Subsurface 2D Image Analyses of the Uyangha Basement Area, South-Eastern Nigeria

Abstract

Geo-electric soundings were made in Stella Maris Secondary School, in Uyangha, Nigeria to image the subsurface and obtain thicknesses and resistivities of different layers. A quantitative interpretation of the data obtained clearly reveals the presence of four (4) geo-electric sections which are interpreted to be dry laterite, moist laterite, weathered basement, and saturated basement. The depth probed is about 100m. The saturated basement is the aquifer unit. Depth to aquifer unit in the area is at about 65m to 80m.The thickness of the aquifer unit ranges from 20m to 35m. For ground water exploitation, boreholes in the area should therefore be drilled to the depth of 91m, for reasonable groundwater yield. The lateritic layer makes the study area suitable for building construction in the area. Keywords: 2-D imaging, electrical resistivity, vertical electrical sounding, pseudo-section, saturated basement. I. Introduction Geophysical surveys are aimed at detecting and locating subsurface geological structures and to measure their dimensions and their physical properties. Geophysical exploration methods are many, which can be classed according to the source of energy used in their measurement. To select a suitable method to be used for any survey, a comprehensive knowledge of the geology of the area and site conditions are necessary. This is because in some instances, site conditions may preclude the successful use of most if not all geophysical techniques. The resistivity study requires the introduction of an artificial current into the ground through point electrodes or long line contacts. The potential in the earth (ground) due to the introduction of the current into the ground is measured using two potential electrodes which are planted on the ground surface. Since the current is measured as well, it is possible to determine the apparent resistivity of the subsurface. This however makes the resistivity method more superior (at least theoretically) to all the other electrical methods (Keary and Brooks 1991). It is important to note that, deviations from the pattern of potential differences expected from homogenous medium, provides information on the form and electrical properties of subsurface inhomogenities. The depth soundings or a vertical electrical sounding (VES) is also known as electrical drilling. It is the process by which depth investigations are made (Zohdy, et al., 1974). The technique is best adopted to determining depth and resistivity for flat laying layered rock such as found in sedimentary terrains. The Schlumberger configuration is mostly commonly used for VES investigations. The technique further finds application in geotechnical survey to determine overburden thickness and also in hydrogeology, to define depth and thickness of porous strata which are potential aquifers (Burger, 1992). The objectives of the study are to attempt to identify the different rock types in the area and their vertical thickness using the electrical resistivity method. This result will help to detect saturated layers for ground water exploitation in the area. The apparent resistivity of the ground is a function of the measured impedance (ratio of potential to current) and the geometry of the electrode array (Burger, 1992). II. Geologic Setting