Very Low Frequency Electromagnetic Research Articles

Application of Very Low Frequency (VLF) Method for Underground River Estimation in Donorojo Sub-District, Pacitan

<p class="Keywords"><span lang="EN-US">Pacitan is included in the Gunung Sewu karst area in the Southern Mountain Zone, and carbonate rocks dominate the constituent rocks. Karst has a unique drainage system because it is dominated by subsurface flow. This research was conducted in Cemeng and Klepu villages, Donorojo sub-district, Pacitan, with 4 track data. Data were collected using the very low frequency electromagnetic (VLF-EM) method with a track length ranging from 200-450 m, a measurement spacing of 5 m, and a transmitter frequency of 19.8 kHz. Data processing uses filtering and inversion, resulting in a cross-section of resistivity values. Based on the subsurface resistivity cross-section profile, the cavity in the carbonate rock layer with a resistivity value of 0-500 Ωm is identified as an underground river. The underground river is found on tracks 1, 2, 3, and 4 near the surface, with a depth of about 30 m below the surface.</span></p>

Estimation of top depth to underground targets of Karous-Hjelt and Fraser filtering of VLF-EM measurements: The Thumb's rule approach

Prediction of the exact location and depth of underground targets with the very low-frequency electromagnetic (VLF-EM) technique is one of the most important and difficult tasks in geophysical investigations. This study examined and compared the conventional 2D KIFFILT inversion pseudo-section and the use of Thumb's rule technique in the Fraser filter plot to estimate the top depth of underground targets. The VLF-EM measurement was performed over several empirical buried target models to identify anomalies or geophysical responses corresponding to subsurface targets. The Karous-Hjelt and Fraser filtering techniques were applied to estimate the depth of the identified anomalies using Thumb's rule and the conventional 2D KIFFILT inversion. The signal behaviours of the VLF-EM current density pseudo-sections and the application of Thumb's rule effectiveness in delineating empirical buried target models were examined. Thumb's rule shows 65 % accuracy with the actual depth of the empirical buried target models, while the conventional 2D KHFFILT inversion shows 30 % accuracy with the actual depth of burial. Thumb's rule is more effective and precise in predicting the accurate depth of underground targets. The influences of conductive and resistive materials on VLF-EM signals and the challenges of VLF-EM surveys were discussed. Thumb's rule is suggested as a substantial technique for estimating top depth to the underground target where depth estimation is of prime interest due to its large degree of accuracy. In addition, the total depth of the current density distribution was noted to be increased when the distance between measuring points increased. This means that VLF-EM signals with longer wavelengths indicate deeper depth penetration into the ground than signals with shorter wavelengths. The accuracy of Thumb's rule regarding top depth estimation of the anomalies has been successfully tested and validated, which can be used for VLF-EM investigation where accurate depth estimation is required. The VLF-EM technique can be considered reliable for depth estimation using Thumb's rule approach, which applies to a wide range of subsurface investigations.

Application of VLF-EM response and geoelectrical sounding in groundwater investigation around an active dumpsite

An integrated geophysical method combining very low frequency electromagnetic (VLF-EM) and vertical electrical sounding (VES) methods were carried out around Adaland, Southwestern Nigeria, located within latitude 7° 54’ 0” and longitude 4° 43’ 0”, with a view to investigate the possible impact of dumpsite on groundwater. This is the major study in the environment to offer extensive evidence regarding the suitability of obtaining quality groundwater. In this research, eight VLF-EM and twelve VES data sets were generated, which were then used to estimate the linear structure, bedrock formation, subsurface geological characteristics, and identification of leachate pathways. The VLF-EM data were analyzed by employing Karous_Hjelt and Microsoft Excel, while the VES data were investigated using the WinRESIST software. TheVLF-EM results confirmed the presence of conductive zones, which might be due to fracture, fault and contact zones or weathered basements. The lithological units acquired from the electrical resistivity results revealed four geoelectrical layers such as topsoil, weathered-based, fractured basement and fresh basement. However, the identified weathered layers and fractured basements from the geoelectrical sections and the corresponding Karous and Hjelt (K-H) pseudo sectionresults around the dumpsite, constitute the main passages for the possible impact of the open dumpsite on groundwater quality, since leachates from the dumpsite could slowly percolate downwards from the topsoil to the water table. Therefore, the impact of the dumpsite on the groundwater is caused by the inadequate clay materials, near-surface features such as fractures/faults, and lateral in-homogeneity. Thus, integrating both methods has been recommended in site characterization for accessing quality groundwater around a dumpsite environment.

