Very Low Frequency Electromagnetic Survey Research Articles

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.

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.

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.

Investigation of groundwater potential using integrated geophysical methods in Moloko-Asipa, Ogun State, Nigeria

Water is essential for livelihood, development, and industrial growth. Its exploration in sufficient quantity is required where it does not freely occur on the surface. This research was aimed to delineate aquifer regions and provide information on the subsurface lithology of Moloko-Asipa Southwestern Nigeria. A combination of eight traverses investigated with very low frequency electromagnetic (VLF-EM) method at 5 m constant sampling interval and ten vertical electrical sounding (VES) were carried out in the survey. Measurements from the VLF-EM survey were processed with Karous and Hjelt filtering to give the resistivity contrast across the selected profiles. The VES data processing involved an automatic approximation of the initial resistivity and thickness of the geoelectric layers with IPI2Win and further filtering by WinResist iteration. Estimation of Dar-Zarrouk parameters was also employed to investigate the aquifer protective capacity of the area. The processed VLF-EM results showed the geology of the area to an average depth of 25 m. The geoelectric section of the VES data revealed minimum of 3 layers from sandy top soil to weathered layer and fresh basement with an average resistivity values of 1,816, 926 and 17,503 Ωm, respectively. The integration of VLF-EM and VES in the investigation revealed that the potential for groundwater exploration in the study area is poor due to the thin nature of the weathered layer and its shallow depth to basement. The aquifer protective capacity of the area was likewise inferred to be poor.

Integration of Multi-geophysical Approaches to Identify Potential Pathways of Heavy Metals Contamination - A Case Study in Zeida, Morocco

The tailings of Zeida's abandoned mine are found near the city of Midelt, in the middle of the high Moulouya watershed between the Middle and the High Atlas of Morocco. The tailings occupy an area of about 100 ha and are stored either in large mining pit lakes with clay-marl substratum or directly on a heavily fractured granite bedrock. The high contents of lead and arsenic in these tailings have transformed them into sources of pollution that disperse by wind, runoff, and seepage to the aquifer through faults and fractures. In this work, the main goal is to identify the pathways of contaminated water with heavy metals and arsenic to the local aquifers, water ponds, and Moulouya River. For this reason, geophysical surveys including electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and very low-frequency electromagnetic (VLF-EM) methods were carried out over the tailings, and directly on the substratum outside the tailings. The result obtained from combining these methods has shown that pollutants were funneled through fractures, faults, and subsurface paleochannels and contaminated the hydrological system connecting groundwater, ponds, and the river. The ERT profiles have successfully shown the location of fractures, some of which extend throughout the upper formation to depths reaching the granite. The ERT was not successful in identifying fractures directly beneath the tailings due to their low resistivity which inhibits electrical current from propagating deeper. The seismic refraction surveys have provided valuable details on the local geology, and clearly identified the thickness of the tailings and explicitly marked the boundary between the Triassic formation and the granite. It also aided in the identification of paleochannels. The tailings materials were easily identified by both their low resistivity and low P-wave velocity values. Also, both resistivity and seismic velocity values rapidly increased beneath the tailings due to the compaction of the material and lack of moisture and have proven to be effective in identifying the upper limit of the granite. Faults were found to lie along the bottom of paleochannels, which suggest that the locations of these channels were caused by these same faults. The VLF-EM surveys have shown tilt angle anomalies over fractured areas which were also evinced by low resistivity area in ERT profiles. Finally, this study showed that the three geophysical methods were complementary and in good agreement in revealing the pathways of contamination from the tailings to the local aquifer, nearby ponds and Moulouya River.

Preliminary geophysical investigation for road construction using integrated methods

Abstract Integrated geophysical methods have been used to investigate the competency of the subsoil. The geophysical surveys conducted involve very low-frequency electromagnetic (VLF-EM) and electrical resistivity (ER) methods (dipole-dipole). ABEM Wadi and Ohmega resistivity meter were used to acquire VLF-EM and ER data, respectively, along two traverses. Station interval of 5 m was used for the VLF-EM survey, while inter-electrode spacing for dipole–dipole was 10 m; the inter-dipole expansion factor (n) ranged from 1 to 5. KHFFILT software was used to generate VLF-EM profiles and pseudosection, while DIPRO software was used for ER. Results from the ER method revealed the pattern of resistivity variations within the study area. The low resistivity values (11–25 Ohm-m) observed at the southern part of the study area could be attributed to changes in clay contents and degree of weathering in the subsurface. The results from the VLF-EM investigation revealed the presence of near-surface linear geologic structures of varying lengths, depths and attitudes, which suggest probable conductive zones that are inimical to the foundation of the road subgrade.

