Professional paper	Received: June 5, 2013
Accepted: November 20, 2013
Hydro-geophysical evaluation of groundwater potential in hard rock terrain of southwestern Nigeria
Hidrološko-geofizikalna opredelitev potenciala podtalnice v ozemlju trdnih kamnin v jugozahodni Nigeriji
Ayodeji Jayeoba*, Michael Adeyinka Oladunjoye
University of Ibadan, Department of Geology, Ibadan, Nigeria *Corresponding author. E-mail: a.jayeoba@mail.ui.edu.ng
Abstract
In an attempt to characterize groundwater potential at the recently acquired land for University of Ibadan Cooperative Housing Estate located at Alabata near Ibadan, South-western Nigeria, integrated geophysical survey involving Very Low Frequency-Electromagnetic (VLF-EM) and resistivity methods were adopted. The VLF data measured along eight profiles were processed applying Fraser filtering and Karous-Hjelt filter on measured real components of the field data. Structural features significant to groundwater development were evident in the Fraser filter map and equivalent current density pseudo-sections. Thirteen Vertical Electrical soundings (VES) were carried out across the area using the Schlumberger electrode array configuration, with half-current electrode separation (AB/2) varying from 1m to 100 m. The layer model interpretation obtained from the sounding curves revealed three to four layer earth models categorized into topsoil, lateritic hard-pan, partially weathered layer and the fresh bedrock. The overburden thickness varies from 4.9 m to 19.1 m. Maps of the aquifer resistivity, aquifer thickness, overburden thickness, basement relief, bedrock resistivity, and secondary geoelectric (Dar-Zarrouk) parameters revealed delineated area with prolific aquiferous groundwater potentials.
Key words: Alabata area, geophysical investigation, current density map, geo-electric map, prolific zones.
Izvleček
Pri poskusu opredelitve potenciala podtalnice v zemljišču, nedavno nabavljenem za zadružno stanovanjsko gradnjo pri Ibadanski univerzi v Alabati pri Ibadanu v jugozahodni Nigeriji, so izvedli integrirano geofizikalno raziskavo, pri kateri so uporabili zelo nizkofre-kvenčno elektromagnetno (Very Low Frequency-Electromagnetic -VLF-EM) in upornostno metodo. Podatke VLF-merjenj v osmih profilih so obdelali z uporabo Fraserjevega filtriranja in Karous-Hjeltovega filtra na merjenih realnih komponentah terenskih podatkov. Strukturne značilnosti, povezane s prisotnostjo podtalnice, so se pokazale na kartah Fraserjevega filtriranja in psevdopreseki ekvivalentne tokovne gostote. Na preiskovanem območju so izvedli trinajst vertikalnih električnih sondiranj (VES) po Schlumbergerovem razporedu elektrod s polovičnim razmikom tokovnih elektrod (AB/2) od 1 m do 100 m. Z interpretacijo pla-stovnega modela, dobljenega iz krivulj sondiranja, so postavili model treh do štirih plasti, ki ustrezajo tlom, trdni lateritni plasti, plasti delne preperine in nepre-pereli matični kamnini. Debelina krovnih plasti je od 4,9 m do 19,1 m. Iz kart upornosti v vodonosniku, njegove debeline, debeline krovnih plasti, reliefa podlage, upornosti podlage in sekundarnih geolektričnih lastnosti (Dar-Zarrouk) je bilo mogoče določiti območja obetavne izdatnosti podtalnice.
Ključne besede: območje Alabata, geofizikalna raziskava, karta tokovne gostote, geoelektrična karta, cone izdatnosti
University of Ibadan cooperative recently acquired a parcel of land to serve as housing estate for its members. The basement complex rocks of southwestern Nigeria underlie the estate, which is located at Alabata near Ibadan. In typical hard rock areas, the geological sequence normally encountered is characterized by the existence of basement rock overlain by variable unconsolidated materials referred to as overburden. The groundwater in a typical Basement Complex environment is usually contained in the weathered and/or fractured basement rocks or alluvial deposits within flood plains.[1] However, the discontinuous nature of the basement aquifer system makes detailed knowledge of the subsurface geology, its weathering depth and structural disposition through geologic and geophysical investigations inevitable.[2] In order to evolve a pragmatic and scientific planning for the management of groundwater resources in this estate, a hydro-geophysical
evaluation of the groundwater potential was carefully carried out.
