Ozean Journal of Applied Sciences 7(3), 2014 Ozean Journal of Applied Sciences 7(3), 2014 ISSN 1943-2429 © 2014 Ozean Publication SUBSURFACE STRUCTURAL CHARACTERIZATION USING DC RESISTIVITY METHOD AT INDUSTRIAL DEVELOPMENT CENTRE (IDC), ZARIA. S. D. SALEH*, P. O. SULE, A.L. AHMED K. A. MURANA** *Department of Physics, Ahmadu Bello University, Zaria, Nigeria ** SLT Department, Abdu Gusau Polytechnic, Talata Mafara, Nigeria *E-mails for correspondence [email protected] __________________________________________________________________________________________ Abstract: Electrical resistivity method employing Vertical Electrical Sounding (VES) was carried out at Industrial Development Centre Zaria. 27 Vertical Electrical Soundings (VES) were conducted along predetermined profiles at a station interval of 100m. The survey was carried out to investigate geoelectric and geologic parameters of the subsurface as means of determining the competence of the materials underlie the premises, in aiding structural and environmental works in the area. Terrameter SAS 300 system was used for data acquisition using Schlumberger array. Maximum current electrode separation (AB) of 200m was used. Interpretation was performed using computer software (Ipi2win and surfer 7). The results of the study indicate that the area is underlain by three to four subsurface layers. The resistivity of the first layer ranges from as low as 54 ohm–m to as high as 487 ohm-m with an average thickness of 2m. This layer is the superficial cover composed of sandy, silt, weathered laterite and clays that are confined in the sand in some places. The second layer has resistivity values between 114 ohm-m to 2518 ohm-m which is the weathered basement rock and fresh laterite in some places. The third layer has relatively low resistivity value that ranges between 101 ohm-m to 916 ohm-m. This layer is the fractured basement with varying thickness. The fourth layer is the crystalline basement rock with resistivity values much greater than 1000 ohm-m and an infinite thickness. The investigations show that the study area is underlain by competent material except towards the northern end that has clayey sand in the superficial cover, the aquifer in the area which is confined occurs in the weathered and fractured basement rock. Eastern half of the study area could be favorable for groundwater exploitation. Keywords: DC Resistivity, Geoelectric and Geologic layers, subsurface structures. __________________________________________________________________________________________ INTRODUCTION The knowledge of subsurface structures within the study area will go a long way to assist the centre in knowing suitable location for structural engineering works, locating suitable areas for refuse dumps, and construction of artificial lake or dams. The statistics of failure of structures such as roads, buildings, dams, bridges and even boreholes throughout the nation has increased to an alarming rate. The need for pre foundation study has 87 Ozean Journal of Applied Sciences 7(3), 2014 therefore become imperative so as to prevent loss of valuable lives and properties that always accompany such failure. Moreover, the acute shortage of water especially during the dry season in Zaria and its environs necessitate the intensive use of groundwater, hence the need for sustainable underground water development and utilization. Geophysical techniques can assist the construction industry in many ways: before, during, and after construction to solve construction problems or to facilitate the construction processes. Resistivity sounding or mapping are surface geophysical techniques that involve the measurement and interpretation of physical properties of the earth to determine subsurface condition for engineering and environmental investigations. The study area Samaru, Zaria, is a semi urban city with increasing population growth and infrastructural development. The annual rainfall is inadequate; therefore the need for subsurface study becomes imperative. Subsurface investigation is not an easy task but the advent of technology has made the quest for reliable location for any structural and/or environmental engineering works easy. Also the need for water for all purposes in life has drift from ordinary search for water to prospecting for steady and reliable subsurface or underground water from boreholes. The geoelectric method has been found to be very reliable for environmental and groundwater studies over the years (Shemang, 1990, Murana et al., 2011) The area under study lacks detailed geophysical investigations. Most geophysical works carried out around this area are regional in nature or are localized to specific areas. However, many