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Geophysical Investigation of Subsurface Water of Erunmu and its Environs, Southwestern Nigeria Using Electrical Resistivity Method



Lukuman Abudulawal, Sikiru A. Amidu, Kasim A. Apanpa, Olusola A. Adeagbo and Olukole A. Akinbiyi
 
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ABSTRACT

Acute drinkable-water shortage is an important problem confronting the people of Erunmu, a suburb of Ibadan, southwestern Nigeria. Geophysical survey was carried out to evaluate the groundwater potential of the basement complex and to delineate potential locations for siting boreholes in the study area. A total of twenty five Vertical Electrical Soundings (VES) were carried out. Data acquisition involved the use of the Schlumberger electrode configuration with half current electrode spacing (AB/2) ranging from 1-100 m. Interpretation of the geoelectrical data involved the use of curve matching technique and computer iteration. Geoelectrical cross sections and isoresistivity and 3D isopach maps were constructed based on lateral combination of inverted soundings from the VES surveys. Available lithologic data from hand-dug wells were used to evaluate geophysical results. The interpretation revealed three and four-model curves H-, HA-, A- and KH-types, with the the H-type curves being the dominant types. Four subsurface layers comprising top lateritic soil, weathered layer, fractured basement and fresh basement were inferred from the interpretation. The top soils are of varying thickness and resistivity values. The fractured and weathered basement with relatively lower resistivity are inferred to be the aquiferous zone and could bear productive water for groundwater supply. The basement was inferred as the lowermost infinitely thick layer with resistivity mostly greater than 1500 Ωm. The results of the soundings greatly contribute to the understanding of the hydrogeology of the basement complex. The fractured and weathered basement aquifers can be developed for sustainable water supply to Erunmu and adjacent communities.

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Lukuman Abudulawal, Sikiru A. Amidu, Kasim A. Apanpa, Olusola A. Adeagbo and Olukole A. Akinbiyi, 2015. Geophysical Investigation of Subsurface Water of Erunmu and its Environs, Southwestern Nigeria Using Electrical Resistivity Method. Journal of Applied Sciences, 15: 741-751.

DOI: 10.3923/jas.2015.741.751

URL: https://scialert.net/abstract/?doi=jas.2015.741.751
 
Received: December 08, 2014; Accepted: March 20, 2015; Published: April 02, 2015



INTRODUCTION

Approximately 50% of the Nigerian landmass is geologically underlain by crystalline basement rocks. These crystalline rocks have been shown to be potential target for sustainable groundwater supply (Acworth, 1987; Olayinka, 1992; Olorunfemi and Fasuyi, 1993; Olayinka et al., 2004). However, groundwater in these rocks are both isolated and compartmentalized. The basement rocks in their deformed state possess little or no primary intragranular porosity and permeability and thus the occurrence of groundwater is due largely to the development of secondary porosity and permeability resulting from weathering and fracturing of the parent rocks. Acworth (1987) provided a detail review of the hydrogeological potential of the basement complex. Generally, development of basement aquifer is a complex inter-play of geology, post emplacement tectonic history, weathering process and depth, nature of the weathered layer, groundwater flow pattern as well as recharge and discharged processes (Acworth, 1987; Olorunfemi et al., 1999). Significant quantity of water could be extracted from the aquifers through boreholes. In particular, boreholes that are drilled into weathered or fractured basement, but which penetrate maximum possible thickness of the regolith have been found to provide maximum yield (Acworth, 1987; Olayinka, 1992).

Groundwater exploration in the crystalline basement, however, requires detailed geophysical investigation to effectively characterize the hydro-geologic zones. This is important because of the great variability and the unpredictability of the nature of basement aquifers (Olayinka, 1992). Of all the geophysical techniques, electrical resistivity method is the most widely used technique. Continuous 2D and 3D electrical surveys, providing a large number of measurements using automated acquisition systems, are popular and very well reported in the literature. Nevertheless, the Vertical Electrical Sounding (VES) is still the most widely used technique in the developing countries, because it is inexpensive and very useful when a deep and/or a very large area of coverage is required (Riss et al., 2011). Although, VES provides one-dimensional measure of resistivity variations in the subsurface, effective characterization of basement geology has been reported using this method (Olorunfemi and Fasuyi, 1993; Olayinka et al., 2004; Nwankwo, 2011).