Integrated geophysical tools, especially resistivity and electromagnetic methods, are commonly used in groundwater exploration, mainly due to the close relationship between electrical conductivity and some hydrological parameters. The Very Low Frequency Electromagnetic (VLF-EM] is an effective tool in mapping conductive fault and fracture zones while resistivity method is used for detecting ground-water presence and differentiating subsurface layers. Electrical and electromagnetic geophysical methods have been widely used in ground-water investigations because of good correlation between electrical properties, geological (composition) and fluid content.[3-6] In present paper, Very Low Frequency (VLF-EM] and Vertical Electrical Sounding (VES) methods were employed to evaluate the groundwater potential of University of Ibadan cooperative housing estate in Alabata, Ibadan.
5T £52'30"
LEGEND
Main road ,„ Minor road , Settlement Vegetation ■ Study area
2 4Km
Figure 1: Location map of the study area.
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Figure 2: Geological map of the study area (Afenkhare, 2012).
Site Description and Geological Setting
The study area is located in Alabata, Ibadan, southwestern Nigeria (Figure 1). It is confined within latitudes 7° 34.970 and 7° 35.138 and longitudes 3° 52.180 and 3° 52.0. The study area is characterized by relatively gentle undulating terrain with elevations of between 265 m and 278 m above mean sea level (msl). The vegetation in the area is of rainforest type, characterized by short dry season and long wet season, with high annual rainfall ranging between 1 000 mm and 1 200 mm. Annual mean temperature is between 22°C and 33°C with relatively high humidity.[7] The survey area is underlain by the Precambrian basement complex rock of southwestern Nigeria. Metamor-phic basement rocks, mostly undifferentiated migmatite-gneiss, quartzite-schist, banded gneiss and granite gneiss, underlie the area.[8] Figure 2 highlights the local geology of study area. The study area falls in the area underlain by banded gneiss in Alabata. The coarse-
grained banded gneiss was low-lying with the elevation ranging from 240 m to 290 m (msl). It strikes approximately north-south with minor folds. There are quartz and pegmatite intrusions occurring concordantly with the rock's strike direction.
Materials and methods
The field investigation involved application of both Very Low Frequency Electromagnetic (VLF-EM) measurements and Vertical Electrical Sounding (VES) for mapping fractures in the bedrock and delineating geoelectrical layers in the overburden materials.
VLF measurement
VLF surveying falls into the far-field system of electromagnetic data collection. The VLF transmitter is a military-based communications antenna that emits a very powerful electro-
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magnetic wave, which when detected tens of kilometers from the source, behaves as a horizontally propagated plane wave.[9] The propagating signal has horizontal and linearly polarized magnetic and electrical components of the radio-wave field in the absence of a subsurface conductor. However, eddy currents are generated when the radio-wave field passes through a buried conductor, creating a secondary electromagnetic field. The increase in the flow of induced current causes the magnetic field to tilt in the vicinity of conducting structures.[10] Since this causes a phase shift with respect to the homogeneous primary field, the total field is elliptically polarized and tilts with respect to the horizontal axis. Consequently, tilt-angle variations follow a response across the anomaly and thus the crossover point coincides with the center of the anomaly. Many commercial instruments measure the changes in the different parameters of the total field. For example, some instruments measure the dip of the major axis and the ellipticity of the polarization ellipse; whereas other instruments measure the vertical and horizontal field components. These components of the anomalous field can be converted into ratios of the vertical anomalous field to the horizontal primary field for tilt angle analysis. Further, a current density can be calculated with respect to depth from the measured magnetic field. For example, a buried sheet conductor in a resistive medium in a horizontal primary magnetic field will induce changes in the amplitude and di-

LEGEND A Well
<- VLF Profile
# VES Point
u
rection of the primary field in proximity to the target. Consequently, on one side of the target, the angle between the vectors of the primary and secondary components of the radio wave field will reach a maximum near an object and change to a minimum upon passing a buried target. The point at which the tilt angle passes through zero, the "crossover" point lies immediately above the target.[11] If the target dips, then the tilt-angle measurements on one side of the anomaly are accentuated at the expense of the tilt-angle measurements on the other side of the target. The tilt angle and current density derived from the anomalous magnetic field can be used in subsequent statistical analyses to locate and to image the subsurface target. Linear filtering of the tilt-angle measurements can aid in locating the position of a buried target. Fraser[12] proposed a simple linear statistical filter of tilt-angle data that converts tilt-angle crossovers into peaks for ease of analysis. Fraser filtering consists of averaging the tilt-angle measurement produced by a subsurface conductor. In a linear sequence of tilt-angle data M1, M2, M3 ... Mn measured at a regular interval, the Fraser filter F . is:
0 = (M3 + MJ - M - M2)
(1)
Figure 3: Location map of the study area showing the VLF-EM profiles, VES points and dug well.