geophysical and geological works have been carried out either at Samaru area or at other parts of Zaria. Du Preez (1952) carried out the regional study of Zaria and reported the presence of prominent steeply dipping joints in some outcrops of granite in Kubanni Basin; he also affirmed the presence of groundwater in some joints. Akpoborie (1973) in his study on Kubanni Basin has reported that the older granite icebergs are intensively fractured giving way for easy recharge of the weathered basement aquifer with rain water. McCurry (1970) has studied the basement geology of Zaria and has concluded that the deeply weathered areas of crystalline basement outcrop constitute the useful aquifers; he has affirmed that the aquifers are variable in extent and thickness ranging from broad areas of deeply weathered to narrow joints and fracture zones. Olugbenga (2009) from a resistivity investigation of the groundwater potential at Nuhu Bamali polytechnics, Zaria main campus, has reported that his study area is underlain by rocks of different lithological compositions comprising the top soil, the weathered basement, fractured and fresh basement rock. The depth to the basement varies from an average of 12m to 29m. He has further stated that the low resistivity area is the most promising for groundwater exploitation. The primary aim of this research work is to characterize the subsurface structures by determining the lithology within the area under investigation base on the physical parameters measured and also to map the subsurface rocks, thereby delineating accumulation zones, recharge and drainage pattern. The objectives set to achieve these aims are: (1) To produce sounding curves, geoelectric and geologic sections along the profiles. (2) To delineate the various strata within the subsurface. (3) To determine depth to basement so as to get a picture of basement topography. (4) To determine aquifer thickness in order to delineate favourable area(s) where to site boreholes. Location of the Study Area The study area, Industrial Development Centre (IDC), Samaru, Zaria, Kaduna state is located approximately on latitude 110 09’22.9” N and 110 09’58.5”N and between longitude 70 39’E and 70 40’E along Zaria - Sokoto road. The study area is opposite Ahmadu Bello University, (ABU) main campus. It has a total land area of about 64.80 hectares. The terrain is relatively flat; the area can be accessed by motorable roads see Fig.1 below. 88 Ozean Journal of Applied Sciences 7(3), 2014 Figure 1: Location map of the study area (source; Google image 2010 ) Climate, Relief, Vegetation and Ecomonic Activity of Zaria Area Zaria is located approximately between latitude 11 0 04’N and 110 09’N and between longitude 70 39’E and 70 50’E and is about 686m above sea level. It falls within the tropical savannah climate, according to Koppen’s world climate classification. It lies in the natural vegetation of the Northern Guinea Savannah, some 80km north of Kaduna town, along the major highway from Kaduna to Kano state. The study area, Samaru, is in the tropical wet and dry climate zone, characterized by strong seasonal rainfall and temperature distribution. The climatic condition is controlled by two air masses, the maritime tropical air mass and the continental tropical air mass. When the maritime tropical air mass is prevailing, the Zaria area experiences a rainy season, while the continental tropical air mass controls the dry season with its cold, dry and dusty air which occasionally limits visibility and reduces solar radiation. Daily maximum temperature shows a minor one of 23oc in November/ December and major peak of 39oc in April. The rainy season has a short duration and it is followed by 5-7 months of dry season. The rainy season appears to begin in May and ends in September while the dry season normally starts from around October to April (Olugbenga, 2009). The prevailing vegetation of tall grass and big trees are of economic importance during both the wet and dry seasons in the study area. About 80 percent of the Zaria populations are engaged in peasant farming producing both food and cash crops. During the dry season, many people in Zaria engage in irrigation farming along some major rivers and near dams. General Geology of the Study Area Zaria area is a dissected portion of the Zaria – Kano plain, belonging to the central basement complex. This has been described by (Russ, 1952) as comprising older high grade metamorphosed gneiss interspersed by belt of young metasediments of mainly quartzite and schist. Zaria region is underlain by crystalline metamorphic and igneous rocks of Precambrian to lower Paleozoic age (Wright and McCurry, 1970) occurring on the basement complex. A major part of the