GEOELECTRICAL INVESTIGATION OF AN EXISTING DAM WITHIN A BASEMENT COMPLEX TERRAIN, SOUTHWESTERN NIGERIA

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1 GEOELECTRICAL INVESTIGATION OF AN EXISTING DAM WITHIN A BASEMENT COMPLEX TERRAIN, SOUTHWESTERN NIGERIA 1 OLADUNJOYE H.T., 2 OLASUNKANMI N.K. and 3 OLATUNJI S. 1 OLABISI ONABANJO UNIVERSITY, AGO-IWOYE, NIGERIA, oladunjoye.hamid@gmail.com 2 LADOKE AKINTOLA UNIVERSITY OF TECHNOLOGY, OGBOMOSHO, NIGERIA, kolexytocome@yahoo.com 3 UNIVERSITY OF ILORIN, ILORIN, NIGERIA, sam61ng@yahoo.com ABSTRACT An existing dam site within the main campus of University of Ilorin, located around the southern flank of the Nigerian basement complex in West central part of Nigeria was geoelectrically investigated using Vertical Electrical Sounding (VES) technique of Schlumberger and Horizontal Resistivity Profiling (HRP) of Wenner Array electrode configuration methods. The objectives are to investigate the vertical contact, lateral changes in the geologic setting, the fracture pattern to evaluate possible dam seepage along the dam axis or the river banks. Twenty seven (27) VES stations on eight profiles with maximum electrode spacing (AB) of 100 m and five (5) HRP of 120 m were established along the flank of the dam. The lithology units delineated are the low resistivity layer (top soil), gravelly zone (mostly saturated) and weathered/fresh basement with resistivity range of about Ωm and thickness range of m, Ωm with thickness range of m and Ωm at depth of 10 m downward respectively. Vertical Electrical Sounding (VES) revealed three major lithological units delineated as the topsoil, weak/gravelly zone and the fracture/fresh basement. The geoelectric sections showed low resistivity regions which might be due to percolation of water beneath 4 stations of which two (i.e. VES 1 and 10) are along the dam reservoir and the two starts at the depth of 10 m, extending downward. The geoelectric maps show high resistivity range of about Ωm and thickness range of m. This is underlain by relatively low resistivity layer whose values loiters around 100 Ωm with thickness range of m, (remain values of fresh basement with its depth). This indicates near surface bedrock straddled with basement depression or fracture. Though the weathered basement shows no surface manifestation, but the significant water reduction can be experienced which is considered as responsible for low recharge as the season changes. Different types of vertical contacts were able to delineate with the aid of HRP. The pseudosections also revealed the depth to unexposed granitic intrusion that appeared in form of dyke in some profiles and various vertical contacts were identified in the areas. KEY WORDS: Dyke, HRP, VES, Seepage, Geometric factor, Bedrock Introduction Dams are vital elements which stores water for purposes such as human consumption, food production through irrigation, electricity production, industrial use, recreation, flood protection etc. It is a barrier of concrete or earth that is built across a river or stream to obstruct or control the flow of water, especially in order to create a reservoir. Dam failure is usually fatal which involves destruction of lives, properties and the ecosystem 1. Hence, it is important to note that once a dam is completed, it represents a potential risk in the future, because of several factors acting on the structures 2. History has shown that flood disaster is not a recent phenomenon, and that its destructive tendencies are sometimes enormous. For instance, in Nigeria, report has it that serious flood disasters have occurred in Ibadan (1985, 1987 and 1990), Oshogbo (1992, 1996, and 2002), Yobe (2000), Akure (1996, 2000, 2002, 2004 and 2006) and the coastal cities of Lagos, Ogun, Port Harcourt, Calabar, Uyo, and Warri 3, 4. University of Ilorin Dam is the studied dam which is located within basement complex of Nigeria. (Figure-1) 5. It lies entirely within the basement rocks in the Western part of Central Nigeria bounded by longitudes 4 39'52" to 4 40'0"E and latitudes 8 27'5" to 8 28'5"N. The study site is easily accessible through a moderately wide motorable road that eased the fieldwork (Figure-2). 43

