Editorial for Special Issue “Using Geophysical Inversion for Mineral Exploration: Methods and Applications”

1. Introduction

Today, minerals are playing a critical role in the transition from fossil fuel-based energy systems to renewable and sustainable energy sources (Owen et al., 2023) [1]. Several critical minerals, including lithium, cobalt, nickel, rare earth elements (REEs), and graphite, among others, are essential both economically and technologically, but their supply chains are vulnerable to disruption. More traditional resources, like copper and zinc, are also irreplaceable in advanced material engineering for energy generation, storage, and transmission applications. Advanced geoscientific technologies are needed to exploit these diverse minerals to meet the industrial demand. Geophysics can contribute to the efficient and effective search for mineral deposits, especially when the targets are scattered over a large area or buried deeply underground (Dentith et al., 2018; Wei et al., 2023) [2,3].

In the early days of geophysical prospecting, the interpretation of geophysical data was mostly through so-called “bump hunting” on data plots or other data proxies, which gave only rough qualitative information about the location and depth of the targets sought. Advancements in numerical simulation, optimization algorithms, and computing devices have made quantitative inversions practical, so the physical fields observed on the Earth’s surface can be converted to reliable images of the subsurface characterized by diagnostic physical properties (Phillips et al., 2001) [4]. Inversions have now become a standard procedure in the workflow of the mineral exploration industry to provide key information for resource estimation and drillhole targeting (Zhdanov et al., 2018; Guo et al., 2020) [5,6].

Developing robust inversion techniques for geophysical applications, and for many other fields, can be demanding since they have to properly handle the intrinsic non-uniqueness and ill-posed nature of the involved geophysical methods to obtain physical models that effectively describe the true parameter distribution underground (Ellis, 1998; Yin, 2000) [7,8]. They are also non-trivial when applied in realistic exploration problems, since there are usually a number of restricting factors, such as noise in data, slow computing speed, and the unavailability of proper a priori information. Other challenges that have been studied extensively include subsurface complexity, anisotropy, dispersion, and difficulties in implementing constraints due to the need for sufficient in situ data, particularly from the depth.

In response to the demand for advanced inversion technology for mineral exploration, we have organized this Special Issue, “Using Geophysical Inversion for Mineral Exploration: Methods and Applications”, to capture some of the latest methodological research and applications of geophysical inversion for modern mineral exploration at this strategic moment.

2. Three-Dimensional Inversions

Three-dimensional (3D) inversions have, in the past decade, been the central topic of research in geophysical inversion (Cox et al., 2012; Martinez et al., 2013; Yang & Oldenburg, 2017; Oldenburg et al., 2020) [9,10,11,12] because a rigorous 3D inversion on a structured or unstructured mesh (also known as voxel inversion) can depict arbitrarily complex geologic targets but is often challenged by computational limitations and convergence issues. The importance of 3D inversion, which can offer high-resolution images of subsurface volumes hosting mineral resources, is evidenced by four out of seven articles in this Special Issue.

Semi-airborne electromagnetic methods place the source on the surface and measure the time-varying magnetic field in the air using aircraft (Becken et al., 2020) [13]. Powered by the latest drone technology, semi-airborne methods are becoming popular for mineral exploration and geological studies in general because they are more cost-effective, have higher resolution, and have a greater depth of penetration than traditional ground and airborne EM methods. Article 4 [14], a theoretical contribution to inversion algorithms, presents a 3D inversion scheme of multi-component semi-airborne transient electromagnetic (SATEM) data for mineral exploration in topographically challenging areas. The paper presents development of a 3D inversion code and demonstrates the successful recovery of subsurface resistivity from complex synthetic models in areas with complex terrain. It also advocates for the measurement of multi-component EM data over single-component data to accurately recover the vertical boundaries of the targeted mineralization.

A 3D inversion offers the flexibility to describe geological features in any geometric shape, but it has to deal with non-uniqueness and convergence issues. Article 5 [15] offers an interesting field data example in which negative TEM data were observed in the central loop configuration, a sign usually considered the IP effect. However, through 3D modeling and inversion, they showed that the 3D coupling effect caused the negative data, and 3D inversion was the only way to position the sought geologic objects properly. They also present a generic workflow for 3D TEM inversion, particularly for mineral exploration, in which strong lateral contrast may exist widely.

3. Multi-Physics and Multi-Parameter Characterization

The precise identification and location of mineral deposits from geophysical data can significantly benefit from multi-physical and multi-parameter characterization (Zhdanov et al., 2021) [16]. Thanks to new algorithms integrating information from multiple sources and the continuously increased acquisition efficiency, most mineral exploration projects now employ at least two or three different surveying methods sensitive to the same or different physical properties (Bo et al., 2024) [17]. There are a number of demonstrative sites that have been studied in the published literature. The articles in this Special Issue also reflect the general trend of employing more methods and parameters to better constrain the inversions and enhance the geological interpretation.