rocks is of high grade metamorphism mainly gneisses which suffered intense folding and granitization and have remained stable for millions of years. Others are migmatites, older granites, and more recently matasediments (quartz, schist, laterites and alluvium). The nature of parent material and the long period 89 Ozean Journal of Applied Sciences 7(3), 2014 of weathering under tropical condition have produced a characteristic topography of peneplain, inselbergs and domes of resistant basement rocks. A combination of topography and geology more or less control the groundwater occurrence in this area. Like other parts of Zaria, Samaru area is underlain by differentiated Pre-Cambrian basement complex formation comprising both igneous and metamorphic rocks. Furthermore, geological information indicates that undifferentiated basement complex of migmatites, granites, gneisses and metasediments is overlain by laterites Fig. 2 (Wright and McCurry, 1970). Figure 2: A generalized geological map of Nigeria (after Kogbe, 1989) Hydrology and Hydrogeology of the Study Area The urban Zaria is blessed with abundant water resources both ground and surface and the distribution of these resources have very little variation in both time and space amongst the sub-settlements (Yusuf et al., 2007). There are two major river systems; the Kubanni and Saye, joined at a confluence to form river Galma. These rivers together with their tributaries (Kamacha and Shika) drain the land area of urban Zaria. Du Preez (1952) in a regional study of Zaria reported the presence of prominent steeply dipping joints in some outcrops of granite in the Kubanni basin. He also affirmed the presence of groundwater in some joints. Generally, the hydrogeology of an area is determined by geology and climatic condition of that region, and then geological formations and the structures underlying the area determine the aquifer that would be developed. Groundwater recharge is the 90 Ozean Journal of Applied Sciences 7(3), 2014 process by which aquifers are replenished with water from the surface. Many factors influence the rate of recharge including the soil type, plant cover, slope, rainfall intensity, and the presence and depth of confining layers and aquifers. Most of the groundwater recharge in the study area occurs during the wet season when precipitation is highest. Choice of Electrode Configuration Many different electrode spreads have been used in the past but only few are still in use today. These include; Schlumberger array, Wenner array and double – dipole array. However, the most widely and commonly used arrays are Schlumberger array and Wenner array. The choice of array and its dimension largely depend upon the target; its size, depth and resistivity contrast with its surroundings. For this present study, Schlumberger array was adopted because of the following advantages; 1) The relatively small separation of the potential electrodes reduces noise due to ground current (from industrial and telluric sources) which may limit the useful depth of penetration. 2) The Schlumberger array has a greater depth of penetration than the Wenner. 3) In general VES method with Schlumberger array assumes considerable importance in the field of groundwater exploration because of its ease of operation (only the current electrodes need to be frequently moved), relatively low cost and its capacity to distinguish between saturated and unsaturated layers (Hadi, 2009). Schlumberger array In Schlumberger symmetrical array the current and potential pairs of electrodes have a common mid- point. All the four electrodes are arranged collinearly, the current electrodes are usually much further apart than the potential electrodes, (Fig.3). The smallest current - potential electrode distance is always much greater than the distance between the two potential electrodes (Telford et al., 1990). In depth probing the potential electrodes are fixed while the current electrode separation is increased symmetrically about the centre of the spread. However, when expansion of the current electrodes causes the potential difference to become so small that it cannot be measured precisely, the potential electrodes are moved further apart, while keeping the current electrodes fixed, and then further readings are taken by expanding the current electrodes using the new potential electrodes positions. The apparent resistivity is plotted against electrode spacing on log- log scale to obtain a sounding curve. Figure 3: Schematic diagram of Schlumberger Array Vertical Electrical Sounding (Ves) Vertical Electrical Sounding (VES) is a geoelectric common method that measures vertical variation of electrical resistivity (Kelly, 1993). It is well known that