Inverted Resistivity Sections Research Articles

Assessment of three mixed arrays dataset for subsurface cavities detection using resistivity tomography as inferred from numerical modelling

The present study deals with the evaluation of a three-mixed array dataset for the detection of subsurface cavities using conceptual air-filled cavity model sets at different depths. Cavity models were simulated using the forward modelling technique to generate synthetic apparent resistivity data for three common individual arrays. These arrays are dipole–dipole (DD), pole–dipole (PD), and Wenner–Schlumberger (WS). The synthetically apparent resistivity data obtained from two different individual arrays were merged to form a high-resolution single model. Consequently, three possible mixed arrays datasets can be obtained: the dipole–dipole-Wenner–Schlumberger (DD+WS), pole–dipole, and Wenner–Schlumberger (PD+WS), and dipole–dipole and pole–dipole (DD+PD). The synthetically apparent resistivity data for both the individual and mixed arrays were inverted using Res2dinv software based on the robust constrain inversion technique to obtain a 2D resistivity model section. The inverted resistivity sections were evaluated in terms of their recovering ability of the model’s parameters (e.g. resistivity, and geometry). The results show that the individual arrays can resolve the location and dimensions of the cavity within reasonable accuracy only at a depth not exceeding 6 m below the surface. On the other hand, a significant resolution enhancement in model resistivity with increasing depth was observed when the mixed arrays were used. The (DD+WS) mixed arrays dataset brings up better model resistivity and shows closer parameters to the true actual model among the other mixed arrays. So it is strongly recommended for cavity detection studies.

New Insights into the Gold Mineralization in the Babaikundi–Birgaon Axis, North Singhbhum Mobile Belt, Eastern Indian Shield Using Magnetic, Very Low-Frequency Electromagnetic (VLF-EM), and Self-Potential Data

The North Singhbhum Mobile Belt (NSMB) in the eastern Indian shield is an excellent example of a diverse lithological domain with multiphase shear/fracture zones. It is a well-acknowledged prospective region known for various mineralizations, e.g., Cu, Au, U, and Fe. The present study was conducted in the Babaikundi–Birgaon Axis (a shear and fracture zone within NSMB) within Babaikundi village. The study suggests that anomalous zones are possibly related to gold-associated sulfide mineralization using photomicrography and geophysical methods such as magnetic, self-potential (SP), and very low-frequency (VLF) electromagnetic (EM) methods. Photomicrography on quartz samples and Scanning Electron Microscope (SEM) Backscattered-Electron (BSE) images show that gold particles are present in the quartz reef vein in Babaikundi. Magnetic data are presented in the total magnetic intensity (TMI), upward continuation, and analytical signal maps. VLF-EM data were analyzed using different processing techniques to delineate anomalous auriferous zones. Furthermore, the SP data were inverted using the Particle Swarm Optimization (PSO), Very Fast Simulated Annealing (VFSA), and Genetic Algorithm (GA) approaches for comparative study of the resultant model parameters and their suitability considering a 2D inverted resistivity section derived using VLF-EM. The photomicrography study indicates the existence of gold/ pyrite/pyrrhotite within the Babaikundi–Birgaon axis, which corresponds to a low magnetic anomaly based on a magnetic study; a high current density, a high Fraser value, and a low resistivity based on the VLF-EM study; and a negative SP value was found based on the SP study. The combined study generally supports mapping auriferous zones and demarcates the Babaikundi–Birgaon axis within the Babaikundi–Birgaon area.

Shallow Permafrost at the Crystal Site of Peaceful Underground Nuclear Explosion (Yakutia, Russia): Evidence from Electrical Resistivity Tomography

The site where a peaceful underground nuclear explosion, Crystal, was detonated in 1974, at a depth of 98 m in perennially frozen Cambrian limestones, was studied by electrical resistivity tomography (ERT) in 2019. The purpose of our research, the results of which are presented in this article, was to assess the current permafrost state at the Crystal site and its surroundings by inversion and interpretation of electrical resistivity tomography (ERT) data. Inversion of the ERT data in Res2Dinv verified against ZondRes2D forward models yielded 2D inverted resistivity sections to a depth of 80 m. The ERT images revealed locally degrading permafrost at the Crystal site and its surroundings. The warming effect was caused by two main factors: (i) a damage zone of deformed rocks permeable to heat and fluids, with a radius of 160 m around the emplacement hole; (ii) the removal of natural land cover at the site in 2006. The artificial cover of rock from a nearby quarry, which was put up above the emplacement hole in order to prevent erosion and migration of radionuclides, is currently unfrozen.

