Very Low Frequency Electromagnetic Method Research Articles

Subsurface Mapping of Groundwater Contamination Pathway at Using Very Low Frequency Electromagnetic Method at Waste Site of Kubanni Basin, Zaria, Kaduna State

Subsurface Mapping of Groundwater Contamination Pathway at Using Very Low Frequency Electromagnetic Method at Waste Site of Kubanni Basin, Zaria, Kaduna State

Near surface assessment of New capital city using geophysical techniques

ABSTRACT The Egyptian government announced the creation of a brand new capital city to address Cairo’s rapid population expansion and improve the quality of life for citizens, which is thought to have a strong demand for housing. In this study, we focused on the application of geophysical techniques, specifically Electrical Resistivity Tomography (ERT) and Very Low Frequency Electromagnetic Method (VLF-EM), to investigate the shallow subsurface in the New Capital city in Egypt. The study area is located in the eastern part of Cairo and is characterised by Oligocene and Miocene rocks. The ERT profiles reveal four geoelectrical layers based on resistivity values, with the first layer (Resistivity >300 Ω.m) interpreted as sand and conglomerate on the surface or basalt below. The second layer (100–300 Ω.m) is identified as sandstone, while the third layer (15–100 Ω.m) is interpreted as humid sandstone. The fourth layer (1–15 Ω.m) is characterised as shale. The abrupt changes in resistivity values horizontally indicate the probability of subsurface structures, including faults. The VLF-EM method is used to identify faults and cracks in the earth’s crust. The results provide information on the size, shape, and depth of both shallow and deep subterranean conductors. Anomalies of high conductivity suggest fault or fracture zones influencing underlying strata.

Investigation of the Source of Groundwater Contamination Near a Farmland Using the VLF-EM and Magnetic Methods

Groundwater pollution continues to be a major problem in society where there is high level of urbanization, industrialization and conventional/inorganic farming. A detailed ground magnetic survey and VLF-EM was carried out on a site just beside a farmland to investigate the possible causes of the groundwater contamination in the area which lies within the Precambrian Basement Complex of Southwestern Nigeria. The survey was carried out using a proton precision magnetometer kept at a constant height during the survey, accompanied by Very Low Frequency - Electromagnetic Method (VLF-EM). Data for the two methods were acquired on five (5) traverses with the length of 100 m each. The distance between each traverse (inter traverse spacing) is 5 m. The VLF-EM data were interpreted qualitatively, using the Fraser and the Karous–Hjelt filters. The Fraser filtered data and relative current density pseudosections from the VLF-EM data indicate the presence of shallow and deep conductive zones that cross the farmland which are indicative of fractures. The quantitative interpretation of the data was done with a 2-D code that performs the inversion of the data. Magnetic maps were generated using Oasis Montaj, and profiles were plotted using Microsoft Excel software. From the results obtained, some major geological features were identified while linear features suspected to be fault/fractured zones, weathered zones, and bedrock depressions were delineated. Low magnetic intensity indicating presence of weak and incompetent zones were discovered. Earlier chemical analysis of both surface and groundwater collected at some depth on the farmland confirms the presence of water contamination. This knowledge supported the interpretation of the anomalies detected by VLF-EM and magnetic data as a possible cause of the contaminated groundwater flowing in connected fractures. The study concluded that the groundwater contamination originates from the farmland where some samples of the soil were found to contain nitrate chemicals resulting from fertilizer and pesticide used to enhance plant growth. Keywords: Nitrate contamination, groundwater pollution, multiple fractures, contaminant transport, fractured zone, inorganic farming DOI: 10.7176/JEES/12-6-04 Publication date: June 30 th 2022

IDENTIFIKASI PERSEBARAN AIR LINDI MENGGUNAKAN METODE VERY LOW FREQUENCY ELECTROMACNETIC (VLF-EM) DI TPA PUUWATU, KOTA KENDARI