Acute water shortage, especially during the dry season is an important problem facing the people of Erunmu and its environment. The area appears to have benefitted minimally from government developmental programs, possibly, because of location and relatively small population (2000 people, 80% of whom are peasant farmers). Since 1978 the State and Local Government had installed water reservoir tanks in neighboring villages. These are filled with water drawn from a treated municipal supply and conveyed to the villages by water tankers. This supply of water, nevertheless had been very sporadic and grossly inadequate and most villagers had continued to draw water from the traditional sources-ponds and hand dug wells. Many of the ponds are possibly infected, hence the people depend on hand dug wells for drinkable water supply, most of which also dry up during the dry season. This lack of good and drinkable water had been linked to outbreak of guinea worm and other diseases in the past (Kale, 1982). Even though incidence of diseases have reduced greatly (Barry, 2007; Callahan et al., 2013), the lack of adequate drinking water, together with observable low level of awareness about public health in the area, implies water-borne disease outbreak is very likely.

In this study, hydrogeology of the basement complex areas around Erunmu, southwestern Nigeria is presented, based on results of twenty five vertical electrical soundings in the area. This study is the first effort in evaluating the hydrogeological potential of the basement rocks in the study area. Meeting the ‘Water for Life by 2015’ millennium development goal of the United Nations (United Nations, 2006; Callahan et al., 2013) requires effective evaluation of freshwater and groundwater resources for sustainable development, especially for people in the rural areas. The present study is well aligned with this millennium goal. The objective of the present study is to evaluate hydrogeological potential of the basement rocks in the study area as well as assessing reliability of the electrical-resistivity geophysical method for delineating potential locations for siting boreholes in the basement complex.

MATERIALS AND METHODS

Description of the study area: The project area with its access roads and location of sounding stations is presented in Fig. 1. A network of motorable roads and footpaths makes access to the area possible. The study area is approximately 16 km north-east of Ibadan metropolis, southwestern Nigeria. It lies within latitudes 7°26’15" N and 7°27’51"N and longitudes 4°3’00"E and 4°4’45"E. It lies within the humid and sub-humid climate region with mean annual rainfall of about 1230 mm and mean maximum temperature of 32°C. The area is drained by network of streams. There is fluctuation in the discharge of rivers and streams with the weather conditions, the highest being recorded during, the rainy season between the months of April and October. From November to February, the water level is low while some streams are completely dry. Generally, the villagers use the river and streams for fishing and farming, whereas, they depend on hand dug wells for drinkable water supply. Although, these hand dug wells yield appreciable water during the raining season, during the dry season, the yield is low while some wells are completely dry.

Geologically, the study area lies within the Nigerian basement complex characterized by crystalline rocks of Pre-Cambrian age (Rahaman, 1976). Local geological mapping which was part of the field campaign for this study, showed that the rock groups in the area are porphyritic granite, porphyroblastic gneiss and undifferentiated gneiss complex (Fig. 2). The rocks have distinct difference in the size of the crystals with at least one group of crystals obviously larger than another group. Foliation planes are well developed and the general strike is north-south. The rocks are westerly dipping, with dip values ranging from 45-60°C. Visual description of lithology profile down the hand-dug wells shows that the top soils consist of reddish brown laterites and silty clay which are well leached and drained. The underlying weathered layer consist of clayey and silty sand which grades into more sandy and medium grained saturated clay at deeper sections (Fig. 3). Generally, thickness of the weathered profile is variable. Whereas, some hand dug wells penetrate appreciably thick overburden and yield appreciable water, others penetrate thin weathered profiles and yield little water or are completely dry during the dry season. The depth to the water table at the time of this study ranged from 0.3-2 m with total thickness of the weathered profiles penetrated by the wells ranging from 5-7 m.