VLF-EM data conversion into apparent resistivity using the Hilbert transform and its application for shallow fault investigation at Pinggirsari, Bandung Regency, Indonesia

This research presented the ability of the Hilbert transformation of The Very Low Frequency-Electromagnetic (VLF-EM) data to investigate the presence of faults. VLF-EM data was collected in the Pinggirsari village area, Arjasari District, Bandung Regency, West Java, Indonesia. Through the Hilbert transformation, the VLF-EM data was converted into apparent resistivity data and then inverted into a resistivity cross-section. For comparison, qualitative interpretation was also conducted using both the Fraser and the Karous-Hjelt filters. Interpretation results using both the Fraser and Karous-Hjelt filters indicate the presence of faults through tilt-angle curve peaks and apparent current density anomalies. The resistivity cross-section resulting from the Occam inversion of the apparent resistivity data, derived from the Hilbert transformation, accurately delineated fault positions. Resistivity changes around 700–800 m indicated fault proximity to a depth of approximately 150 m, with a good match to the geological map. Within a distance of 0–700 m, low resistivity values were associated with the Malabar tuff, while >700 m, higher resistivity values were associated with the Waringin-Bedil andesite. The Hilbert transform helps in clarifying VLF data interpretation for precise fault detection through resistivity parameters.

Geophysical characterization of lamproite fields in the Dharwar craton using VLF-EM and advanced filtering techniques: insights from conductivity analysis and analytical signal mapping

Abstract This study presents a geophysical investigation of the lamproite fields located in the Dharwar craton, aiming to map conductivity variations using contemporary techniques. The study employs very low-frequency electromagnetic (VLF-EM) methods, applying Hilbert transformations, and first-order vertical derivatives to the Fraser and Karous–Hjelt filtered contoured of VLF-EM data. The peninsular gneissic complex (PGC) granitic rocks in the study area experienced tectonic forces, resulting in fractures along specific WNW–ESE to NW–SE trends. Within these crustal weak zones, these lamproites are emplaced. The lamproite pipes are volcanic rocks. As a result, the top portions are weathered and tend to conductive, and the conductivity tends to decrease with depth. The volumetric size of lamproites ranges from centimetres to hundreds of metres, unlike kimberlites that are larger. Hence, the exploration of lamproites poses challenges. The contours of in-phase and quadrature components were used to identify the cluster of lamproite zones within the study area. From this study, the boundaries of the lamproite pipes were clearly identified using real component's analytical and first-order vertical derivative signal contour maps. The VLF-EM pseudo-depth current density section was used to identify anomalous lamproite, pipes, and their subsurface extensions, along with the surrounding formations. The current investigation findings specify that the lamproites exhibit weak conductive. These results provide valuable insights for exploration efforts within the Dharwar craton, and can aid in the identification and mapping of the lamproite fields.

Integration of Geological and Geophysical Approaches in Meeting the Challenges of Uranium Exploration in the Cuddapah Basin — a Case Study from Chitrial Area, Nalgonda District, Telangana

