Effects of anisotropic changes on the conductivity of sedimentary aquifers, southeastern Niger Delta, Nigeria

Abstract

Sundry soils/rocks are characterized by electrical properties with clearly or obscurely expressed anisotropies. These anisotropic effects may be low, moderate or high depending on the coefficient of anisotropy (λ). The vertical electrical sounding technique employing Schlumberger electrode configuration and lithological information from boreholes were deployed to characterize the lithological diversity in homogeneous and anisotropic geologic units that serve as aquifer systems and their overlaying layers in the coastal region of Akwa Ibom State. Based on the $\lambda$, the assessed volume of sedimentary formation is classified into low anisotropy λ < 1.2, moderate anisotropy $\left( {1.2 < \lambda \le 1.3} \right)$ and high anisotropy $\left( {\lambda > 1.3} \right)$ with alluvium (64.3%), inter-bedded shale and sandstone (14.3%) and shale and slate (21.3%). The estimated percentage of respective compositional coverage indicates that alluvium is dominant, while the blended inter-bedded shale and sandstone as well as the shale and slate are minor geologic units in the Benin Formation. Inferred index of spread of alluvium indicates that the homogeneous and anisotropic units assessed are intrinsic/microscopic in nature as identified by the impressed current that passed through geologic system. The results showcased that the plot between the strike-dependent resistivity $\left( {\rho_{\theta } } \right)$ at arbitrary chosen strike and geometric mean resistivity $\left( {\rho_{m} } \right)$ can be used as a yardstick for inferring the degree of consistency of geologic compositions in homogeneous and anisotropic media. Both the quantitative (graphic) and qualitative (contour) results portend the thin possibility of having anisotropy free geologic units. The finding proposes that ground resistivity measurements and interpretations of geologic structures should be constrained by borehole information in order to firm up the intended plans for obtaining clearer, defendable and well-resolved subsurface structures.