VLF-EM scanning is a valid method for locating groundwater fractures.
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Peer-reviewed hydrogeophysical literature confirms that very low frequency electromagnetic (VLF-EM) scanning is a robust and valid method for identifying conductive zones and locating groundwater-bearing fractures.
Abstract An integrated geophysical investigation using very low frequency electromagnetic (VLF-EM) and electrical resistivity methods has been carried out for detection of groundwater bearing fracture zones in hard rock area of Sangod Block, Kota District, Rajasthan. A total of seven VLF-EM profiles and three vertical electrical soundings have been carried out to identify the fracture zones to locate the possible borehole positions for meeting the demand of drinking and irrigation water supply in the area. The interpretation of VLF-EM data is carried out qualitatively using Fraser and Karous-Hjelt filters and the interpretation of sounding data is done using IPI2win software. The interpretation of VLF-EM data usually represents the appropriate fracture zones based on higher values of relative apparent current density that correspond to the subsurface fracture. The results of the study showed the presence of fracture zones which are prominently oriented in the NE-SW and NW-SE direction. The results also showed that most of the fracture zones are located at a depth range of 10–30 m within the subsurface. The vertical electrical sounding data are collected at suitable locations corresponding to subsurface fracture zones, identified on basis of VLF data. The interpretation of VES data also suggest subsurface fracture zone at these locations. The VLF-EM and VES at suitable locations along the VLF profile suggest the existence of fracture zones in part of Sangod Block of Kota district which indicate that the study area has good prospects for groundwater. Therefore, on basis of this study the drilling of productive boreholes are recommended at sounding locations VES 3 along the VLF profile P2 at depth range of 10–20 m, VES 1 along the VLF profile P5 at depth range of 10–40 m and VES 2 along the VLF profile P7 at depth range of 5–20 m.
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.
The research area, site “G” is located in the city of Ibadan and is surrounded by the basement complex rock mainly migmatite-gneiss and been a hard rock, it is not alienated from the problem of water scarcity. In this region, water is usually encountered either in weathered zone or fractured basement which serve as a conduit for groundwater passage. It is also along this fracture that materials such as quartzo-feldsparthic minerals occur and tend to heal the fracture, hence the need for intensive geological and geophysical exploration to locate this important basement fracture. Very low frequency Electromagnetic (VLF-EM) alongside with geological survey was adopted and conductivity data were acquired along fifteen VLF traverses utilizing very low frequency of about 15 KHz using ABEM WADI survey equipment. 13 profiles were in the east-west (E-W) direction which is the dip direction of the structural elements, while the 2 profiles were in north-south (N-S) direction to serve as a control. The acquired data were analyzed, filtered and displayed inform of profiles and Karous-Hjelt (K-H) filtered pseudo-section to show conductivity and resistivity in 2-D. The results reveal pocket of conductive zones which have been considered as fractures with different dimension and orientation. Most are single fractures, while some are closely spaced double fractures, some of which are connected to each other in depth. Also, some of these fractures contain water while some are dry, some are even suspected to be healed with quartzo-feldsparthic minerals and all of these depend on the conductivity / resistivity signature exhibited by the fractures. Therefore, it can be concluded that VLF has effectively characterise the basement into fractures that contain water, barren fracture and even fracture that contain quartzo-feldsparthic vein.
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