The first value F1 is plotted half way between positions M2 and M3; the second value is plotted halfway between M3 and M4. Many instruments can calculate a current density from the magnitude of the measured magnetic field.[13] Karous and Hjelt[14] developed a statistical linear filter, based upon[12] and linear field theory of Bendat and Piersol[15] This filter provides an apparent depth profile from the current density (H0) which is derived from the magnitude of the vertical component of the magnetic field at a specific location (Figure 3). The depth profile can be calculated from:
Ia (0] = 2n (-0.102^ + 0.059H-2 - 0.561H-1 +
+ 0.561H1 - 0.059H2 + 0.102H3)/Z (2)
Where, the equivalent current density Ia at a specified horizontal position and depth Z is based upon a symmetrical filter of the measured current (from the measured magnetic component of the anomalous field).
In this study, VLF-EM method was employed to map the study area with the object of isolating fracture zones which are likely to be filled with water. ABEM Wadi VLF electromagnetic equipment with in-built digital display unit and powered by battery was used. For the VLF-EM measurements, radio signal from station GQD in Rugby UK was the main signal station tuned / selected. This corresponds to frequency values of 18.8 kHz and was employed to generate the primary electromagnetic field around the buried conductors in order to induce the detected secondary field and measured as a fraction of the primary field by the VLF-meter. Eight profiles were measured with three (3] trending approximately N-S and five (5] trending approximately E-W with measurement station intervals of 10 m. The profiles ranges between 170 m and 250 m long and the majority of the profiles run perpendicular to the general N-S geologic strike in the study area (Figure 3). A sub-meter-accurate Global Positioning System (GPS] was used for exact spatial positioning of collected data.
Geoelectric resistivity measurement
Electrical resistivity data were acquired using the Campus Ohmega resistivity meter. The survey involved 1-D Vertical Electrical Sounding (VES]. The VES utilized the Schlumberger electrode array with half-current electrode separation (AB/2) ranging from 1m to 100 m and thirteen (13] VES stations were occupied (Figure 3). The coordinates of each VES station were taken with the Garmin handheld Global Positioning System (GPS] device to ensure accurate future geo - referencing
Data Processing and Evaluation
The VLF-EM data as well as those of the VES measurements were subjected to data processing and evaluation as the basis for interpretation.
For VLF-EM, the acquired field data were processed to simplify the obtained complex information into a profile in which the displayed function is directly related to physical property of the underlying rock. Thus, measured raw real and imaginary components were subjected to Fraser[12] and Karous-Hjelt[14] filtering operations to suppress noise and enhance signal.
The Fraser filter[12] converts crossover points into peak responses by 90° phase shifting. This process removes direct current bias that reduces the random noise between consecutive stations resulting from very low frequency component of sharp irregular responses.[16] The Karous-Hjelt filter[14] uses the linear fit theory to solve the integral equation for the current density. This forms the basis of the overall interpretation and delineation of potential fracture zone.