2 The topography of the area is relatively plain with minor undulations (Figure). The city is drained mainly by Asa River and its tributary river such as Aluko, Alalubosa, Okun, Osere, Agba and Oyun River which forms the drainage basin. The main river within the campus is river Oyun which is dendritic in nature flows from South-East to North- West. Despite series of geotechnical studies preceding the construction of dams, there are still number of problems that dams are prone to. Such problems can be caused by existence of concealed fracture/faults, fissures, joints or shears which can greatly reduce the reservoir capacity of the dam. Thus, dams require ongoing maintenance, monitoring, safety inspections, and sometimes even rehabilitation to continue safe service. The objectives of this work are to investigate the vertical contact, lateral changes in the geologic setting, the fracture pattern and possible dam seepage along the dam axis and its banks for safety of the dam. different dam breach parameter estimators that are normally used to describe the physical characteristics of a dam can be compared in the study of behavior of dam. These parameters comparison must go along with their computation if they are within the specific model. the empirical and embankments erosion process models had been used as case study for this comparison 6. integrated geophysical methods to test a concrete dam, in which Seismic geoelectrical, and GPR methods were used. The geoelectrical method was used to detect a possible water saturation of the area which might have effect of degradation of the mechanical parameters of the dam body. Its geoelectrical study makes use of dipole-dipole array at 2 m electrode spacing 7. Vertical Electrical Sounding (VES) was used to reveal a network of lineaments pressured to be fractured in the geologic section of a dam situated in a basement complex 8. Momoh et al used geoelctrical investigation of a dam site in Maro area of central basement terrain of Nigeria using about 64 VES of Schlumberger array. The interpretation revealed 4 geoelectric layers viz: top soil, laterite, weathered layer, resistive bedrock. They were able to suggest the dam should be along investigated transverse without reservoir. This study makes use of the combination of Profiling and Sounding method in studying the the static and dynamic properties of this dam. Likewise the lithological arrangement in conjunction with seepage characteristics was observed from modeling and its interpretation 9. This study was embarked upon to integrate sounding and profiling along the dam axis and the river bank. Schlumberger array using vertical electrical sounding was employed to compliment and probe the profiling information acquired through Wenner array of the profiling. The methods was able to give high resolutions in the profiling and the sounding technique. This is because a lot of vertical contacts were observed from the profiling technique due to the configuration employed. Figure-1: Geologic Map of Nigeria (After Oyawoye 1964) 44

3 4 39' 4 40' WORKS (ADMIN) PG HOTEL 8 29' 8 29' MAIN CAMPUS GATE SUG BUILDING SCHOOL CLINIC SENATE BUILDING SUGAR RESERCH INSTITUTE FACULTY OF ENGINEERING UNILORIN CENTRAL MOSQUE GT BANK SKY BANK STAFF QUARTERS UNJLORIN PRY SCHOOL RECEPTION GROUND 8 28' UNILORIN DAM 8 28' UNILORIN SECONDARY SCHOOL 4 39' 4 40' Figure-2: Road Map of University of Ilorin Showing the Study Area (Olasunkanmi et al) MATERIALS AND METHODS Electrical resistivity method of geophysical survey using Vertical Electrical Sounding (VES) technique of Schlumberger and Horizontal Resistivity Profiling (HRP) of Wenner array electrode configurations was used to achieve the stated objectives of this work. Twenty seven VES stations on eight profiles with maximum electrode spacing (AB) of 100 m and five HRP of 120 m were established along the West-East flank of the dam. HRP involves the process where current and potential electrodes are maintained at a fixed separation and progressively moved along the traverse after each measurement. VES involves the process where the current and potential electrodes are maintained at same relative spacing and the whole spread is progressively expanded along the profile. Theory: The fundamental equations are derivable from Ohm s laws. The electric potential V r at any point P distance r from a point electrode emitting an electric current I in an infinite homogenous and 10, 11 isotropic medium of Resistivity ρ is given by For a semi-finite medium, this is the simplest Earth model, and with both current and potential pointelectrodes placed at the Earth s surface. ( ) Irrespective of surface location and electrode spread, the resistivity is constant in a homogenous and isotropic ground. However, it does vary with the relative positions of electrodes when there is presence of subsurface inhomogeneities and any computed value is known as apparent resistivity ρ a. ( ) For a semi-finite medium, which is the simplest earth model, and with both current and potential point-electrodes placed at the earth s surface, 45

