Comparison of Conventional Schlumberger and Modified Schlumberger Arrays in Estimating Aquifer Parameters in a Typical Basement Complex, Southwestern Nigeria

Abstract

This study compared the interpretation results of the Vertical Electrical Sounding (VES) data acquired using the conventional Schlumberger and modified Schlumberger arrays with a view to assessing the effectiveness of the modified Schlumberger arrays of VES as alternatives to the conventional Schlumberger array at sites with space constraint during geophysical exploration. A total of thirty (30) VES data for both conventional Schlumberger and modified Schlumberger arrays were collected across different rock units within Federal University Oye-Ekiti campus and Irare estate in Oye-Ekiti metropolis, south-western Nigeria, with electrode spacing (AB/2) ranging from 1 to 100 m. The field data were interpreted qualitatively by partial curve matching, computer iteration methods and statistical comparison. The VES interpretation results (layer resistivities and thicknesses) were used to generate six (6) geoeletric sections and statistical correlation plots for the study area. From the VES interpretation results, four (4) different VES type of curves were delineated. These are the H, A, HA and KH types. The geoelectric sections delineated four geologic layers which are topsoil, weathered layer, fractured basement and fresh basement. The topsoil resistivity varies from 10 to 282 Ωm and thickness ranging from 0.3 to 2.1 m. The weathered layer varies from 7 to 253 Ωm and thickness ranging from 0.6 to 14.9 m. The resistivity values of the basement vary from 781 to 15007 Ωm. 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 geoelectric parameters obtained from the different arrays across varied lithologic units show that strong relationships exist between the different arrays. The coefficient of correlation, R, with values ranging from 0.78 to 0.99 implies that a good similarity exists between the different arrays employed in this study. Hence, modified Schlumberger arrays can be good alternatives to the conventional Schlumberger array for the estimation of aquifer parameters especially in urban settings where space constraint is a major challenge.