Leachate which is a liquid released from waste due to biological degradation processes. If the leachate flow is allowed to flow to the surface of the soil, it will have negative effects on the surrounding environment. One of the geophysical methods that can be used to detect leachate distribution patterns is the VLF-EM (Very Low Frequency-Electromagnetic) method. The VLF-EM (Very Low Frequency-Electromagnetic) method is a geophysical method that utilizes electromagnetic principles using a low frequency radio wave source of 10-30 kHz. The research location is in TPA Puawatu, Kendari City. The analysis was carried out by looking at the 2D cross section of the flow density gained from the data processing. Qualitative interpretation can be carried out by the method developed by Karous-Hjelt with the producing of a 2-dimensional cross section with current density parameters. The high current density value can be identified as the most conductive anomaly. The direction of leachate distribution in the Puuwatu TPA tends to flow from the north of the TPA to the south of the TPA, which indicates that many swamps in the South of the TPA have been contaminated with leachate.

Investigation of Groundwater Potential Using Electromagnetic Method at a Basement Complex Area of University of Abuja, Gwagwalada

The Very Low Frequency Electromagnetic Method (VLF-EM) was used in view of detecting fractured or weathered zones within the University of Abuja Staff Quarters, Gwagwalada, Federal Capital Territory. The VLF -EM data measured along seventeen profiles of 400m at inter profile distance of 25m and interstation separation of 10m were done using the Scintrex ENVI Instrument. The VLF-EM survey revealed features significant to groundwater potential as conductive zones in the Fraser Filter maps and current density pseudosections. Three distinct zones were delineated based on the current density distribution. The fresh basement terrain corresponds to the highly resistive zone with current density value less than -20. The intermediate zone has current density value range of -20 to 25 typical of rocks and soil component which are slightly resistive to slightly conductive and corresponds to the partially saturated units. The third zone is highly conductive with current density value greater than 30 which includes the saturated weathered or fractured basement, fault zones, clay units and saturated sandy units within the study area. The north eastern, north western (profiles 1-8) and some parts of the southern region (profiles 16 and 17) of the study area show higher conductive zones than the central parts of the study area. This survey has helped in detecting sites that are suitable for groundwater exploration by identifying water bearing fractures and weathered zones in the study area. The areas with high conductivity response are areas with conductive overburden material such as clayey soil, saturated soil, water filled fractures and faults or weathered zone within the basement.

Mapping the near-surface geoelectrical structure of the Mottled Zone using the very low frequency–electromagnetic method

Mottled Zones (MZ) in south Levant regions show discrete, varied-dimensional rock complex exposures and many unexplored areas in the vicinity of the Dead Sea Transform Fault (DSF). The research area, located south of the Daba–Siwaqa Mottled Zone complex in Jordan, was surveyed using the very low-frequency electromagnetic method (VLF-EM) for the purpose of resolving the geoelectrical setting of MZ rocks, the associated structural features, and the underlying sedimentary layers. The VLF-EM tipper data acquired along five profiles was analyzed using Fraser, Karous-Hjelt, and apparent resistivity linear filters. The quantitative interpretation was conducted using a 2D inversion of the VLF-EM tipper data, and a proposed synthetic model was tested and analyzed at MZ exposure. The majority of MZ rocks are likely to be located in a region of negative Fraser values (≥ −10.0%) and negative Karous-Hjelt values (≥ −8.0%). The apparent resistivity map exhibits MZ rocks at high values (1000–2500 Ω·m).The derived resistivity models recovered from the 2D inversion of the VLF-EM data showed a relatively resistive MZ rock body immersed in relatively less resistive (conductive) sedimentary layers composed of clayey marl and marl. The known and inferred locations of MZ rocks derived from the 2D resistivity section are compatible with the locations proposed by the filtering techniques. Furthermore, the 2D resistivity sections showed a stressed underlying sequence of sedimentary rocks demonstrating sub-vertical contacts and possible displacement along fault planes. A synthetic 2D resistivity model revealed a wedge-shaped formation of 38.0 m thick MZ rocks that can be resolved from the underlying sedimentary layers.

