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CHAPTER ONE

INTRODUCTION

1.1   Background of the Study

Geoelectrical resistivity imaging has played an important role in addressing a wide variety of hydrogeological, environmental and geotechnical issues. The goal of geoelectrical resistivity surveys is to determine the distribution of subsurface resistivity by taking measurements of the potential difference on the ground surface. For a typical inhomogeneous subsurface, the true resistivity distribution is estimated by carrying out inversion on the observed apparent resistivity values. In environmental and engineering investigations, the subsurface geology is usually complex, subtle and multi-scale such that both lateral and vertical variations in the petrophysical properties can be very rapid. Two-dimensional (2D) geoelectrical resistivity imaging has been widely used to map areas with moderately complex geology (e.g. Griffiths and Barker 1993; Griffiths et al. 1990; Dahlin and Loke 1998; Olayinka 1999; Olayinka and Yaramanci 1999; Amidu and Olayinka 2006). In the 2D model of interpretation, the subsurface resistivity is considered to vary both laterally and vertically along the survey line but constant in the perpendicular direction. The major limitation of the 2D geoelectrical resistivity imaging is that measurements made with large electrode spacing are often affected by the deeper sections of the subsurface as well as structures at a larger horizontal distance from the survey line. This is most pronounced when the survey line is placed near a steep contact with the line parallel to the contact (Loke 2001). Potential field surveys are relatively inexpensive and can quickly cover large areas of ground. The primary goal of studying potential fields is to provide a better understanding of the subsurface geology. If groundwater is to be exploited, it is essential that the entire project be conducted in most efficient and cost-effective way possible. Rushing into borehole drilling will probably result in the incorrect location of aquiferous zone. The geoelectric techniques has been successfully used in investigating groundwater potential in various geological setting even in areas of complex geology in different parts of the world (Sabet, 2015). The most usual parameters are the porosity, the permeability, the transmissivity and the conductivity (Bernard, 2013). The heavy reliance on groundwater as a source of affordable water for both industrial and domestic use throughout the world demands that the water occurrence within the subsurface is of significant quantity and high quality. Subsurface geological characterizations using surficial geoelectrical resistivity technique are sufficient to address variety of problems related hydrological investigations in complex geological terrains such as crystalline basement. Several works have been carried out on the assessment, abstraction, development and management of groundwater within the hard rock terrain of Nigeria. Olayinka, A. I., Weller, A. (1995). Groundwater is that water found within the saturated voids beneath the ground. It is the major source of potable water supply in the Makurdi area. Groundwater is the subsurface water which fully saturates the pores and behaves in response to gravitational force (Straher, 1973). Water is an indispensable resource and the concern of many earth scientists and researchers have been on the acquisition of a reliable source of drinking water (Akinbinu, 2015). Surface and groundwater resources are abundant in Nigeria. The water resources master plan for Nigeria which was prepared by the Japan International Co-operation Agency (JICA) in 2006 indicates an estimated surface water resource of about .67 and groundwater storage of about (Oteze, 2006). These figures greatly outweigh the country’s total water demand of about (Oteze, 2006). Surface water is frequently found to be grossly degraded in quality because of its exposure to physical, biological or chemical contaminants (Edet, 2004). Groundwater on its own has less of a degree of contamination when compared with surface water and this has contributed to an increase in the number of boreholes drilled by the government, non-governmental organizations and individuals in Nigeria. Resistivity imaging method has improved the chance of drilling successfully by identifying the fractured and weathered zones in hard and compacted terrain (Loke, 2001). The flow potential of groundwater is a measure of the transmissivity of the aquifer, which is the product of the aquifer thickness and hydraulic conductivity. The knowledge of aquifer characteristics is important in determining the natural flow of water through an aquifer, it response to withdrawal of fluid, the availability, quantity and quality of the groundwater. Hydraulic conductivity (k) is a measure of the ease with which a fluid will pass through a medium (Heigold et al., 1979). By definition, hydraulic conductivity depends not only upon the medium but also on the fluid (Heigold et al., 1979). Geophysical site investigations for groundwater exploration are scanty and inadequate in the study area and the hydrogeology is not well developed. Among several geophysical methods employed in groundwater exploration (electrical resistivity, gravity, seismic, magnetic, remote sensing and electromagnetic), the electrical resistivity method is the most effective for locating productive wells. It is an effective and a reliable tool in locating viable aquifers for continuous and regular water supply (Todd and Mays, 2005). It has the advantage of non-destructive effect on the environment, cost effective, rapid and quick survey time and less ambiguity interpretations of results when compared to other geophysical survey methods (Todd, 1980). The vertical electrical sounding (VES) technique provides information on the vertical variations in the resistivity of the ground with depth (Ariyo, 2005). It is used to solve a wide variety of groundwater problems, such as determination of depth, thickness and boundary of aquifers (Bello and Makinde, 2007; Asfahani, 2006), determination of zones with high yield potential in an aquifer (Akaolisa, 2006; Oseji et al., 2005) and determination of groundwater quality (Arshad et al., 2007). VES has been employed extensively in groundwater investigations in many basement complex terrains of Africa (Adeniji et al., 2013; Olayinka and Olorunfemi, 1992; Barongo and Palacky, 1991; Palacky, 1989). Resistivity is a principle that is governed sorely by pore fluid content or matrix mineral. If the matrix mineral is highly conductive (gold, clay, galena, etc.), the resistivity will be low. If pore fluid is water, resistivity will also be low. There is a clear absence of conductive minerals on the outcrops; therefore, low resistivity response can only be due to the presence of groundwater in the fractures. Hence, the study aims at examine the application of Geo-electrical resistivity imaging to investigate groundwater potentials.