Integration of Surface Electrical and Electromagnetic Prospecting Methods for Mapping Overburden Structures in Akungba-Akoko, Southwestern Nigeria

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Author(s) Oluwafemi, O. | Oladunjoye, M. A.
Pages 122-147
Volume 2
Issue 1
Date January, 2013
Keywords Very Low Frequency Electromagnetic method (VLF-EM), electrical resistivity, groundwater development, depth-to-bedrock, overburden structures, basement depressions.
Abstract

Integrated geophysical techniques involving Very Low Frequency (VLF) electromagnetic survey and the Electrical Resistivity Sounding method have been used to assess the effectiveness of these methods in mapping variations in overburden composition and bedrock lithology, that is, to characterize the overburden materials and delineate the concealed basement structures within the bedrock of Akungba-Akoko southwestern Nigeria with a view to determine the groundwater potential of the town and its implication to engineering site development. The study area is underlain by migmatite-gneiss complex with the predominant rock type being granite gneiss, which is concealed in many places. Nine VLF-EM traverses were occupied at 20 m interval. The VLF normal and filtered real component anomalies identify major geological interfaces suspected to be faults/fractured zones which were used to pick points thirty six VES stations. These VES stations were occupied using Schlumberger electrode array configuration, with half-current electrode separation (AB/2) varying from 1 m to 100 m. The resistivity sounding curves obtained from the study area vary from 2-layer (A type) earth model where the depth to bedrock is shallow to 3-layer earth model (A, H, Q, and K types) show the subsurface layers categorized into the topsoil, weathered/fractured layers and the fresh bedrock which are characteristics of the basement complex terrain. The H-type is the most dominant curve type in the study with percentage occurrence of 36.11%. Three subsurface layers were identified which are topsoil, the weathered/fractured basement and the fresh bedrock. The resistivity of the topsoil ranges from 58 Ωm to 2300 Ωm while the thickness varies from 0.1 m to 2.9 m. The weathered/fractured layer resistivity values range from 33 Ωm to 1977 Ωm with the resistivity values of the weathered layers ranging from 33 Ωm to 499 Ωm while the thickness varies from 1.9 m to 6.2 m, the overburden thickness, that is, depth to basement varies between 1.0 m to 7.3 m. The fractured layer resistivity values range from 560 Ωm to 1977 Ωm while the thickness varies from 1.3 m to 7.3 m. The fresh bedrock has infinitely high resistivity values between 1998 Ωm to 13197 Ωm. The low resistivity, thick overburden and fractured bedrock constitute the aquifer units and the series of basement depressions identified from the geoelectric sections are potential points for groundwater development The results obtained from this study have emphasized the usefulness of geophysical methods in establishing variation in lithology and if properly employed could greatly assist in choosing productive borehole locations, thus reducing financial losses associated with a poorer choice of boreholes location outside the potential points for groundwater development. The study also revealed that depressions, overlain by relatively moderate to thick overburden mostly constitute high groundwater potential zones with competent materials for probable engineering development schemes. Hence, not only groundwater development scheme but also the engineering scheme in a basement setting requires a quantitative knowledge of hydro-geophysical parameters of the hydrogeologic unit which reveals the superficial materials overlying the crystalline bedrock and the bedrock structures to recommend a probable engineering site.

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