Integrated Geophysical Investigation of River Ogun Floodplain, Papalanto, Southwestern Nigeria

A geophysical investigation that involves the integration of Vertical Electrical Sounding (VES), Electrical Resistivity Imaging (ERI), and Ground Penetrating Radar (GPR) methods was conducted on Papalanto, Ogun River floodplain, a location underlain by sedimentary terrain of Southwestern Nigeria. This research aimed to image the underground lithological units and delineate the shallow geologic structures in order to characterize the area for agricultural suitability throughout the dry season. VES results typically outlined three geologic layers which are topsoil, saturated loamy clay, and alluvium. From 2D inverted resistivity results, three major geologic layers, namely topsoil, saturated loamy clay, and alluvium, were outlined and are in very good agreement with the results attained through VES. The first three layers of 3D inverted resistivity sections display a great amount of variation in the distribution of resistivity at superficial depth, made up of low resistive content. From the GPR survey, three geologic layers were also outlined from the results, namely the topsoil, saturated loamy clay, and alluvium. Consequently, the study location can be said to be semi-competent to competent luxuriant farming land in consideration of the resistivity distribution of the floodplain subsurface. Therefore, VES, ERI, and GPR are very effective geophysical methods for describing and classifying the shallow subsurface in reference to the measured physical properties. Hence, they should be applied in related geophysical investigations for better insight into the geology of the subsurface. Keywords: Agricultural suitability; Dry season; Floodplain; Geophysical investigation; Underground lithological units.

2-D Electrical Resistivity Imaging Survey for Lithological Assessment at Igwete Primary School, Amai, South-South Nigeria

Electrical Resistivity Imaging (ERI) is a useful near-surface imaging technique, which mainly include data acquisition, numerical modelling and tomographic inversion. Within the study area, only one – dimensional (1-D) Electrical Resistivity survey has been carried out for Geophysical investigations. Therefore, 2-D ERI survey was carried out at the Igwete Primary School, Amai to provide electrical picture of the subsurface from which discrete bodies and lithology are better revealed vertically and in lateral extent. The 2-D ERT survey data were acquired using the Petrozenith Earth Resistivity meter while employing the Wenner electrode array. The 2-D apparent resistivity data were inverted to obtain true resistivities of the subsurface using res2dinv software running on personal computer. The subsurface resistivity models were displayed as pseudo sections and inverted resistivity section in the form of colour shaded contour maps. The inverse resistivity model images indicate that at a lateral extent in the range (15.00-21.00) m and (33.00-39.00) m, anomalies suspected to be gravel mixed with sand is in place with resistivities of about (254.00-948.00) Ωm. From the geologic section we can infer that a geological formation is observed at a lateral position of (27.00-32.00) m of resistivity in the range (90.00-93.00) Ωm. This structure is inferred to be a clay pocket. The sandy nature of the formation requires that underground water development be sought for at (9.00-15.00) m over a depth (2.30-8.00) m in the sandy environment. Results of 2D resistivity imaging has helped to delineate the lithology which comprise mainly of; sand, sandy clay, clayey sand depositional environment. The resistivity of these lithology falls in the range (90.00-93.00) Ωm with depth to formation of about (2.30-6.00) m.

