Dipole-Dipole Resistivity Measurements of Weathered Soils Over Major Rock Types in a Humid Climate - A Case Study of Ashanti Region-Ghana

Abstract

Ninety six (96) dipole-dipole resistivity sounding curves acquired with Mac-Ohm II resistivity meter over the Birimian, Intrusives, Tarkwaian and Voltaian rocks underlying Ashanti Region have been analyzed qualitatively to identify their characteristic shapes over each rock type. Geoelectrical sections have also been modeled from the curves and drill logs obtained close to the sounding points. Four (4) major three (3)-geoelectrical sections were identified. These were characterized by the H, A, Q and K- type curves which corresponded to the resistivity sequence r1>r2 r2>r3, r1 r3 respectively. Over the Birimian rocks, near surface resistiv-ity values ranged from100-600 ohm-m within the depths from 0-9 m. A moderately resistive stra-tum with values ranging from 200-500 ohm-m formed the second layer, and could be found to the depths of about 25 m. The low resistivity (conductive) zones were encountered at depths be-tween 25-40 m with values ranging from 180-250 ohm-m. The Intrusive rocks indicated resistiv-ity values in the range from 500-600 ohm-m for the first layer with thickness 11 m, overlying a more resistive second stratum with values ranged from 800-1700 ohm-m and of thickness 11 m, This in turn overlay a low resistive zone between 22-40 m depth with values 180-1200 ohm-m. The Tarkwaian rocks recorded the range from 500-1300 ohm-m as the first layer resistivity val-ues between 0-9 m depth, the second layer resistivity values were relatively lower than the upper stratum and ranged from 400-850 ohm-m to the depth of 23 m. The bottom layer resistivity indi-cated values from 200-400 ohm-m within the depths of 23-40 m. Formations over the Voltaian rocks recorded the lowest range of resistivity generally from 60-200 ohm. The top layer values ranged from 60-110 ohm m, the middle layer from 160-200 ohm m and the bottom layer re-cording values within the range from 60 -100 ohm m. The old rocks such as the Intrusives, Tark-waian and the Birimian seem to exhibit higher values of resistivity measurements. However the low values of resistivity close to 40 m depth as well as the significant rise to higher values could indicate the approach of the fractured basement rock which could indicate potential target in groundwater exploration programs.