Abstract Uranium exploration in the Cuddapah Basin is both scientifically and logistically challenging. An integrated multidisciplinary approach is adopted to infer the subsurface geology. Very Low Frequency Electromagnetic (VLF-EM) survey, conducted over parts of Chitrial outlier, Nalgonda district, Telangana, has indicated thirteen subsurface conductors. Two prominent trends, viz. NE-SW and NNE-SSW/N-S, are revealed from the stacked profiles and pseudosections. Six of these conductors (2 NE-SW and 4 NNE-SSW/N-S trending) are found to have appreciable strike extent (>300 m) with steeply dipping (60°–75°) to vertical attitude. These conductors are interpreted to extend ≥75 m into the basement from the overlying sediments. The ballooned-up zones of low apparent resistivity especially in the sediments, associated with one NNE-SSW/N-S and two NESW trending conductors, are interpreted as zones of hydrothermal alteration and, therefore, demarcated as potential targets for subsurface exploration by inclined boreholes. This study indicates that integration of geophysical data with geological features will help in optimizing the exploration input and narrowing down the target zones for uranium exploration even in deeper reaches of the Cuddapah Basin.

Exploration of suspected magnetite ore minerals in Emure – Ekiti, Southwestern Nigeria using integrated geophysical methods of very low frequency electromagnetic (VLF-EM) and magnetics

Investigation on the occurrence of magnetite ore minerals in Emure – Ekiti, Southwestern Nigeria was carried out by integrating magnetic and very low frequency electromagnetic (VLF-EM) methods on the suspicion that the major part of the magnetite ore being sought lies in the subsurface. Eight traverses in total were established with five traverses occupied in the northeast–southwest direction and the other three in the northwest–southeast direction. Six traverses were established across the exposed part of the pegmatite (magnetite) veins to serve as a control. Inter-station spacing of 1 m was chosen, so as not to miss any significant ore occurrence. The VLF-EM profile is a plot of the real and filtered real components. The KH-filter geophysical software was used to invert real data into a two-dimensional subsurface map that revealed variations in conductivity. The results of the geophysical interpretation for both methods indicate anomalous signatures observed on the exposed section of the pegmatitic veins, which are indicative of magnetite ore. These anomalous signatures align with certain zones along the Tr4 and Tr8 transects. Some of the high magnetic anomalies observed on traverse 5, 6, and 7 coincide with low conductivity suggesting the concentration of minerals such as quartz, microcline, plagioclase, and biotite, which are commonly associated with magnetite. The study has not only enhanced the mapping of the subsurface characterisation of the deposit but also confirms the reliability and suitability of using magnetics and VLF-EM geophysical methods for mapping magnetite deposits.

The effectiveness of very low-frequency electromagnetics (VLF-EM) method in detecting buried targets at a controlled site

The ever-increasing anthropogenic activities that pose a significant threat to environmental security and sustainability have spurred geophysicists to review enhance geophysical techniques for shallow geophysical investigations, especially in identifying illegal buried materials. This article applied very low-frequency electromagnetic (VLF-EM) at an experimental geophysical site (EGS) to examine the geophysical response over various buried targets. The VLF-EM data acquired on the site with and without buried targets demonstrate the nature of the anomalies and the characteristic signals of the buried targets. There are significant variations in the anomalies source-bodied between the site with and without buried targets. The result of the pre-burial investigation shows no major contrast in the equivalent current density values across the site without targets. Minors and major anomalies were encountered after burying the targets corresponding to the buried targets. Some signals become stronger over a large buried target. However, there were a few undetected targets and some cases of unsatisfied results, which were also discussed. The performance of the VLF-EM method in terms of depth estimation was also examined. A significant variation was noted due to the presence of the buried targets and it was noted that the current density seems to always emerge maximally and minimally around the conductor targets and non-conductive targets, respectively. The failure of the VLF-EM to detect the buried pipes in various orientations was examined. The VLF-EM method is more consistent at small spacing and it can be very useful for identifying underground metallic and non-metallic targets. The study successfully provides useful information to complement the complexity of the use of geophysical methods while enhancing the subsurface information and understanding of VLF-EM anomalies or responses generated by various targets such as subsurface geological structures, buried waste contaminants and underground utilities to boost environmental studies and engineering investigations.

