1. Introduction. Adeeko Tajudeen Olugbenga 1, *, Ojo Emmanuel Osiewundo 2. Physics Journal. Abstract

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1 Physics Journal Vol. 1, No. 2, 2015, pp Determination of Subsurface Delineation Using Electrical Resistivity Sounding in Zuba and Environs of Gwagwalada Area Council, Abuja, North Central, Nigeria Adeeko Tajudeen Olugbenga 1, *, Ojo Emmanuel Osiewundo 2 1 Department of Physics, Faculty of Science, University of Abuja, Abuja, Nigeria 2 Science Infrastructure Department, National Agency for Science and Engineering Infrastructure, Abuja, Nigeria Abstract The electrical resistivity investigation of Zuba and Tungamaje area, of Gwagwalada Area Council, Abuja, was carried out with a view to providing geology and geophysical information on the different sub-surface layers, depth, thickness, and distribution of the fractured basement as potential sources of groundwater. The basement rocks consist of a migmatite-gneisses, granite gneiss, and granite. The granite occurs in several locations of the study area. Twelve vertical electrical sounding stations were established utilizing the Schlumberger electrode configuration. The electrical resistivity data obtained where interpreted using IPI2win software. The results obtained from the analysis of the geophysical data showed that the study area is underlined by three geo-electrical layers. These layers are the topsoil, weathered layers, and fractured basement. The top soil layer of thickness and resistivity values ranging from m and ohms meters, weathered layer ranging from m and ohms meters and the fractured basement ranging from infinity in thickness and ohms meters. Also, the study area lacks sufficient fractures and the thickness of the overburden was also thin for groundwater exploration activities. Keywords Gwagwalada, Granite Gneiss, lithology, potential, Schlumberger, Tungamaje, Zuba Received: July 3, 2015 / Accepted: August 9, 2015 / Published online: August 19, 2015 The Authors. Published by American Institute of Science. This Open Access article is under the CC BY-NC license Introduction Groundwater occurrence is greatly influenced by the geology, topography and climatic factors that prevailed in a given area. By the same fact, the hydro-geologic condition of Gwagwalada area council is mainly controlled by the geology and geological structure. Geological structures (faults, fractures and lithologic contacts) play a great role in the movement occurrence of groundwater in the study area, the area is characterized by rocks like granite, pegmatite, shiest and gneiss which are Precambrian in nature groundwater occurs in the basement complex in the weathered mantle or in the joint and fracture systems in the unweathered rocks. The area under study belongs to the Precambrian era. It is underlain by the Nigerian basement complex rock of the Precambrian age. Weathering and other denudational activities have made parts of the under laying rock mass to be slightly thicker in some areas than others. The area has a fairly plain topography with sparsely distributed medium size hills and highlands that may have been formed by outcropping basement rocks. The basement rocks consist of a migmatite-gneisses, granite gneiss, and granite. The migmatite-gneiss is the most wide spread rock unit. The granite occurs in several locations. Detailed reports * Corresponding author address: adeeko.olugbenga@uniabuja.edu.ng (A. T. Olugbenga)

2 Physics Journal Vol. 1, No. 2, 2015, pp of the lithological description, age, history, structure and geochemistry of the Basement Complex of Nigeria are given in Oyawoye, 1972; Black et al., 1979; Rahaman, 1988; Caby, 1989, and Dada, In Fig. 1 the blue arrow indicates the location of Abuja on the geologic map of Nigeria. In the study area, all the three major rock categories mentioned above are well represented in Fig. 2. The rocks are generally weathered into reddish micaceous sandy clay to clay materials, capped by laterite (Obaje, 2009). The choice of a particular method is governed by the nature of the terrain and cost considerations [Emenike, 2001]. Fig. 1. Geological map of Nigeria, showing the position of Abuja (blue arrow) in the basement complex of north central Nigeria (modified from Obaje, 2009). 2. Geology of Study Area The study area is zuba and Tungamaje in Gwagwalada Area Council, is part of the basement complex of Nigeria considered by various workers to be Precambrian to lower Paleozoic in age (Oyawoye, 1970 and Rahman, 1976). Zuba is located in the North Central part of Nigeria. It is situated along Kaduna Lokoja road, it is located at an elevation of 432 meters above sea level and its population amounts to 536,068 (cencus,2006). Its coordinator are latitude N and longitudes E. The Abuja Guide, a National Space Research and Development Agency Atlas of 2002, Abuja is located between latitudes and North and longitudes and East. The area of study forms part of the Basement Complex of north central Nigeria; with lithologic units falling under three main categories, which include (1) Undifferentiated migmatite complex of Proterozoic to Archaean origin, (2) Metavolcano-Sedimentary rocks of Late Proterozoic age and (3) Older Granite Complex of Late Precambrian - Lower

