This research compared the interpretation results of the Vertical Electrical Sounding data acquired using the conventional Schlumberger and modified Schlumberger arrays with a view to assessing the effectiveness of the modified Schlumberger arrays of vertical electrical sounding as an alternative to the conventional Schlumberger array at sites with space constraint during groundwater exploration. A total of thirty-seven (37) sounding locations were occupied and one hundred (100) sounding data for both conventional Schlumberger and modified Schlumberger arrays were collected across different rock units within Ibadan metropolis, south-western Nigeria, with electrode spacing (AB/2) ranging from 1 to 75 m. The field data were interpreted qualitatively by curve matching and computer iterative methods. Also, statistical analysis of subsurface units and the coefficient of correlation “R” of the statistical plots of the field data shows the relationship between the different arrays. The raw data plot of the different arrays shows significant similarities while statistical analysis of the geo-electric parameters obtained from the different arrays across varied lithologic units show that strong relationships exist between the different field methods. The coefficient of correlationRwith values ranging from 0.7 to 0.99 implies that a good similarity exists between the different field methods employed in this study. Hence, modified Schlumberger arrays can be said to be a good alternative to the conventional Schlumberger array for groundwater exploration especially in urban settings where space constraint is a major challenge.

Naturally occurring bitumen seeps migrating within the shallow subsurface contaminates soil and groundwater in communities within the eastern segment of the Dahomey Basin in southwestern Nigeria. Managing these contaminations require an understanding of the distribution of the bitumen seeps to isolate contaminated regions to avoid farming activities or drilling shallow drinking water wells within them. Several studies have assessed the shallow occurrence of bitumen in the region, focusing on their economic potentials but ignores the challenges with soil and groundwater contamination in these communities. This study focuses on using electrical resistivity imaging techniques with limited soil cores to delineate the distribution of bitumen seeps within the shallow subsurface in Imakun-Omi community in southwestern Nigeria. We used both 1D and 2D numerical and field approaches in this study. 13 vertical electrical soundings using a Schlumberger electrode array were first acquired to obtain the bulk resistivities and layer thicknesses. The 1D resistivity alongside well lithologic data served as prior data for a forward modelling study to assess the capability of resolving the bitumen seeps within electrically resistive coastal sands. Results of the forward modelling studies guided the acquisition of five 2D electrical resistivity profiles used to image the distribution of bitumen seeps using a Wenner electrode array with a unit electrode spacing of 2 m. Results of this study show that bitumen seeps with resistivities greater than 3000 Ohm-m are distributed within the top 4 m in the area and extends down to 10 m in some locations. Our numerical studies show that despite difficulties in imaging bitumen seeps within sediments overlain by higher electrical resistivity layers, reliable results can be derived, thus proving the versatility of electrical resistivity in delineating areas contaminated by bitumen seeps within shallow coastal sands.

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