water-covered areas. The study analyzes and discusses the characteristics of the relative error of the magnetic field through forward simulation. The observation and profile experiments were conducted at the estuary of a river in Liaoning Province, China, and high-quality data in the river and the pseudo-geoelectric section of the underwater space were successfully obtained. The results demonstrate the feasibility and effectiveness of the AMT observation technology over water-covered areas, emphasizing the importance of locating the magnetic channel in a quiet zone at a certain distance from the shore. This configuration helps reduce the influence of resistivity differences between water and shore, ultimately improving data quality and accuracy. The research suggests that the AMT observation technology, utilizing separated electric and magnetic channels, has the potential for further improvement and can serve as a valuable guide for mineral exploration over water-covered areas. Full article

(This article belongs to the Special Issue Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration)

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20 pages, 8831 KiB

Open AccessArticle

Conversion of Induced Polarization Data and Their Uncertainty from Time Domain to Frequency Domain Using Debye Decomposition

by Joost Hase, Grigory Gurin, Konstantin Titov and Andreas Kemna

Minerals 2023, 13(7), 955; https://doi.org/10.3390/min13070955 - 17 Jul 2023

Cited by 1 | Viewed by 1565

Abstract

The time-domain (TD) induced polarization (IP) method is used as an extension of direct current (DC) resistivity measurements to capture information on the ability of the subsurface to develop electrical polarization. In the TD, the transient voltage decay is measured after the termination of the current injection. To invert tomographic TD IP data sets into frequency-domain (FD) models of complex electrical resistivity, a suitable approach for converting TD IP transients and their corresponding uncertainties into the FD is essential. To apply existing FD inversion algorithms to TD IP measurements, a conversion scheme must transform the measured decay curves into FD impedances and also propagate the corresponding measurement uncertainty from the TD to the FD. Here, we present such an approach based on a Debye decomposition (DD) of the decay curve into a relaxation-time distribution and the calculation of the equivalent spectrum. The corresponding FD data error can be obtained by applying error propagation through all of these steps. To accomplish the DD we implement a non-linear Gauss–Newton inversion scheme. We test the conversion scheme in a synthetic study and demonstrate its application to field data on a tomographic TD IP data set measured on the Maletoyvaemskoie ore field (Kamchatka, Russia). The proposed conversion scheme yields accurate impedance data for relaxation processes, which are resolved by the TD measurements. The error propagation scheme provides a reasonable FD uncertainty estimate, as confirmed by a Monte Carlo analysis of the underlying parameter distributions. Full article

(This article belongs to the Special Issue Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration)

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17 pages, 13409 KiB

Open AccessArticle

Three-Dimensional Inversion of Long-Offset Transient Electromagnetic Method over Topography

by Xinyu Wang, Hongzhu Cai, Lichao Liu, André Revil and Xiangyun Hu

Minerals 2023, 13(7), 908; https://doi.org/10.3390/min13070908 - 5 Jul 2023

Cited by 1 | Viewed by 1874

Abstract

The long-offset transient electromagnetic method (LOTEM) is widely employed in geophysical exploration, including environmental investigation, mineral exploration, and geothermal resource exploration. However, most interpretations of LOTEM data assume a flat Earth, and the commonly used one-dimensional (1D) interpretation encounters significant challenges in achieving reliable geological interpretations when topography is ignored. To address these challenges, this study presents an effective three-dimensional (3D) LOTEM inversion method. In this study, we discretize the simulation domain using unstructured tetrahedra to accurately simulate complex geological structures. The finite-element time-domain (FETD) method is utilized to calculate the LOTEM forward responses, and the limited-memory BFGS (L-BFGS) optimization method is employed for 3D LOTEM inversion. To avoid explicit calculation of sensitivity, we obtain the product of the transposed sensitivity matrix and the vector through adjoint forward modeling. Several synthetic models are used to verify the developed program, and the influence of topography on LOTEM inversion is examined. The numerical results demonstrate that topography can significantly impact the inversion result, potentially leading to incorrect geological interpretations. Finally, the developed inversion algorithm is applied to a realistic ore model from Voisey’s Bay, Labrador, Canada. The 3D inversion successfully reconstructs the spatial distribution of the ore body, further confirming the effectiveness of the developed algorithm. Full article