Exploring groundwater resources in southwestern Nigeria: An integrated geophysical approach

Groundwater potential regions were delineated using Very-Low Frequency Electromagnetic (VLF-EM) and Vertical Electrical Sounding (VES) geophysical techniques. The VLF-EM measurements were taken as fast reconnaissance to delineate shallow conductive features along seven traverses with 10 m station intervals. The data were qualitatively analyzed and indicated zones of hydrogeological relevance, pointing to regions for VES investigations. Thirteen VESs were mapped with 100 m intervals between current electrodes, and the data were processed manually and using WinResist software to generate sounding curves and geo-electric sections that revealed 3 (84.6%) and 4 (15.4%) layers of earth models. The aquifer units were found in the weathered and fractured layers. The subsurface was classified into high, medium, and low groundwater potential regions, with a larger area indicating low prospects. Regions with high potentials had overburden thicknesses above 18 m, while medium and low yields had overburden thicknesses of 13–18 m and below 13 m, respectively.

The Geophysical Investigation of Leachate Formation in Mgbuka Obosi, Idemili North Local Government Area of Anambra State, Nigeria

The contamination of leachates (a polluted liquid made up of various toxic substances) from landfills is a significant environmental concern, especially in developing countries.This contaminated fluid is formed when rainwater interacts with refuse and travels through the pore spaces in soil. Constant migration of this fluid poses a significant threat to the quality of both surface and groundwater in and close to the landfill area. To determine the extent of leachate formation, the study used both Very Low-Frequency Electromagnetic (VLF-EM) and Electric Resistivity Tomography (ERT) to locate and define the spatial distribution of the leachate plume migration pathway at Mkpuka Obosi Dumpsite in Idemili North LGA of Anambra, Nigeria. Seven profiles were surveyed for both the VLF-EM and ERT. The VLF-EM survey, with a profile length of 100 to 200m and a 10m interstation spacing, revealed the presence of a conductive pollutant (leachate plume) in the subsurface. The results from the 2D ERT survey that employed a Wenner array with a profile length of 100 to 200m and an electrode spacing of 5m, divided the subsurface into six zones with an unusually low resistivity ranging from 0-250\(\Omega\)m. The leachate plumes were interpreted as the zone with the lowest resistivity of 0 to 25\(\Omega\)m, having an average thickness of approximately 10m and extending beyond the probed depth of 50m. Additionally, the percentage of leachate was found to be concentrated primarily at the center of the landfill and gradually decreasing proportionally from the center. Thus, emphasizing the importance of addressing the issues in landfill management.

VERY LOW FREQUENCY ELECTROMAGNETIC (VLF-EM) SURVEY FOR GROUNDWATER DEVELOPMENT IN BICHI/BAGWAI AREA, NORTH WESTERN NIGERIA

Abstarct
 Detailed Very Low Frequency Electromagnetic (VLF-EM) hydrogeophysical survey was undertaken to identify conductive zones and recommend potential areas for possible groundwater development in the Bichi area of Kano state northwestern Nigeria. Geologically, the area is characterized by porphyritic granite, coarse grained granite and medium to fine grained granite. The VLF-Electromagnetic method was adopted as a fast reconnaissance tool to map possible linear fractures. During the survey, measurements were taken at a station interval of 20 m along each profile line ranging from 0 – 420 m, making a total of six VLF-EM traverses which were mapped in the study area. The filtered components for both real and imaginary parts of the VLF-EM data were plotted against distance for each profile using the Karous–Hjelt filter® computer software to interpret and identify the top of linear fractures. The Very Low frequency (VLF) normal and filtered component anomalies identified ten (10) major geological interfaces suspected to be faults/fractured zones (f1 – f10). These suspected zones were marked as targets for future groundwater development in the area since these anomalous zones are areas of high conductivity and this parameter is one of the physical characteristics of water saturated zones. Therefore, this work has proven that VLF method is robust in tying down good locations for groundwater development in rural areas.
 Keywords: Electromagnetic, Hydrogeophysical, Groundwater Development, Conductive Zones, Fault, Fractures.