Identification, Monitoring, and Assessment of an Active Landslide in Tavan-Hauthao, Sapa, Laocai, Vietnam – A Multidisciplinary Approach

The active landslide located in the Tavan-Hauthao, Sapa district, Laocai province, Vietnam was investigated using geophysical methods (2D Electrical Resistivity and Tomography), geotechnical investigations, and a ground survey to assess the geologic condition of the sliding block and surrounding ground. Landslide displacement was measured using 15 surface monitoring points. Numerical modeling was done to assess the behavior of an active landslide. This multi-disciplinary approach helped in interpreting landslide stratigraphy, geotechnical characteristics of the sliding groundmass, depth, and nature of the sliding plane. The surface area of the slide is approximately 1200 m2. Studying this active landslide is important as it affects the road No. 152, which is an important road connecting the Sapa Ancient Rock Field. This study involved surface topographical survey, surface and sub-surface geological, and geotechnical investigations including Standard Penetration Test and Electrical Resistivity Tomography. Geologic and geotechnical data were used to characterize an active landslide block, which is composed of different soil layers underlaid by granitic rock. The surface electrical-resistivity measurements across the Sapa landslide resulted in inverted-resistivity sections with distinct resistivity contrasts that correlated well with the geology and geo-hydrology observed in boreholes.

Geophysical Investigation Using 3-Dimensional Grid-Formation for Subsurface Lithology Characterization (A Case Study of Ovia North East, Edo State, South South Nigeria)

Geophysical investigations using three-dimensional (3D) grid formation was carried out in Ovia North East Local Government Area of Edo State, Nigeria for subsurface lithology characterisation so as to generate a comprehensive basemap of the study area. Twelve (12) traverses in form of a rectangular grid were occupied for the 2D Electrical Resistivity Imaging (ERI) using the Wenner array. The 2D were all collated to form the 3D grid. The 2D Electrical Resistivity data was processed by the inversion of the 2D apparent resistivity data using the DIPRO software to generate the 2D inverted resistivity section while the 3D inverted resistivity model was done by inverting all the twelve traverses using 3DEarthimager software to model the 3D cube. The results of the 2D ERI revealed three (03) to five (05) resistivity structures across the twelve traverses indicating clay/clayey sand, sand and sandstone on a 200 and 300 m lateral distance and corresponding depth of 39.6 and 57.3 m across each traverses. Resistivity values generally varies from 16.8 – 45302 Ωm across Traverse 1 – 12. The layer horizontal depth slices of the 3D inverted resistivity distribution are in six layers, which are; 0 - 5 m, 5 – 10.8 m, 10.8 – 17.4 m, 17.4 – 25 m, 25 – 33.7 m and 33.7 – 43.8 m. The 3D inverted resistivity model within the study area covered lateral plane (the roll axis), 300 m, in the x plane (the pitch axis), 200 m lateral distance was covered and in the depth plane (the yaw axis), a maximum depth of 66 m is imaged. The inverted 3D Resistivity values generally vary from 189 - 6149 Ωm across the study area. The resistivity structures delineated from the 3D model are clayey sand and sand.

Integration of electrical resistivity and soil analysis for Agricultural soil characterization—a case study

Integration of 2D electrical resistivity method and soil analysis was used to characterize the topsoil in the farmland of Federal University Oye-Ekiti, Nigeria, to determine its suitability for agricultural purposes. A 2D electrical resistivity survey was carried out along four traverses using Wenner array electrode configuration with electrode spacing ranging from 1 to 5 m and station interval of 1 m. The acquired 2D electrical resistivity data were processed and inverted using DIPRO version 4.01 software. Forty (40) soil samples were collected along the established traverses at depth intervals of 0.5 and 1.0 m to estimate the moisture contents and grain size distributions. The 2D inverted resistivity sections revealed three probable geologic materials namely clay, clayey sand, and sand. Grain size analysis showed that the farmland is made up of gravel, sand, and fines (silts and clay) with mean percentage distributions of 8.77%, 80.08%, and 11.15% respectively. The moisture content of the forty (40) soil samples analyzed ranges from 10.2 to 18.6%, with an average value of 11.91%. Areas of low electrical resistivity values were observed to coincide with zones of fine particles and high moisture content and are identified to be favorable for crops in the farmland. This study shows the effectiveness of the electrical resistivity method and soil analysis in determining the variability of soil moisture content at shallow depth and in selecting soil that can aid the optimum production of crops.