4 Figure-3: The Generalized Form of the Electrode Configuration Used in Resistivity Measurements. {( ) ( )} But, {( ) ( )} For the HRP; since, ( ) Thus, Where R is the measured resistance and configurations employed during the survey. is the Geometric Constant which is a function of the electrode RESULT AND DISCUSSION Field Curves: The result of the Horizontal Resistivity Profiling (HRP) and the Vertical Electrical Sounding (VES) conducted around the Unilorin dam is presented both in Linear Plots and 2-D Resistivity Structure along the traverses surveyed. The geologic equivalence models of lateral and depth resistivity variations are juxtaposed with the linear plots along all the Horizontal Resistivity Profiles as shown in Figure- 4 below. The linear plots were obtained by plotting the profiling midpoints against their instantaneous apparent resistivity value. These plots were able to depict the weak zones as profiling progresses on the transverse. Field inspection gives/ helps in detailed understanding of these plots. From the plots, profiles beside the dam bank depicted low resistivity values compared to profiles along the river bank. This might be due to saturation of these profiles as a result of the pressure of the volume of the water in the dam bank Graph of Apparent Resistivity(ohm-m) against Midpoints(m) Apparent Resistivity(m) Midpoints(m) Laterite Wet Sand 43 Laterite Wet Sand

5 Figure-4: HRP Curves with Inferred Geologic Equivalence. Figure-5: Typified Resistivity Field Curves of Sampled VES stations The Vertical Electrical Multilayer Sounding field curves obtained after curve matching and computer iteration showed various types of curves which were determined by the relationship existing between the layer resistivity values ρ 1, ρ 2, ρ 3... ρ n. It was discovered that the curves are mostly H-type (23 VES stations), while 2 VES stations showed A- type, 1 VES station showed HA-type and 1 VES station showed HKH-type. Summary of the formation layer thickness, classification of the resistivity sounding curves and samples of the curves are presented on table-1 below TABLE-1: Depth Sounding Interpretation Result Location Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 h 1 (m) ρ 1 (Ωm) h 2 (m) ρ 2 (Ωm) h 3 (m) ρ 3 (Ωm) ρ 4 (Ωm) h 4 (m) ρ 5 (Ωm) h 5 (m) VES VES VES VES VES VES VES

6 VES VES VES VES VES VES VES VES VES VES VES VES VES VES VES VES VES VES VES VES TABLE-2: Classification of the Resistivity Sounding Curves. TYPE CURVES RESISTIVITY MODEL MODEL FREQUENCY LOCATION (VES) A ρ 1 < ρ ,23 H ρ 1 > ρ 2 < ρ ,2,3,4,5,6,7,8,9,11,12,14,15,16,17,18,19,21,22,24,25,26,27 HA ρ 1 > ρ 2 < ρ 3 < ρ HKH ρ 1 >ρ 2 <ρ 3 >ρ 4 <ρ TOTAL 27 The Topography of the area: The Global Positioning System (GPS) reading which gives the coordinates and elevation of an area was used to obtain the surface topographic map of the area. The map shows the undulation, rugged troughs and crests due to erosions which characterize the topography of the area. The flow net of the dammed river, the dam axis, the dam reserve and the overflow part of the dam is shown on the map (Figure-6). 48

7 Figure-6: Topographic Map of the Study Area Pseudosections: This is a phase diagram showing the fields of different equilibrium mineral assemblages for a single bulk-rock composition. Apparent resistivity pseudosections were obtained by contouring the apparent resistivity values, in ohm-meters, in the vertical and horizontal directions.the lithological arrangement predicted by the linear plots shows that area around the dam comprises of wet sand underlain by lateritic sand, saturated gravel, weathered basement and basement complex. In the same vein, the lithological arrangement of profiles beside the stream flank depicts combination of literate soil, gravelly layer, and basement complex. In the case of VES, the horizontal location of the point is placed at the mid-point of the set of electrodes used to make that measurement. The vertical location of the plotting point is placed at a distance which is proportional to the separation between the electrodes. 49

