Determination of Subsurface Delineation Using Electrical Resistivity Sounding in Zuba and Environs of Gwagwalada Area Council, Abuja, North Central, Nigeria
Adeeko Tajudeen Olugbenga1, *, Ojo Emmanuel Osiewundo2
1Department of Physics, Faculty of Science, University of Abuja, Abuja, Nigeria
2Science Infrastructure Department, National Agency for Science and Engineering Infrastructure, Abuja, Nigeria
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 1.31-2.16m and 236-990 ohms meters, weathered layer ranging from 1.22-5.19m and 33.8-213 ohms meters and the fractured basement ranging from infinity in thickness and 397-966 ohms meters. Also, the study area lacks sufficient fractures and the thickness of the overburden was also thin for groundwater exploration activities.
Gwagwalada, Granite Gneiss, lithology, potential, Schlumberger, Tungamaje, Zuba
Received: July 3, 2015
Accepted: August 9, 2015
Published online: August 19, 2015
@ 2015 The Authors. Published by American Institute of Science. This Open Access article is under the CC BY-NC license. http://creativecommons.org/licenses/by-nc/4.0/
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 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, 2008. 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].
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 90051 4711 N and longitudes 70 121 4611E. The Abuja Guide, a National Space Research and Development Agency Atlas of 2002, Abuja is located between latitudes 80 101 and 90 451 North and longitudes 60 301 and 70 451 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 Palaeozoic age, also known as Pan-African Granites Ajibade et al., 1987. 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.
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, weathered layer, and fractured basement. The top layer resistivity values ranges from 236 to 990 ohm-m, with mean resistivity of 528.25 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 1.7892m. 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 ohm-m, with mean resistivity of 539.42 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.
|VES||Layers||ρ ( Ω-m)||Thickness(m)||Depth(m)||Probable Geological Section||Curve types||Coordinates|
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.