In the conventional workflow of mineral exploration, geophysicists predominantly use gravity, magnetic, electric, and EM methods. Interestingly, exploration geophysicists have recently paid more attention to seismic methods for mining purposes (Roots et al., 2017) [18]. Article 6 [19] in this Special Issue is an excellent example of how local earthquake tomography is used to study a metallogenic Maricunga belt in Chile on a regional scale. They found a clear spatial correlation between low Vp/Vs anomalies and the locations of gold deposits. In contrast, high Vp/Vs anomalies coincided with the main regional faults, which may act as the main fluid pathways. The authors explain the observed low Vp/Vs anomalies with the genesis of porphyry-type deposits and demonstrate, through their study, that earthquake tomography can provide additional information to characterize mineral potential at a regional scale.

The importance of utilizing multiple methods and physical parameters is further highlighted in Articles 1, 3, and 7 [20,21,22]. Article 1 [20] presents a successful case study for graphite exploration in North Norway, where multiple geophysical and geological surveys at multiple scales were integrated. They first quickly surveyed large areas using airborne geophysics, including frequency-domain electromagnetic, magnetic, and radiometric methods, confirming the low resistivity and low magnetic anomalies observed at previously known graphite deposits. Based on the airborne survey findings, new locations were followed by ground geophysical surveys, e.g., EM31, charge potential, self-potential, and electric resistivity tomography, to successfully find buried high-quality graphite with a high total carbon of up to 40% by trenching and drilling. The article is a perfect example of the use of airborne geophysical surveys as reconnaissance surveys to mark areas for ground follow-up surveys and finally confirm graphite deposits by drilling.

Article 3 [21] is another excellent example of a multi-methodological study involving magnetic, VLF, electric and induced polarization (IP), and self-potential (SP) from the North Singhbhum Mobile Belt in eastern India to map the geological settings controlling gold mineralization along a shear zone. It is another example of the importance of conducting geophysical surveys at different scales and using multiple methods sensitive to different physical properties to resolve structures relevant to ore-forming processes and extract parameter information to pinpoint potential exploration targets. They applied both deterministic and global inversion schemes (particle swarm optimization), which shows a general trend of both types of optimization approaches being applied today, alongside each other, and benefitting from the flexible use of inversion strategies.

Article 7 [22] presented a convincing example of the simultaneous interpretation of the resistivity model from CSAMT inversion and the frequency dispersion parameter from SIP inversion to better understand the mineralization system of a large gold deposit. CSAMT and SIP have widespread applications in the practice of mining geophysics because they characterize the ore-controlling and ore-bearing objects with two different electric properties—resistivity and chargeability. The combined signature of low resistivity and high dispersion is a good tracer of the mineralization in the ductile shear zone outside the alkaline magmatic rock.

Most geophysical surveys are carried out for exploration and to locate resources. It is worth noting that Article 2 [23] in this Special Issue presents a novel application of geophysical inversions in mine development. The geotechnical parameters (e.g., cohesion, plastic index, moisture content, friction angle) are vital for the planning and operation of coal mining. Traditional methods use a large amount of borehole data to estimate these parameters. However, Article 2 [23] demonstrates an alternative method to calculate them using the empirical relations between these parameters and ERT resistivity. The presented method is much more cost-effective than traditional methods, although such empirical relations from laboratory measurements may not be valid globally, and local conditions must be carefully considered.

4. Summary

The seven articles introduced above are a timely snapshot of current technology and practices using inversions for mineral exploration. The Special Issue is not a comprehensive collection. Nevertheless, it has good coverage of different geophysical methods, the latest theoretical developments, and inspiring practical applications. Finally, we would like to draw the reader’s attention to some cutting-edge technologies for geophysical inversions that are expected to emerge in future Special Issues. The first is the rapidly developing artificial intelligence (AI) opportunities boosted by huge computing power. AI-augmented geophysical information handling has evolved from being purely data-driven in the early days to a hybrid mode incorporating data and physics-driven mechanisms for improved accuracy (Colombo et al., 2021; Sun et al., 2021) [24,25]. AI’s potential can be fully exploited when working with 3D inversions and high-resolution subsurface imaging when the number of unknowns is prohibitively large for traditional methods (Huang et al., 2021) [26]. The second is the extensive applications of joint inversion in practice (Zhdanov et al., 2024) [27]. Joint inversion has gained significant theoretical and algorithmic advancements but has not been a necessary part of the industrial workflow. Thanks to the latest instrumentational developments and reduced acquisition costs, large volumes of multi-physical data are becoming available, paving the way for the practical applications of joint inversion with high reliability.