Hydrogeological Implication of Electrical Resistivity Sounding in Ita-Eku, Ado Ekiti, Nigeria

Electrical resistivity prospecting method was used in evaluating the groundwater potential of Ita-Eku, southeastern part of Ado-Ekiti, Ekiti state, Nigeria using the vertical electrical sounding technique. A total of seven VES using the schlumberger electrode array was carried out. The VES acquired were interpreted qualitatively and quantitatively and were presented as table, chart, section and maps. The interpreted models revealed three to four geo-electric layers namely; topsoil, clay/lateritic soil, weathered/fractured layer and fresh basement. The topsoil which is relatively thin has thickness ranging between 0.8 and 2.5 m while the resistivity values range from 26 to 171 Ω-m. The second layer has thickness ranging from 2.5 – 5.9 m and resistivity values from 5to 19 Ω-m. The third layer is the weathered/fractured layer with thickness ranging from 5.9 – 35.3 m and the resistivity values range from 171to 404 Ω-m. The basement resistivity values range between 717 and 3466 Ω-m. First and second order (Dar Zarrouk parameters) data were used to generate the geo-electric section, overburden thickness map and longitudinal conductance map. These section and maps were considered in evolving the groundwater potential map.The groundwater potential mapshows that about 24 % of theentire study area falls within zone rated as having high groundwater potential, while about 25 % of the study area constitutes the low groundwater potential rating and the remaining 51 % has moderate groundwater potential rating.


INTRODUCTION
Geophysical methods especially the resistivity technique have been successfully used in groundwater exploration, since it is usually a non-invasive, relatively cheap and a quantitative evaluation technique [1].Electrical resistivity measurements in geophysicalsurveyusing the vertical electrical sounding (VES) technique is a well-established approach for solving a variety of geotechnical, geological, hydrological and environmental problems [2]. In recent years, there has been an increased awareness that subsurface characterization using standard drilling methods which offers a point measurement does not provide the information toaccurately evaluate the true distribution of geologic parameters beneath ground surface at many sites [3]. Hence as complement to drill-hole, geophysical technique can provide broad composite images of the subsurface over large areas at relatively lower cost and higher speed.Geophysical techniques make siting of buried utilities, massive engineering structures and recreation facilities easier and convenient [4]. In this study, only seven (7) vertical electrical soundings were carried out to cover a particular area in Ita-Eku area of Ado-Ekiti. The data obtained were processed and their outputswere presented as table,chart, section and maps. According to [5], earth resistivity can be related to important Tel: +2348030402302 geologic parameters of the subsurface such as type of rocks and soils, porosity and degree of saturation. This study will highlight properties of rocks within the study area, depth to bedrock and thickness of potential aquiferous horizonso as to determine the groundwater potential and desirable points for optimal groundwater exploitation.

Geological and Geographical Setting of the Study Area
The study area is locatedaround Odo-Ado area of Ado-Ekiti within Latitude 7.60°and 7.61°and Longitude 5.22°and 5.24°. It is situated within Igirigiri village along Ado-Odo Local Government Area, Ado-Ekiti. The topography is approximately flat with elevation ranging from 1374.7 to 1450.1ft above sea level. The geology of the study area can be explained within the context of the geology of the Precambrian basement Complex of southwestern Nigeria which form a part of the basement complex of Nigeria [6]. The major rock type within the area is typically Migmatite-gneiss comprising charnockitic rocks. The vegetation in the area is of rainforest type, characterized by short dry season and long wet season, with high annual rainfall of about 1,300 mm. Annual mean temperature is between 18 ℃ and 33℃ with relatively high humidity [7].