Integration of geophysically derived parameters in characterization of foundation integrity zones: An AHP approach

An integrated geophysical investigation involving Very Low Frequency Electromagnetic (VLF-EM), and electrical resistivity methods using Schlumberger Vertical Electrical Sounding (VES) techniques was conducted at Deeper Life Camp ground, Ipinsa, Akure, Southwestern Nigeria with the aim of developing a model map that will enhance the knowledge of the subsurface geology viz-a-viz foundation integrity for appropriate location of building within the study area. The effect of four factors including soil apparent resistivity at different depths, depth to bedrock, geology and fracture density on engineering foundation were considered. In order to achieve this, a total of ten traverses were established in approximately NW-SE, NE-SW, N-S and E-W directions with station interval of 5 m and inter-traverse separation of 10 m. One hundred and forty eight (148) station positions were occupied in all for the VLF-EM profiling. Also, fifty four (54) VES stations were occupied across the study area with current electrode spacing (AB/2) varying from 1 to 65 m. Hilbert transform, Amplitude analysis, Fraser technique and Q-Factor performed on the VLF-EM method assisted in the delineation of conductive zones that could be inimical to foundation integrity. The VES result delineated four major geo-electric layers within the study area which are: topsoil, weathered layer, fractured bedrock and the fresh bedrock. The thickness of the layers generally ranges from 0.5-19.6 m. Based on geological and geophysical investigations, foundation integrity map of the area was produced using the Multi-criteria Decision Analysis, approach of Analytical Hierarchy Process (AHP). The model map classified the foundation integrity of the study area into very low, low, moderate, high and very high foundation integrity zones. The competency model at a depth of 3 m is adjudged most suitable for foundation in the study area.

English

In this study, effort has been made to achieve a comprehensive geophysical investigation on immediate and remote causes of structural/foundation failures, specifically highway(s) within the southwestern basement complex region of Nigeria. Geophysical survey involves the use of Very Low Frequency Electromagnetic method (VLF-EM) which was complemented with electrical resistivity method using two techniques; 2D Electrical Resistivity Tomography (ERT) and Vertical Electrical Sounding (VES) along Akure/Oba-Ile Airport Road, which covers a pilot test of 600 m. The VLF-EM survey produced EM anomalies and identified possible linear features along the study area. The results obtained from the VES were used to determine the second order parameters for determination of subsurface integrity/competence. The geoelectric sections identified three to five geoelectric/geologic subsurface layers along the traverse. The ERT gave information on the subsurface characteristics whose section delineated five major geologic layers comprising of the topsoil, weathered basement, fractured zone, partly fractured basement and the fresh basement. This shows that fracture and weathering are responsible for the structural failures along the study area which are products of probability functions of geodynamics. The methodology can be adopted as a model to proficiently and quickly detect changes in subsurface resistivity, integrity and identify anomalous zones which can serve as useful tool in the understanding of the probability functions of geodynamics that can bring structural and foundation failures within the basement complex region. Key words: Road, longitudinal conductance, subsurface integrity, dipole-dipole, geoelectric section, very low frequency.

A comparative study for the source depth estimation of very low frequency electromagnetic (VLF-EM) signals