Data acquisition: Twenty five Schlumberger Vertical Electrical Soundings (VES) were made in the study area using a maximum spread (AB) of 200 m. The data was collected by 4 person team using a digital averaging equipment (ABEM Terrameter) as the resistant-measuring device.

Fig. 1:Location map showing road network and VES locations

A portable 12.5 V car battery was used as current source while four stainless metal stakes were used as electrodes. A saline solution was used to reduce the electrode resistant at each location. Also, a GPS receiver was used to determine the coordinates and elevations of the sounding points. Progress rates of 5 VES per day were achieved because of the thick vegetation and hard terrain. The orientations of the survey, for 10 out of the 25 VES were constrained by accessibility as it was required to manually cut foot paths through thick vegetation to make way for the electrode spread. Where possible the soundings were made close to existing hand-dug wells. We used four cycle stacking and set the standard error of measurements to 5% in the instrument. At each measurement, the resistivity meter displayed resistance and the corresponding RMS (root mean square) error of the reading which were generally less than 5% throughout the survey. The recorded resistance values were used to compute apparent resistivity values. The locations of the VES points and existing hand dug wells are shown in Fig. 1. VES were numbered in the order in which soundings were carried out.

Data interpretation: The apparent resistivity data obtained from the field was plotted against half of the current electrode spacing (AB/2) on a log-log scale. The data was first interpreted using conventional partial curve maching and drawing of auxiliary point diagram, results served as input for computer iteration procedure. The prime motive of the VES interpretation method is the determination of the number of layers denoted as “n”, layer thickness in meters denoted by “h” and layer resistivity in Ωm denoted by “ρ”. The interpretation of the curve was based on the principle that all points of maxima and minima are indicators of different lithologies. Similarly, where the resistivity values tends to infinity, it is an indication of the fresh basement rock (Zohdy, 1989; Telford et al., 1990).

Based on the partial curve matching, initial estimates of the resistivities and thickness of the various geoelectric layers were obtained. These were used as starting models for a computer based inversion procedure. The commercially available WINRESIST software was used for the quantitative interpretation. The inversion procedure uses a least square approach (Zohdy, 1989; Loke and Barker, 1996; Amidu and Dunbar, 2008) to minimize the difference between the input data and the theoretically derived curves.

Fig. 2:Geological map of the study area


Fig. 3(a-d): Examples of Lithologic profile down the hand-dug wells in the study area at location (a) L1, (b) L2, (c) L6 and (d) L7

Due to handy controls, the WINRESIS was able to choose from a set of equivalent solutions the one that best fit both the geophysical and geological data.

Geoelectric cross sections were constructed based on lateral combination of inverted soundings of the VES results. Next, isopach and isoresistivity contour maps were constructed using results of the resistivity interpretation. The geoelectric cross sections and various maps were constructed using the Surfer 9 software (Golden, Colorado). This program uses various mathematical models to generate maps and cross sections, even when only a limited number of measured data points are available. In this study, Kriging algorithm using the default setting in the program was used. Details of the use of Krigging method for gridding and contouring are explained by Oliver and Webster (1990).

RESULTS

VES curves: Figure 4 shows examples of the model curves for the VES data. The sounding curves reflect possible presence of three and four geoelectric layers. The three layer models are of H (ρ123) and A (ρ123) types, whereas, the four layer curves are of HA (ρ1234) and KH (ρ1234) types. The dominant curves are H-types, comprising of 18 out of the 25 interpreted VES curves and are found in various parts of the survey area. The other curve types, each comprising of two curves are each localized in different part of the survey area, except for the KH type curves comprising of VES 23 and VES 10 that are about 20 m apart. The detail interpreted resistivity values, layer thicknesses and layer depths are summarized in Table 1. The first layer in all cases are interpreted as the top soil with model resistivity ranging from 16.9 Ωm (VES 4) to 207 Ωm (VES 12). The thickness is variable and mostly less than 2 m, except for VES 18, where thickness of 3.4 m was interpreted for the top layer. For the middle weathered layer, the modeled resistivity values range from 10.2 Ωm (VES 4) to 223 Ωm (VES 23) with depth to the base of the layer less than 20 m. Generally, the hand dug wells in the area terminate at this layer. Underlying the weathered layer are the fractured layers and fresh basement. Whereas, the fractured layers have interpreted resistivity less that 1000 Ωm, the fresh basement mostly have interpreted resistivity greater than 1500 Ωm.