The VES, field data were interpreted through the following steps:
—	smoothing of the apparent resistivity field data curve and removing the electrical noises superimposed using an appropriate filter operator;[17]
—	matching the smoothed field curve with the standard curves of the auxiliary method;[18, 19]
—	preparing an initial geo-electrical model (thicknesses and corresponding resistivities] for a limited number of layers and incorporating the geological background and well information in the study area;[6]
—	entering the initial geo-electrical model into the Vander Velpen[20] modeling package. Iterations were carried out to reach the best fit between the smoothed field curve and the calculated one. The root mean square (RMS) errors of the resulting models ranged between 2.3 % and 3.2 %. The final VES interpretation results (layer resistivities and thicknesses] were used to generate secondary geoelectric (Dar-Zarrouk) parameters, weathered layer thickness map, weathered layer resistivity map, overburden thickness map, bedrock resistivity map and the basement topography map of University of Ibadan cooperative housing estate in Alaba-ta, Ibadan. The spatial representation of the data was done using surfer 9.0 software with Kriging employed as the gridding method. The data were ranked using the overburden thickness, aquifer resistivity, aquifer thickness and bedrock topography inferred from the first order geoelectric parameters and total longitudinal unit conductance, total transverse resistance unit and electrical an-isotropy inferred from second order geoelectric parameter to generate the groundwater potential map of the study area.
Geoelectric (Dar-Zarrouk) Parameters
A geo-electric layer is described by two fundamental parameters: its resistivity (p) and thickness (h.), where the subscript i indicates the position of the layer in the section. Other geoelectric parameters can be derived from its resistivity and thickness.[21] For i = 1, 2 ... n-layer, these parameters are:
— Total longitudinal conductance (S) S/S = hi/pi + hJp2 + ... + hJpn
Total transverse resistance (T) T/(n m2] = hi pi + h2 P2 + ... + hn pn
Maillet[22] has defined S and T as Dar-Zarrouk parameters. They can be defined for individual layers, or as a summation for a multi-layer section.
—	Average longitudinal resistivity (pL) pL/(n m) = H/S = 2 h/( Shi/Pi)
Where H = 2 h. (h. is the thickness for each layer i)
—	Average transverse resistivity (pt) pt/cn m) = T/H = (2hi p)/ 2hi
—	Electric anisotropy (A)
* = (pt /pL)1/2 = (T S/H2)1/2 (dimensionless)
—	Root means square resistivity (pm) pm/(n m) = (pt X pl)1/2 = A X pl = (1/A) X pt
In this study area, the above geoelectric parameters (S, T, pl and pm) are calculated to the top of the basement rock as shown in Table 1.
Table 1: Calculated geoelectric (Dar-Zarrouk) parameters
VES NO	LATITUDE*	LONGITUDE*	Elevation (m)	l1 (n m)	l2 (n m)	l3 (n m)	l4 (n m)	h1 (m)	h2 (m)	h3 (m)	S (1/n)	T (n m2)	X	M (m)
1	596129.48	838398.80	278.5	113.8	115.8	1823.6	_	1.7	5.6	_	0.063 3	841.94	1.00	267.3
2	596107.62	838297.41	277.1	221.8	57.2	3643.6	_	1.1	7.3	_	0.132 6	661.54	1.11	259.6
3	596058.17	838199.66	270.6	294.8	740.9	620.5	2 489.9	1.2	7.1	10.8	0.031 1	12 315.55	1.02	248.9
4	595993.71	838252.97	268.3	326.8	53.5	5365.4	_	0.8	4.1	_	0.079 1	480.79	1.26	268.1
5	595936.63	838293.39	264.9	110.6	72.8	1155.4	_	1.1	7.0	_	0.106 1	631.26	1.01	257.9
6	595958.47	838402.14	270.4	440.0	35.4	1146.0	_	2.2	8.3	_	0.239 5	1 261.82	1.66	256.5
7	595987.68	838509.07	272.0	58.0	158.8	37.9	1 906.6	1.8	5.1	11.3	0.361	1 342.55	1.00	247.8
8	595989.92	838509.07	268.3	427.0	46.6	1230.4	_	1.2	6.2	_	0.135 9	801.32	1.44	276.6
9	596045.18	838308.23	269.9	357.9	41.9	739.6	_	1.2	7.0	_	0.170 4	722.78	1.35	263.8
10	596026.65	838260.44	273.2	702.3	51.5	2803.9	_	1.0	6.2	_	0.121 8	1 021.6	1.01	269.8
11	596076.30	838328.58	271.2	153.4	29.0	1813.3	_	1.7	6.1	_	0.221 4	437.68	1.26	267.2
12	595998.88	838422.49	274.8	237.4	57.3	792.4	_	2.0	11.5	_	0.209 1	1 133.75	1.14	264.5
13	596055.81	838457.61	274.0	127.6	148.1	133.3	2 709.9	1.5	10.7	4.2	0.115 5	2 335.93	1.00	263.6
*Date for geographic coordinates is Universal Transverse Mercator (UTM) ¿1 - ¿^ = Resistivity values for each layer (0 m) hi - h3 = True thickness for each layer (m)