(This article belongs to the Special Issue Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration)

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13 pages, 2933 KiB

Open AccessArticle

Measurement of Dried Seafloor Massive Sulfides

by Malte Wichmann and Andreas Hördt

Minerals 2023, 13(7), 867; https://doi.org/10.3390/min13070867 - 27 Jun 2023

Viewed by 1234

Abstract

We carried out spectral-induced polarization (SIP) measurements on a set of dried seafloor massive sulfide samples and compared the results with those obtained with the same samples fully saturated with NaCl solution. We find that the conductivity and polarizability are generally high for both dried and saturated samples, i.e., exhibiting phase shifts in the order of 100 s of mrad and imaginary conductivities up to 1 S/m. Depending on the particular sample, the polarizabilities of the dried samples remain as high as for the saturated samples or are slightly reduced. The high polarizability of dried samples and the fact that polarizability cannot be destroyed by drying are significant observations because most of the existing theories to explain the polarization of mineralized rock assume a pore space filled with an electrolyte. We also found that the often-used agar gel is unsuitable for coupling the dried samples to the electrodes because it releases water into the sample. Coupling with plasticine is a feasible alternative because significantly less fluid is absorbed by the sample when it is incorporated into the sample holder. Full article

(This article belongs to the Special Issue Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration)

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21 pages, 10406 KiB

Open AccessArticle

Optimizing Semi-Airborne Electromagnetic Survey Design for Mineral Exploration

by Saeed Nazari, Raphael Rochlitz and Thomas Günther

Minerals 2023, 13(6), 796; https://doi.org/10.3390/min13060796 - 10 Jun 2023

Cited by 10 | Viewed by 2516

Abstract

As semi-airborne mineral exploration has a limited budget, it is critical to design experimental procedures that generate data to maximize desired information. We investigated the effects of transmitter–receiver geometries for a variety of anomalies and semi-airborne layouts. Our simulations indicated that flight line spacing of 200 m and a point distance of 100 m provides the optimal trade-off between coverage and survey progress for various targets. Based on the target size and distance between the transmitter and the target, the transmitter length should be at least equal to the length of the target. However, where the distance between the transmitter and the target is more than 1 km, the transmitter length should be at least two or three times the target size. Of similar importance are the location and direction of the transmitter cables, which can have a significant impact on the result of inversion and should be parallel to the target strike. By using more than one transmitter, better results can be obtained. If the strike of the target is known, transmitters should be parallel to each other, and if not, it is better to use perpendicular transmitters. The results of this study showed that the optimal distance between transmitters is 3 km. Our simulations showed that it is even possible to recover targets just below the transmitter in corresponding areas of masked data. Full article

(This article belongs to the Special Issue Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration)

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25 pages, 8280 KiB

Open AccessArticle

Self-Potential as a Tool to Monitor Redox Reactions at an Ore Body: A Sandbox Experiment