3 99 Adeeko Tajudeen Olugbenga and Ojo Emmanuel Osiewundo: Determination of Subsurface Delineation Using Electrical Resistivity Sounding in Zuba and Environs of Gwagwalada Area Council, Abuja, North Central, Nigeria Palaeozoic age, also known as Pan-African Granites Ajibade et al., All these rocks have been affected and deformed by the Pan-African thermotectonic event. The rocks of the area are generally quartz-rich acidic types which account for the generally sandy nature of the soil. There is however, one major advantage about the type of rocks and soils found in the area because of the availability of construction materials in the form of building stones, quartz and pistolitic gravel, building sands and earth for use as foundation materials, as well as pottery raw materials. Figure 3 shows the geological map of Abuja showing Gwagwalada area council. The amount of rainfall in the area is moderate between the months of April and October and the dry season which begins from October November and last until March-April, although there could be some scanty flashes of rain during this period. However, within these seasons is a brief harmattan season that is occasioned by the north east trade wind and the attendant dust haze, increased cold and dryness. Weather conditions in the area are influenced by its location within the Niger Benue trough on the windward side of the Jos Plateau and at the climate transition zone between the essentially humid south and sub-humid north of the country. The high temperatures and the relative humidity in the Niger-Benue trough give the area a heating effect. Fig. 2. Geological map of the study area and showing positions of VES points.

4 Physics Journal Vol. 1, No. 2, 2015, pp Fig. 3. Geological Map of Abuja (source: Geology Unit of Bwari Area Council August, 2014). 3. Material and Method The electrical resistivity method utilized the Vertical Electrical Sounding (VES) is used to measure vertical variations in electrical properties beneath the earth surface involving the schlumberger array, ABEM Terrameter SAS 300C was used to acquire resistivity data. Field equipment, include: Terrameter, current and potential electrodes, long conductors with crocodile clips, hammers, field survey tapes and mobile phones for communication. The Resistance measurements are made at each expansion and multiplied by the respective geometric factor (K) to give the resistivity. A total of twelve VES soundings were carried out along the study area. The electrode separation (AB/2) varied from 1.5 to 150 m was used with the aim of probing a depth of at least 1/3 of AB. The VES station were marked, two current electrodes (C1 and C2) of equal distances on the opposite side of the VES station were measured and hammered into the ground. Similarly, two other electrodes (P1 and P2) of equal distances at VES point between the current electrodes were measured. The obtained field dates were subjected to analysis and interpretation by computer iterations using Ipi2Win Software. 4. Result and Discussion A total number of twelve (12) VES were carried out, six in each of the two locations. The results of the interpretation were used to determine the expected subsurface geologic and hydro-geologic features of the water bearing rocks. The interpreted result of the vertical electrical sounding data revealed the different geo-electric layers in terms of their resistivities and depths in the study area. The software used in the processing of the raw data works in such a way that it merges neighbouring layers of slightly different resistivities in order to minimize the layers detected. The sounding curves show three layer earth models. The three layer curve Characterized by H curve types covered 100% of the study area. The rocks within this basement complex are grouped into three categories; these are the older granites, gneiss and mignetite; the older metasediments; and the younger metasediments. According to Ajibade and Wright (1980), the rocks of the basement complex are believed to have evolved in at least four orogenic events namely: the pan African (600±150My), The Kibaran (1100±200My), The Eburnian (2000±200My) and the Liberian (2800±200My). The migmatite gneiss complex dominates the basement complex in the study area consisting of fairly uniform biotite and biotite hornblende gneisses with locally intercalated bands of amphibolites and quartzite (Geological Survey of Nigeria, 1986). The result of the geophysical investigation revealed three subsurface geo-electrical layers in all the VES stations. The observed geo-electric sections include the top soil layer,

