Geophysical Study of Limestone Attributes At Abudu Area of Edo State, Nigeria

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1 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (5): Scholarlink Research Institute Journals, 2011 (ISSN: ) jeteas.scholarlinkresearch.org Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (5): (ISSN: ) Geophysical Study of Limestone Attributes At Abudu Area of Edo State, Nigeria 1 F.O. Ezomo and 2 C.N. Akujieze 1 Department of Physics, Faculty of Physical Sciences, University of Benin Ugbowo, Benin City, Edo State, Nigeria 2 Department of Geology, Faculty of Physical Sciences, University of Benin, Ugbowo, Benin City, Edo State, Nigeria. Corresponding Author: F.O. Ezomo Abstract The need to prospect for limestone geophysically became inevitable in view of the fact that limestone rock is a vital building stone normally employ for the manufacture of cement and concrete which is of economic importance to Nigeria in particular and the world in general. The investigated limestone attributes in this study therefore include its depth, thickness below sea level or earth s surface. Geophysical study of limestone attributes was investigated by electrical resistivity survey method in Abudu Area of Edo State, Nigeria. This investigation actually entailed carrying out electrical resistivity survey of vertical electrical sounding (VES) employing Schlumberger array. Ten (10) VES, fairly distributed in eight different stations of Abudu area were carried out. The software IP12WIN utilizing computer iteration was used for interpretation of apparent resistivity data. The result of the electrical resistivity survey showed that limestone was intercepted at a depth varying from about 10.0m to 60.0m while the thicknesses varied from about 5.0m to 50.0m. The resistivity of the limestone formations varied from about 30,000 to 400,000 ohm-m. Area of probable limestone formations and their thicknesses have been identified especially for future mining of industries foundation, operations and drilling. Keywords: limestone, geophysical, electrical resistivity, Abudu, Nigeria INTRODUCTION Geophysical exploration is usually conducted to locate significant accumulations of limestone deposit, clay deposit, oil, natural gas and other minerals, including ground water which are of economic importance to Nigeria as a nation in particular and the world in general (Ezomo and Ifedili, 2007). The use of limestone products for construction and building purposes has greatly aided many developed and developing nations of the world e.g. Nigeria, China, Spain, Brazil etc. This however made the researcher as a Nigerian to prospect for limestone (Ezomo and Akujieze, Limestone is a carbonated sedimentary rock composed mainly of calcite (CaCO 3 ) usually precipitated either by the action of organisms or directly as the result of organic processes. The great majority of limestone is biochemical limestone usually formed from wave-broken fragments of algae, corals and shells (William, 1997). The fragments may be of any size (gravel, sand, silt and clay). The action of these waves and subsequent cementations of these fragments into rocks will give the limestone relatively coarse grained with recognizable fossils or uniformly fine-grained and dense from the acclamation of microscope fragments of corraline algae (Koeford, 1979). 795 Limestone is particularly susceptible to recrystallization, the process by which new crystals, often of the same composition as the original grains developed in a rock (Clinton and Foster, 1995). This research paper tends to estimate the thickness of limestone deposit and its depth below sea level in Abudu Area using geophysical survey method with the intention of providing detailed documentation of known limestone deposits and recommend possible set up of an industry in Abudu that rely on limestone for manufacture purposes. The research area was Abudu and is underlained by Benin formation (Ezomo, 2010) which contribute to the Niger Delta basin having limestone as one of its basic lithologies (Information Centre, 2011). Abudu lies approximately on latitude of about N and longitude of about E. Abudu is hilly and has little natural topographical variations. The terrain is generally low lying with gentle slope (Information Centre, 2011). EXPERIMENTAL WORK Schlumberger electrode configuration of vertical electrical sounding (VES) was employed for this research, full detail of the method have been documented (Ezomo, ).