Investigating the Susceptibility of Tunnel Erosion in Southern Nigeria using Integrated Geophysical Methods

There has been a growing incidence of soil subsidence in various regions of Anambra State, located in southern Nigeria. These subsidence events primarily stem from the formation of natural tunnels or soil pipes within the subsurface. To delve deeper into this issue, the present study employed two geophysical techniques, namely the Very Low Frequency Electromagnetic (VLF-EM) method and the Electric Resistivity Method utilizing the Dipole-Dipole array. These methods were utilized to investigate the characteristics and spatial distribution of soil pipes at two specific sites: Awka Site 1 and Awka Site 2, both situated within Anambra State, Nigeria. Four profiles with transverse lengths of 100 m and 5 m spacing were surveyed. On each profile, the inphase and outphase were collected using the Abem Wadi Meter after a confirmed connection to the external satellite, while nine dipole-dipole profiles were carried on the same location. Four profiles with transverse lengths of 100 m and 5 m spacing were surveyed (Figures 3 and 4). The data analysis from the VLF-EM survey reveals that a significant portion (80%) of the low conducting zones observed in the pseudosection originates from the top of the profile, indicating a downward trend in the formation of soil piping. The VLF-EM result also inferred that the subsurface voids in the study areas extend vertically downward up to 10 meters, with an average horizontal extension exceeding 0.5 meters. This highlights the extensive spatial reach of the subsurface voids and emphasizes their potential impact on the surrounding environment. The dipole-dipole survey conducted in the study area identified six distinct structures, with the eroded formation being particularly noteworthy. This structure, characterized by resistivity ranging from 1200–30000 Ωm, plays a significant role in creating favorable conditions for soil piping. Furthermore, the presence of strong dispersive soils increases the likelihood of soil piping occurrences within this structure.

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.

Application and comparison of very low frequency electromagnetic and electrical resistivity techniques to investigate a Karstic Region: A case study of EL Hajeb Municipality, Morocco

Geophysical methods are widely used to determine subsurface karst structures' depth because of their risks. However, their qualitative or quantitative interpretations involve a degree of uncertainty. Consequently, several methods are used to improve the credibility of drawn conclusions. In this study, we used two well-known geophysical methods, namely Very Low Frequency Electromagnetic (VLF-EM) and electrical resistivity methods, to characterize the observed or suspected sinkholes in an area of the municipality of El-Hajeb, Morocco, where no previous research had been conducted. This built-up zone is characterized by soluble carbonate rocks and sinkholes, which qualifies it as a high-risk area. We also compared four interpretation methods to estimate the depth of anomalies from VLF-EM data: Derived Euler Deconvolution (DED) and Extended Euler Deconvolution (EED) which, to authors' knowledge, have not been applied before, Karous-Hjelt (K-H) pseudosections, and Classic Euler Deconvolution (CED). Electrical resistivity data were used to confirm the VLF-EM results. The findings indicated that some conductive anomalies, manifested by decreased real component and apparent resistivity, are in good agreement with shallow sinkholes and depressions observed in the study area. In addition, other conductive anomalies indicate hidden sinkholes. The estimated depths of an observed sinkhole from K-H pseudosections, CED, DED, and by electrical resistivity tomography are comparable. The finding of this study revealed the effectiveness of the used geophysical methods to identify karst features. It also demonstrated the efficacy of the VLF-EM data processing tools, particularly CED and DED, to estimate the depth of these features, with the exception of the EED, which unexpectedly provided large depth values.

Frequency-Domain Electromagnetic and VLF Data with An Application to Modeling of Basement Structures: A Case Study Within, Ibadan Area, Southwestern Nigeria

The study area, the church camp is within Ibadan which fall within southwestern basement complex of Nigeria and it is mainly by migmatite-gneiss. The hard rocks are usually characterized by basement fracture which may serve as a conduit for groundwater passage. This has necessitated detail geological and geophysical investigation to accurately and precisely delineate this structure. Geological mapping alongside very low frequency Electromagnetic (VLF-EM) techniques were adopted in which conductivity data were acquired along Fifteen (15) VLF profiles using ABEM WADI. Thirteen (13) of these profiles were in the East-West (E-W) direction which is the dip direction of the structural elements, while the remaining two profiles were in North-South (N-S) direction. The data obtained from the field were processed, filtered and presented inform of profiles and Karous-Hjelt (K-H) filtered pseudo-section to visualise conductivity in (2-D). Pockets of conductive structures were delineated and were interpreted as fractures of various dimension and orientation. Some are single fracture and others are closely-spaced double fractures which some of them joined or fused together at depth with some suspected to contain water while some are dry base on their conductivity/resistivity signature exhibited. In conclusion, VLF-EM has proven to possess the capability to characterize and model the basement fractures and define their dimension, axial orientation and indicates their parallelism which also signifies their origin as regard stress regime that produces the fracture.

