VLF Method Research Articles

Characterization of the Sulfide Deposits in the Southeastern Nigeria Using VLF Method: Insights from Numerical Modeling and Field Examples

A priori geologic and geophysical information has been used to construc conceptual VLF experiments on conductively and inductively coupled overburden geological models of the lead-zinc (Pb-Zn) mineralization zone found in southeastern Nigeria.This is based on the finite element approach to (1) simulate different geologic situations of overburden occurrence, (2) examine the roles played by overburden in modifying and masking VLF responses of a buried conductor target, and (3) confirm the effectiveness of VLF method in mapping lead-zinc lodes found in sedimentary terrains.The computed theoretical model curves and field examples are expected to serve as guide for VLF anomaly pattern recognition due to overburden thickness, resistivity and width of conductor in similar terrain as the study area.

Current state and prospects of development of passive radio engineering monitoring of storm activity

Passive radio engineering monitoring means of storm activity in VLF range are considered in the work. The comparative analysis of characteristics of modern Russian and foreign lightning location systems which have a broad practical application is carried out. Possibilities of improvement of precision and probabilistic characteristics of VLF methods and techniques for location of various types of lightning high current components are estimated.

Application of vlf method to identify flow patterns of underground river in the karst area Ngargoharjo, Wonogiri

Study about potency of underground river is indispensable as an effort to resolve the problem of drought in the karst area of Ngargoharjo Wonogiri. This study aimed to identify the pattern flow of underground river in the Village Ngargoharjo based subsurface structures. The method used is very low frequency (VLF) method with tilt angle mode which measurement has five tracks, each track is 700 meters and consists of 37 measurement points. Based on the results obtained ekivalen current density values in the range of 10% to 35% which are interpreted as a conductive anomaly in the form of an underground river. Underground river in the village of Ngargoharjo have a flow pattern that starts from the spring in the Gedangan flowing to the Southwest toward Pakel Kopek Sinkhole. Underground river over there is a upstream of the river, so the water debit is small.

Investigation Geological structural morphology of Quaternary sediment on coastal Bac Lieu

Georadar is a high-resolution near-surface geophysical method which employ radio waves, typically in the 1 to 1000 MHz frequency range tomap structure and features buried in the ground (or in man-made structures). Georadar method with Pulse EKKO 1 OOA instrument offers the highest resolution and rapidly provides continuous graphicformat which permits rapid semi-quantitative interpretation for in-field analysis. VLF method with Wadi instrument records the ratio of the strengths of the vertical and horizontal fields at the ground surface. Then this method can be used to reveal steeply dipping structures with large cross-sections and low resistivities as fructure/ conductivity zones.This paper is presented the result of using Georadar, VLF and electrical profiling methods for geological structural morphology of quaternarysediment investigation on coastal Baclieu. The structural cross-section of morphology of quaternary sediment is established by nine layers of georadar data for shallow structure and several interfaces of conductivity for deeper structure.

Identification of Manganese Distribution Using Very Low Frequency (VLF) Method at Karangsari Village, Pengasih Sub-District, Kulonprogo Regency, DIY

Research of manganese distribution was conducted in the village of Karangsari, Pengasih sub-district, Kulonprogo Regency, DIY which is the former area exploration and exploitation of manganese. The purpose of the research is known the value of relative conductivity and the distribution of manganese. The method used very low frequency tilt mode. The tools used 2 set of T-VLF BRGM (sensor and monitor) and the data processing software are Ms. Excel and KHFILT. T-VLF operated with 2 frequencies that are 19800 Hz from Australia and 22.200 Hz from Elbino Japan. The total lines are 6 lines with 75 m space. Distance between measured points is 15 m, whereas line length is 300 m. The parameters measured from VLF method are tilt (%) and ellipticity (%). The raw data VLF is processed by Ms Excel to obtain a tilt, ellipse, fraser graph vs distance and the RAE from Karous Hjelt filter. While the results of processed by KHFILT software is the equivalent current density contour map. Relative conductivity of area research are line 1 (-10 to 10) mho/m, line 2 (-40 to 30) mho/m, line 3 (-20 to 20) mho/m, line 4 (-30 to 10) mho/m, line 5 (-40 to 30) mho/m and line 6 (-30 to 40) mho/m. The spread of manganese were in the high conductivity, toward west and east of research area with depth up to 45 m.

