HYDROGEOPHYSICAL CHARACTERIZATION OF AQUIFER UNITS IN PART OF JOS YOUNGER GRANITE PROVINCE, NORTH CENTRAL NIGERIA.
1Yinusa Ayodele Asiwaju-Bello,2Sunday Bayode,3Joy Omowumi Ololade and4Olusegun Adekoyejo Dada.
1,3Department of Applied Geology, Federal University of Technology, Akure, Nigeria.
2Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria.
4Department of Marine Science & Technology, Federal University of Technology, Akure, Nigeria.
E:mail:;; ; ,
ABSTRACT
This work was carried out in the non-orogenicYoungerGranite terrain of North Central Nigeria. Geoelectric method involving horizontal profiling usingWenner array and Vertical Electrical Sounding using Schlumberger array, was employed in field data collection. Twenty-nine traverses, at spacing of 25m, were established for horizontal profiling study with station separation of 25m along each traverse. A total of 371 stations were occupied.Apparent resistivity data from horizontal profiling were used to generate iso-resistivity mapsfor different horizons and these were used to characterize the study area into Low (ρ<700Ω-m), Medium (ρ=700-1400Ω-m), and High (ρ>1400Ω-m) apparent resistivity zones. The low (ρ<700Ω-m) apparent resistivity values of a = 40m found in the central, north east and north western parts of the investigated site are indicative of fractured/highly weathered rocks in the area. Based on profiling results, twenty-one VES stations were established.
The geoelectric sections from VES result were used to delineate six subsurface geoelectric layers. These are topsoil, weathered rock material, hard fresh rock (boulders), partly weathered rocks, fractured/highly weathered rock material and fresh bedrock. The delineated weathered rock material, partly weathered rocks and the fractured/highly weathered rock material horizons were identified with resistivity values ranging from 300 – 600, 600 – 1,000 and 100 – 600 ohm-metres, respectivelyand thickness values varying from 2 – 20, 5 – 22 and 6.5 – 56metres, respectively. The investigation revealed existence of limited discrete zones of weathered and fractured rocks at deep depths which can support medium to high groundwater abstraction in the investigated area.
Keywords:Geoelectric, Fracture, Weathered, Aquifer, Groundwater, Younger Granite
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1.0INTRODUCTION
Hydrogeophysical surveywas carried out at the permanent site of National Institute of Mining & Geosciences campus, Jos, Nigeria, an area that is underlain by Younger Granite rocks. The survey was meant to delineate all areas within the site having substantial accumulation of groundwater in the subsurface.
1.1Geology and Geomorphology of the Locality
Jos, the study arealies within the Younger Granite Province of Nigeria (Fig. 1). The Younger Granite rocks are non-orogenichigh-level plutonic igneous rocks that were emplaced in ring forms in parts of central Nigeria around the Jurassic period, about 150 - 162 million years
ago (Buchanan et al, 1971) The dominant lithologic unit in the locality is porphyritic hornblende biotitegranite which is part of the Jos-Bukuru ring complex rocks (Buchanan et al, 1971). The top soil at the study site wasthe reddish to brownish lateritic materialwhich usually harden during the dry season.
1.2Hydrogeology of the Crystalline Rock Environment
Crystalline rocks are generally non-porous and depend on acquisition of secondary porosity to be able to store water. The presence, extent and characteristics of groundwater storage units in this area are dependent on the depth and degree of weathering as well as intensity of fracturing of the underlying rocks. Thickness of the weathered overburden, i.e. depth to the fresh bedrock, is normally directly proportional to groundwater storage capacity of any available aquifer unit in the subsurface (Olorunfemi, 1990). Availability of fractures at depth is expected to complement the overburden effect and determine borehole yield in the terrain (Olayinka and Olorunfemi, 1992).
Younger Granite rocks are known for their hardness and induration. They have not been subjected to significant weathering to deep depths except in few areas where fracturing must have occurred. An investigation for groundwater in such rocks is more of a survey of the available fracture zones that are expectedly limited in the rocks (Anuduet al, 2008).
2.0METHOD OF STUDY
A total of twenty nine (29) traverses of about 100 – 500m long with inter-traverse separation of 25m were established in an approximately east – west directions across the study area. The geoelectric study involved the horizontal profiling (HP) and a follow-up investigation with the Vertical Electrical Sounding (VES) technique (Fig. 2). The Wenner Array was used for the horizontal profiling while the Schlumberger Array was adopted for the VES technique. Horizontal profiling measurements, with electrode spacing of a = 20m and a = 40m, were carried out along the 29 traverses.
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Figure 1: Simplified map of Younger Granite Province of Nigeria showing Jos locality (after MacLeod et al, 1971). Inset is map of Nigeria showing Jos.
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Generally, this profiling technique was employed as a means of delineating possible areas with characteristiclow apparent resistivity values that are diagnostic of deep-seated weathered layer and fractured zoneswhich can be probed further with the VES technique (Keller and Frischnecht, 1966;
Asiwaju-Bello, 1999).The horizontal profiling data was used to generate the iso-resistivity maps using SURFER, a graphic computer software.
