Subsurface Characterisation Using the Electrical Resistivity Method: A Case Study of Itohan Girls Grammar School, Benin City, Nigeria
Abstract
To map the subsurface for borehole development, a 2D electrical resistivity inversion survey was conducted at Itohan Girls Grammar School, located on Sapele Road in Benin City. The researchers used the ABEM Terrameter SAS 2000 to perform Wenner and Schlumberger configurations over different areas. The inter-electrode spacings were 10 m, 20 m, 30 m, and 40 m, while the AB/2 values were 300 m for VES 1 in the NW–SE direction, 392m for VES 2 in the SE–NW direction, and for VES 3 in the N–S direction, with a traverse length of 600 m. Using DIPRO software, a pseudosection of the apparent resistivity values acquired along a 140 m (W–E) traverse was created to process the data. Thereafter, the researchers performed computer iterations. They generated a columnar geosection for the three VES datasets. VES 1, VES 2, and VES 3 revealed resistivity values of 283.4 Ωm, 153.0 Ωm, and 145.6 Ωm, respectively, indicating sandy clay formations with soil thicknesses of 3.5 m, 5.3 m, and 1.3 m, respectively. The second layers of VES 1 and VES 2 correspond to the third layer of VES 3, showing high resistivity values of 2601.6 Ωm, 1528.0 Ωm, and 4600 Ωm, respectively. These resistivity values depict lateritic sand formations with thicknesses of 23.6m, 29.6m, and 11.7m, respectively. The second layer of VES 3 has a resistivity of 87.0 Ωm, indicating a clayey sand formation with a thickness of 1.6m. VES 1 has a sandy clay formation as its basement, with a resistivity of 475.3 Ωm. The basement layers of VES 2 and VES 3 have resistivity values of 18,640.0 Ωm and 100,000.0 Ωm, respectively, indicating sandstone as the bedrock. The 2D resistivity structure revealed four distinct soil formations along the traverse from west to east, extending to depths of about 50 meters. These correspond to the geoelectric layers identified by the vertical electrical soundings conducted in the area. The results suggest that the area is suitable for large, shallow engineering structures and borehole drilling.
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