resistivity methods can be successfully employed for groundwater investigations where a good electrical resistivity contrast exists between the water – bearing formation and the 91 Ozean Journal of Applied Sciences 7(3), 2014 underlying rocks. Vertical Electrical Sounding (VES) also called depth sounding or sometimes electrical drilling is used when the subsurface approximates to a series of horizontal layers each with uniform but different resistivity. The goal is to observe the variation of resistivity with depth. Schlumberger configuration is most commonly used for VES. The mid-point of the array is kept fixed while the distance between the current electrodes is progressively increased. This causes the current lines to penetrate to ever greater depth depending on the vertical distribution of conductivity. A typical subsurface current distribution is illustrated in Fig. 4. Figure 4: Schematic illustration of Basic Concept of Electrical Resistivity Measurement METHODOLOGY Instrumentation In this survey the instruments used for data collection are; Terrameter SAS 300 system and its components, magnetic compass, field hammer, cutlass, ranging pole, pegs, electrodes, cables and reels, measuring tape, global positioning system (GPS), and other accessories. The Terrameter SAS 300 which stands for (Signal Averaging System) that is consecutive readings are taken automatically and results are averaged continuously and presented automatically on the display. Field Procedures and Data Collection Vertical Electrical Sounding (VES) using D.C resistivity method was carried out in the study area. The data were acquired using Schlumberger array. Profiles of 400m length that are not necessarily parallel to each other were established at different locations to cover the whole area under study as shown in Fig. 5. 92 Ozean Journal of Applied Sciences 7(3), 2014 Figure 5: Location map of the study area showing positions of profiles Measurements were taken along the profiles following the technique outlined by (Telford et al., 1990). A station interval of 100m was used to establish various sounding points. Measurements were taken at each VES point by expanding the current electrodes symmetrically about the centre of the spread. The maximum exploration depth (also known as depth of penetration) of the Schlumberger array is 1/4 to 1/3 of the maximum distance of AB (Frohlich et al., 1996). That is, for AB = 200m the depth probed is about 50m to 60m. This depth is considered good for subsurface structural investigation and other parameters of interest in the study area. Data Processing The data were reduced, and the computed apparent resistivity values were then plotted against their corresponding AB/2 values on log-log graph paper using computer software (IPi2win). A typical example of such plots is shown for VES station IDC 20 in Fig. 6. The computer software used (Ipi2Win) is designed for Vertical Sounding and/or induced polarization data 1D interpretation. Targeting the geological result is the specific feature distinguishing Ipi2win from other popular programs of automatic inversion. Special attention is paid to user- friendly interactive interpretation thereby providing low fitting error. 93 Ozean Journal of Applied Sciences 7(3), 2014 Figure 6: A Typical resistivity curve and interpreted model for VES station IDC 20 Basis of Interpretation Available geological controls such as borehole data, resistivity values of earth materials compiled from previous works were used in order to have a meaningful interpretation. Table1shows the resistivity values of rocks materials compiled from previous work, while table 2 shows resistivity values adopted for this present study. Assumption has been made in the interpretation of Vertical Electrical Sounding (VES) data that: (a) the various geoelectric layers encountered are electrically homogenous and isotropic. However, because of the existence of lateral variation in resistivity within a layer, possibility of error in interpretation is present. (b) the principle of equivalence that is non- unique solution for a number of types of a typical three layer curve which are only distinguished by the resistivity value of their second layer in a relation to the value of either the first or third layer. Figure 7: Borehole log data drilled near the study area. (After hydro drilling and engineering ltd Zaria) 94 Ozean Journal of Applied Sciences 7(3), 2014 RESULTS The data analysis for the Vertical Electrical Sounding (VES) was performed using computer software (Ipi2win) as stated above. Different geophysical and geologic work carried out in Zaria area were considered in order to arrive at the resistivity values used for the interpretation of this present