by André Revil, Zhaoyang Su, Zhongmin Zhu and Alexis Maineult

Minerals 2023, 13(6), 716; https://doi.org/10.3390/min13060716 - 24 May 2023

Cited by 4 | Viewed by 2140

Abstract

Ore bodies generate natural electrical fields that are measurable at the ground surface. The ground surface signature of this electrical field is called a self-potential anomaly. We developed a sandbox experiment to monitor the evolution of a self-potential anomaly associated with redox processes mediated by bacterial activity at the surface of a buried metallic object crossing the water table. A Bio-Electrochemical Cell (BEC) is formed by a metal bar connecting the upper, oxygen-rich, part of the tank and an aquifer containing an electron donor in the form of acetate. The self-potential response was observed during a period of 327 days. The tomography of the self-potential signature confirms that self-potential tomography is able to locate the metallic target acting as a BEC. In addition, we performed redox potential, pH, and electrical potential measurements over a vertical cross-section of the tank at several time steps to obtain an idea of where the redox front is located. The distributions of the redox potential and pH further demonstrated the development of the oxidation-reduction chemical processes facilitated by the BEC as bacterial communities developed around the metallic bar. The electrical potential anomaly shows that the bacterial communities followed a short period of exponential growth, then a longer period of a sustained population. These results demonstrate the usefulness of the self-potential method in monitoring redox processes at the surface of a buried ore body. Further works will need to combine such self-potential anomalies with induced polarization anomalies through joint inversion. Full article

(This article belongs to the Special Issue Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration)

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20 pages, 3316 KiB

Open AccessArticle

Estimation of Electrical Spectra of Irregular Embedded Samples

by Malte Wichmann and Andreas Hördt

Minerals 2023, 13(3), 412; https://doi.org/10.3390/min13030412 - 15 Mar 2023

Cited by 1 | Viewed by 1267

Abstract

In order to interpret spectral induced polarization (SIP) data measured in the field for the purpose of mineral exploration, laboratory investigations are necessary that establish relationships between electrical parameters and mineral properties. For massive ores, and in particular for seafloor massive sulfides (SMSs), samples may be difficult to obtain, and it is often not desired to cut cylindrical plugs out of the available hand pieces. We suggest a method to obtain the electrical spectra of hand pieces from measurements on the samples embedded in a non-polarizing medium, in our case quartz sand. As such, destroying potentially precious samples is not necessary. The frequency-dependent phase spectrum of the sample is calculated by dividing the bulk spectrum with a so-called dilution factor, which is obtained from numerical simulation and has a real and constant value. We evaluate the method with a set of SMS samples where conventional cylindrical plugs are available. We can estimate the phase shift maximum of 73% of the samples with a deviation less than 50% from the reference. The estimation quality slightly decreases if the dilution factor is approximated by the volumetric share of the sample. We consider the performance acceptable if the general difficulty to obtain reproducible and representative laboratory measurements for massive sulfides is taken into account. Full article

(This article belongs to the Special Issue Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration)

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23 pages, 22414 KiB

Open AccessArticle

Integrated Interpretation of Electrical Resistivity Tomography for Evaporite Rock Exploration: A Case Study of the Messinian Gypsum in the Sorbas Basin (Almería, Spain)

by Alberto Pérez-López, Miguel García-López and Miguel González-Gil

Minerals 2023, 13(2), 136; https://doi.org/10.3390/min13020136 - 17 Jan 2023

Cited by 3 | Viewed by 3291

Abstract

In this study, we conduct an investigation of the Sorbas Basin (Almería, Spain) on the Messinian gypsum unit using geophysical prospecting methods. Geophysical electrical resistivity tomography (ERT) methods were applied to study the subsurface of this gypsum unit, the exploitation of which could be of interest economically, with different commercial specifications for alabastrine and selenitic gypsums. For the interpretation of the different ERT images, the data for the surface geology, borehole cores, and seismic refraction conducted at a point within the ERT profiles were used. The results obtained from this investigation can be used as a reference for other similar studies in other regions. It was observed that selenitic gypsum is more resistive than alabastrine gypsum; therefore, the diagenetic processes of dehydration (anhydritization) and hydration (gypsification) increase the “percolation” phenomenon through possibly ensuring a greater connectivity of the shale matrix. Fracturing and moisture can be used to fully determine the resistivity of the purest and most resistive gypsum, to the point of considerably lowering the resistivity in an entire area affected by fracturing. The use of different tests with different lengths for the same profile can help one better understand the structure of the gypsum body in the subsurface, especially when there are shale intercalations or more- or less-pure levels of gypsum that do not reach a value of a few meters in thickness, because these thinner levels of a few meters are not defined in the ERT images when the test is performed at depths of up to 75 m. Full article

(This article belongs to the Special Issue Development of Geoelectrical and Electromagnetic Methods in Mineral Exploration)

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