8 50

9 Figure-7: HRP Pseudosections Fig. 5: Vertical Psuodosections 51

10 52

11 Figure-8 VES Pseudosections Figure-9: Resistivity of Layered Model at 10m Depth The sections show apparent resistivity value variations and the characteristics at different depths. The resistivity variation from the horizontal profiling in the surface layer have the range of 28.8 Ωm to 2612 Ωm, which shows that the texture of the rock constituent is hard and dry due to high exposure to intense radiation from the sun. At a depth of about 4 m to 35 m, a relatively weak zone with apparent resistivity value ranging from 100 Ωm to 342 Ωm is shown in most profiles and a Dyke-like structure was observed at depth of about 10.0 m with resistivity ranges between 1515 Ωm Ωm as shown on the 2D structure for profile 1. It forms a laterally elongated structure in the form of void. The weak zone could be as a result of water percolation or as the tip of a weathered rock of the basement rock underlying the area. Conclusion 53 The results revealed the presence of near surface basement which are expected to serve as the sealing barrier for water retention in the reservoir. These basements show no fracture signature which might be a threat of the failure of the dam. Some vertical contacts which look like fractures were observed along the river bank, these features aligned and agreed with past work on River Oyun which suggested to have controlled fracture 13. The anomalous percolation zone/weak zone obtained at near-surface in the HRP resistivity structure showed the adverse effect of careless domestic activities around the study area. These activities if not properly checked or controlled might tamper with the sealing barrier which will be dangerous to the dam. It can be concluded that Wenner configuration with multiple spacings (a) ranging from 5m, 10m, 15m, 20m, 30m is good enough in identifying and precisely locating

12 geologic vertical contacts, weak zones, faults, and fractures. The fracture pattern of the study area is unevenly distributed. Also the depth to the basement in the study area is shallow. The low resistivity areas, weak zones, delineated by the geoelectric map present little or no risk of reservoir water seepage but may be the water table or the area is extensively marshy, capable of retaining water to the ground surface level. The suspected fractured basement observed beneath some sounding points along the dam bank, started at the depth of 10 m and proceeds downward as explained in figure-9. These can be considered inimical to the continued water retention or zone of anomalous seepage but no surface manifestation and significant water reduction experienced. The presence of fracture is generally accompanied by high fluid streaming potential and substantial water can be lost through the fractures thus initiating the weakening of the dam foundation REFERENCES 1. Akanmu J.O. Management of the Downstream Impacts of Dams Operation orbed Experience and Hydropower Dams as a Case Study, 22 nd ICOLD proceeding Volume II, Barcelona, Spain, pp (2006) 2. ICOLD World Register of Dams, Computer Database, Paris, International Commission on Large Dams (1998) 3. Daily Sketch. Kano to spend N17 million on drainage, 16th November, p.3. Ibadan (1988) 4. African Concord. Chaining the Flood Monster, Vol.2, No.24, pp Concord, Lagos. (1988) 5. Nigerian Compass.. Combating flood disasters in Nigeria 6 th September 2010 p.20 (2010) 6. Oyawoye M.O, The Basement Complex of Nigeria, Geological survey of Nigeria. Vol.1and 2, pp (1964) 7. Gee, D. Michael and Brunner, Gary W., Comparison of Breach Predictors Association of State Dam Safety Officials (ASDSO), Dam Safety 2007, Austin TX 9-13 September (2007) 8. Karasthathis V.K, Karmis P.N Drakatos V,.Stavrakakis G, geophysical methods contributing to the testing of concrete dams; application of Marathon Dam Journal of applied geophysics 50 pp (2002) 9. Oyeneye O., Oladapo I. and Folami S. Geoelectrical study of Dam Site of Federal College of Agriculture, South Western Nigeria, Journal of Medwell online Vol., 2, No. 10, PP ; (2007) 10. Grant, F.S.and West G.F., Interpretation Theory in Applied Geophysics, McGraw- Hill, New York (1965) 11. Dobrin, M.B. and C.H. Savit, Introduction to Geophysical Prospecting 4th Edition., McGraw Hill Book Co., New York (1988) 12. Olasehinde, P.I. Elucidating fracture patterns of the Nigerian Basement Complex, using electrical resistivity method. Z. Angew. Geowiss. Heff., 8, 5, pp (1989) 54