Application of Three Electrical Resistivity Arrays to Evaluate Resolution Capacity of Fractured Zones at Apatara Farms, Iwo, Osun State, Nigeria

L Adeoti ,O Seun Afolabi,S Kehinde Ishola,Adebayo Oluwaseun Ojo

doi:10.4314/jasem.v21i6.36

Abstract

The study applied three different electrical resistivity arrays (Wenner, dipole-dipole, and Pole-dipole) based on their resolution capacity to delineate fractured zones at Apatara Farm in Iwo, Osun State, Nigeria. Theoretical apparent resistivity data were computed for each model and contaminated with 5% Gaussian noise as a further concession to real field conditions. The simulated results revealed that the Wenner array gave the least error in trying to reconstruct the true model when the fractured zone is placed near the subsurface. However, when the fractured zone is placed at a depth beyond 5 m, the Dipole-Dipole array gave a better resolution than Pole-Dipole and Wenner array in decreasing order of resolution. The study further revealed that the Wenner array is less susceptible to edge effect at shallow depth while Dipole-dipole is more susceptible to edge effect at deeper depth followed by the Pole-dipole array. 2D electrical resistivity field measurements were carried out to confirm the results of the numerical simulation in the same field using the same parameters. The inverted resistivity images showed that the fractured zones are well delineated by the Dipole-dipole and Pole-dipole arrays but poorly resolved by the Wenner array. The study has demonstrated the usefulness of numerical modelling for imaging of fractured zone necessary for hydrogeological purpose and through modelling, the user has unlimited power to image or simulate a real-world scenario seamlessly before carrying out the actual field survey.Keywords: Electrical resistivity array; fractured zones; finite element method; 2-D models; resolution, mean absolute error

Highlights

Ambiguity in the interpretation of electrical resistivity dataset as well as other geophysical datasets can be reduced by numerical modelling
Fig. 2a. shows model consisting of a fractured zone denoted with a resistivity value of 200 Ωm placed at the surface depth of 0 m to 15 m
The dimension of inverted DipoleDipole anomaly is smaller in relation to the actual model follow by Pole-Dipole but the resolution of the image is higher using Wenner array

Summary

Introduction

Ambiguity in the interpretation of electrical resistivity dataset as well as other geophysical datasets can be reduced by numerical modelling. The detectability of various two-dimensional earth models using multi-electrode systems in a noisy environment has been studied by (Sandor et al, 2011; Szalai et al, 2014) Such models are representative of fractured zones which are discontinuities in crystalline basement rocks generated by tectonic forces or intrusion of magmatic bodies (George et al, 2013). The appropriate electrode array can be determined and an idea of the anomaly responses can be obtained at the planning stage of the survey using forward modelling rather than trial by error on the field The usefulness of this was demonstrated by (Xianjin and Lagmanson, 1999) for mapping horizontal and vertical conductor using different electrode arrays. In this study, we investigated the resolution capacity of these three conventional electrode arrays to delineate fractured zones at Apatara Farms, Iwo, Osun state using finite element Method (FEM) modelling approach

Methods

Results

Conclusion