Identifying the Distribution of Alteration Zone Using Very Low Frequency Method in Candi Gedong Songo, Ungaran, Semarang, Central Java

The alteration zone could be a key link and a proof to know where the paleo heat source was previously located. An electromagnetic survey to identify and to map the lateral and vertical distribution of alteration zone had been done at geothermal area Candi Gedong Songo, Ungaran, Central Java, from 9th - 19th of June 2014, using VLF method. The survey consisted of 6 profiles, with NW - SE direction, which were located nearby the fumaroles spots and then went down to the observed alteration zone. Each profile was 600 m long and the distance between each profile was 20 m. The space between each measurement point of a profile was 20 m. In this study, tilt and ellipticity data with frequency of 19.8 kHz (Japan) and 24 kHz (Panama) were used. First, the data was processed to get the cross features anomaly between tilt and ellipticity data on the chart. Then, the derivative fraser and the relative current density pseudosection were also made to support the cross features anomaly. The interpretation of this data was done qualitatively using fraser and relative current density pseudosection. The result shows that the alteration zone gives high response of conductivity compared to its surrounding area. This is supported by the anomaly cross features between tilt and ellipticity data on the chart, also by high value of fraser and relative current density. Thus, the alteration zone are located in meter 150 - 250 in V1 and V2 profiles, also in meter 180 - 250 in V5 and V6 profiles. This result indicates that the ancient heat source was previously located nearby the fumaroles area and it is physically shown by the presence of sulphuric clay mineral content at the alteration surface area.

Application of gamma ray spectrometric measurements and VLF-EM data for tracing vein type uranium mineralization, El-Sela area, South Eastern Desert, Egypt

This study is an attempt to use the gamma ray spectrometric measurements and VLF-EM data to identify the subsurface structure and map uranium mineralization along El Sela shear zone, South Eastern Desert of Egypt. Many injections more or less mineralized with uranium and associated with alteration processes were recorded in El Sela shear zone. As results from previous works, the emplacement of these injections is structurally controlled and well defined by large shear zones striking in an ENE–WSW direction and crosscut by NW–SE to NNW–SSE fault sets. VLF method has been applied to map the structure and the presence of radioactive minerals that have been delineated by the detection of high uranium mineralization. The electromagnetic survey was carried out to detect the presence of shallow and deep conductive zones that cross the granites along ENE–WSW fracturing directions and to map its spatial distribution. The survey comprised seventy N–S spectrometry and VLF-EM profiles with 20m separation. The resulted data were displayed as composite maps for K, eU and eTh as well as VLF-Fraser map. Twelve profiles with 100m separation were selected for detailed description. The VLF-EM data were interpreted qualitatively as well as quantitatively using the Fraser and the Karous–Hjelt filters. Fraser filtered data and relative current density pseudo-sections indicate the presence of shallow and deep conductive zones that cross the granites along ENE–WSW shearing directions. High uranium concentrations found just above the higher apparent current-density zones that coincide with El-Sela shear zone indicate a positive relation between conductivity and uranium minerals occurrence. This enables to infer that the anomalies detected by VLF-EM data are due to the highly conductive shear zone enriched with uranium mineralization extending for more than 80m.