METHODOLOGY
Electrical resistivity method which involves the use of vertical electrical sounding techniques was carried out inIta-Eku area of Ado Ekiti. The geophysical data were acquired with the resistivity meter (Geo-pulse Meter) which is a high quality earth resistance meter capable of accurate measurements over a wide range of conditions. It contains both the transmitter unit, through which current enters the ground and the receiver unit, through which the resultant potential difference is recorded. Other materials include: two metallic current and two potential electrodes, two black coloured connecting cables for current and two red colouredcables for potential electrodes, two reels of calibrated ropes, hammer for driving the electrodes in the ground, compass for finding the orientation of the traverses, cutlass for cutting traverses and data sheet for recording the field data. In the study area, the electrode spread of AB/2 was varied from 1 to a maximum of 100 m. The electrical resistivity data were processed by plotting the apparent resistivity values against the electrode spread (AB/2). This was subsequently interpreted quantitatively using the partial curve matching method and computer assisted 1-D forward modeling with Win-Resist 1.0 version software [8].

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Corresponding Author. E-mail: seungbemi77@yahoo.com Second order geoelectric parameters called Dar Zarrouk parameters were determined from the iterated geoelectric parameters [9]. The second order parameter of interest in this study is the longitudinal conductance (Si). This second order parameters are derived using equations developed by [10]. For n layers, the total longitudinal unit conductance is given by:

RESULTS AND DISCUSSION
The results of this work are presented as table, chart, geo-electric section and map.

Table and Chart
Curve types identified were A, H, KH and HA ( Table 1). The predominant curve type is the HA curve type having percentage frequency of 57.1 % while the H, A and KH curve types has 14.3 % each respectively. The high percentage recorded by the HA curve type within this study area is an indication of a predominant low groundwater potential of the study area.Typical curve types are as shown in Fig.3.    Second order geoelectric parameters called Dar Zarrouk parameters were determined from the iterated geoelectric parameters [9]. The second order parameter of interest in this study is the longitudinal conductance (Si). This second order parameters are derived using equations developed by [10]. For n layers, the total longitudinal unit conductance is given by: where, S = longitudinal unit conductance, n = number of layers, ℎ = layer thickness, = layer resistivity.

RESULTS AND DISCUSSION
The results of this work are presented as table, chart, geo-electric section and map.

Table and Chart
Curve types identified were A, H, KH and HA ( Table 1). The predominant curve type is the HA curve type having percentage frequency of 57.1 % while the H, A and KH curve types has 14.3 % each respectively. The high percentage recorded by the HA curve type within this study area is an indication of a predominant low groundwater potential of the study area.Typical curve types are as shown in Fig.3.  Second order geoelectric parameters called Dar Zarrouk parameters were determined from the iterated geoelectric parameters [9]. The second order parameter of interest in this study is the longitudinal conductance (Si). This second order parameters are derived using equations developed by [10]. For n layers, the total longitudinal unit conductance is given by:

RESULTS AND DISCUSSION
The results of this work are presented as table, chart, geo-electric section and map.

Table and Chart
Curve types identified were A, H, KH and HA ( Table 1). The predominant curve type is the HA curve type having percentage frequency of 57.1 % while the H, A and KH curve types has 14.3 % each respectively. The high percentage recorded by the HA curve type within this study area is an indication of a predominant low groundwater potential of the study area.Typical curve types are as shown in Fig.3.  Fig.4 is a geo-electric section generated to connect all the VES points within the study area. The geoelectric section shows the variations of resistivity and thickness of geo-electric layers below VES 7, 6, 5, 1, 2, 4 and 3 in that order. The geo-electric sections revealed four subsurface geo-electric layers consisting of topsoil, clay/lateritic soil, weathered/fractured layer and fresh basement. The topsoil which is relatively thin has thickness ranging between 0.8m and 2.5 m while the resistivity values range from 26 to 171Ω-m. The low resistivity values obtained within this layer indicates that the topsoil is composed of clay and vegetable soil. The second layer has thickness ranging from 2.5 -5.9 m and resistivity values from 5to 19 Ω-m. This layer predominantly composed of clayey soil. The third layer is the weathered/fractured layer with thickness ranging from 5.9 -35.3 m and the resistivity values range from 171to 404 Ω-m. This contains weathered and fractured rocks that are partially saturated with water. This layer forms the major aquifer unit within Ita-Eku area. The basement resistivity values range between 717 and 3466 Ω-m, it is partially fractured under VES 5 but very fresh under VES 7, 6, 1, 2, 4 and 3. The bedrock also form basement ridges under VES 7 and 2 with a depression which underlain VES 6, 5 and 1. The depth to bedrock within this study area ranges between 6.6 mand 36.5 m.