Investigation of incessant road failure in parts of Abeokuta, Southwestern Nigeria using integrated geoelectric methods and soil analysis

A flexible pavement devoid of discontinuities allows for smooth movement of a vehicle load on the roadway. This study involved the use of integrated geoelectric methods comprised of 1D and 2D Electrical Resistivity Tomography (ERT) as well as soil analysis to investigate causes of unceasing road failures along busy Camp—Alabata Road, Abeokuta, Southwestern Nigeria. Four road sections (two failed portions and one fair section and one good section) were identified along which four resistivity traverses were established along the investigated roadway. Four 1D Vertical Electrical Sounding (VES) points were also carried out on the 2D ERT lines. Apparent resistivity data were measured along the four traverses using Schlumberger and Wenner arrays with the aid of a Campus Ohmega resistivity meter. The VES and 2D resistivity data were processed and inverted using WinResist and RES2DINV softwares, respectively. Twenty soil samples at a sampling depth (0–1.0 m) with an interval of 0.2 m were also collected on all designated road sections and analyzed for selected hydraulic/geotechnical properties related to pavement durability. Penetration resistance (PR) was measured in situ by a penetrologger while other subgrade soil properties were evaluated in the laboratory. The VES results delineate three geoelectic layers comprising topsoil, weathered basement (clayey), and fractured/fresh basement with their corresponding resistivity values ranging between 71 and 282 Ωm, 12–76 Ωm, and 261–10,094 Ωm. The thicknesses range between 0.9 and 3.2 m for topsoil and 4.5–19.1 m for weathered basement. 2D resistivity inverted sections delineate two lithologic layers: topsoil with resistivity values > 200 Ωm, devoid of linear geological structures as competent topsoil on a stable road section while incompetent topsoil on failed road sections were characterized by resistivity values < 200 Ωm. The weathered layers as depicted by 2D inverted resistivity sections were generally of resistivity less than 100 Ωm while fractured/fresh basement were not depicted in the 2D model. Failed road sections are underlain by topsoil with a resistivity (< 200 Ωm), shallow weathered (clayey) layer, and differential settlement of saturated subgrade materials. Soil analyses results showed that the stable portion depicts lowest mean values of plasticity index (5.12%) and liquid limit (22.21%), uniform sandy-loam textural class, highest mean value (1.24 cm/h) of saturated hydraulic conductivity (Ksat); % sand content (73.2%) and Penetration Resistance (PR) values that increased within the sampling depth (0–40 cm). However, one of the failed sections had lowest mean value (0.81 cm/h) of Ksat, % sand content (64.9%) and PR values that decreased within the sampling depth (0–40 cm). Analysis of variance (ANOVA) showed that there were significant differences at the 5% level among all designated road sections with respect to percent silt and moisture content (MC). Adequate engineering construction approaches should be adopted considering the thick layer of incompetent formation (clay) observed across the area investigated. Integrated interpretations present a better resolution and detailed characterization of the substratum underlying the pavement.

Analyzing sensitivity and resolution of some electrical resistivity configurations for detecting subsurface cavities using inverted synthetic models by 2D electrical resistivity tomography technique

It is important to determine the location, depth and shape (dimensions) of cavities under subsurface in site investigation phase before construction. This study aims to make a study for properties, analysis and comparison of the results of different electrodes configurations that used in 2-D electrical resistivity Tomography surveys to detection subsurface cavities to determining the appropriate and suitable configurations type that must be conduct in the field to obtain best and accurate results. Two-dimensional synthetic models were created for Geoelectrical resistivity of five different geological features, which reflects the common cases of cavities in nature. These models are (narrow, broad, shallow and deep) cavities adding to one model for multiple cavities. RES2DMOD program was used to create these models, while RES2DINV program used in Inversion method to obtain the true 2-D inverted resistivity sections for six configuration types of electrode array for 2D electrical resistivity technique, after adding a Gaussian noise ratio of (5%). Generally, we are noticed the possibilities of most tested configuration types for determine electrical anomalies, because the high contrast between the cavities and the surrounding rocks resistivities values. In addition, RMS ratio overall does not exceed 5%, hence, this ratio is an acceptable and indicates to the quality of the inverse process. The Wenner array have high sensitivity to vertical resistivity variations, and Dipole-dipole array have high sensitivity to lateral resistivity variations. While the Schlumberger-wenner have high sensitivity in both lateral and vertical resistivity variations Also, it was noted that the efficiency of all types decreasing with increasing depth of cavities detecting, due to the low percentage of contrast between the cavities and the surrounding rocks. The Wenner beta (WB) configuration is the best choice in terms of inversing true resistivity values, Secondly the Pole-Dipole (PD). While in term of determining the location and shape (approximately dimensions) of cavities, Dipole-Dipole (DD), Wenner-Schlumberger (WS), Wenner Alpha (WA), Dipole-Pole (DP) respectively. However, in case of noisy areas, Wenner Alpha (WA) configuration gives the best result.