In general, a quantitative interpretation of geophysical data yields information on the nature of subsurface structures of geologic interest besides depth of the source and the associated physical property contrast. However, all geophysical interpretation invariably incurs an inherent ambiguity including VLF-EM method. Techniques such as Karous-Hjelt (KH) current density, Hilbert transform etc. are in vogue to estimate the depth to the source that are either qualitative or semi quantitative. Here, we present a comparative analysis of the depth derived from VLF-EM signals by different methods over a uranium rich basement fractures of Raigarh District, Chhattisgarh, India. The obtained results are based on techniques that are not very common in VLF-EM data interpretation such as Euler deconvolution (ED), Hartley spectral analysis and analytical signal approach of denoised in-phase component realized by Empirical Ensemble Mode Decomposition (EEMD). The estimated depth from ED and Hartley spectral analysis range 10–62 m and 12–40 m respectively are compared with KH pseudo section, Hilbert transform and drilling depth. Overall, the results of the aforesaid techniques have shown satisfactory comparison wherein the Hilbert transform of EEMD de-noised in-phase component of traverse T1 (35 m), Hartely power spectrum of the principal profile PPQ (40 m) and the radially averaged power spectrum (38.8 m and 40 m) are close to the drilling depth. Therefore, the results obtained by methods presented emphasize that these techniques are equally applicable to VLF-EM signals for estimation of depth to subsurface conductors.

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.

The effectiveness of the very low frequency electromagnetic method (VLF-EM) in the exploration of sulphide mineralization in arid environments, case study from South Sinai Peninsula, Egypt

Very low frequency electromagnetic (VLF-EM) measurements were carried out in Wadi Isbayia area, south Sinai Peninsula, to test the efficiency of the VLF-EM method in the exploration of sulphide mineralization in arid environments. The VLF-EM field measurements, including tilt angle, real and imaginary components of the received VLF field, were carried out along fifteen profiles covering a quartz monzonite bedrock. Interpretation of the VLF measurements, in the light of geological information, has showed that sulphide minerals in the Wadi Isbayia area extend from the ground surface to a depth of about 200 m. However, the structural lineaments, especially faults, have controlled the distribution of the sulphide mineralization. A few of polished sections for rock samples, collected from the sites of the VLF-EM anomalies in the study area, have been prepared and examined by ore microscopy which confirmed the presence of pyrite and chalcopyrite as well as iron oxides, disseminated in the quartz monzonite bedrock. These results showed that the VLF-EM method is an effective tool in the exploration of sulphide minerals in the arid environments.

Integrated geophysical investigation of the causes of road pavement failure along Ibadan-Lagos dual-carriage, Southwestern, Nigeria

An integrated geophysical study has been conducted in a segment of Ibadan- Lagos Dual-carriage road to examine the geological factors responsible for highway failure. Schlumberger Vertical Electrical Sounding (VES) and Very Low Frequency Electromagnetic Method (VLF-EM) to evaluate the subsurface integrity, mapping out the subsurface structural features within the sub-grade soil and delineating the bedrock relief as a means of establishing the causes of road pavement failure of 1300 m stretch of road from Ibadan axis. Results from VLF-EM showed the presence of near surface linear geologic structures from the surface to a depth of 15 m which suggests probable conductive zones that have devastating effects to the foundation of the road pavement (g1, g2 and g3). VES results showed the presence of four geologic layers while fractured basement were observed with resistivity value generally less than 100 Ωm which indicates the presence of the water table. The unstable segment is characterized by low resistivity of the near surface materials and shallowness of the aquifer zone on which the road pavement was constructed. The failure of the road is caused mainly by geologic sequence and structures such as clayey subgrade soil under the road pavement and some suspected geological features such as faults and fractures. Keywords: Lithologic contacts, aquifferous zone, geoelectric section, linear contact, sub-grade soil

Geophysical investigation on the fracture distribution of Iwo-Olupona Area south-western Nigeria using VLF-EM techniques