Overburden Thickness Chart
The chart shows variation inthickness of theoverburden from points "sounded"within the study area (Fig.5).It varies from 6.6 to 36.5 m, with a mean of 20.9 m. The regions around VES points 2 and 7 have thin overburden (6.6 to 8.4 m), while areaswhich underlain VES 1, 3, 4, 5 and 6 have relatively thick overburden (21.1 to 36.5 m). Zones within the study area whose overburden thickness is less than10 m are considered in this study to have limited (low) groundwater potential, sinceit also show near-surface basement ridges, high aquifer protective capacity (thick clay layer above the aquifer) and low values of total longitudinal conductance (0.12 -0.31 mhos). On the other hand, zones identified as having overburden thickness ≥ 21 m are considered to have moderate to high groundwater accumulation potential, due to the favourable basement structure and relatively high longitudinal unit conductance values (0.15 to 1.23 mhos). The low groundwater potential zones cover about 28 % of the entire study area, while the remaining parts fall within the medium to high groundwater potential (about 72 % of the study area).

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Corresponding Author. E-mail: seungbemi77@yahoo.com  Fig. 7 shows the groundwater potential map of the study area. Important parameters to be considered when evaluating groundwater potential of an area within the basement complexare the thickness of the overburden, the resistivity and thickness of the weathered layer [11], [12]. A horizon is regarded as significant with respect to its water-bearing potential, if it is relatively thick and has low resistivity valueswhich suggest a saturated condition [13]. The groundwater potentialof the study area was classified into high, moderate and low potentials. Zones (under VES 5 and 6)with thick overburden(29.3 -36.5 m) with high longitudinal conductance valuesranging from 0.15 to 1.23 mhos (Fig. 6), significantly thick weathered layerof resistivity values between 15 and 404 Ω-mand within the basement depression are classified as high groundwater potentials. Zones (under VES 2 and 7)with thin overburden (thickness between 6.6and 8.4 m), near-surface basement ridges, high aquifer protective capacity (thick clay layer above the aquifer) and low values of total longitudinal conductance (0.12 -0.31 mhos) are considered to have low groundwater potential. Whilezones(under VES 1, 3 and 4)having overburden thickness between 21.1 to 22.1m with thin weathered layer of resistivity values between 19 to 300 Ω-mand longitudinal unit conductance value between 0.36 to 0.50)are considered to have moderate groundwater potential.The groundwater potential map shows that about 28% of theentire study area falls within zone rated as having high groundwater potential, while about 28 % of the study area constitutes the low groundwater potential rating and the remaining 44% has moderate groundwater potential rating. The aquifer in this area occurs within a basement depression,highly localizedand is protected against infiltration of polluting fluid(delineated beneath VES 5 and 6).