Combined electrical resistivity imaging and ground penetrating radar study for detection of buried utilities in Federal University of Agriculture, Abeokuta, Nigeria

The knowledge of the exact location and depth of buried utility such as pipes assists greatly for prevention of accidents and wastage of resources. This study addresses the application of integrated geophysical methods for detections of buried utility in student’s residential hostels, Federal University of Agriculture, Abeokuta, southwestern Nigeria. The goals of the surveys are to determine the effectiveness of the applied methods in detection of buried utilities locations and their depths as well as verification of obtained depths to the top of target by excavation method. 2D electrical resistivity imaging (ERI) using Wenner array with electrode separation ranging from 0.25 to 1.25 m and ground penetrating radar (GPR) survey using a common-offset profiling and center frequency of 400 MHz were carried out along ten traverses in the study area. The field data were processed using RES2DINV and RADAN 7 softwares for resistivity and GPR data, respectively. Inverted resistivity sections and Radar Images generated information up to a depth of 0.80 m and 3.00 m, respectively. The position, size, and depth of buried pipes cannot be determined precisely by ERT compared to GPR survey. The depths to the top of buried pipes by ERT ranges from 0.20 to 0.46 m, while the GPR measured depths ranged from 0.16 to 0.55 m from the surface. Integrated interpretation showed that GPR can effectively provide information about locations and depths to the top of buried pipes with a higher precision than ERT method. Comparisons of measured GPR depths with in-situ verification using excavation method correlate fairly well.

Interpretation of resistivity data using 3D Euler deconvolution and Radially Averaged Power Spectrum

Different electrode configuration in resistivity measurements over the same geologic structures generally produce different anomaly patterns. This is related to the position of the structure concerning the electrodes. Given the above, a unified approach has been proposed using the concept of analytical signal to interpret the resistivity data as electric potential follows Laplace’s equation. We interpret the data sets using Euler deconvolution and Radially Averaged Power Spectrum (RAPS) to determine both lithological boundaries and units and compare the results with inverted resistivity section. We analysed the resistivity data using electrical resistivity tomography (ERT) technique over conducting dyke and vertical fault for Wenner and dipole–dipole arrays, respectively. The obtained structural indices for dyke and fault are 1.0 and 0.6, respectively. The results from Euler depth solutions match well with the inverted resistivity section. Subsequently, two field examples one each over ground water and mineral exploration were analysed. The delineated lineaments over ground water exploration matches fairly well with the available results. In addition, some additional lineaments are also mapped. These new features could be a zone of interest for a detailed survey. The depths from Euler depth solutions and RAPS are in agreement. However, no depth information was available earlier. The data for mineral exploration have been acquired by ERT technique with a profile length of ~500 m over Dhanjori Basin, Jharkhand, India. The Euler depth solution and RAPS indicated the presence of two interfaces at an average depth of 8 and 20 m and 7 and 21 m, respectively. The first interface is present all along the profile, whereas the second interface lies between a part of the profile. The location and depth of these are in broad agreement with the resistivity sections obtained from ERT/AMT and borehole data. A borehole in the vicinity of the survey area indicated the presence of two interfaces. The first interface coincides with the soil depth and the second interface coincides with the sulphide mineralization.