Geophysical and geochemical investigation for structures favorable to uranium mineralization in Dhofar region, Sultanate of Oman

The very low frequency electromagnetic (VLF-EM) method is a rapid and reliable geophysical method was used in the investigation for uranium bearing structures in the Proterozoic basement of the Dhofar region that may be a potential source and trap for uranium and associated minerals in Sultanate of Oman. Accordingly, the in-phase and out of phase components of VLF-EM field along 11 traverses were recorded at two different transmitter frequencies viz. 16 kHz and 16.4 kHz at an approximate line interval of 20 m with a measurement interval of 10 m. These measured components were subjected to Fraser and Karous-Hjelt filtering in order to decipher the nature and depth to the sub-surface conductor. The Fraser filtered in-phase VLF-EM components yield a strong amplitude almost over the top of the conductor. On the other hand, the Hjelt filtered responses yield the depth to the source in the range 10–80 m. Spectral analysis of selected a few traverses result the depth to source in the range 31–39 m. Analytical signal approach of total magnetic field of line-I substantiates this depth. The peaks of Fraser filtered stacked profiles delineate approximately the trend of the sub-surface fault which mimic the trend inferred from the combined interpretation of VLF-EM and magnetic anomalies. The results of Fraser and Karous-Hjelt filter output suggest a mineralized conductive sub-surface fault aligned along NW-SE. It may be noted that the random sampling of gamma ray spectrometer has recorded a maximum uranium concentration of up to 30–40 ppm prior to VLF-EM survey.

Groundwater resources evaluation in calcareous limestone using geoelectrical and VLF-EM surveys (El Salloum Basin, Egypt)

Understanding and developing groundwater resources in arid regions such as El Salloum basin, along the northwestern coast of Egypt, remains a challenging issue. One-dimensional (1D) electrical sounding (ES), two-dimensional (2D) electrical resistivity imaging (ERI), and very low frequency electromagnetic (VLF-EM) measurements were used to investigate the hydrogeological framework of El Salloum basin with the aim of determining the potential for extraction of potable water. 1D resistivity sounding models were used to delineate geoelectric sections and water-bearing layers. 2D ERI highlighted decreases in resistivity with depth, attributed to clay-rich limestone combined with seawater intrusion towards the coast. A depth of investigation (DOI) index was used to constrain the information content of the images at depths up to 100 m. The VLF-EM survey identified likely faults/fractured zones across the study area. A combined analysis of the datasets of the 1D ES, 2D ERI, and VLF-EM methods identified potential zones of groundwater, the extent of seawater intrusion, and major hydrogeological structures (fracture zones) in El Salloum basin. The equivalent geologic layers suggest that the main aquifer in the basin is the fractured chalky limestone middle Miocene) south of the coastal plain of the study area. Sites likely to provide significant volumes of potable water were identified based on relatively high resistivity and thickness of laterally extensive layers. The most promising locations for drilling productive wells are in the south and southeastern parts of the region, where the potential for potable groundwater increases substantially.

Geophysical evaluation of groundwater potential in part of southwestern Basement Complex terrain of Nigeria

The geophysical assessment of groundwater in Awa-Ilaporu, near Ago Iwoye southwestern Nigeria was carried out with the aim of delineating probable areas of high groundwater potential. The area falls within the Crystalline Basement Complex of southwestern Nigeria which is predominantly underlain by banded gneiss, granite gneiss and pegmatite. The geophysical investigation involves the very low frequency electromagnetic (VLF-EM) and Vertical Electrical Sounding (VES) methods. The VLF-EM survey was at 10 m interval along eight traverses ranging between 290 and 700 m in length using ABEM WADI VLF-EM unit. The VLF-EM survey was used to delineate areas with conductive/fractured zones. Twenty-three VES surveys were carried out with the use of Campus Ohmega resistivity meter at different location and at locations areas delineated as high conductive areas by VLF-EM survey. The result of VLF-EM survey along its traverse was used in delineating high conductive/fractured zones, it is, however, in agreement with the delineation of the VES survey. The VES results showed 3–4 geoelectric layers inferred as sandy topsoil, sandy clay, clayey and fractured/fresh basement. The combination of these two methods, therefore, helped in resolving the prospecting location for the groundwater yield in the study area.

Can Very Low Frequency Electromagnetic Survey Detect Graphite Deposits in the Subsurface? Initial Results from Daltanganj, Jharkhand

Abstract: Very low frequency electromagnetic (VLF-EM) survey has been carried out near an open cast mine of graphite deposit in Daltanganj, India. The area has a small mineable open cast mine where mining companies are digging randomly to excavate graphite deposits. The miners fail to find a good amount of graphite deposit as they do not have any prior knowledge about the actual structure and deposition. Since graphite is a non-metallic mineral but holds high conductivity contrast with its neighboring host rocks (Granite gneiss/schist), electromagnetic survey is suitable to map the area. Therefore VLF-EM survey was conducted near the open cast mine to determine the sub-surface graphite deposit and their structural control. The present study shows that there are thick conducting zones associated mainly with graphite mineralization located near to the old mine with a depth of around 40 m and width of ~ 80m. VLF-EM effectively utilized to target the sub-surface graphite mineralization and associated structures in reducing the cost and time for further geophysical surveys. However, other geoelectrical methods can be used to validate the graphite deposits.

