Fracture Detection in a Hard Rock Terrain Using Radial Geoelectric Sounding Technique

Abstract

Groundwater exploration in the hard rock terrain of Nigeria primarily focused on the extent and type of the weathered mantle and fracturing. Fractures are lineaments in the subsurface which host a lot of mineral resources and groundwater. The present study is aimed at detecting fractures that were caused by recent tectonic activities around Lupma Hill in Paikoro Local Government area of Niger State, Central Nigeria. The tremor was reported to have a magnitude of 3.2 on the Richter's scale. The lateral extent of this fracturing was estimated to be about 400 m. A combination of detailed geologic mapping and Radial Geoelectric Sounding Techniques were utilised to characterise the subsurface structural trends. The area is composed primarily of granitic rocks. These rocks occur as outcrops, hills and ridges. Joint directions observed on outcrops were measured, plotted as a Rosette diagram and revealed a predominant NE-SW trend. Radial geoelectric sounding was done using Schlumberger array configuration in three different orientations: 0°, 060° and 120° to a depth of 80m to obtain apparent resistivity values. Apparent resistivity values were plotted along three azimuths; 0°, 060° and 120° to obtain electrical resistivity anisotropy polygons. The sounding data were interpreted using WINRESIST software. Analysis of electrical resistivity anisotropy polygons indicated that fractures detected at shallow depths are oriented in NE-SW direction and at greater depths, fractures are oriented in the N-S direction. These results are similar to results obtained from the plot of the Rosette diagram. Generally, three geoelectric layers, consisting of thin topsoil, whose depth values range from 1 m to 3 m and resistivity values ranging from 5 ohm-m to 63 ohm-m, weathered mantle whose depth values range from 3 m to 19 m and resistivity values ranging from 10 ohm-m to 512 ohm-m and finally fresh basement having resistivity ranging from 200 ohm-m to 1539 ohm-m were found. These sets of results greatly favour greatly favour groundwater development in the area. The use of Radial Geoelectric Sounding methods of investigating the subsurface with a combination of geologic mapping for unravelling fractures in the Basement Complex terrain has proven to be a very useful contribution in groundwater exploration in the Basement Complex.