work. The work of Hassan (1987), Shemang (1990) and Olugbenga (2009) were considered, these were compared with resistivity values given by Telford et al. (1990). Table 1 shows resistivity value from previous work. Table 1: Resistivity values of rock materials within the Basement area. (Shemang, 1990) Rock type Fadama laom Clay, Silt and sandy Weathered Laterite Fresh Laterite Weathered basement Fractured basement Fresh basement Resistivity Value (Ωm) 20-90 100-200 200-2700 850-3000 20-200 500-1000 > 1000 A comparative Analysis of the soils and rocks materials from north central area of the basement complex (Zaria, Kaduna, & kano) was carried out in order to arrive at the resistivity values adopted for this work, as presented in table 2 below Table 2: Resistivity values adopted for this present study. Rock type Fadama Loam Clay, Sandy silt, Sandy clay, Fresh Laterite Weathered Laterite Weathered basement Weathered basement (lateritic) Fractured basement Fresh basement Resistivity Value (Ohm-meter) 30-90 100-200 850-3000 30-300 20-300 300-1500 500-1000 >1000 The resistivity values given in table 2 were used for the interpretation of this present work. The resistivity models at the sounding points were used to produce geoelectric and geologic sections for the profiles. Example of such plots for profile EF is shown in Fig. 8. DISCUSSION The Geoelectric and Geologic Section along Profile Ef Figure 8 shows geologic section along profile EF. This profile suggests that the region is underlain by three layers. The superficial cover (first layer) has resistivity values ranging between 104 ohm-m and 303 ohm-m. The thickness varies from 1.2m to 2m, this layer probably is made up of sandy – clay and silt (lateritic) soil. The second layer has resistivity values between 119 ohm-m to 310 ohm-m with an average thickness of 3m. This 95 Ozean Journal of Applied Sciences 7(3), 2014 layer probably is the weathered basement that is mostly lateritic. The third later has resistivity value of 2072 ohm-m which is the crystalline basement rock. Figure 8: A geoelectric and geologic section for a profile EF Maps Produced from Interpreted Data In order to look at some subsurface structural trends in the study area, and to reveal the lithological sequence of the subsurface formation, specialized maps were produced from the interpreted resistivity data obtained for all the VES stations with the aid of computer software (surfer 7). These are the isoresistivity map, aquifer thickness map and depth to the basement (overburden thickness) map. The essence of these maps is to show the lateral variation of resistivity over a horizontal plane at certain depths. In other words these maps indicate the distribution of resistivity in the study area. Iso Resistivity Map of the Suface Layer (Top Soil) The map has been produced in order to see the major composition of the top soil and to know if there is water saturation in the superficial cover. The map was produced by contouring the resistivity of the surface layer obtained at all VES points within the study area. Fig. 9 96 Ozean Journal of Applied Sciences 7(3), 2014 Resistivity value (ohm-m) 11.158 idc 27 idc 26 latitude 11.156 11.154 idc 25 idcidc 2322 idc 21 11.152 idc 20 idc 17 idc 16 idc 19 idc 15 idc 12 idc 18 idc 11 idc 08 idc 14 idc 04 idc 10 idc 07 idc 13 idc 03 idc 09 idc 06 idc 05 7.662 7.664 7.666 idc 02 7.668 longitude 0 0.002 0.004 0.006 N 460 440 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0.008 Figure 9: Iso- resistivity map of the surface layer Aquifer Thickness Map The Aquifer thickness map was produced by subtracting the thickness of the first layer from the total depth to the basement. The thickest part of the aquifer was found to be region around VES points IDC 04, 16 and26 with thickness of 22.41m, 19.51m and 15.36m respectively. These points were recommended for drilling of borehole. Fig. 10 shows the aquifer thickness map. 97 Ozean Journal of Applied Sciences 7(3), 2014 N Thickness(m) 21 20 11.158 19 18 idc 26 17 11.156 16 idc 25 15 idc 22 idc 23 idc 20 idc 17 idc 16 idc 19 idc 21 11.154 idc 15 idc 18 11.152 13 idc 12 11 idc 04 idc 07 idc 10 idc 06 idc 09 12 idc 08 idc 11 idc 14 idc 13 14 idc 03 10 9 8 idc 05 idc 02 7 6 7.662 7.664 7.666 7.668 5 4 3 0 0.002 0.004 0.006 0.008 Figure 10: Aquifer thickness map Depth to The Basement Map (Overburden Thickness Map) The overburden thickness map was produced from the interpreted depth to the basement at each sounding point (Fig. 11). This map was prepared to view the geometry or topography of the basement under the study area in order to enable a general view of the variation in the overburden thickness. The map suggests that the depth to the basement in the study area range