Investigation of Leachate Plume and Groundwater Pollution using Geophysical and Physio-chemical Methods in an Open Dumpsite at Idugosa, Federal Capital Territory (FCT), Abuja, Nigeria

A geophysical study has been carried out using the electrical resistivity method, Very LowFrequency Electromagnetic (VLF-EM) and physiochemical analysis to investigate the effects of groundwater contamination in the vicinity of an open dumpsite at Idugosa. A Vertical Electrical sounding was carried out at 100 m away from the dumpsite and the results revealed an H-type curve with 3- layers, with a low resistivity value of 90.0 Ωm, thickness of 9.8 m and depth of 11.3 m at the weathered basement. The control VES revealed 5 –layers of HK-type curve. The resistivity of the weathered and fractured basement ranged from 241.8 Ωm to 2908.4 Ωm with a thickness range of 3.5 m to 13.6 m and a depth range of 5.6 m to 19.1 m. Results of the horizontal profiling using Wenner profiling revealed a low resistivity value of 1.6 Ωm to 3.8 Ωm at a depth of 15 m to 30 m, concentrated at stations 20 to 100. The bottom of the western part showed that the leachate plume had infiltrated and contaminated the groundwater in the zone. The VLF-EM with the three profiles revealed high current density values on the KarousHjelt filter plot at distances of 20 m to 38 m for profile 1, 25 m to 50 m and 75 m to 95 m for profile 2, revealing the presence of contaminants which is mainly from decayed organic matter from waste bodies and heavy metals from the dumpsite. Profile 3 which is further away from profile 1 revealed that the contamination is reduced as we move further away from the dumpsite at about 10 m to 20 m, 50 m to 60 m and 90 m respectively. The physio-chemical analysis results showed a pH range of 6.73 – 7.17 which is within the World Health Organization (WHO) standard (6.5 - 8.5). The Electrical Conductivity (EC) value of the Idugosa dumpsite was found to be 804.9 μS/cm, which is above the WHO standard for EC values, which should not exceed 400 μS/cm. This high value can be attributed to the infiltration of the leachate plume from the dumpsite. The EC for Idugosa control which is 188.0 μS/cm is within the WHO limit for groundwater, thus its safe for consumption.

Integrated geophysical investigation for lead and zinc mineralization in Wase, middle Benue Trough, Nigeria

There is an increasing demand for solid industrial minerals worldwide, as countries seek to support their economic growth. Wase, situated in Plateau State, is notable for lead and zinc mineralisation due to tectonic and hydrothermal activities in the region. The study utilises geophysical surveys to map mineralised zones of lead-zinc occurrence. These include ground magnetic survey, very low frequency electromagnetic, electrical resistivity tomography, and induced polarisation (IP). The field data were acquired using a GEMSys Overhauser magnetometer with VLF option and ABEM LS 2 Terrameter. twenty-nine (29) magnetic profiles with 50m spacing and a total length of 24.0 km were obtained and fifteen (15) very low frequency electromagnetic (VLF-EM) profiles with 10 m spacing in the E-W direction. A total length of 12.5 km was acquired across target areas of the magnetic profiles. The multiple-gradient electrode array of the ERT/IP surveys was used for data acquisition, and RES2DINV software by Geotomo was used for the inversion of the acquired data. High chargeability values corresponding to low resistivity values characterised zones with suspected lead-zinc mineralisation. Following this preliminary finding, we advise that further trenching, pitting, or core drilling be carried out to ascertain ore grade, possible beneficiation, and estimate of the reserve.