S = Total longitudinal conductance (1/0) to the top of the basement rock
T=Total transverse resistance (0 m2)
X = Electric anisotropy (dimensionless) to the top of the bedrock M = Bedrock relief (m)
Results and discussion
VLF-EM Survey
Fraser filtering responses ranged in value from -105 % to 160 % along the profiles. Figure4 shows the Fraser filtered data (real or in-phase components]. The in-phase profiles show positive peaks of different intensities and sharp-
ness, suggesting the presence of shallow and deep conductors.[23]
Lower values of relative current density correspond to higher values of resistivity.[24] All the VLF-EM profiles in this study were processed using the Karous-Hjelt filter.[25] Conductors (coloured red] were delineated from equivalent current density pseudo sections along
277
traverse 1, 3 and 7 (Figure 4). A higher value of relative current density is regarded as conductive subsurface structures, such as fractures^23, 26] which often store groundwater in hard rock terrains.
The 2-D inversion shows the variation of equivalent current density, and change in conductivity with depth. With such equivalent current density cross-section plots, it is possible to qualitatively discriminate between conductive and resistive structures where a high positive value corresponds to conductive subsurface structure and low negative values are related to resistive materials.!24, 26] In addition, equivalent current density cross-section also gives an idea about the dip direction; however, exact dip angle cannot be estimated due to the vertical axis variable being a pseudo depth only.[26, 27]
The equivalence current density pseudo-section of profile 1 (Figure 4a] reveals the presence of major anomaly at the southern section between 125 m and 162 m, which can be referred to as fracture zone.[28] Furthermore, two high current density zones between 17 m and 26 m, and 75 m along the profile can also be referred to as indications of the potential subsurface fracture system[26] with the fracture at 75 m dipping southwest (Figure 4a]. Asymmetry in the observed real and imaginary anomalies suggests the dipping nature of a subsurface conductive body.[29, 30] The Fraser filtering data plots and the Karous-Hiljet current density plot for profile 3 as presented in Figure 4a reveals a number of anomalies, which reflects conductive subsurface structural trends of inferred fractures zones. In addition, profile 7 shows
5 596025 —
O
CL iyi
Profile 1
Profile 8
Profile 3
V
Profile 1

^ 838350 —
838500 UMT Coord.
I I
838500 838500
838500 UMT Coord.
--«es*"*"
—I (a)
838500
Profile "
Profile
DISTANCE in m
r
T
1

Real component, unnormalized
r
T
-1 (b)
596100
UMT Coord.	UMT Coord.
Figure 4: Fraser filtering graph and equivalent current density pseudo-sections: (a): N-S direction, (b): E-W direction.
N
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high equivalent current density between stations 42 m and 68 m and station 110 m with the latter dipping southeast. Other closures of conductive bodies are present on different section with each conductive body coinciding with points identified on the profiles as fractures and or geological features (Figures 4a and b).
Resistivity Sounding Curves
The resistivity sounding curves obtained from the study area varied from the 3-layer (A and H types] to 4 layer (KH) with the H type being the predominant. The typical curve types are as shown in Figure 5. Table 1 gives the summary of the VES interpretation. The thickness and characteristics of the aquifer are fairly known due to the well dug in the centre of the study area. The key to success of any geophysical survey is the calibration of the geophysical data with hydro-geological and geological ground truth information; therefore, geoelectric station 10 was purposely located near the well (Figure 3). Measurements from existing well dug in the area reveal lateritic top soil, sandy clay, and basement rock. The depth of the well measured is 6.2 m and the potentiometric surface is at 5.3 m. The depth measurement correlates fairly well with the interpreted values of VES-10 (Figure 5].