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Corresponding Author. E-mail: seungbemi77@yahoo.com  Fig. 7 shows the groundwater potential map of the study area. Important parameters to be considered when evaluating groundwater potential of an area within the basement complexare the thickness of the overburden, the resistivity and thickness of the weathered layer [11], [12]. A horizon is regarded as significant with respect to its water-bearing potential, if it is relatively thick and has low resistivity valueswhich suggest a saturated condition [13]. The groundwater potentialof the study area was classified into high, moderate and low potentials. Zones (under VES 5 and 6)with thick overburden(29.3 -36.5 m) with high longitudinal conductance valuesranging from 0.15 to 1.23 mhos (Fig. 6), significantly thick weathered layerof resistivity values between 15 and 404 Ω-mand within the basement depression are classified as high groundwater potentials. Zones (under VES 2 and 7)with thin overburden (thickness between 6.6and 8.4 m), near-surface basement ridges, high aquifer protective capacity (thick clay layer above the aquifer) and low values of total longitudinal conductance (0.12 -0.31 mhos) are considered to have low groundwater potential. Whilezones(under VES 1, 3 and 4)having overburden thickness between 21.1 to 22.1m with thin weathered layer of resistivity values between 19 to 300 Ω-mand longitudinal unit conductance value between 0.36 to 0.50)are considered to have moderate groundwater potential.The groundwater potential map shows that about 28% of theentire study area falls within zone rated as having high groundwater potential, while about 28 % of the study area constitutes the low groundwater potential rating and the remaining 44% has moderate groundwater potential rating. The aquifer in this area occurs within a basement depression,highly localizedand is protected against infiltration of polluting fluid(delineated beneath VES 5 and 6).    Fig. 7 shows the groundwater potential map of the study area. Important parameters to be considered when evaluating groundwater potential of an area within the basement complexare the thickness of the overburden, the resistivity and thickness of the weathered layer [11], [12]. A horizon is regarded as significant with respect to its water-bearing potential, if it is relatively thick and has low resistivity valueswhich suggest a saturated condition [13]. The groundwater potentialof the study area was classified into high, moderate and low potentials. Zones (under VES 5 and 6)with thick overburden(29.3 -36.5 m) with high longitudinal conductance valuesranging from 0.15 to 1.23 mhos (Fig. 6), significantly thick weathered layerof resistivity values between 15 and 404 Ω-mand within the basement depression are classified as high groundwater potentials. Zones (under VES 2 and 7)with thin overburden (thickness between 6.6and 8.4 m), near-surface basement ridges, high aquifer protective capacity (thick clay layer above the aquifer) and low values of total longitudinal conductance (0.12 -0.31 mhos) are considered to have low groundwater potential. Whilezones(under VES 1, 3 and 4)having overburden thickness between 21.1 to 22.1m with thin weathered layer of resistivity values between 19 to 300 Ω-mand longitudinal unit conductance value between 0.36 to 0.50)are considered to have moderate groundwater potential.The groundwater potential map shows that about 28% of theentire study area falls within zone rated as having high groundwater potential, while about 28 % of the study area constitutes the low groundwater potential rating and the remaining 44% has moderate groundwater potential rating. The aquifer in this area occurs within a basement depression,highly localizedand is protected against infiltration of polluting fluid(delineated beneath VES 5 and 6).

4.CONCLUSION
Electrical Resistivity Method involving Vertical Electrical Sounding (VES) has proved useful in the delineation and characterization ofaquifer unit as part of the preliminary investigations to assess groundwater resource potential and development in Ita-Eku, Ado-Ekiti,southwestern Nigeria. The first order geo-electric parameters obtained from the interpretation of the vertical electrical sounding data and the second order (Dar Zarrouk) parameter (longitudinal conductance Si) were used to generate the geoelectric section, overburden thicknessand longitudinal conductance charts.These are reflective of the hydraulic properties of the aquifer unit in the study area. From the interpreted results, zones with high overburden thickness, high longitudinal conductance values and significantly thick weathered layer are classified as having high groundwater potentials, zones with thin overburden, near-surface basement ridges, high aquifer protective capacity and low values of total longitudinal conductanceare considered to have low groundwater potential, whilezones having moderate overburden thickness with thin weathered layer are considered to have moderate groundwater potential. About 28% of the area falls within the high groundwater potential rating, while about 44% constitutes the medium groundwater potential rating and the remaining 28% constitutes low groundwater potential rating. Hence the groundwater potential of the area is generally rated to be moderate. The VES stations underlain by high and medium groundwater potential zones are envisaged to be viable for groundwater development within the area. Thus, groundwater resource development and management can be effectively planned for in the study area.