Combined self-potential, electrical resistivity tomography and induced polarisation for mapping of gold prospective zones over a part of Babaikundi-Birgaon Axis, North Singhbhum Mobile Belt, India

The Palaeo- to Meso-Proterozoic (1.0–2.4 Ga) North Singhbhum Mobile Belt (NSMB) is an assembly of multiphase folded, low- to medium-grade meta-sedimentary and meta-igneous rocks, lying within the north Singhbhum crustal province. The mobile belt is situated between the Archean Singhbhum craton in the south and the Meso/Neo-Proterozoic (0.9–1.7 Ga) Chotanagpur Gneissic Complex in the north. Gold mineralisation of moderate concentration is reported from different parts of NSMB along shear /fracture zones within the volcano-sedimentary and meta-sedimentary rocks sequence. The mineralisation is structurally controlled and is associated with higher order shear zones. Gold occurs in association with sulfides like pyrite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, etc. This work attempts to map the auriferous shear/fracture zone along the Babaikundi-Birgaon axis (BBA), which lies within the NSMB. In the present study, combined geophysical approach comprising self-potential (SP), electrical resistivity tomography (ERT) and induced polarisation (IP) methods were used for mapping of subsurface gold prospects. SP data were collected along 7 profiles at 10 m station interval with approximate profile separation of 100 m. The anomalies of possible ore bodies were found at depth of about 18–25 m using Euler deconvolution of SP data. The ERT and IP data were collected along four profiles at 10 m electrode spacing with approximate profile separation of 100 m. The inverted resistivity sections indicate very low resistivity value in the depth range of ∼20 m inferring highly conducting, while relatively high chargeability are also observed at the depth range of ∼25 m. The possible existence of Babaikundi-Birgaon lineation and the occurrences of sulfide minerals in quartz reef associated with gold mineralisation were mapped based on the horizontal locations and extensions of negative anomaly in SP data, low resistivity in 2-dimensional (2D)-inverted resistivity section and high chargeability in 2D-inverted IP section. Previous studies based on electron probe micro-analyzer (EPMA) and scanning electron microscope (SEM) analysis over the study area substantiate our combined geophysical interpretations for the presence of gold prospective zones along the BBA. Highlights Integrated geophysical approach comprising SP, ERT and IP methods Delineation of BBA Mapping of subsurface gold prospects/ mineralisation Validation of results using borehole data, EPMA and SEM–SED analysis of rock samples (a) Map showing profiles AA/, BB/, CC/ and DD/across the Babaikundi-Birgaon Axis (BBA). (b) Upward continued (60 m) self-potential (SP) anomaly map showing possible extension of a part of BBA. Schematic resistivity (c) and chargeability (d) distribution models showing possible extension of a part of the BBA. (e) mineralised sample with sulfide hosted quartz reef in BBA. The possible existence of BBA and the occurrences of gold-bearing sulfide minerals in quartz reef are mapped based on the horizontal locations and extensions of negative SP, high conducting / low resistivity in electrical resistivity tomography (ERT) and high chargeability in Induced Polarization (IP). Note that profiles C-C/ and D-D/ are not parallel to each other; their field orientation is shown in Figure 1(c) and Figure 2.

Long-term hydrogeophysical monitoring of the internal conditions of river levees

To evaluate the vulnerability of the earthen levee of an irrigation canal in San Giacomo delle Segnate, Italy, a customized electrical resistivity tomography (ERT) monitoring system was installed in September 2015 and has been continuously operating since then. Thanks to a meteorological station deployed at the study site, we could investigate the relationship between the inverted resistivity values and different parameters, namely air temperature, rainfall and water level in the canal. Air temperature seems to have a minor but not negligible influence on resistivity variations, especially at shallow depth. A model of soil temperature versus depth was used to correct resistivity sections for air temperature variations through the different seasons. Changes of the water level in the canal and rainfall significantly affect measured resistivity values. At the study site, the most important variations of resistivity are related to saturation and dewatering processes in the irrigation periods. Although we explored the effect of drawdown procedures on resistivity data, this process, causing rapid variations of resistivity values, is still not completely understood because the canal is rapidly emptied during rainfall events. Therefore, the effect of variations of the water level in the canal on levee resistivity cannot be distinguished from the effect of rainfalls. To study the effect of water level variations alone, we considered the beginning of the irrigation period when the dry canal is gradually filled and we observed a smooth trend of resistivity changes. The effect of rainfall on the data was studied during different periods of the year and at different depths of the levee so that the resistivity variations could be evaluated under different conditions. To convert the inverted resistivity sections into water content maps, an empirical and site-dependent relationship between resistivity and water content was obtained using core samples. Water content data can then be used for the implementation of stability analysis using custom modeling. This study introduces an efficient technique to monitor earthen levees and to control the evolution of seepage and water saturation in pseudo-real time. Such a technique can be exploited by Public Administrations to reduce hydrogeological risks significantly.