Weathered Basement (Aquifer unit) Resistivity Map
The weathered basement resistivity map (Figure 6] shows the resistivity variation within the aquifer units of the study area. The resistivity value range is between 29Hm and 621Hm; with a mean value of 104 H m. The resistivity is least at the centre towards the western part of the study area with values ranging between 29Hm and 100 H m. The northern part has resistivity values ranging from 100 Hm to 150 H m with the resistivity increasing towards the north. The resistivity also increases toward the south of the study area with the values ranging between 100 H m and 620Hm (Figure 6). The classification of the groundwater potential of the aquifer units based on resistivity was premised on the findings of Adiat et al.[2] Zones characterized by resistivity value less than 100 Hm or greater than 400 H m was recognized as area of least groundwater prospect. Zones of medium yield
Figure 5: Representative VES curve in Alabata with their respective interpreted resistivity log. (a): KH curve, (b): A curve & (c): H curve.
for groundwater prospect are characterized by resistivity values ranging between 300Hm to 350 H m. Areas with resistivity values between 100 H m and 300 H m are classified as zones of high groundwater yield potential. In addition, Olayinka et al.[5] classified weathered basement in the basement complex of Nigeria with resistivity values ranging from 100 Hm to 800 H m as good groundwater aquifer. Barker et al.[31] also observed that the highest yielding boreholes in the basement complex of Zimbabwe were associated with weathered layers resistivity values between 100 H m and 600 H m. Based on these, the northern and southern part of the study area is presumed to have good groundwater aquifer.
t
Figure 6: Aquifer resistivity map of the study area.
Weathered Basement (Aquifer Unit) Thickness Map
The thickness of the weathered basement varies between 4.1m and 11.5 m (Table1). The aquifer unit in the study area has a mean thickness of 7.4 m. The aquiferous zone is relatively thick around the northwest and southeast portion of the study area (8 m to 11.5 m) while the thickness of the remaining part is relatively thin (4.1 m to 8 m) (Figure 7). The aquifer unit in the entire area is generally characterized by low thickness between 4 m to 8 m. However, some areas have relatively thick aquiferous unit with thickness varying between 8 m and 12 m.
Overburden Thickness Map
The overburden thickness map (Figure 8) shows that the overburden thickness of the area varies from 4.9-19.1m, with a mean of 10.5 m. The overburden thickness map show zones of relatively thick overburden (greater than 13 m) and zone of relatively thin overburden (<13m). Appreciable overburden thickness zones are possible groundwater collecting zones; therefore, unconsolidated material could contain reliable aquifer if thick and san-dy.[5,32] Geophysical studies in southwestern
basement complex of Nigeria have identified
thick overburden as zones of high groundwater potentials.[33-35] The overburden is relatively thick (13 m to 20 m) in the northern and southeastern portions of the study area. These zones are suggestive of possible groundwater potential zones in the area. Such zones cover about 38% of the entire study area.
Figure 7: Aquifer thickness map of the study area.
Figure 8: Overburden thickness map of the study area.
Aquifer protective capacity evaluation The total longitudinal conductance (S) to the top of the basement rock ranges between 0.0311S and 0.361 S. The maximum value was recorded at VES7 (0.361 S) with gradual decrease towards the south (Figure 9). The minimum value was recorded at VES 3 (0.0311 S). A marked increase in S may correspond to an average increase in the clay content and consequently a decrease in transmissivity.[6] The total longitudinal unit conductance values can also be utilized in evaluating overburden protective capacity in an
N
area.[36] This is because the earth medium acts as a natural filter to percolating fluid. Its ability to retard and filter percolating fluid is a measure of its protective capacity.[36,37] The highly impervious clayey overburden, which is characterized by relatively high conductance, offers protection to the underlying aquifer.[38] The protective capacity of the overburden has been zoned into good, moderate and weak protective capacity.[39] They classified the longitudinal conductance above 0.7 S as good protective capacity zone, the portion having conductance values ranging from 0.2 S to 0.69 S were classified as zone of moderate protective capacity while zone with 0.1S to 0.19 S was classified as weak protective capacity and where the conductance value is less than 0.1S were considered poor. The above classification has revealed that the overburden materials of the study area ranging between moderate to poor protective capacity zone. The moderate protective capacity covers the northwestern part of the study area and extends towards the central part, which has weak protective capacity. The northeast through the eastern part to the south of the study area have poor protective overburden (Figure 9). From the longitudinal conductance map of the area, about 30 % of the area falls within the moderate protective capacity while about 70 % constitutes the weak/poor protective capacity rating. This suggests that materials of weak/poor protective capacity underlie the area.