Delineation of fracture zones in the part of Vindhyan fringe belt of Ahraura region, Mirzapur district, Uttar Pradesh, India using integrated very low frequency electromagnetic and resistivity data for groundwater exploration

Integrated electrical and electromagnetic geophysical methods were used to delineate fracture zones in the part of Vindhyan fringe belt of Ahraura region of Mirzapur district, Uttar Pradesh, India for groundwater exploration. Groundwater occurs in fractures in hard rock area. A detailed survey of the area was done using satellite imagery to extract the lineament, and appropriate locations were selected for integrated electrical (resistivity method) and electromagnetic (VLF-EM) geophysical surveys. Very low frequency electromagnetic (VLF-EM) survey was conducted along ten traverse profiles. Qualitative interpretation of VLF-EM was done using Fraser and Karous-Hjelt filtering techniques which generate Fraser-filtered real data curves as well as pseudo-current density cross section along each profile. The interpretation result (anomaly) of VLF-EM may or may not delineate the appropriate fracture zone based on a number of higher values of relative current density which correspond to the subsurface feature. Four gradient profiling (GP) surveys of DC resistivity method were carried out at the position of the VLF-EM anomaly and to correlate the anomaly of gradient profile and VLF-EM anomaly which located the position of vertical electrical sounding (VES) and also the fracture zone. VLF-EM and subsequent resistivity surveys (GP and VES) at suitable locations suggest the existence of a fracture zone in the part of Vindhyan fringe belt of Ahraura region of Mirzapur district for the purpose of groundwater exploration.

Groundwater Exploration in Ago-Iwoye Area of Southwestern Nigeria, using Very Low Frequency Electromagnetic (VLF-EM) and Electrical Resistivity Methods

Very Low Frequency Electromagnetic (VLF-EM) and Vertical Electrical Sounding (VES) method were used to delineate fracture zones for groundwater exploration within the permanent site of Olabisi Onabanjo University, Ago-Iwoye, Southwestern Nigeria which is underlain mainly by suites of gneissic rocks. The VLF-EM survey was made at 10 meters interval along eight (8) traverses at East - West direction ranging from 230 - 500 meters in length. The plot of filtered real and filtered imaginary identified eight (8) prominent fracture zones close to the surface or sub surface which was further delineated by VES method. Seventeen (17) VES soundings were carried out at locations of prominent VLF anomalies presumably typical of basement fracture using AB/2 of 100m. The results shows 3 distinctive geo-electric interpreted layers of sandy top soil, (0.6m to 8.6m, 107.8 m to 662.72 m), underlain by sandy clay/clayey sand (5.7m to 43.5m, 34.7Ωm to 346.8Ωm) which is underlain by fractured/fresh basement (484Ωm to 5651Ωm). The basement fractures identified from VLF-EM anomaly curves were confirmed by geo-electric interpretation of the VES, therefore, the use of the two methods help in identification and delineation of prominent prospective groundwater areas.

Geophysical signature of the vien-type uranium mineralization of Wadi Eishimbai, Southern Eastern Desert, Egypt

The application of various geophysical tools with different responses succeeded in fixing U-mineralization in Wadi Eishimbai area. The area was studied using detailed ground spectrometric, magnetic, and filtered very low-frequency electromagnetic (VLF-EM) surveys. The interpretation of the obtained spectrometric maps clearly reflects the sharp increase of equivalent uranium (eU) content. Meanwhile, K and Th contents show sharp decreases. The eU/equivalent thorium (eTh) ratio correlates positively with eU concentrations and negatively with eTh concentrations, indicating an increase in U potentiality than the surrounding granite. The N–S shear zone displays an eU content ranging from 20 to 140 ppm. The ENE-trending lamprophyre is characterized by elongated uranium anomalies trending in the E–W direction, with values >90 ppm. Equivalent uranium content of the brecciated granite attains values up to 700 ppm. The ground magnetic and VLF-EM surveys played important roles in providing structural information which are proven useful in geological mapping and mineral exploration for the discovery of uranium mineralization in the study area. This study follows the expected subsurface extension of the Sela shear zone under Wadi sediments. The ground total magnetic intensity map shows a relatively narrow and an elongated shape for the lamprophyre anomaly extending for about 600 m in the Wadi toward the western direction. VLF-EM contour maps of the two used frequencies (17.1 and 28.5 kHz) show excellent agreement, indicating that the shear zone is distinguished with slightly strong conductivity westwards as an extension of the main shear zone. It is elongated in an ENE–WSW trend and extends in the western direction, referring to the existence of conductive materials. Most of the NW/SE-trending faults cause sudden changes in the magnetic and VLF-EM contour spacing over an appreciable distance, which suggests a discontinuity in depth due to their left-lateral strike-slip displacements. The interpreted faults, with an ENE–WSW trend representing the main trend of Sela shear zone through which hydrothermal solutions flowed, cause high alteration and uranium mineralization.

