Modelling the trends of resistivity gradient in hydrogeological units: a case study of alluvial environment

Abstract

Generally, longitudinal resistance is identified as one of the second order geo-electric indices used to assess aquifer vulnerability to contamination; mapping of geologic sequence for location of engineering sites and evaluation of subsurface for mineral explorations based on the varying electrical characteristics. Geo-resistivity of vertical electrical sounding (VES) was performed to examine the variations of resistivity gradient for surficial geological lithologies in the alluvial environments. Fourteen VES employing Schlumberger electrode configuration with a maximum current spacing AB of about 300 m were performed. The VES data were reduced to its first order geo-electric indices, representing geologic equivalents of the layers penetrated by current with the constraints of nearby logged boreholes. The first order geo-electric indices were converted to second order geo-electric indices, which include longitudinal conductance and resistance. The injected current at its maximum electrode separations penetrated four sedimentary units characterized by resistivity range of 95.2–3455.5 Ωm and mean value of 834.8 Ωm in the first layer. In the second layer, the gamut of resistivity was found to be from 8.8 to 3606.6 Ωm while its average value was 1327.8 Ωm. In the third and fourth layers, the mean values of resistivities were 657.2 and 1551.2 Ωm while their ranges were 72.5–1464.5 Ωm and 117.1–4893.0 Ωm respectively. Two sets of reflection coefficients estimated between layers 1 and 2 and 2 and 3 were determined and used to assess the trend of distribution of the geologic units, which indicated that the layers are alternating with argillites and arenites. Both the longitudinal conductance and resistance estimated indicate that the assessed alluvial environments are on the average, vulnerable to contaminations.