Based on areaswhere low resistivity values were observed for electrode spacing of a = 20 and a = 40m, a total of 21 controlled VESwas conducted within the study area (Fig. 2).ABEMTerrameter model SAS-1000 instrument was used for the data acquisition. Schlumberger electrode array configuration was employed for the VES using a maximum current electrode spacing (AB/2) of 100m. The VES data were interpreted using manual partial curve matching technique to obtain the initial geoelectric parameters which were later subjected to 1-D computer iteration using WinResist, a data modelling computer software. The results obtained were used to construct geo-electric sections.
3.0RESULTS AND DISCUSSION
The results obtained were presented as summary table, iso-resistivity maps (Figs 3 & 4),, depth-sounding curves (Fig.5) and geoelectric sections (Figs. 7 and 8). Figure 3 shows the general apparent resistivity distribution for the electrode spacing a = 20m.
The obtained values were grouped into Low (ρ<700Ω-m), Medium (ρ=700-1400Ω-m), and High (ρ>1400Ω-m) apparent resistivity zones. The low (ρ<700Ω-m) apparent resistivity values found in the central, north east and north western part of the investigated site are indicative of materials in the upper most 7m of the earth in the area. The map is therefore indicative of features at shallow depths.
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Figure 2:Location and Data Acquisition Map of the Study Area.
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The a = 40m apparent resistivity map (Fig. 4) also display low apparent resistivity values (ρ<700Ω-m) which correlated well with the areas delineated as low apparent resistivity values for a = 20m. The lowapparent resistivity (ρ<700Ω-m) zones is straddled with medium tohigh apparent resistivity (ρ700Ω-m) zonesallignedin the southwest – northeast directions andoccupy a small area in the north western part of the study area. The medium tohigh apparent resistivity zones suggest near-surface bedrock while the low apparent resistivity zones are indicative of relatively deep bedrock, weathered layer and deep-seatedfractures.
Typical depth sounding curves obtained for the area are H, KH, QH, HKH, KQH, KHKH and QHKH. A summary of the VES interpretation results are in Table 1. Three geoelectric sections wereconstructed across the study area (Figs. 6, 7 and 8). These are along the line of section A – A”, which runs east – west,and sections B – B” and C – C” running in the north –south directions (Fig. 4). The three geoelectric sections delineated six subsurface geoelectric units. These are topsoil, weathered rock material, hard fresh rock (boulders), partly-weathered rocks, fractured/highly weathered rock material and fresh bedrock.
The first layer is the Topsoil. Its resistivity values generally range from 320 – 5500 ohm-m and its thickness varies from 0.8 – 2.2m. It is made up of sandy and clayey materials which may be indurated in some cases.The second layer is the weathered rock material. The resistivity value range from 300 – 600 ohm-m and the thickness varies from 2 – 20m. The layer is composed of sandy clay and clayey sand.
The third layer is the hard fresh rock (boulders). The resistivity values range from > 1500 ohm-m and its thickness varies from 2 – 42m. It is composed of fresh rock material in boulder forms.
The partly weathered rock is the fourth layer with the resistivity value ranging from 600 to 1000 ohm-m and its thickness varying from 5 – 22m. The layer is made up of partly-weathered granitic rocks.
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Figure 3: Apparent Resistivity Distribution for HP of ɑ=20m.
Figure 4: Apparent Resistivity Distribution for HP of ɑ=40m.
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The fifth layer is the fractured/highly weathered rock material. The resistivity values range from
100 – 600 ohm-m and the thickness varies from 6.5 – 56m. The layer is the most reliable aquiferous horizon within the subsurface in the area.
The last layer is the fresh granite bedrock. The resistivity values are generally above 1500 ohm-m. Depth to bedrock ranges from 16.5 to over 72 metres.
Both the weathered rock material and the partly-weathered rocks occur at shallow depths and their resistivity values will allow area only limited hydraulic conductivity characteristics. The small amount of storage in them will be prone to easy pollution from the ground surface. However, where found, they may constitute low to medium
hydrogeological significance and will readily serve the needs of shallow hand-dug wells.
The fractured/highly weathered rock material has thickness varying from 6.5 to over 56 metres, with layer resistivity from 100 to 600Ω-m. This unit is encountered everywhere along all the traverses but at varying depths which is not less than 15m below ground surface. It is a unit that is of highest hydrogeological significance in the area and it is expected to be a large reservoir of groundwater at deeper depth. Areas with large thickness are expected to support boreholes with high yields (Olorunfemi, 1990). In most places, the hard rock unit of low permeability overlies this aquiferous zone making it more difficult for pollution by infiltrating water.
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Table 1: Summary of VES Interpretation Results for the Study Area.