Joint Interpretation of Geological, Magnetic, AMT, and ERT Data for Mineral Exploration in the Northeast of Inner Mongolia, China

The integrated interpretation of data from various technologies has the potential to obtain a more accurate estimate of subterranean earth properties. In this paper, we implement the joint interpretation of geological and geophysical data for mineral exploration in the northeastern region of Inner Mongolia, China. The joint application of several methodologies reduces the exploration risk. We first determined an approximate and large potential area for mineral exploration with geological data and magnetic data interpretation in Gaoerqi. Results from the two types of data analysis show that the ore deposit strikes roughly east in the northern part of the Gaoerqi mining area. Next, we employed the audio-magnetotelluric (AMT) method to study the subterranean electrical resistivity distribution and divide the earth into four layers. Inverted resistivity sections from the AMT data illustrate that the ore deposits are likely developed in the low-resistivity zone of the survey area from the land surface to 300-m depth. Finally, the high-resolution borehole-to-surface electrical resistivity tomography (ERT) method was employed for further investigation of the location and attitude of the potential ore deposits. Inverted resistivity sections from the ERT data show that two prospective areas for mineral exploration were observed in the west of the survey area and that the eastern portion of the survey area warrants further investigation.

Soil Structure Evaluation Across Geologic Transition Zones Using 2D Electrical Resistivity Imaging Technique

This study utilizes the electrical resistivity values obtained using 2-D Electrical resistivity imaging (ERI) technique to evaluate the subsurface lithology across different geological units. The primary objective was to determine the effect of subsurface lithology on the integrity of a road pavement, which had developed cracks and potholes at various locations. The dipole-dipole configuration was utilized and a total of nine traverses were established in the study area, whose geology cuts across both the basement and sedimentary complexes. The inverted resistivity section obtained showed significant variation in resistivity along established traverses and also across the different rock units, with the resistivity value ranging from about 4 ohm-m to greater than 7000 ohm- m. The lithology as interpreted from the resistivity section revealed the presence topsoil, clay, sandy clay, sand, sand stones/basement rocks, with varying vertical and horizontal arrangements to a depth of 40m. Results suggest that the geologic sequence and structure might have contributed to the observed pavement failure. The capability of the 2D ERI as an imaging tool is observed, especially across the transition zones as depicted in this study. The study further stressed the ability of this technique if properly designed and implemented, to be capable of providing a wealth of information that could complement other traditional geotechnical and geologic techniques.

Controlled Source Audio Magneto Telluric (CSAMT) studies for uranium exploration in Durgi area, Palnad sub-basin, Cuddapah basin, India

Cuddapah basin is known for hosting unconformity proximal uranium deposits viz., Lambapur, Peddagattu, Chitirial and Koppunuru along the northern margin of the basin. It is well known that these deposits are mostly associated with basement granitoids in Srisailam Sub-basin, and with cover sediments in Palnad subbasin where basement topography and fault/fracture system influence the fluid flow causing basement alteration and ore deposition. Geological setup, surface manifestation of uranium anomalies and association of the hydro-uranium anomalies near Durgi area in southern part of the Palnad sub-basin, have prompted detail investigation by geophysical methods to probe greater depths. Controlled Source Audio Magneto Telluric (CSAMT) survey conducted over five decades of frequency (0.1-9600 Hz) delineated the various lithounits of Kurnool and Nallamalai Groups along with their thicknesses as there exist an appreciable resistivity contrast. Interpretation of CSAMT sounding data are constrained by resistivity logs and litholog data obtained from the boreholes drilled within the basin indicated three to four layered structure. Sub-surface 2-D and 3-D geo-electrical models are simulated by stitching 1-D layered inverted resistivity earth models. Stitched 1-D inverted resistivity sections revealed the unconformity between the Kurnool Group and Nallamalai Group along with basement undulations. The faults/fractures delineated from the CSAMT data corroborated well with the results of gravity data acquired over the same area. Simulated 3-D voxel resistivity model helped in visualising the faults/fractures, their depth extent, thickness of the Banganapalle quartzite and basement configuration. Integrated interpretation of CSAMT, gravity and borehole data facilitated in delineating the unconformity and the structural features favourable for uranium mineralisation in deeper parts of the Palnad sub-basin.

