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Vertical electrical resistivity soundings were conducted in order to delineate the groundwater potentials at some locations in Ezza North Local Government of Ebonyi State. Twelve vertical electrical soundings were obtained using the Schlumberger configuration with the aid of the OHMEGA terramenter (SAS1000). The field data were subjected to interpretation by employing the method of partial curve matching techniques using the master curves and the corresponding auxiliary curves. A computer programme (RESOUND) was used to interpret the resistivities and the thicknesses of the subsurface. The parameters obtained were used to determine the resistivities and thicknesses of the subsurface layers. Two profiles, VES11 and VES 12 indicated eight geoelectric layers. Five geoelectric layers were evident at five locations namely VES 3, 4, 7, 8 and 10. Data from the remaining five locations (VES 1, 2, 5, 6 and 9) revealed six layers each. The major lithologic units of the area are shales, sandstone and mudstone. The water bearing rocks were interpreted to exist at depths between 20m and 130m in most of the VES locations. The results fairly correlated with some logged boreholes close to the survey area. The aquifers have resistivities ranging between 9Ωm and 110Ωm.The geophysical search for groundwater has shown that the survey area has good groundwater potentials which if exploited would go a long way in reducing the problems of seasonal water shortage and possible health problems associated with the consumption of unhygienic surface water in the area.


1.1 Background

Groundwater is one of the very important natural resources. Though it is true that greater percentage of the earth’s surface is composed of water including seas, oceans, rivers, streams, ponds and others, yet none of these surface sources is as hygienic or as economical for exploitation as the groundwater (Singh, 2007). The amount of fresh water available for human use is less than 0.08% of all the water on the planet (BBC Sci/Tech News, 2000). Groundwater is recommended for its natural microbiological quality and its general chemical quality for most uses (McDonald et al., 2002). Due to its scarcity, water related diseases are found in many parts of the world. In Nigeria, for example, Okoronkwo (2003) attributed the guinea worm infestation in some parts of Ebonyi State to ignorance and lack of safe drinking water. The people, according to him, lacked boreholes and depended only on ponds and other existing contaminated sources.
Over the years, boreholes have usually been drilled with or without previous knowledge of the subsurface stratification in search of water. As a result of multiple failed boreholes, researches grew towards minimizing failed wells, thereby reducing the risk as well as cost of drilling (Adetola, and Igbedi, 2000). Tremendous breakthroughs have been recorded in the use of electrical methods used in the exploration of the subsurface minerals (Selemo et al., 1995). Geophysics involves the measurement of contrasts in the physical properties of materials beneath the surface of the earth and the attempt to deduce the nature and the distribution of the materials responsible for these observations at the surface.

It involves the application of the principles of physics to the study of the earth. The geophysical methods used in the investigation of the shallow features of the earth’s crust vary in accordance with the physical properties of rocks. In seismic method of exploration, seismic waves travel with different speeds through different materials due to variations in their elastic moduli and densities. Variation of densities in the subsurface can as well lead to change in gravitational acceleration at the surface (gravity method). Measurable differences in magnetic field can be obtained at field sites due to variations in magnetic susceptibilities, referred to as magnetic method.
Similarly, variations in the electrical conductivities of rocks and sediments can produce different values of apparent resistivities as the distances between measuring probes are increased or as the position of the probe is changed on the surface (electrical resistivity method). Electrical resistivity is one of the physical properties which can be used to distinguish among different rocks. This is because the resistivities of different rocks and minerals vary widely. While igneous rocks containing no water have very high resistivities, metallic ores have very low resistivties (Telford et al., 1990).

The apparent resistivity of the subsurface as measured on the surface is a function of the current, the recorded potential difference and the geometry of the electrode array. Presence of water substantially controls the variation of the conductivities in the shallow subsurface. The measurements indicate water saturation and connectivity of pore spaces because water-bearing rocks and minerals have lower resistivities and electric current usually follows the path of least resistance (Ezema, 2005). Resistivity methods have been found successful for locating and accessing groundwater. It is cost effective and subject to careful study of the geology of the survey area.



Hence, the geology of the study area must be well known before embarking on resistivity survey. In electrical resistivity survey, current is passed into the ground through two current electrodes. Two other electrodes are used to measure the resulting potential difference produced by this current. The information is used to calculate the apparent resistivity of the rock. All substances act to retard the flow of electric current so that energy must be expended to move charged particles. The extent to which a substance restrains this movement is described by its electrical resistivity. The principal goal of electrical resistivity surveying is to measure this physical property as a basis for distinguishing layering and structure of the earth.
The two main types of procedures employed in resistivity surveys are vertical electrical sounding VES, and constant separation traversing CST. In constant separation traversing, which is used to determine lateral variation in resistivity, the current and potential electrodes are maintained at a fixed separation and progressively moved along a profile. In vertical electrical sounding, the current and the potential electrodes are progressively expanded about a fixed central point. By progressively expanding the current electrodes, readings of the potential difference are taken as current reaches to greater depth. This gives the information on the resistivities and thicknesses of the underlying horizontal strata.

The modern equipment for measuring the potential difference and the current is the signal averaging system (SAS) terrameter. The resistivity of the subsurface material is a function of the magnitude of the current, the recorded voltage and the geometry of the electrode configuration. The electrical resistivity obtained is termed “apparent” because it is not likely that the subsurface materials beneath the survey area are homogeneous. The apparent resistivities are subject to interpretation techniques including the curve matching and/or computer interpretation. Based on the resistivities and the thicknesses of the underlying formations and the available geology of the area, the depth to water bearing rocks (aquifer) may be estimated.

1.2 Location of the study area

The area under survey lies between latitudes 060081 and 060171 north of the equator and longitudes of 070521 and 080001 east of the Greenwich Meridian. Figure 1.1 is the map of Nigeria showing the location of Ebonyi State. The map of the area under survey, Ezza North Local Government Area, is shown in figure 1.2. The area which covers about 246 squared kilometres lies in the south eastern part of Abakaliki, off Enugu-Ogoja highway. Abakaliki is about 62km South East of Enugu and about 22 kilometres West of Afikpo in Ebonyi State.

The global positioning system (GPS) receiver was used in the field to obtain the global grid positions of the vertical electrical sounding points, including the longitudes, latitudes and the elevations. This instrument receives its data from the GPS satellite. The GPS locations of field stations are shown on table 1.1. In addition to Enugu-Ogoja Road, the survey location can equally be accessed through the Onueke market along the Abakaliki – Afikpo Expressway.

1.3 Geology of the area

The study area belongs to the Asu River group shales. According to Reyment (1965), the sediments of the Asu River



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