Abstract
Very Low Frequency Electromagnetic (VLF-EM) and electrical resistivity surveys were conducted at Modomo/Eleweran, along Ede-road, south western Nigeria, with a view to delineate the hydrogeophysical characterization of the study area. The area is underlain by the Precambrian Basement Complex rocks. The VLF-EM traverses were established along 6 traverses with a station interval of 10 m with lengths ranging from 130 to 360 m. Linear features presumed to be geologic fissures inferred from the filtered real pseudo-sections helped in selecting twenty-nine VES points that were further probed using ABEM SAS 300 C Resistivity Meter. The spreading were carried out using the convectional Schlumberger electrode configuration with half-current electrode separation (AB/2) varying from 1 to 100 m was used for the sounding. The VES data were presented as depth sounding curves and were appropriately iterated using RESIST version (1.0) software. The VLF filtered real profile displayed a low peak trend depicting poor or no fracture signature. Four lithological formations were delineated which included the topsoil, weathered layer, partly weathered/fractured basement and fresh bedrock. The delineated weathered and fractured basement columns constituted the aquifer units. Additionally, from the geophysical parameters viz a viz thin overburden thickness, clayey weathered layer and low fractured frequency characterized by the study area, it is inferred that the groundwater potential of the area varies between poor and low. However, the study justified the use of a combined geophysical investigation as a better tool in evaluating the groundwater potential in the basement complex. Key words: Weathered layer, geological fissures, aquifer, electrical resistivity, geoelectric section, electromagnetic.
Highlights
Groundwater has been described as water which occurs in the vadoze zones (Fitts, 2002) which filled the pore space of soil and fissure below the water table (Freeze and Cherry, 1979)
The EM profiling shows that the Very Low Frequency Electromagnetic (VLF-EM) anomaly varies from -42 to -4 and -16 to 18 in the raw real and filtered real components, respectively
Eight Vertical Electrical Sounding (VES) curve types were identified in the area revealing four geo-electric layers consisting of the topsoil, weathered layer, partly weathered/fractured basement and fresh basement
Summary
Groundwater has been described as water which occurs in the vadoze zones (Fitts, 2002) which filled the pore space of soil and fissure below the water table (Freeze and Cherry, 1979). This can be extracted by boreholes and hand-dug wells occur in a highly permeable geological formation known as aquifers which have properties that allow storage and movement of water (Eduvie, 2006). The highest groundwater yield in basement terrain is found in areas where thick overburden overlies fractured zones (Olorunniwo and Olorunfemi, 1987) These zones are often characterized by relatively low resistivity. The indiscriminate sinking of boreholes without employing systematic scientific approach, that is, pre-drilling geophysical investigation has led to unsuccessful boreholes with poor or low yield (Bayode et al, 2007)
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