Effects of anisotropic changes on the conductivity of sedimentary aquifers, southeastern Niger Delta, Nigeria
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RESEARCH ARTICLE - HYDROLOGY
Effects of anisotropic changes on the conductivity of sedimentary aquifers, southeastern Niger Delta, Nigeria N. J George1 · N. E Bassey2 · A. M Ekanem1 · J. E Thomas1 Received: 18 August 2020 / Accepted: 13 October 2020 © Institute of Geophysics, Polish Academy of Sciences & Polish Academy of Sciences 2020
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 𝜆 , the assessed volume of sedimentary formation is classified into low anisotropy λ 1.3) 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 ( ) 𝜌𝜃 at that passed through geologic system. The results showcased that the plot between the strike-dependent resistivity ( ) arbitrary chosen strike and geometric mean resistivity 𝜌m 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. Keywords Anisotropy · Hydrogeological units · Diversity · Strike-dependent resistivity · Sensitivity
Introduction
* N. J George [email protected]; [email protected] N. E Bassey [email protected] A. M Ekanem [email protected] J. E Thomas [email protected] 1
Department of Physics (Geophysics Research Group), Akwa Ibom State University, PMB 1162, Mkpat Enin, Uyo, Nigeria
Department of Geology, Akwa Ibom State University, PMB 1162, Mkpat Enin, Uyo, Nigeria
2
Most ground inhomogeneity and anisotropy induce strong directional effects on resistivity distributions and sounding curves realized from measurements (Caglar and Avsar 2007). Lithological diversity, a dynamic process, which causes electrical anisotropy of soils/rocks, is an effect of alternating beddings of arenites and argillites within the subsurface.
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