Geoelectric cross sections: Figure 5 shows examples of interpretive cross-sections that have been constructed using the results of the VES (Fig. 4). Corresponding sounding points are indicated on each section. These cross-sections are interpretive of two-dimensional geometry of the possible geo-electrical layers in the subsurface and can be used to infer the geometry of the various layers (Mbonu et al., 1991; Nwosu et al., 2013; Oladunjoye et al., 2013). Cross-section AB, connecting VES 6, 5, 4 and 3 shows presence of four geoelectric layers. The top two layers correspond to the reddish brown top soil with overlying lateritic clay layer. The underlying geoelectric layer consists of the weathered profile which is clay rich and represents region of progressive chemical degradation from fresh basement rock to soil (Acworth, 1987; Amidu and Olayinka, 2006). The layer is thickest, around 8 m, at the central part of the section at VES 4. The deepest geoelectric layers observed by our soundings are interpreted as fresh basement rocks. This fresh basement are of infinite thickness and higher resistivities (greater than 1500 Ωm) (Olayinka et al., 2004).

Fig. 4(a-f): Examples of the model curves from the VES data on schlumberger configuration, (a) VES 1, (b) VES 4, (c) VES 10, (d) Ves 12, (e) VES 15 and (f) VES 23

In some of the interpretations (such as VES 20), however, the interpreted resistivities for the lowermost layers are found to be relatively low (<1500 Ωm) for a fresh basement. In this case the sounding is adjudged to have encountered the fractured basement, where presence of water within the fracture contribute to the lower resistivity (Olayinka, 1992). These fractured basement, with exceptionally low resistivities are inferred to be saturated and are potential targets for groundwater supply (Olayinka et al., 2004; Oladunjoye et al., 2013).

Cross sections CD and EF show similar lateral variations in the subsurface layers. The top two layers are the lateritic top soils. The thickness of the weathered layer is variable. The layer is thickest at VES 15 (about 23 m) in profile EF which, along with its relatively low resistivity is a potential target for groundwater.

Table 1:Results of computer modelling interpretation of the VES curves


Fig. 5:Example of geoelectric cross sections from the VES data

Fig. 6(a-b): (a) 2D and (b) 3D isopach maps of the overburden thicknesses inferred from the resistivity data interpretation

Isopach map of the overburden and isoresistivity map: Figure 6a shows the isopach map of the overburden as inferred from the resistivity data interpretation. The results show that overburden thickness is highly variable, being thickest in the vicinity of VES 22 and 23 to the north and VES 13 and 15 in the central part of the study area. Relatively thin overburden occurs in the vicinity of VES 5, 7, 17 and 24. This variable overburden thickness in the area is easily discernible in Fig. 6b where the depth interpretation from the VES data has been combined with the GPS data to generate 3D view of the overburden thickness. The great variability and the unpredictability of the nature of basement aquifers (Olayinka, 1992) make such three-dimensional view highly important in evaluating groundwater potential of the basement complex and developing sustainable water supply program. A band of relatively thick overburden zone extending from southern part of the area to a point towards the north are easily identifiable. Groundwater development in this area can concentrate in this zone, where relatively thick overburden can sustain good water supply and yield (Olayinka et al., 2004).