5 101 Adeeko Tajudeen Olugbenga and Ojo Emmanuel Osiewundo: Determination of Subsurface Delineation Using Electrical Resistivity Sounding in Zuba and Environs of Gwagwalada Area Council, Abuja, North Central, Nigeria weathered layer, and fractured basement. The top layer resistivity values ranges from 236 to 990 ohm-m, with mean resistivity of ohm-m. Its highest value was observed at VES 12 and the lowest at VES 01 (as seen in fig. 4). The top layer thicknesses range from 1.31 to 2.16m, with mean thickness of m. It highest value was observed at VES 07 and the lowest at VES 06. The second layer constitutes the weather layer and its resistivity values range from 33.8 to 213Ωm with mean resistivity of 67.33Ωm (as seen in fig. 5). The highest value was observed at VES 06 and the lowest at VES 02. Its thicknesses range from 1.22 to 5.19m, with mean thickness of 2.992m. The highest thickness was observed at VES 06 and the lowest at VES 02 (as seen in fig. 7 and fig. 8). The third layer which constitutes the fractured basement which has resistivity values that range from 297 to 966 ohmm, with mean resistivity of ohm-m. Its highest value was observed at VES 8 and the lowest at VES 1(as seen in fig. 6). The lithology was used as a preliminary basis for rock type identification, in the (FCT) (Edetand Okereke, 1985), malomo et al, 1982/83, omeje et al, 2013 and the lithology of the study area agree with the earlier study mention. Table 1. Simulated Result of Resistivity Data from the Study Area. VES Layers ρ ( Ω-m) Thickness(m) Depth(m) Probable Geological Section Curve types Coordinates Top soil N Weather layer H Fractured basement E Topsoil N Weather layer H Fractured basement E Topsoil N Weather layer H Fractured basement E Topsoil N Weather layer H Fractured basement E Topsoil N Weather layer H Fractured basement E Topsoil N Weather layer H Fractured basement E Topsoil N Weather layer H Fractured basement E Topsoil N Weather layer H Fractured basement E Topsoil N Weather layer H Fractured basement E Topsoil N Weather layer H Fractured basement E Topsoil N Weather layer H Fractured basement E Topsoil N Weather layer H Fractured basement E

6 Physics Journal Vol. 1, No. 2, 2015, pp LONGITUDE LONGITUDE LATITUDE Fig. 4. Topsoil Layer Iso-Resistivity LATITUDE Fig. 5. Weather Layer Resistivity LONGITUDE LONGITUDE LATITUDE Fig. 6. Fracture Basement Fig. 7. Weather Thickness LATITUE

7 103 Adeeko Tajudeen Olugbenga and Ojo Emmanuel Osiewundo: Determination of Subsurface Delineation Using Electrical Resistivity Sounding in Zuba and Environs of Gwagwalada Area Council, Abuja, North Central, Nigeria Fig. 8. 3D of Weather Thickness. 5. Conclusion This study has been able to shown the importance of resistivity method in determine the lithology of an area. The geophysical investigation carried out delineates the presence of three subsurface layers which comprised the top soil which is composed of sandy clay or clayey sand, weathered layer which due to the low resistivity of the VES point for this layer, there is indication of possible clay materials within the weathered layer, and fractured basement rock cannot sustain boreholes. The basement rocks consist of a migmatite-gneisses, granite gneiss, and granite. The migmatite-gneiss is the most wide spread rock unit. In zuba and tungamaje, the rock compositions are made up of granites; granite gneiss gave relatively higher yields in the faulted zones or apparently fractured which could be the evidence of volcanic activity marked by the occurrence of flat toped lateritised basalt. There should be a thorough investigation of rock to determine its characters and properties within the area. References [1] Abuja Guide, (2002). A National Space Research and Development Agency Atlas, 10. [2] Adeeko Tajudeen Olugbenga and Ojo Emmanuel Osiewundo (2015). Underground Water Distribution System in Gwagwalada Area Council Abuja, Nigeria, using Resistivity Geophysical Method. International Journal of Scientific Research in Agricultural Sciences, 2(4), [3] Adeeko Tajudeen Olugbenga and Muhammad Hassan (2015). A Geophysical Survey Involving Twelve Vertical Electrical Sounding (VES), Lokoja Area Council Kogi State. International Journal of Scientific Research in Environmental Sciences, 3(7), [4] Ajibade AC, Woakes MW, Rahaman MA (1987). Proterozoic crustal development in the Pan-African regime of Nigeria. In Proterozoic lithospheric evolution geodynamics. (17): [5] Ajibade AC, Woakes MW, (1980). Proterozoic crustal development in Pan-African regime of Nigeria. In C.A..Kogbe (ed).geology of Nigeria, 2 nd revised edition, rock view nig. Ltd [6] Ariyo SO and Banjo AA (2008). Application of electrical resistivity method for groundwater exploration in a sedimentary terrain. A case study of Ilara-Remo, Southwest Nigeria. Continental Journal of Earth Sciences, (3): [7] Balogun O (2001). The Federal Capital Territory of Nigeria: A Geography of its Development; Ibadan University Press [8] Barongo JO, Palacky GD (1989). Investigation of electrical properties of weather layers in the Yale area, Western Kenya, using resistivity soundings. Geophysics 56:

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