Ground geophysical survey for studying the potentiality of uranium mineralization in rhyolite zone—Um Safi area, Central Eastern Desert, Egypt

Um Safi area is located in the southern part of the Central Eastern Desert of Egypt. The total count airborne radiometric survey data of this area have been treated qualitatively and quantitatively and correlated with the geological map of the study area. The total count image of this area shows some radiometric anomalies related to the younger granites and are structurally controlled by the dominating faults trending in NW–SE, NE–SW, and E–W directions. The map also shows some anomalies associated with the rhyolite flow tuffs. This survey indicates that the study area possesses gamma radiation ranging between 0.5 and 14 Ur as a total count. Field investigations reveal that the basement rock units of Um Safi area are extruded by rhyolite stocks and trachyte plugs and these rocks show high potentiality of radiometric elements. Detailed ground geological and geophysical studies are made over the rhyolite zone of Um Safi area by the authors; these surveys of detailed ground survey have been conducted along 16 parallel profiles oriented at N–S direction with profile separation of 40 m and with stations every 20 m along the profiles. The location of each station was determined using GPS. The analysis of the gamma-ray spectrometric readings for the rhyolite zone of Um Safi area by using the Geosoft programs indicates that there is an increase in the eTh content from 18 to 397 ppm eTh at the rhyolitic volcanic rocks. Moreover, the eU content shows an increase from 2 to 95 ppm eU at these rocks. The main trend of their eU and eTh anomalies is the NW–SE direction. The VLF method was used and the data were interpreted qualitatively to determine the location of subsurface structures. The highly conductive zones, that are consistent with rhyolite rocks trending in the NW–SE and NE–SW direction, are due to mineralization and they are interpreted as fractured mineralized zones. The integration of the results of the ground gamma-ray and VLF-EM interpretation reveal that all the radiometric anomalies lie over the rhyolite rocks and are associated with elongated positive conductivity anomalies having NW–SE and NE–SW direction. The NW–SE direction is considered as the main trend which plays an effective role in the distribution of the radioactive elements in the rhyolite zone of Um Safi area.

Correlation of VLF-EM Data with Radiometric Measurements: Implications for Uranium Exploration around Beldih, South Purulia Shear Zone, India

This study is an attempt to correlate VLF-EM data with the radiometric measurements to decipher the subsurface structure and to locate uranium mineralization in the shear zone. The study area is around Beldih mine which is an open cast apatite mine located on the South Purulia Shear Zone. VLF method has been applied to map the structure and the presence of radioactive minerals has been delineated by the detection of highαandγcounts with respect to the background radiations. High radiation counts and high surfaceγactivity are found just above the higher apparent current-density zones in all the profiles studied, at various locations, indicating uranium and/or thorium mineralization as well as good correlation between these techniques.

Geophysical Anomalies Associated with Uranium Mineralization from Beldih Mine, South Purulia Shear Zone, India

Abstract Beldih mine at the central part of the South Purulia Shear Zone (SPSZ) has been reported with low grade uranium-bearing formation within quartz-magnetite-apatite host in kaolinized formation. Therefore, the present integrated geophysical study with gravity, magnetic, radiometric, very low frequency electromagnetic (VLF) and gradient resistivity profiling methods around the known mineralized zones aimed at identifying the exact geophysical signatures and lateral extent of these uranium mineralization bands. The closely spaced gravity-magnetic contours over the low to high anomaly transition zones of Bouguer, reduced-to-pole magnetic, and trend surface separated residual gravity-magnetic anomaly maps indicate the possibility of high altered zone(s) along NW–SE direction at the central part of the study area. High current density plots of VLF method and the low resistive zones in gradient resistivity study depict the coincidence with low gravity, moderately high magnetic and low resistivity anomalies at the same locations. Moderate high radioactive zones have also been observed over these locations. This also suggests the existence of radioactive mineralization over this region. Along profile P2, drilled borehole data revealed the presence of uranium mineralization at a depth of ∼100 m. The vertical projection of this mineralization band also identified as low gravity, low resistivity and high magnetic anomaly zone. Thus, the application of integrated geophysical techniques supported by geological information successfully recognized the nature of geophysical signatures associated with the uranium mineralization of this region. This enhances the scope of further integrated geophysical investigations in the unexplored regions of SPSZ.

