Investigation of groundwater seepages in a basement complex terrain using ground penetrating radar and vertical electrical sounding techniques.
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Date
2015
Authors
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Publisher
Obafemi Awolowo University
Abstract
Ground Penetrating Radar (GPR) and Electrical Resistivity (ER) surveys were carried out in the Basement Complex environment at Erinle area within the Ede metropolis, Osun State. This was with a view to investigating groundwater seepage and its effects on buildings in the study area.
Ground Penetrating Radar (GPR) profiles with lengths of between 80 m and 100 m each were acquired along eight (8) traverses; seven of which ran West-East, while the remaining one ran North-South to the general strike of the geology. The data were acquired at a station interval of 10 m along the 100 m spread. The data were subjected to processing by the RADprowin (GPR Software) to produce radargrams. Thirty two Vertical Electrical Sounding (VES) stations were acquired using the ABEM SAS300C Resistivity Meter along eight (8) traverses with the Schlumberger array and a maximum spread of 100 m. The data obtained from the electrical resistivity survey were interpreted using the partial curve matching and computer assisted 1-D forward modelling. The VES interpretation results were used to generate geoelectric sections.
The Ground Penetrating Radar (GPR) and Electrical Resistivity delineated three subsurface layers. These include the topsoil, the weathered layer and the basement bedrock. On the geoelectric sections, the topsoil had resistivity range of 69 Ωm to 426 Ωm and thickness range of 1.2 m to 7.2 m. The weathered layer had resistivity range of 19 Ωm to 281 Ωm and thickness range of 7.0 m to 17.4 m. The basement bedrock had resistivity range of 459 – 5289 Ωm and thickness range of 4.9 m to 8.6 m. On the radargrams, the topsoil was characterized by planar to smooth reflections. The weathered layer was characterized by nearly smooth reflections while the basement bedrock was characterized by chaotic reflections with micro-diffractions. The weathered layer revealed anomalously low resistivity values (< 65 Ωm). These anomalously low resistivity values were probably due to increase in clay and water content. This thus made Traverses 1, 2, 3 and 5 more susceptible to groundwater seepages as compared to the other traverses, as these traverses revealed anomalously low resistivity values (< 65 Ωm). Linear and near – vertical structures (subsurface structures) were delineated at a depth range of 2 m to 6 m along GPR profiles represented by Traverses 1, 2, 3 and 5. These subsurface structures which included micro – fractures, joints and mud cracks could serve as conduits for groundwater seepages.
The Study concluded that low resistivity/high water moisture content was due to increase in clay and water content in the weathered layer and some sections of the topsoil. This made such zone susceptible to groundwater seepages through micro-fractures, joints and mud cracks.
Description
xv,170p
Keywords
Ground penetrating radar, Electrical resistivity, Topsoil, Geoelectric, Micro-diffractions, Resistivity
Citation
Salako, A.O. (2015). Investigation of groundwater seepages in a basement complex terrain using ground penetrating radar and vertical electrical sounding techniques. Obafemi Awolowo University.