This work uses Very Low Frequency Electromagnetic (VLF-EM) method to investigate the subsurface structure on a proposed Housing estate in Iwo, South-Western Nigeria with coordinates 7° 36’ 36.3” N and Longitude ≈ 4° 12’ 06.6” E. The average VLF frequency employed during the field work is 20.8 kHz with average signal strength of 17dBm and the measuring interval of 20m. The data obtained by the VLF-EM method were analysed by 2Dline Plotting. Raw real, R (In phase) and the imagining, I (quadrature) components were measured on the field. The Raw Real and filtered real data were plotted against stations on the same graph using excel, the graphs obtained are sinusoidal in shape with the peak indicating a conductive zones, resulting from weak geologic materials that may be due to fracture, joints, crack, and the trough indicates a non-conductive zones, associated with competent material which may be attributed to fresh basement material or resistive geological formation. Keywords: Fracture, conductive zone, weak zone, structural displacement, frequency

Effects of very low frequency electromagnetic method (VLFEM) and physicochemical change of Zango abattoir

The study examined the impact of livestock dung on ground water status in the study area. To achieve this, a very low frequency EM survey was conducted; the aim and objective was to detect fractures in the subsurface. VLF data were acquired at 5m intervals along two profiles, with maximum length of 60m in the North-South direction. The fraser filtered real component of the processed VLF data detected anomalous zones/ litho logical boundaries that may possibly serve as conduit for the movement of solid waste contaminant into the ground water. Water sample from the hand dug wells were collected in prewashed 250ml plastic container and analyzed in the Laboratory. Result obtained shows that physicochemical analysis deviate from WHO and SON standard and complement with VLF EM survey. This has shown that it is not free from certain heavy metals, as such, the water is not suitable for human and livestock consumption. It is recommended that the ground water in Zango should be treated using coagulants before use. Keywords : Abattoir, Livestock, SON, Water quality, WHO, Zango

Mapping of subsurface fault structures by VLF-EM method in Al Khoud area, Muscat, Sultanate of Oman

The subsurface fault structure near Al Khoud area of Muscat city, Sultante of Oman, was investigated using very low frequency electromagnetic (VLF-EM) method at two different transmitter frequencies namely 15.1 and 16.0 kHz. The measured in-phase and out-of-phase components were processed and interpreted in order to determine the depth to the top and width of the fault by Kharous-Hjelt filtering and the amplitude of analytical signal approach. In addition, the principal profiles from the contour maps of in-phase and out-of-phase components when processed and interpreted yield similar results for depth and width of the fault structure. The over all results suggest that the nature of fault structure is dipping as it lies at a wide range of depth from different profiles. The results are presented highlighting the merits of the methods.

Mapping Groundwater Contamination around a Dumpsite in Benin City, Nigeria Using VLF-EM Method

An electromagnetic survey was used to delineate conductive and non conductive zones in order to accurately locate electrical resistivity tomography profiles at Ikhueniro refuse dumpsite. The survey comprised four roughly N ‐ S, and E ‐ W direction, very low frequency electromagnetic (VLF-EM) profiles around the perimeter of refuse dumpsite where contaminants/conductive bodies were observed using resistivity imaging and soil analysis method in a previous study. The Fraser and the Karous-Hjelt filters were used in the interpretation of the VLF-EM field data. The data inversion was carried out using a 2-D code which was constrained using previous resistivity survey. The results of the resistivity models calculated from the pseudosections indicate the presences of low resistivity zone up to a depth of 50 m within the subsurface that is in a good agreement with the results obtained from the previous resistivity imaging survey which indicated leachate contamination at the refuse dumpsite.

Resistivity mapping as a tool for identification and characterisation of weakness zones in crystalline bedrock: definition and testing of an interpretational model