Bedrock Topography Map
The bedrock topography map (Figure 10) reflects the topography of the bedrock underlying the area and its structural disposition. The map shows that the basement structures in the area include both basement ridge and depressions. The ridge occupies the eastern and central parts of the study area while the depressions occupy the northern, western and southern parts of the study area. Naturally, the groundwater flows from areas of high pressure (such as bedrock ridge] to area of low pressure (such as bedrock depression]. It is then expected that areas identified as depressions on the map are the groundwater collection points, which have significant role in groundwater development.
t
277 275 273 271 269 267 265 263 261 259 257 255 253 251 249 247
Figure 9: Total longitudinal conductance unit map of the study area.
595950 596000 596050 596100
Figure 10: Bedrock topography map of the study area.
Electrical anisotropy (X)
In the study area, the coefficient of anisotropy to the top of the basement rock ranges between 1.00 and 1.66. The maximum A value was recorded at VES-6 (1.66] and the minimum A values are recorded at VES-1, 7, and 13 (1.00) (Table 1). The coefficient of anisotropy map (Figure 11) shows that A is high around the western part of the study area and decreases towards other zones. Singh and Singh[40] pointed out that lower values of anisotropy correspond to high aquifer potential zones. Based on this, the northern portion through the east to
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the southern part of the study area is characterized by higher groundwater potential. These areas coincide with the areas with weak/poor protective layers (Figure 9); part of it falls on the ridge in the bedrock topography map (Figure 10), which is the groundwater-diverting zone. This is confirmed by the Fraser filter and equivalent current density distribution in profile 4 (Figure 4b) with high response on Fraser filter graph but with no corresponding accumulation of current density distribution on the equivalent current density pseudo-section. This fracture may contain unsaturated material. However, the depression indicated on the bedrock topography map (Figure 10) contains saturated material presumed to be groundwater as indicated on Fraser filter and equivalent current density pseudo-section of profile 6 (Figure 4b). In addition, the southern part of this area has the requirements that favour groundwater abstraction (Figure 11).
Figure 11: Electrical Anisotropy map of the study area.
Bedrock resistivity map Figure 12 shows the contour map of the bedrock resistivity. The resistivity values of the bedrock vary from 740 Om to 5 3650m. According to Olayinka and Olorunfemi,[41] the resistivity values that exceed 1 000 Om is fresh bedrock but where the resistivity reduces to less than 1 000 Om, the bedrock is fractured
Figure 12: Bedrock resistivity map of the study area.
and saturated with fresh water The fractured zone constitutes a major component of the aquifer in a basement complex area. From the map (Figure 12), the bedrock in the southwestern, southeastern, northeastern and around the central parts is highly resistive (2 000 0 m to 5 400 0 m). This coincides with the low conductivity values displayed on the equivalent current density pseudo-section of profiles 1, 2, 5, 7 and 8, which confirms the presence of highly resistive features in those zones. The resistivity values for the bedrock in the northwest to the major parts of the central portion towards the south central ranges between 12000m to 2 000 Om. There is good correlation between bedrock resistivity map and the equivalent current density pseudo-sections crossing this area, which show shallow conductive features. The resistivity value of the western and minute portion of south central is less than 1000 0 m (700 Om to 1000 0m). This is revealed by the current density pseudo-sections of profile 1, which shows a highly conductive feature between stations 122 m and 167 m, and profile 7, which has high current density reflection at station 110 m with the fracture dipping southeast. The prominent current density reflects the presence of localized fractures containing groundwater.[26] There is good correlation between the equivalent current density pseudo-sections and the bedrock resistivity map.