2 Ten (10) VES, fairly distributed were conducted using the ABEM signal averaging system (SAS) 300 terrameter and its 2000 booster for deeper penetration. In electrical resistivity sounding, four electrodes are earthed along a straight line in the other AMNB, where A and B are the current electrodes, M and N, the potential electrodes. The calculated apparent Measurements were taken at increasing current electrodes distance such that the electric current passed into the earth s surface penetrates greater depths. The greatest current electrodes separation (AB) was 632m in a six (06) points per decade operation. The operational efficiency of six points per decade in subsurface geophysical study have been documented (Ezomo, Ifedili and Akujieze, ). THEORETICAL ANALYSIS There are different types of electrical resistivity theoretical approach based on electrodes array for interpreting resistivity data. The techniques of data interpretation used involved seeking a solution to the inverse problem namely the determination of subsurface apparent resistivity distribution from surface measurement (Ezomo and Ifedili, ). There is a function called Kernel function which represents a very good solution to the inverse problem. It describes the apparent resistivity measurements in terms of subsurface lithological variation with depths. The function assumes the earth to be locally horizontally stratified, inhomogenous and isotropic layers, and unlike apparent resistivity function, it does not depend on electrode configuration. It cannot be measured in the field but has to be obtained from the transformation of measured apparent resistivities. The kernel function utilized in this work is derived after (Ezomo and Ifedili, ), if the observed apparent resistivity is such that 2 r r T J r d Where the Kernel function is given as T J r d r 0 1 J 1 represent Bessel function of first order, first kind and T() is the transformed restivity data (Ezomo and Ifedili, ). However, when the earth is approximately composed of horizontally stratified isotropic, and homogenous media such that the change of resistivity is a function of depth, the Schlumberger configuration is the most widely used array and may provide useful information in solving subsurface geophysical problems. A vital aspect of the Schlumberger is the less sensitivity of the array to the effect of near surface lateral heterogeneities and easy recognition of their effects (Ezomo and Ifedili, ). 796 resistivty ( ) according to Schlumberger array condition of AB 5 MN is 2 2 AB MN 2 2 V... MN AB = Current electrodes spacing in metre MN = Potential electrodes spacing in metre V = Potential differences in volts, I = electric current in Amperes, = 22 / 7. RESULTS AND DISCUSSION After the collection of data from the resistivity field survey operation in the VES stations, the apparent resistivity values ( ) are plotted against half the current electrode spacing ( AB / 2 ) with both axis on a log scale. The surrounding curves generated shown in figures 1 8 were interpreted using computer software IP12WIN to provide a model for the layer thicknesses and resistivities shown in typical table 1 for all VES stations of the various lithologies or rock types. The lithologies of the different layers were then confirmed by using nearby borehole/drillers log of the Abudu area (Abudu Bore-hole, 2011). The method of obtaining various lithologies of all the VES stations have been documented in previous research work (Ezomo, Ifedili and Akujieze, ). In VES station one (1), dense limestone / hard rock was not intercepted. This is however due to the non uniformity in stratigraphy. In VES station two (2), dense limestome / hard rock was intercepted at a depth of about 31.00m below sea level, with a thickness of about 15.20m. In VES station three (3), dense limestone / hard rock was intercepted at a depth of about 52.40m below sea level, with a thickness of about 40.60m. In VES station four (4), dense limestone / hard rock was intercepted at an infinite depth below sea level, with a thickness of about infinity. In VES station five (5), dense limestone / hard rock was intercepted at a depth greater than 84.79m below sea level, with a minimum thickness of about 50.90m. In VES station six (6), dense limestone / hard rock was intercepted at a depth greater than 167.3m below sea level, with a minimum thickness of about m. In VES station seven (7), dense limestone / hard rock was intercepted at a depth of about 11.60m below sea

3 level, with a thickness of about 5.22m. Finally, in VES station eight (8), dense limestone / hard rock was intercepted at a depth greater than 68.10m below sea level, with a minimum thickness of about 27.70m. Close examination of all the VES stations lithologies revealed that limestone / hard rock occurred in all the stations of the study area except VES station one (1). The thickest limestone / hard rock formation was observed in VES station three (3) with a thickness of about 40.60m intercepted at a depth of about 52.50m below sea level while the thinnest limestone / hard rock formation was seen in VES station seven (7) with a thickness of about 5.22m, intercepted of a depth of about 11.60m below sea level. CONCLUSION Geophysical survey based on electrical resistivity method has proved successful in achieving reliable results arising from delineation of limestone / hard rock in Abudu area of Edo State, Nigeria because of its existence in all the VES stations. The depths below sea level of the detected limestone / hard rock varied from about 10.00m to 60.00m with thicknesses varying from about 5.00m to 50.00m. The resistivity of the limestone / hard rock formation varied from about 30,000 ohm-m to 400,000 ohm-m. Abudu is therefore rich in limestone / hard rock and hence industry that uses limestone as its major raw material for construction and building purposes should be sited in Abudu, to create job opportunity for the unemployed citizen. This will therefore improve the economy of Edo State in particular and Nigeria in general. Fig. 1: Interpretation of VES (location 1): Fig. 2: Interpretation of VES (location 2): Fig 3 : Interpretation of VES (location 3): 797

