Subsurface electrical resistivity modelling over a suspected fault zone at Ojirami, Southwestern Nigeria
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ORIGINAL ARTICLE
Subsurface electrical resistivity modelling over a suspected fault zone at Ojirami, Southwestern Nigeria Akinola B. Eluwole1 · Olusola A. OlaOlorun2 · Oladimeji L. Ademilua2 · Abel O. Talabi2 · Adeyinka O. Aturamu2 · Yemisi C. Ajisafe2 · Olufemi F. Ojo2 · Christopher A. Ajayi2 Received: 5 May 2020 / Accepted: 14 June 2020 © Springer Nature Switzerland AG 2020
Abstract The subsurface of a suspected fault zone at Ojirami Southwestern Nigeria was investigated using the four-electrode array model of the electrical resistivity method of geophysical prospecting. Measurements were taken along four traverses using the 1-D and 2-D electrical resistivity field techniques. The Schlumberger and dipole–dipole electrode configurations were respectively utilized for each of the techniques. The 1-D data were interpreted by partial curve matching and computer iteration using the IPI2Win® software. The data obtained from the 2-D ERI were processed and migrated using the Dipro for Windows® 4.0 inversion software. Inversion was done using the finite element modelling (FEM) method. The secondorder smoothness constraint was used and five iterations were carried out on each data set. Three 2-D resistivity models and one geoelectric section were generated from the inverted resistivity data. A resistivity depth slice map of the depth range of 2.5–5 m was also generated for the assessment of the spatial configuration of the suspected fault zone. Geoelectric units which include the topsoil/outcrop, weathered/faulted bedrock and the fresh bedrock were delineated by the 2-D resistivity models and the geoelectric section. The suspected fault zone manifested as zone of comparatively low resistivity (8–81 Ω-m) within two resistive (800 to > 5000 Ω m) basement bedrocks on the resistivity images and geoelectric section. The resistivity depth slice map also revealed the fault zone to be a low resistivity discontinuity within two flanks of fresh bedrocks. The study concluded that the study area is indeed a fault zone. Keywords Model · Finite element modelling · Resistivity models · Fault zone · Resistivity discontinuity
Introduction The surface of a phyllite outcrop (Fig. 1) at Ojirami, Edo State, Southwestern Nigeria is contemplated to manifest some evidences of faulting. This is because apart from the solution holes which are products of chemical weathering, the surface of the outcrop appears somewhat planar with multiple joints. The outcrop is assumed to be the remnant of ancient faulting episode(s). However the other block is not in view anywhere in the immediate vicinities of the outcrop—a situation of scientific curiosity for several experts. All visual observations on the outcrop agreeably suggest that * Akinola B. Eluwole [email protected] 1
Department of Geophysics, Federal University Oye-Ekiti, PMB 373, Oye‑Are Road, Oye‑Ekiti, Nigeria
Department of Geology, Ekiti State University, Ado‑Ekiti, Nigeria
2
the environment is a fault zone. To get a clue to this concern, geophysical investigations becam
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