Integration of VLF-EM and VES data for pavement failure investigation in a typical basement complex terrain of southwestern Nigeria

Abstract Geophysical investigations involving very low frequency electromagnetic (VLF-EM) and vertical electrical sounding (VES) techniques were deployed to study the pavement failure along a major road in a typical basement complex terrain of southwestern Nigeria. This study was designed to assess the failure and provide a basis for ultimate engineering design. The study delineated a 3- to 4-layer geoelectric sequence: a topsoil of thickness varying from 0.4 to 1.2 m, a spectrum of clayey materials of thickness varying from 1.9 to 14.5 m and a weathered/fractured basement occurring at depths of between 3.6 and 15.0 m. The shallow basement indicated low bearing capacity, with resistivity values ranging from 62.1 to 377.9 Ωm. The presence of clay/clayey materials, linear features and the effect of rising water table within the influence zone underlined the pavement failure. This information will facilitate appropriate designing, soil improvements and selection of materials for road construction that can stand the test of time.

Geoelectrical model of geothermal spring in Ie Jue Seulawah deriving from 2D VLF-EM and DC resistivity methods

Seulawah Agam is one of the volcanic areas in Aceh province, Indonesia, which planned for a powerplant constriction with an energy capacity expected to be approximately 230 MWe. This volcano has seven manifestations in the form of craters, hot water, and heated soil. The hydrothermal system in this volcano is controlled by a fault system which acts as a medium for the entry and exit of fluids. Therefore, understanding the local geology is required for geothermal power plant development, especially for the determination area for injection and production wells. In this research, we use the Very Low-Frequency Electromagnetic (VLF-EM) methods combined with electrical resistivity tomography data on the Ie Jue manifestation area to determine the shallow structure related to the manifestation. The VLF was made for 4 profiles with 700 m length for each VLF-EM profile and 300 m for electrical resistivity lines. We utilized the Karous Hjelt filter for qualitative interpretation, while Occam's algorithm was applied for 2D inversion of data for quantitative analysis of VLF-EM data. Based on the current density model, several vertical conductive anomalies can be well demonstrated at a distance of 300-400 m from the four VLF profiles. The conductive anomaly can also be seen in the resistivity data from the electrical resistivity. The results of the Occam model show that the depth of faults and fractures is seen at 30 m depth with low resistivity (below 100 Ωm). This anomaly is generally associated with outcrops in the field, such as fumarole and warm ground on the east side of the manifestation area. In addition, the 2D inversion model of VLF also shows the contrast of several fracture zones as a place for fluid to enter and exit the Seulawah volcano. Therefore, based on our result, it can be summed up that this method is effectively applied to geothermal in high terrain areas such as in Indonesia and can be used to suggest safe locations for injection wells and production of geothermal drilling.

Pre-foundation geophysical investigation of a site for structural development in Oka, Nigeria

ABSTRACT Frequency of structural failure globally has necessitated geophysical investigation of subsurface geology of a site for engineering construction works. Combined very low frequency electromagnetic (VLF-EM) and electrical resistivity methods were used to provide detailed information on subsoil profile for documentation and references for durable and sustainable construction works. Thirteen traverses were established from which geophysical data were acquired. Major conductive geological interfaces suspected to be faults/fractured zones were identified from the plots of VLF-EM data. These points serve as 50 sounding stations further investigated using Schlumberger electrode array with vertical electrical sounding technique and electrical resistivity imaging on selected four traverses of the site. The acquired data were processed, inverted and interpreted. VLF-EM 2-D inverted models revealed conductive zones at some locations suggesting incompetent zones, responsible for structural instability. Saturated clayey subsoil and uneven bedrock topography with depressions at some points could cause differential settling which has negative impact on engineering structures. Structural failure may arise from existence of concealed geological structures, deep weathering/fractured bedrock, heterogeneous and structurally deformed (F1–F16) subsurface geological setting. Thus, classified unstable sections are considered priority in structural design and construction to mitigate unforeseen challenges. Deep foundations in form of piers and piles are encouraged to avert structural failure.