from about 4.72m to 21.3m around VES IDC 25 and 16 respectively. The deepest part is the region already suggested for groundwater exploitation in this work. 98 Ozean Journal of Applied Sciences 7(3), 2014 N Depth (m) 21 11.158 idc 27 19 idc 26 11.156 17 latitude idc 25 idcidc 23 22 idc 21 11.154 11.152 idc 09 idc 06 idc 05 7.664 7.666 13 idc 04 idc 07 idc 10 idc 13 7.662 15 idc 20 idc 17 idc 16 idc 19 idc 15 idc 12 idc 18 idc 11 idc 08 idc 14 idc 03 11 9 idc 02 7.668 7 longitude 5 0 0.002 0.004 0.006 0.008 Figure 11: Depth to the basement (overburden thickness) map. Fresh Basement Rock Map The resistivity values of the fresh basement obtained in the course of the interpretation were used to plot the iso resistivity map of the fresh basement rock underlying the study area (Fig. 12). This map was produced by contouring the resistivity values af the fresh basement. The map was produced to veiw the distribution of fractured and unfractured rocks underlying the weathered basement. The map shows that the north- western part of the study area is underlain by unfractured granitic rock mostly at shallow depth. 99 Ozean Journal of Applied Sciences 7(3), 2014 Resistivity(ohm-m) N 8500 8000 11.158 idc 27 7500 7000 idc 26 6500 latitude 11.156 6000 idc 25 idcidc 2322 idc 21 11.154 11.152 5500 idc 20 idc 17 idc 16 idc 19 idc 15 idc 12 idc 18 idc 11 idc 08 idc 14 idc 04 idc 10 idc 07 idc 13 idc 03 idc 09 idc 06 idc 05 7.662 7.664 7.666 5000 4500 4000 3500 3000 2500 idc 02 2000 7.668 1500 1000 longitude 500 0 0.002 0.004 0.006 0.008 Figure 12: Iso resistivity map of the fresh basement DISCUSSION & CONCLUSIONS The geoelectric and geologic sections for the profiles suggest that the study area is underlain by three to four layers of different lithological compositions; namely the superficial cover consisting of clayey sand, silt, weathered laterite with an average thickness of 2m. The second layer which mainly composed of fresh laterite / weathered basement has varying thicknesses ranging from 1.86m to 22.4m, the thickness of the fractured basement ranges between 4.0m to 19.0m, depth to the basement ranges between 4.72m to 21.3m, and the fourth layer is the fresh basement rock with an infinite thickness. The investigation shows that the area is underlain by competent material, however, the superficial cover is loose and humus which contained clayey sand in some places thus need to be totally excavated in any structural and engineering works in the study area. The fresh basement rock map (fig. 12) indicates that the north - western part of the study area could be used for refuse dump because of unfractured / unfissured nature of granitic rock underlying the area at shallow depth. The laterite in the first layer as shown in some locations is of great importance as it reduces surface run off and aids infiltration into the underlying aquifer. The thickness of the weathered basement in the eastern half of the study area is large enough to harbour substantial quantity of water; therefore, this area could be suitable for hand dug wells or boreholes. The drainage pattern in the study area corresponds with the basement topography (towards the depressed area) this result in the effective recharge of the aquifer in the areas recommended in this study. 100 Ozean Journal of Applied Sciences 7(3), 2014 RECOMMENDATION The study area lack detailed record of any geophysical investigation and no borehole log is available; as a result of this conclusion derived cannot be compared with any previous results. It is recommended therefore; that further geophysical and geological investigations employing other methods should be carried out in the study area so as to have detailed information about the subsurface. ACKNOWLEDMENT The authors acknowledge the permission given by the authority of Industrial Development Centre, (IDC) Zaria, to carry out this research work on IDC and for the assistance rendered during the fieldwork. The authors equally thank Physics Department of Ahmadu bello university, Zaria for providing the equipment for the study. The contribution of Late Mr. B. Nwosu of Physics department, Ahmadu Bello University Zaria, during Data acquisition is also acknowledged. REFERENCES Akpoborie, I.A. (1973). Hydrological Control of Structures along Samaru Creek. Unpublished B.Sc. Thesis, Geology Department, Ahmadu Bello University, Zaria, Nigeria Du Preez, J.W. (1952). The Regional Rechnical College Site Zaria Water Supply. Unpublished Report, Geological Survey of Nigeria, No. 1005. Frohlich. R.K., Fisher, J.J. and Summerly, E. (1996). Electric- hydraulic conductivity correlation in fractured crystalline bedrock: central landfill, Rhode Island, USA. 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