Exploration Geophysics is published by CSIRO PUBLISHING on behalf of the Australian Society of Exploration Geophysicists. Papers published report the results of significant case histories and relevant original research in geophysics, with emphasis on the Australian and similar environments.

The Yeoncheon titanomagnetite deposit formed by Precambrian magma differentiation is located in Gyeonggi-do, South Korea. Our team conducted an airborne magnetic survey for multiscale mineral exploration and then selected a promising survey area. An electrical resistivity survey was carried out in the potential area to image subsurface structure. Because ore minerals are mainly distributed in gabbro monzodiorite rather than quartz monzodiorite, we applied three-dimensional inversion of electrical resistivity tomography (ERT) data to identify lithology boundaries related to magma differentiation. The resistivity criterion distinguishing the lithologies of gabbro and quartz monzodiorite was determined from laboratory resistivity experimental results performed on drilling cores. The selected region for gabbro monzodiorite extends to the northeast direction, which is consistent with the geology map, magnetic anomaly, and drilling data. The inversion results of ERT can help in selecting the location of geophysical survey or drilling.

Understanding the physical flow mechanisms in aquifer systems is essential in effectively protecting groundwater resources and preserving subsurface environments from a wide range of contaminants. A conceptual model is a simplified representation of a groundwater system and gaining knowledge about the geological features and parameters controlling the flow and transport processes is a crucial first step towards properly constructing a site-scale conceptual model. In this study, we present a multi-step workflow that involves integrated borehole techniques to gain information concerning groundwater flow. Measurements from core-scale to field-scale enable us to better build a subsurface geological structure divided into the unconsolidated layer and the fractured bedrock. In addition, neutron logging and mercury injection capillary pressure techniques allow for the development of vertical porosity distribution in the alluvial layer. For fracture characterization, the fracture geometry is delineated using a series of borehole imaging techniques and single-hole tests to differentiate the individual permeable fractures from other hydraulically inactive fractures. Combining the hydraulic and geometric evaluations, the presence of large-scale connective fracture networks is identified. Our high-resolution three-dimensional (3D) site-scale conceptual model is expected to contribute to improving the reliability and availability of numerical groundwater models.

Our research group has been studying the modeling and inversion algorithms of complex resistivity (CR) method since several years ago and recently applied developed algorithms to various field applications. Because of tough and complex terrain in our country, simple profile survey and 2D interpretation were generally used. In this study, we developed three dimensional modeling and inversion algorithm for complex resistivity method to get more precise interpretation. We adopt the adaptive lagrangian multiplier determination method in the 3D inversion which is automatically set based on the size of error misfit and model regularization norm. It was tested on the real data acquired for two test sites. Data was acquired with the Zeta system, the complex resistivity measurement system manufactured by Zonge Co. In the interpretation only the lowest frequency data is used considering its quality. After that, developed 3D inversion was applied to the each data set. Its results were compared to those of resistivity data conducted at the same site. Resistivity image sections of CR and conventional resistivity method were almost identical. Anomalies in phase section showed good agreement with those identified by the interpretations of resistivity monitoring data and other geological information. Phase and resistivity anomalies were more clearly discriminated in 3D interpretation than 2D. CR method is very effective if the target has strong IP property and can be used for various purposes. But it has some difficulties in data acquisition that it takes more time and efforts compared to normal resistivity survey.

The direct current electrical resistivity survey or electrical resistivity tomography (ERT) is a geophysical exploration technique employed in various fields, including mineral exploration, environmental studies, and subsurface safety assessments. ERT, primarily conducted at the surface, can also be conducted between adjacent boreholes to yield high-resolution subsurface images at greater depths. Compared to surface exploration methods, cross-hole ERT employs distinct electrode configurations with varying sensitivities, making it crucial to consider these differences in electrode configuration or measurement environment conditions. This study elucidates electrode configurations and subsequently analyzes the characteristics of cross-hole ERT data for each electrode configuration. Following this, we explore the peculiarities stemming from borehole ERT data. We also describe distortions in ERT data caused by survey geometry and provide an examination of potential solutions to mitigate these distortions. Additionally, we introduce several case studies involving borehole ERTs.

Recently, due to the active spread of electric vehicles, the demand for batteries is increasing fast, and for this reason, the exploration for lithium that is an essential mineral for battery production, is increasing. In Korea, lithium exploration is also being conducted around deposits where lithium was identified in the past. However, most lithium mines are located in very rough terrain, so it is not easy to conduct a surface geological and geophysical exploration. Without considering complex topography, errors may occur in the inversion of surface geophysical exploration data, and in particular, it is necessary to use precise topographic information for the three-dimensional inversion. In this study, we would like to introduce a case study using high-resolution topographic data obtained from a drone-mounted LIDAR in the three-dimensional inversion of surface resistivity and IP data conducted for lithium exploration. The target area is the Boam Mine, located in the Middle East of Korea. Surface geophysical exploration was conducted along a road and ridge of the mountain, which are relatively easy to set up the survey line. Because existing topographic maps that are publically available did not include mining traces related to mining development and topographical changes formed by nearby roads, it is not adequate for the 3D inversion of surface resistivity and IP data. To acquire precise topographical information, aerial photography and LIDAR measurements using drones were performed. A numerical topographic model was constructed using the obtained high-precision DEM (digital elevation map). By applying this to the three-dimensional inversion, the distribution of the underground mineralization zone was estimated. The interpreted results were compared with the existing drilling results performed near the mine. Comparing the two results, drilling surveys using only surface geological information proceeded in the direction in which the mineralization zone did not develop. Drone LIDAR measurement is a costly exploration method and is difficult to use actively at all exploration sites. However, if three-dimensional inversion is required where the surface topography is very complex, as in this survey area, it could give more reliable inversion results.