LABORATORY MEASUREMENTS AND COMPARISON OF 2-D RESISTIVITY IMAGING OVER TWO SIMULATED GEOLOGIC TARGETS

Abstract

This research examines the relative effectiveness and imaging capabilities of laboratory 2D resistivity imaging over two geologic targets with five most commonly used electrode arrays.This is undertaken to establish the array(s) that is/are most suitable for mapping the geologic targets. Arrays considered include the Wenner (WNN), Wenner-Schlumberger (WSC), pole-dipole (PDP), pole-pole (PPP), and dipole-dipole (DPD). A model tank was carefully built in the Department of Applied Geophysics, Federal University of Technology Akure. The tank was made of transparent glass and housed within a wooden plank to withstand the pressure of the sand fill. The tank was filled with well-sieved sand, saturated with water. Two geological models that simulate an underground tunnel and buried thin resistive dyke were utilized to examine the imaging capabilities of these arrays. A wall test effect, involving Wenner array, was carried out to determine the area within which geoelectric measurements could be taken without significant wall effect. One electrical traverse was established within the tank. Unit electrode spacing used for the imaging was 2 cm while expansion factor (N) ranges from 1 to 8 for simulated tunnel and 1 to 6 for the simulated resistive dyke. The simulated tunnel and dyke show up as high resistivity anomaly within a relatively low resistivity host medium for all the arrays.Pole-pole array offers the best horizontal resolution (0.38 m) with considerable vertical resolution of 0.10 m for the tunnel. The value of the total resolution (0.16 m) suggests that WSC least resolves the modelled tunnel. The result obtained for the dyke suggests that WNN and WSChave total resolution of 0.17 m and 0.15 m respectively. Considering the data coverage, anomaly effect and imaging efficiency of the five arrays, pole-pole presents higher mapping efficiency over a tunnel while Wenner- Schlumberger and Wenner present higher mapping efficiency over a buried dyke.