Total transverse resistance unit
The total transverse resistance unit (T) to the top of the basement rock ranges between 12 315.55 and 437.68Hm2. The maximum T value was recorded at VES-3(12 315.55 Hm2) with gradual decrease towards the central part. The minimum T value was recorded at VES-11 (437.68 H m2] Figure 13 shows the map of total transverse conductance unit of the study area. The southern part has the highest value, which ranging from 1 000 H m2 to 12 300H m2. The central part has value ranging between 400 Hm2 to 1 000 H m2 while the northern zone has value ranges from 1 000 Hm2 to 2 500 H m2. Transverse resistance unit map has been used in determination of zones with high groundwater potential.[42] According to Braga et al.[43] high values of T can be associated with the zones of high transmissivity and areas with high values on T map indicate unconfined aquifer. Hence, the southern zone is suitable for groundwater exploitation.
Figure 13: Total transverse resistance unit map of the study area.
Groundwater Potential Evaluation
The groundwater potential evaluation of the area was based on the integration of the equivalent current density pseudo-sections, aquifer resistivity, aquifer thickness, overburden thickness, total longitudinal conductance unit, total transverse resistance unit, and electric anisotro-py and bedrock topography maps. The ground-
Good Groundwater potential
Moderate Groundwater potential
I Poor Groundwater potential
695950 596000 596050
Figure 14: Groundwater potential map of the study area.
water prolific area will have high accumulation of current density, which reflects the presence of fracture zones,[26] thick overburden >10 m[5] and aquifer resistivity ranging from 100Hm to 800 H m.[2, 5' 31] In addition, groundwater potential area should have low longitudinal conductance unit, which indicate an increase in trans-missivity,[6] high transverse resistance unit,[43] low values of electrical anisotropy < 1.2[40] and areas characterized by depressions on the basement topography map. These maps were synthesized and integrated for the evolvement of the groundwater potential map, which was eventually used to categorize the study area into good, moderate and poor groundwater potential zones (Figure 4). The central/western portion is characterised by decrease in overburden thickness (7.2 m at VES 10), weathered layer resistivity (35Hm at VES 6), total transverse resistance unit (723 H m2 at VES 9) and increase in electric anisotropy (1.66 at VES 6) and total longitudinal conductance (0.24 S at VES 6), reflecting low aquifer potential. On the other hand, northeastern/southeastern region is characterized by increase in overburden thickness (19.1m at VES-3), weathered layer resistivity (602 Hm at VES-3), total transverse resistance unit (12 316Hm2at VES3) and decrease in electrical anisotropy (1.00 at VES-1, 7 and 13) and total longitudinal conductance unit (0.0311S at VES-3), reflecting high aquifer potentials. In this regard, the northern and south-eastern parts of the study area are
N
categorised as good groundwater potential;	Acknowledgements moving towards the central from the northern
and southern parts, groundwater potentiality	The authors acknowledged Mr. Isaac O. Baba-
changes from good to moderate while the west-	tunde with profound appreciation for the as-
ern/central part is categorised as area with	sistance during all the data acquisition. Sincere
poor groundwater potential (Figure 14).	thanks are given to the anonymous reviewers.
Conclusions
A comparative integrated interpretation of VLF-EM and VES data enabled the evaluation of the groundwater prospect of University of Ibadan cooperative housing estate in Alabata, Ibadan; a basement complex terrain of south-western Nigeria. With the additional information obtained from existing well in the area, the spatial distribution of the regolith/weathered layer, containing the near-surface or overburden aquifers, was reliably delineated from the bedrock housing the bedrock aquifers. The geoelec-tric parameters (layer resistivities and thicknesses) which are known to be of hydrogeologic relevance, gathered from the VES interpretation were used to generate maps (weathered/ fractured layer resistivity map, weathered/ fractured layer thickness map, overburden thickness map, basement topography map and Dar Zarrouk parameters maps). The maps were interpreted individually by identifying geoelec-tric parameters favourable to groundwater occurrence. The maps were combined to form a composite entity from which the groundwater potential of the study area was evaluated. The groundwater potential map was used to classify the study area into good, moderate and poor groundwater zones. The hydrogeologic importance of the equivalent current density pseudo-sections, the basement depressions identified on the basement topography map, maps generated from the primary and secondary (Dar Zarrouk) parameters corroborated the deductions from the groundwater map. Zones identified to have moderate and good ground-water potential can be considered for ground-water development at University of Ibadan cooperative housing estate in Alabata, Ibadan. The southern/northern portion was considered as good groundwater prospect area.
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