Noise reduction and data recovery for a VLF-EM survey using a nonlinear decomposition method

Geologic noise and background electromagnetic (EM) waves often degrade the quality of very low frequency electromagnetic (VLF-EM) data. To retrieve signals with significant geologic information, we used a new nonlinear decomposition technique called the empirical mode decomposition (EMD) method with the Hilbert transform. We conducted a 2D resistivity model study that included inversion of the synthetic data to test the accuracy and capabilities of this method. Next, we applied this method to real data obtained from a field experiment and a geologic example. The filtering procedure for real data starts with applying the EMD method to decompose the VLF data into a series of intrinsic mode functions that admit a well-behaved Hilbert transform. With the Hilbert transform, the intrinsic mode functions yielded a spectrogram that presents an energy-wavenumber-distance distribution of the VLF data. We then examined the decomposed data and their spectrogram to determine the noise components, which we eliminated to obtain more reliable VLF data. The EMD-filtered data and their associated spectrograms indicated the successful application of this method. Because VLF data are recorded as a complex function of the real variable distance, the in-phase and quadrature parts are complementary components of each other and could be a Hilbert transform pair if the data are analytical and noise free. Therefore, by comparing the original data set with the one obtained from the Hilbert transform, we could evaluate data quality and could even replace the original with its Hilbert transform counterpart with acceptable accuracy. By application of both this technique and conventional methods to real data in this study, we have shown the superiority of this new method and have obtained a more reliable earth model by inverting the EMD-filtered data.

Integrated geophysical investigations in the Hisar geothermal field, Demirci, western Turkey

The shallow, low-temperature geothermal field of Hisar, in western Turkey, was studied by means of vertical electrical sounding (VES), Wenner, self-potential (SP) and very low-frequency electromagnetic (VLF-EM) profiling surveys. The VES survey, conducted along the valley in an E–W direction, provided lithological and structural information that is in good agreement with the well data, and suggests that the field is characterized by low-resistivity values (≤30 Ω m). Because the resistivity structure in a N–S direction could not be investigated due to the rough topography towards the north, SP, Wenner and VLF-EM measurements were made to identify and characterize a possible hot-fluid carrying fault/fracture zone that forms one of the boundaries of the valley. There is a good correlation between the results of the SP, VLF-EM and Wenner surveys, which confirm the existence of the NE–SW striking fault zone that had been inferred from geologic information. Tensional fractures that developed perpendicular to the fault zone were also identified. Consistent with the results of the SP and Wenner profilings, the two-dimensional model derived from VLF-EM data and a Karous–Hjelt current density pseudo-section detected the conductive fault zones bearing the geothermal fluids.

Interpretation of ground VLF-EM data in terms of vertical conductor models

The very-low-frequency electromagnetic (VLF-EM) method has been used extensively for the detection and delineation of weak conductors such as those formed by water and/or clay-filled fracture and shear zones in the Precambrian rocks of the Canadian Shields as part of the Canadian Nuclear Fuel Waste Management Program. However, no satisfactory method of interpreting VLF anomalies over such conductors was available when the field data became available in the early 1980s. A study was undertaken to develop a method for quantitative interpretation of ground VLF-EM data over two-dimensional (2D) sheet-like conductors of low conductance, finite depth and depth extent, embedded in a resistive host rock of finite resistivity, using characteristic curves. Numerical modelling of the VLF response of such conductors revealed that the axes-ratio (inappropriately termed ellipticity in VLF literature) variations are much smaller than the corresponding tilt angle variations, and much less sensitive to the depths of the conductors. Extensive ground VLF-EM surveys over weakly conductive fractures and shear zones at several test areas in Ontario also indicated that the axes-ratio response is not only much smaller than the corresponding tilt angle response, it is often difficult to identify in the field data. An interpretation scheme was devised to determine the conductance, depth and depth extent of such weak vertical conductors located in a host rock of finite resistivity which depends on the peak-to-peak tilt angle response and the horizontal separation between the peaks. A complete interpretation of such conductors requires knowledge of the host rock resistivity and the approximate depth, if the depth extent of the conductor is required. The interpretation method was applied to interpret a VLF anomaly over a complex conductivity structure at East Bull Lake, Ontario which illustrated the limitations of the technique. A ground VLF anomaly from Chalk River, Ontario, was interpreted using the interpretation scheme described, which agrees well with the information from geological mapping and with the numerical forward modelling response.