4 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (5): (ISSN: ) Fig. 4: Interpretation of VES (location 4): Fig. 5: Interpretation of VES (location 5) Fig. 6: Interpretation of VES (location 6) Fig. 7: Interpretation of VES (location 7) Fig. 8: Interpretation of VES (location 8) 798

5 Typical Table 1: Lithologies of all VES stations VES Layer Resistivity (m) Lithology Thickness Depth (m) (m) Laterite Laterite Very fine sand Clayey sand Dry sand/gravel Laterite Gravel Wet sand Hard rock/dense limestone Very fined sand Infinity Infinity Clayey sand Laterite Hard rock/dense limestone Gravel Infinity Infinity Laterite Medium sand Medium sand/gravel Hard rock/dense limestone Infinity Infinity Fine sand Laterite Gravel Clayey sand Hard rock/dense limestone Laterite Hard rock/dense limestone Medium sand/gravel Hard rock/dense limestone Dry sand Medium sand / gravel Hard rock /dense limestone Laterite Laterite Coarse dry sand Laterite Hard rock/dense limestone Wet silty sand Hard rock/dense limestone REFERENCES Abudu Borehole / Driller s Log (2011). Courtesy of Edo Water Board, Sapele Road, Benin City, Nigeria. Clinton, P.J. and Foster, S.S.D. (1995). Evaluation of Groundwater using Schlumberger array. Hydrogeological Journal, 3: Ezomo, F.O. (2010). Geophysical investigation of groundwater in Agbor Area of Delta State, Nigeria. Journal of the Nig. Association of Maths Physics, 16(1), Ezomo, F.O. (2010). Geophysical survey as a useful instrument for determining subsurface lithology in Igarra, Edo State, Nigeria. Journal of the Nig. Association of Maths. Physics 17(1), Ezomo, F.O. (2010). Schlumberger Array as a useful instrument for determining water bearing formations in Agbor and its environs in Nigeria. African Journal of Science 11(1), Ezomo, F.O. and Akujieze, C.N. (2009). Geophysical Determination of buried structural features in Ovbiogie Village, Edo State, Nigeria. Journal of the Nig. Association of Math. Physics 14(1), Ezomo, F.O. and Ifedili, S.O. (2005). Drilling as a useful tool for water bearing formations investigation in Uhiele, Ekpoma. Journal of Applied Science. Vol. 9, No. 3, Ezomo, F.O. and Ifedili, S.O. (2008). Schlumberger Array of vertical electrical sounding (VES) as a useful tool for determining water bearing formation in Iruekpen, Edo State, Nigeria. African Journal of Sciences 9: Ezomo, F.O. and Ifedility, S.O. (2007). Vertical electrical sounding as a useful instrument for investigating aquifer in Eguare-Egoro, Edo State. Journal of the Nigeria Association of Math. Physics 11(1),

6 Ezomo, F.O., Akujieze, C.N. (2010). Geophysical study of the Aquifer attributes at Ika North Area of Delta State, Nigeria. Journal of Science, Engineering and Technology 17(3), Information Centre (2011). Palm House, Sapele Road, Benin City, Nigeria Introducing Abudu town. Koeford, O. (1979). Geosounding Principles, Resistivity Sounding Measurements. Elsevier Science Publishing Co. Amsterdam, Oxford: New York, 276P. Okwueze, E.E. and Akpotu, N. (1979). Standard table of approximate ranges for electrical resistivities of rocks and soil. Calabar, Nigeria. William Lowrie (1997). Fundamental of Geophysics, Cambridge University Press, Zondy, A.A.R. Eaton, G.P. and Mabey D.R. (1974). Application of surface geophysics to ground water investigation. Tech water resources investigation Washington U.S.A. Geo / Survey. 800