3D geoelectrical model of geothermal spring mechanism derived from VLF measurements: A case study from Aggistro (Northern Greece)

The most important task of a survey concerning the exploitation and protection of a thermal spring is the understanding of the mechanism of the outflow of thermal fluids. In this concept, geophysical methods are conscripted to contribute to the understanding of the hydrogeological model, since they can provide information without interferences which could potentially react on the flow of the hot water (i.e. wells, pits).In the most of the cases, the occurrence of such springs is related to the local faulting system that allows the upwards flow of the thermal fluids.In this concept, the VLF method is suggested since it is a very fast, low cost and can detect faulting zones with high accuracy.In this work, VLF method is applied in order to study the faulting system that is related to the mechanism of the hot springs in Aggistro (Northern Greece). VLF measurements have been done on parallel lines in order to examine the possible existence of conductive zones, such as faults, that potentially could be used for the outflow of the thermal fluid. VLF data have been inverted allowing the calculation of 2D resistivity models where conductive zones can be accurately located. Finally, a pseudo-3D presentation of the resistivity model allows the understanding of the mechanism of the hot spring.Electrical resistivity tomography method has been used to cross check the inversion results of VLF method and the correlation was very good. This fact, supplemented by hydrogeological data deriving from adjacent wells, verified the resistivity model revealed by VLF data, suggesting that VLF method can be a fast and accurate application for the study of a hot spring.A productive well, drilled at a location suggested by the geophysical data, found groundwater of a temperature of 34.3°C and discharge of 200m3/h.

VLF探査およびVES探査による津波浸水海岸平野での地下水塩水化調査事例

VLF探査およびVES探査による津波浸水海岸平野での地下水塩水化調査事例

Two-dimensional resistivity imaging in the Kestelek boron area by VLF and DC resistivity methods

Abstract A VLF and DC resistivity investigation was conducted in the Kestelek area, western Turkey, to determine the two-dimensional images of the boron deposits. The two-dimensional resistivity images were obtained by the inversion of tipper and resistivity data for VLF and DC resistivity methods, respectively. The VLF tipper data also were improved applying the Fraser and Karous & Hjelt (K&H) filtering to delineate the boundaries of the subsurface boron deposits. The main findings are: (1) moderate (> 25 Ωm) and relatively high (> 40 Ωm) resistivity zones in the two-dimensional models, which is mostly supported by the K&H real part of tipper as the negative current density peaks, may be interpreted as middle level of potatoes type colemanite and lower level of crystal type colemanite boron deposits inside the conductive units, respectively. (2) Transition from positive peaks (conductive zones) to negative peaks (resistive zones) in the K&H real part of tipper current density pseudosections may indicate the potential locations of the boron deposits. (3) Drilling well results obtained around two profiles of the study area are consistent with distribution of the resistive boron deposits in the two-dimensional resistivity models and K&H real part of tipper filtering images.

Delineation of subsurface structures using resistivity, VLF and radiometric measurement around a U-tailings pond and its hydrogeological implication

The hydrogeological characteristics of the uranium mill tailings pond in the vicinity of Jaduguda (Jharkhand, India) were investigated to examine possible contamination and suggest suitable remedial measures, if required. As the hydrogeological characteristics of subsurface geology are closely related to the electrical properties of the subsurface, geophysical measurements using electrical resistivity coupled with Very Low Frequency electromagnetic method and radiation study were used to investigate the geophysical and geological condition of mill tailings in order to characterize the subsurface structures of the tailings pond. The resistivity interpretation depicted the thickness of the soil cover and thickness of tailings in the pond, as well as the depth to the basement. It also suggested the possible flow direction of leachate. It was observed that the resistivity of the top layer decreases in the direction opposite to the dam axis, which in turn, indicated that the groundwater movement occurs in the opposite direction of the dam axis (in the northwest direction). The VLF method depicted the fractures through which groundwater moves, and also showed the current density alignment in the northwest direction at 10 m depth. The radiation measurement showed relatively higher counts in the northwest direction. This correlated well with the resistivity measurement. The current density at a depth of 20 m showed a closed contour suggesting no groundwater movement in the area at this depth, and that high conductivity material was confined to the tailings area only. It was concluded that groundwater moves in opposite direction of the dam axis at shallower depth only. It was found that continuation of fractures do not extend to deeper depths, which suggested that the tailings storage facility at Jaduguda was reasonably safe from any downward contamination.