In recent years, the focus on feasibility studies for tunnels has increased in Norway. Traditionally, the refraction seismic method and the very low-frequency electromagnetic method (VLF-EM) have been used. The Geological Survey of Norway introduced the electrical resistivity traversing method (ERT) in feasibility studies for tunnel construction purposes. Resistivity modelling shows that the method has the potential to characterise fracture zones geometrically; i.e., the thickness, dip, and depth extent. Based on previous studies, a model for mineralogical characterisation is proposed. This model, and the possibility for geometrical characterisation, is critically tested with success on three tunnel projects. The results of the comparison study, with regards to weakness zones, show that VLF-EM is a method that is capable of locating fracture zones, while refraction seismic is capable of locating and indicating the width of the zone, and can be used to imply the thickness of the soil cover above bedrock. The 2D resistivity method is able to locate the weakness zone, indicate the width, depth extent, and the dip of the zone, and in addition, characterise the zone with respect to stability or water problems. The crystalline bedrock characterisation is divided into three groups: resistivity values above 3,000 Ω m indicating good rock quality, values between 3,000 and 500 Ω m indicating bedrock with mainly water problems, while values <500 Ω m indicate clay-bearing, unstable rock with fewer water problems. From our investigations, we conclude that the 2D resistivity method is a very good supplement to traditional methods for feasibility studies on tunnelling purposes in crystalline rock.

Detection of basement fractures favourable to uranium mineralization from VLF-EM signals

The amplitude of the analytic signal of the in-phase component of very low frequency electromagnetic method (VLF-EM) data is used in the delineation of the direction of basement fractures which host uranium. The images of the 2D and 3D amplitude of the analytic signal, particularly 3D images, bring out precisely the basement fractures. The in-phase component with the Hilbert transform yields an approximate depth to the basement fractures. The entire study is exemplified with the in-phase component of VLF-EM data to map the spatial distribution of basement fractures where uranium mineralization is reported in Sambalpur granitoids in the Raigarh district, Chhattisgarh, India.

Application of the VLF-EM method with EEMD to the study of a mud volcano in southern Taiwan

A three-year geophysical investigation was carried out in the Wu-Shan-Ding mud volcano nature reserve in southwest Taiwan. This study utilized the very low-frequency electromagnetic (VLF-EM) method with a recent data processing technique, ensemble empirical mode decomposition (EEMD), to investigate the subsurface structure of the mud volcanic site through the single profile modeling and the surface 2D contour analysis. With the help of this new data processing technique, the signal of VLF-EM data was enhanced to an extent that a regularized inversion with finite-element forward modeling scheme can be applied to map the subsurface resistivity image of a single profile with satisfactory accuracy. This study successfully predicted the evolving of a gryphon, and proposed a model showing that the mechanism of fissure eruptions is more appropriate than a point source model for the shallow structure of the Wu-Shan-Ding mud volcano. The results also indicate that the Chi-shan fault is still active and that it traverses the study area with a nearly vertical up-thrown angle.

Detecting leachate plumes and groundwater pollution at Ruseifa municipal landfill utilizing VLF-EM method

A Very Low Frequency-Electromagnetic (VLF-EM) survey was carried out in two sites of domestic waste of old and recent landfills. The landfill structures lie on a major highly fractured limestone aquifer of shallow groundwater less than 30 m, which is considered as the main source of fresh water in Amman–Zarqa region. A total of 18 VLF-EM profiles were conducted with length ranges between 250 and 1500 m. Hydrochemical and biochemical analysis of water samples, taken from wells in the region, has also been conducted. The integrated results of previous DC resistivity method of the same study area and the outcomes of the 2-D tipper inversion of VLF-EM data proved the efficiency of this method in locating shallow and deep leachate plume with resistivity less than 20 Ω m, and enabling the mapping of anomalous bodies and their extensions down to 40 m depth. The sign of groundwater contamination was noticed in many surrounding wells resulting in the high number of fecal coliform bacteria and total coliform bacteria and the increase in inorganic parameters such as chloride (Cl). The pollution of groundwater wells in the landfill area is attributed to the leachate bodies which flow through the upper part of Wadi Es Sir (A7) or Amman–Wadi Es Sir Aquifer (B2/A7). Furthermore, several structural features were detected and the direction of local groundwater movement has been determined. The structural features have been found to have critical effects on the flowing of leachate plume towards north-northeast and west-southwest of the potable aquifer in the area.