Abstract
<p>In a hard rock environment with surface or near surface expression of outcrops, the use of VES only becomes difficult due to its associated problems and limitations. However, the use of VLF-EM becomes imperative in mapping basement structures relevant to groundwater development. A total of 2100 VLF-EM data plus selected VES points were covered over 7 traverses with length varying from 3000 to 4500 m. The VLF-EM result mapped basement structures relevant in groundwater development. This is further studied using VES. Results of the investigation indicate that a HKH sounding curve is obtained with a maximum of five subsurface layers comprising: the top soil, clay/sandy clay unit, the fresh basement, fractured basement and the basement bedrock. The fractured basement layer constitutes the main aquifer unit with thickness of 20-25 m. The layer resistivity and thickness ranges are 60-100 ohm-m; 40-80 ohm-m, 1600-2100 ohm-m, 250-420 ohm-m, and 2200-7500 ohm-m; and 0.3-1.2 m, 1.9-2.5 m, 16.5-22.5 m and 22-30 m for the five layers respectively. The geoelectric results compared favorably well with drilling information to above 90%. The yield of the well is 2 L/s with good recharge capability.</p>
Highlights
The Federal Housing Estate off Irese road, Akure is located on longitudes 5°11 ́ E to 5°14 ́ E and latitudes 7°17 ́ N to 7°19 ́ N (Figure 1)
Very Low Frequency electromagnetic (VLF-EM) has been applied successfully to map resistivity contrast at boundaries of fractured zones having a high degree of connectivity [Parasnis 1979]
The existence of a high resistive basement rock (1830-2500 Ωm) with thickness of 15.818.5 m) underlain the weathered layer at shallow depth, indicated that VLF-EM and vertical electrical sounding (VES) would be appropriate techniques to explore the possibility of delineating water bearing fractures where significant a b c d e f g h
Summary
The Federal Housing Estate off Irese road, Akure is located on longitudes 5°11 ́ E to 5°14 ́ E and latitudes 7°17 ́ N to 7°19 ́ N (Figure 1). The area has not been connected to the municipal water supply system, the inhabitants rely solely on water from hand-dug wells, boreholes and at times streams This area is characterized by surface manifestations of rocks that at times form high elevations. The advantage of VLF-EM is that it is relatively fast compared to many other geophysical methods This method has probably been the most popular electromagnetic tool for quick mapping of near surface geologic structures in mineral exploration. It is being increasingly used for shallow groundwater exploration [Palacky et al 1981] as a reconnaissance tool for weathered layer investigations. VLF-EM has proved to be successful in identifying deep water bearing fractures in bedrock [Sundararajan et al 2007]
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