Integrated geophysical exploration for the Longtoushan Ag-Pb-Zn deposit in the southeast of the Da Xing’an Ling mountains, Inner Mongolia, northern China

Geophysical and geological studies attempting to search and explore new mineralisation resources were carried out in the Longtoushan area, Inner Mongolia, northern China. The Longtoushan area is located within a large alteration zone, which is covered by a variety of altered lava, bioclast limestone and Quaternary sediments. In this case study, ground geophysical surveys played an important role in defining the mineralised structures beneath Quaternary sediment cover. The aim of this study was to investigate the subsurface mineralisation in the area using a combination of very low frequency electromagnetic (VLF-EM), Stratagem EH4 and controlled source audiofrequency magnetotelluric (CSAMT) measurements. These successful surveys in the area revealed that the VLF, Stratagem EH4 and CSAMT methods were effective in detecting the unseen mineralised system and results from the different methods confirmed each other. VLF measurements at 2875 stations along 35 lines identified the principal mineralised shear system as two sub-parallel linear conductive belts, tracing the principal mineralised system for a further 650 m of strike length beneath cover loess, in a NW–SE-trending direction. The subsequent Stratagem EH4 soundings along four parallel traverses perpendicular to the mineralised trend indicated that the principal mineralised shear structure, extending for more than 600 m in the dip direction, was almost vertically dipping above the 200 m level but changed to the SSW direction at a steep angle at depth. A CSAMT survey on two lines confirmed that the ore-bearing mineralised zone was a conductive belt and contained a low resistivity anomaly of less than 500 Ωm.

An Amendment of the VLF tanδ Criteria to Improve the Diagnostic Accuracy of the XLPE-insulated Power Cables

VLF diagnosis technology is introduced in IEEE Std 400 and proposed as evaluation criterion in an effective way of detecting water tree which mainly causes the failure of XLPE insulated cables. In order to inspect the accuracy of the VLF method for XLPE insulated power cables in Korean distribution system, diagnosis for 41 cables which were being serviced in the fields has been carried out and they were removed for AC breakdown voltage test after. Regarding the 41 cables, it was hard to confirm any relation between the VLF values and AC breakdown voltages and also water tree in the insulation was not detected. However, the other cables were failed several days after the diagnosis of the 41 cables. Water trees were found and their VLF values were also much higher than the criterion of IEEE standard. It has been ascertained that we need to change the IEEE criteria in order to improve the accuracy of detecting water trees by additional analyzing of field examples of failure and case studies from overseas countries and therefore amended VLF test voltage and evaluation criteria have been proposed.

A VLF survey using current gathering phenomena for tracing buried faults of Fethiye–Burdur Fault Zone, Turkey