Resistivity imaging technique to delineate shallow subsurface cavities associated with old coal working: a numerical study

Subsidence is an environmental hazard due to subsurface cavities associated with abandoned coal workings in the Eastern Indian coalfields. These cavities pose danger to coal mining of deeper coal seams. Most of these subsurface cavities are unapproachable as no mine plans are available. Here, an attempt has been made to detect and possible delineation of air and as well as water filled cavities applying Electrical Resistivity Imaging (ERI) technique using dipole–dipole, pole–dipole and Wenner–Schlumberger arrays. Models are simulated with a reasonable resistivity value of formations considering board and pillar mining environment in multilayer earth. The inverted resistivity section corresponding to water filled cavities without a barrier could be able to detect three cavities for dipole–dipole and Wenner–Schlumberger arrays, whereas pole–dipole array could detect all four cavities. Later on, with inclusion of 1 m coal as a barrier in the model, the dipole–dipole array could be able to bring the signature of cavities, whereas pole–dipole and Wenner–Schlumberger configuration is unable to provide any signature. Similar resistivity responses are noticed for air filled cavities with and without barrier conditions. When all three arrays data are jointly inverted, there is a significant improvement in resistivity response which enables the detection of water filled as well as air filled cavities with and without a barrier.

Application of two geophysical methods to characterize a former waste disposal site of the Trabzon-Moloz district in Turkey

Radionuclide variations, the vertical and lateral extent of a waste mass in the former Trabzon municipal solid waste dumpsite were investigated by combining in situ gamma-ray spectrometric measurements with 2D resistivity imaging methods. In the first step, the natural radioelement concentrations on the surface of dumpsite were measured using a portable gamma-ray spectrometer. The average activity concentrations of 238U, 232Th and 40K in the dumpsite are 42.68, 49.88 and 417 Bq/kg, respectively. In addition, radiation hazard parameters were calculated and compared with the international standard values. As a result of the evaluation of the radiological data, it was found that there are no significant radiologic hazards for humans and the environment. In the subsequent stage, 2D electrical resistivity method, using Wenner array, was carried out in this area. The survey was conducted using a multi-electrode resistivity instrument and the measured resistivity profiles were interpreted using RES2DINV program. Electrical resistivity values were obtained from three parallel lines. Results of the resistivity survey show that the waste masses in the study area reach to depths of about 18 m, with very low resistivity values less than 20 Ωm. According to the 2D inverted resistivity sections, low resistivities ( 160 Ωm) in profiles B and C are associated with non-degradable waste materials, medical wastes and buried construction materials. Also, very high resistivity zone (874 and 2636 Ωm) in profile A are interpreted as landfill gases.

Geologic, Geotechnical, and Geophysical Investigation of a Shallow Landslide, Eastern Kentucky

Shallow colluvial landslides are common in eastern Kentucky, as well as in the east-central Appalachian region. A geological, geotechnical, and geophysical investigation was carried out for a shallow colluvial landslide in Boyd County, KY. The purpose of this project was to assess the geologic conditions, extent, and behavior of a rainfall-triggered landslide in eastern Kentucky and to evaluate the use of electrical resistivity as a tool to characterize a shallow colluvial landslide. This study showed that 1) colluvial landslide movement is correlated to the rainfall and 2) inverted resistivity sections with distinct resistivity contrasts that correlated to landslide stratigraphy, depth of the failure surface, and groundwater regimes.