2D Geo-electrical Resistivity Imaging of Clay Deposit in Agbonmwoba Village, Edo State, Nigeria

Abstract

This study involves delineation of clay deposits in Agbonmwoba village area of Obaretin town, Edo state using 2D geo-electrical resistivity tomography technique to image the extent and economic quantity of the deposit, in order to appraise its commercial viability. The Wenner-Schlumberger electrode configuration was employed in five different locations within the study area, and a total of ten (10) Wenner-Schlumberger soundings were acquired in the area with a spread length of 200m along each traverse. The field data was inverted using the Earth Imager 2D inversion software program. Low resistivity geologic formations (ρ < 50 Ωm) were interpreted as clay formations using knowledge of regional geology of the area as reported from previous studies. Along traverses 1, 2 and 8 sampled at old Sapele road Ekiosa clay deposits was imaged at depth of 8.9-15m at lateral spread of 30m and 70m in TR1, 8.9-17.9m in TR2 and 4m in TR8. In traverses 3 and 4 sampled along Presco road in Obaretin, clay deposits were imaged at 8.9m from the surface in TR3 and 8.9-10m in TR4 at lateral spread of 70m and 100m respectively. Traverses 5, 6 and 7 were sampled at Agbonmwoba primary school, and clay deposit was imaged at 17.9-21m in TR5 at lateral spread of 80-90m, clayey sand was imaged at 7.5m (< 8.9m) and sand deposit was imaged at 24.0m (< 26.8m) which underlies clay. In TR6, clay deposit was imaged at 8.9m from the surface and in TR7, clay deposits were imaged in layers at profound depths >30m at lateral spreads of 20-80m and 130-160m respectively. In TR8 clay deposit was imaged at 4m from the surface. In TR9 clay deposit was imaged at 8.9-17.9m at lateral spread of 90-110m and in TR10 clay deposit was imaged at 17.9-20m at lateral spread of 40m and 100-120m respectively. This results show that clay deposits imaged along traverses 1, 2, 4, 5, 9, and 10 are located at proximal depths within the subsurface and this implies that the subsurface geology distribution along these profiles is slightly homogenous in lithology, and clay deposits along these traverses are exploitable, while clay deposits along traverses 3, 6 and 8 are found at the surface (0-8.9 m) as thin clay deposits and therefore not commercially viable for exploitation. Clay deposits imaged in traverse 7 was relatively massive in size (subsurface distribution) and was observed at profound depth of 26.8-35.7m (> 30m), which makes it commercially viable for exploitation compared with other traverses. The results was correlated with borehole lithologic section drilled along TR5 in the area and both sections agreed in lithologic sequence with respect to depth. Thus, 2D resistivity imaging gives a better vertical and lateral view of subsurface layers than geo-electric sections computed from 1D models, because of its ability to give a continuous record of subsurface formations at greater depth.

Keywords: Electrical resistivity tomography (ERT), Clay deposit, Wenner-Schlumberger configuration, 2D resistivity pseudosection.

DOI: 10.7176/JEES/10-10-06

Publication date:October 31^st 2020