S/N / STATIONNo. / GEOELECTRIC LAYER PROPERTIES
(resistivity, , in ohm-metres; Thickness, T, in metres) / CURVE
TYPE
Layer 1 / Layer 2 / Layer 3 / Layer 4 / Layer 5 / Layer 6
1 / TR 3,4 / = 1400
T= 1 / = 700
T= 7 / = 1420
T= / - / - / - / H
2 / TR 3,7 / = 560
T= 1 / = 168
T= 5 / = 3075
T= 6.5 / = 624
T= ?? / =
T= / - / HKH
3 / TR 3,15 / = 1550
T= 1 / = 7750
T= 1 / = 1560
T= 8 / = 504
T= ?? / =
T= / - / KQH
4 / TR 4,6 / = 320
T= 1 / = 256
T= 3 / = 540
T= 45 / = 204
T= ?? / =
T= / - / HKH
5 / TR 7,1 / = 1040
T= 0.5 / = 2080
T= 13 / = 630
T= 58 / = 1850
T= / - / - / KH
6 / TR 7,8 / = 2900
T= 1 / = 3625
T= 1 / = 1020
T= 20 / = 2750
T= / - / - / KH
7 / TR 8,13 / =1800
T= 1 / = 1440
T= 1 / = 450
T= 34 / = 1150
T= / - / - / QH
8 / TR 9,1 / = 2300
T=1 / = 8050
T= 1 / = 1440
T= 20 / = 300
T= 21 / = 24000
T= / - / KQH
9 / TR 13,16 / = 750
T= 1 / = 1125
T= 1 / = 135
T= 1.5 / = 650
T= / - / - / KH
10 / TR 15,3 / =1400
T= 1 / = 280
T= 11.5 / = 1500
T= 19.5 / = 82.5
T= 2 / = 1050
T= / - / HKH
11 / TR 15,13 / = 3500
T= 1 / = 700
T= 11.5 / = 37500
T= 16.5 / = 650
T= ?? / =
T= / - / HKH
12 / TR 15,15 / = 1050
T= 1 / = 525
T= 2 / = 1240
T= 6 / = 285
T= 6.5 / = 6750
T= / - / HKH
13 / TR 16,12 / = 1750
T= 1 / = 1138
T= 15 / = 1500
T= 14 / = 420
T= ?? / =
T= / - / HKH
14 / TR 17,4 / = 480
T= 1 / = 960
T= 2 / = 487.5
T= 5 / = 2100
T= 16 / = 550
T= ?? / =
T= / KHKH
15 / TR 17,14 / = 1900
T= 1 / = 950
T= 14 / = 3500
T= 12 / = 420
T= 30 / = 31500
T= α / - / HKH
16 / TR 18,1 / = 1500
T= 1 / = 1875
T= 1 / = 850
T= 17 / = 174
T=9 / = 900
T= α / - / KQH
17 / TR 19,9 / = 2050
T= 1.5 / = 820
T= 12.5 / = 2975
T= 7.5 / = 280
T= 8 / = 7500
T= α / - / HKH
18 / TR 19,18 / = 4800
T= 1 / = 1440
T= 7 / = 750
T= 21 / = 3045
T= 21 / = 31.25
T= ?? / = α
T= α / QHKH
19 / TR 21,18 / = 5500
T= 1 / = 825
T= 4.5 / = 1350
T= 28.5 / = 32.5
T= ?? / = α
T= α / - / HKH
20 / TR 29,5 / = 2600
T= 1 / = 1300
T= 2 / = 700
T= 7 / = 1245
T= α / - / - / QH
21 / TR 29,7 / = 5000
T= 1.5 / = 1000
T= 17.5 / = 2725
T= 11 / = 138
T= 14 / = 5625
T= α / - / HKH
Figure 6: Geoelectric Section Along Line A-A”.
Figure 7: Geoelectric Section along Line B-B”.
Figuere 8: Geoelectric Section along Line C-C”.
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4.0CONCLUSION
A hydrogeophysical investigation, employing horizontal profiling and Vertical Electrical Sounding techniques, was carried outat the permanent site of National Institute of Mining & Geosciences, Jos, Nigeria. The apparent resistivity values obtained for the electrode spacing a = 20m and a = 40m were used to characterize the area into Low (ρ<700Ω-m), Medium (ρ=700-1400Ω-m), and High (ρ>1400Ω-m) apparent resistivity zones. The low apparent resistivity values (ρ<700Ω-m) observed for a = 40m suggest the presence of weathered layer/fractured bedrock at deeper depths. KHtype curve and its variants constitute the dominant geoelectric curves that characterise the prevailing weathering pattern of the Younger Granite rocks in the study area. The geoelectric sections constructed were used to delineate six subsurface geoelectric layers. These are topsoil, weathered rock material, hard fresh rock (boulders), partly weathered rocks, fractured/highly weathered rock material and the fresh bedrock.
The delineated horizons of weathered rock material, partly weathered rocks and fractured/highly weathered rock material constitute
the units of hydrogeological significance with moderate to high layer resistivity values of 300 – 600, 600 – 1000 and 100 – 600 ohm-metres, respectively. Their thickness values range from 2 – 20, 5 – 22 and 6.5 – 56 metres, respectively.
The investigation also revealedthat the low apparent resistivity value(<700Ω-m) zones obtained using horizontal profiling correspond with areas of lowlayer resistivities (<600Ω-m) in the geoelectric sections obtained from VES whichare indicative of fractured/highlyweathered zones at deeper depth which can support medium tohigh yieldingboreholes in the study area.
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