DISCUSSION

The results of this study has helped to better understand subsurface geoelectric distribution, basement topography and the possible hydrogeology of the basement complex in Erunmu area. The three and four-layer model curves obtained from the VES interpretations together with resistivity values are consistent with results from previous VES surveys in other areas in the basement complex of Nigeria as reported in the literature (Ajayi and Adegoke-Anthony, 1987; Okhue and Olorunfemi, 1991; Olayinka et al., 2004). For example, the predominance of three-layer H-type curves in the study area agrees with results of Oladunjoye et al. (2013), who reported 16 H-type curves out of 22 sounding points in Awotan area of Ibadan, southwestern, Nigeria. Olorunfemi and Fasuyi (1993), however has shown that various complex combinations of H, A and K curves, yielding multiple geoelectric layers are possible, especially for relatively deep fractured basement aquifers.

Generally, in this study, the subsurface layers are found to comprise of the top lateritic soil, weathered layer, fractured basement and fresh basement. This is well aligned with results of Olayinka et al. (2004). Also, the VES results are consistent with lithologic logs from the hand-dug wells in the study area. The top soils are of varying thicknesses and resistivity values. The fractured and weathered basement with relatively lower resistivity are inferred to be the aquiferous zone and could bear productive water for groundwater supply (Oladunjoye et al., 2013). The weathered layer constitutes the hydrogeologically significant layer because of its water bearing capacity and relatively high porosity, permeability and specific yield (Acworth, 1987), whereas, fractures significantly influence the groundwater yield because of the higher permeability (Olorunfemi and Fasuyi, 1993).

According to Olayinka (1992), a borehole should be sited where it can penetrate the maximum possible thickness of the regolith, such that adequate storativity and transmissivity can be guaranteed. As shown by the VES results, such conditions are met in the central part of the study area around VES 13 and around VES 22 in the north, where in addition to the relatively thick overburden, the weathered profile and fractured basement resistivity values are relatively low which is inferred to be indicative of water-filled fractures in the basement. The groundwater yield from the weathered horizon can be supplemented by accumulated groundwater in the fractured basement and thus enhancing perennial yield. Olorunfemi and Fasuyi (1993) established a relationship between resistivity values and fracture density, where low resistivity are indicative of high fracture density and high water yield and higher resistivity values are diagnostic of fractured zones with low density and potential low yield. Thus, in this study, fractured bedrock (resistivity less than 500 Ωm, such as around VES 13, 19, 20 and 22) are inferred to be indicative of high fracture density with high aquifer potential. Moderate fracturing are inferred in areas with fractured zone resistivity ranging from 500-1000 Ωm (such as around VES 21) and hence moderate aquifer potential. For fracture resistivity greater than 1000 Ωm, such areas are considered to be of low water potential.

Corroborative evidence was provided by the cross sections plots as well as 2D and 3D isopach maps which allow study of possible spatial variations in the subsurface resistivity (Oladunjoye et al., 2013). Generally, it was easier to discern areas with relatively thick overburden and low weathered/fractured basement resistivity which are good potential targets for borehole siting, as opposed to areas with highly resistive weathered and fractured basement with thin overburden which are potential poor target for groundwater supply. A band of relatively thick overburden and low resistivity zone as observed from the 3D plot represents areas where groundwater development in the study area can concentrate, as the relatively thick overburden can sustain good and perennial water supply.

CONCLUSION

The VES survey in Erunmu southwestern Nigeria has contributed greatly to the understanding of the hydrogeology of the basement complex in the area. The subsurface layers are found to comprise of the top lateritic soil, weathered layer, fractured basement and fresh basement. Basement complex aquifers are potentially good resource for sustainable and drinkable water supply. Areas with relatively high thickness of the weathered layer and with low resistivity values of the weathered and fracture zones have been successfully identified as potential targets for siting boreholes. An important limitation to this study is the lack of drilling information to test the locations identified for borehole siting. Thus, further work is recommended in the study area. In particular, government should evolve a sustainable water development program which will involve drilling of boreholes to develop the basement aquifers. Such program will help to alleviate water supply problem of the people in Erunmu and environments.

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