Fethiye–Burdur Fault Zone (FBFZ) produces moderate to large earthquakes which can severely affect the surrounding cities spreading towards the basins of the region. In this study, with the aim of feature city planning, very low frequency electromagnetic (VLF) survey was carried out across (FBFZ) to locate subsurface faults covered by basin fills. VLF parameters such as the apparent resistivity, phase (VLF-R response) real and imaginary parts of tipper (VLF-EM response) were obtained using GBZ, a well known radio station in England at Oxford, which give a strong signal in our survey area with a suitable frequency (19.6 kHz) for our purposes. VLF method is called as VLF-EM, when it is only relying on the measurement of horizontal and vertical magnetic fields. However, when it includes measurement of horizontal electric field, providing resistivity values, it is referred as VLF-R method. In this study, the direction of horizontal electric field is perpendicular to fault strike describing the H-polarization mode. Although this mode produces strong VLF-R (resistivity) response, we also obtained quite clear and characteristic VLF-EM anomalies along some of the survey profiles. Both VLF-R and VLF-EM responses with the contribution of the vortex and gathered (direct) currents, in E and H-polarization modes, are reviewed and VLF anomalies observed over FBFZ are discussed. In FBFZ, current gathering anomalies become stronger in the presence of high conductivity contrast and in the presence of parallel fault branches separating the medium into resistors in series. In these conditions, changing electrical charge at the contacts of faults cause successive variations in E-field. As a result VLF-R response is strengthened and also secondary magnetic field which causes VLF-EM anomalies may be enhanced, in H-polarization mode. Interpretation of resistivity, phase and tipper data with current density pseudosections, over our survey lines, showed the location of buried faults of FBFZ beneath the sedimentary covers of the basins.

Geological mapping by geobotanical and geophysical means: a case study from the Bükk Mountains (NE Hungary)

AbstractGeological mapping of an unexposed area can be supported by indirect methods. Among these, the use of mushrooms as geobotanical indicators and the shallow-penetration electromagnetic VLF method proved to be useful in the Bükk Mountains. Mushrooms have not been applied to geological mapping before. Common species like Boletus edulis and Leccinum aurantiacum are correlated with siliciclastic and magmatic formations while Calocybe gambosa is correlated with limestone. The validity of this correlation observed in the eastern part of the Bükk Mts. was controlled on a site where there was an indicated (by the mushrooms only) but unexposed occurrence of siliciclastic rocks not mapped before. The extent and structure of this occurrence were explored with the VLF survey and a trial-and-error method was applied for the interpretation. This case study presented here demonstrates the effectiveness of the combination of these relatively simple and inexpensive methods.

Elecrical Tomography and VLF methods contribution to underground wastewater pipe construction

In the area of municipality ofKropia (Attica, Athens) the construction of a tunnel for the transportation of treated wastewater has been decided. The length of the tunnel is approximately 1400 m and it is designed to cross an area at a depth of maximum 100 meters. Geological and tectonic mapping showed the existence of limestone and fault systems zones that are crossing the geological formations. Thus, geophysical survey has been designed in order to provide information about the geological and tectonic status at the proposed area. The application of electrical tomography has been designed in addition with VLF electromagnetic method. The first one was expected to show the geological cross section at the axis designed while the VLF method could provide more detailed information about the fault structures zones at the area. Results of the geophysical survey showed lateral changes of the mechanical behavior of the limestone. In parallel, fault zones that are crossing the axis have been mapped. Considering that engineers need to know the nature of probable problems that they have to deal with, geophysical investigation can provide useful information in a low cost and quick application and contribute to the best decision as concern the final positioning of the tunnel and the penetrating method.

INTERPRETATION OF VLF MEASUREMENTS RELATED TO HYDROGEOLOGICAL SURVEYS

The aim of the application of VLF surveys in hydrogeology is the contribution to the decision making as concern the construction of a well. Geophysical survey by the application of VLF method must respond to the questions (a) if the construction of a well should be proposed or not, and if the answer is yes then (b) where exactly must be located. These answers are very crucial ones, since VLF methods is applied mainly in areas where bedrock is close to the surface and no chance has a well if it will not drill an aquiferous fault. Problems arise by the fact that the target to be detected is a conductive body which is interpreted as a fault filled with water. The major question that is posed to the surveyor is if the conductivity of the fault is caused by fresh water or by clay materials or saline water. Forward models of different resistivity contrast between host rock and conductive body have been computed and examined. Results show that interpretation of the imaginary part is very important to the evaluation of the data. Since the conductor -in the meaning of water presence onlymust be a medium one, a high value of the real part should be expected in parallel with a close to 0 value of the imaginary component which also must show one peak to the Fraser filtered data.