3D Electrical Resistivity Tomography (ERT) Survey of a Typical Basement Complex Terrain

Transcription

1 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (4): Scholarlink Research Institute Journals, 2011 (ISSN: ) jeteas.scholarlinkresearch.org Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (4): (ISSN: ) 3D Electrical Resistivity Tomography (ERT) Survey of a Typical Basement Complex Terrain B.S. Badmus, O.D. Akinyemi, J.A. Olowofela and G.M. Folarin Department of Physics, University of Agriculture, Abeokuta, Nigeria Corresponding Author: B.S. Badmus Abstract A 49-electrode system was used in the Electrical Resistivity Tomography (ERT) survey in three locations with each location marked into 7 by 7 square grids for the electrodes using 1.0m, 3.0m and 5.0m unit electrode spacings in succession. The pole-pole electrode array was used with the two remote electrodes placed at distances of 23m, 40m and 65m from the grids with 1.0m, 3.0m and 5.0m unit electrode spacing respectively to reduce their telluric effects on the apparent resistivity values measured. The data obtained were analysed and 3D models of the subsurface were generated using 3D inversion software called RES3DI V and the Slicer Dicer software. The result from this study was found to be similar to those from the studies for which multichannel 3D equipment were used. The results of the interpretation revealed the Lithology to compose lateritic soil, sand, sandstone, sandy clay and clay. The results also showed that the three different unit electrode spacing displayed similar geo-electric models and the electrode spacing determines the depths of investigation. Geological structures are 3-Dimensional (3D) in nature, thus 1-Dimensional and 2-Dimensional surveys cannot adequately model them. However, sophisticated 3D survey equipment is very expensive and not readily available. This prompted the use of a single-channel SAS 300B Terrameter to obtain a 3D model of the subsurface. Keywords: resistivity, tomography, 3D models, electrode spacing, geo-electric layer I TRODUCTIO Geological structures are three-dimensional (3D) in nature and so one-dimensional or twodimensional survey cannot be adequately accurate in modeling them. However sophisticated 3D survey equipments are very expensive. This study was carried out to show that a single-channel Terrameter can be used to obtain a 3D model of the subsurface. There are numbers of mineral resources beneath the earth surface. Detecting them and predicting their qualities and quantities is of importance to both government and individuals, like exploration geophysicists, hydro-geophysicists and the like. A number of methods have been used for this exploration purpose, such as magnetic method, gravity methods, seismic methods - seismic reflection/refraction, electrical and electromagnetic methods, induced polarization, etc. These methods can as well be used for environmental investigations, surveys and monitoring. Of interest in this work is the electrical resistivity method which makes use of measurements on ground surface to reveal subsurface resistivity distributions. This could be one-dimensional (1D), two-dimensional (2D) or three-dimensional (3D). Extensive work has been done using 1D resistivity surveying (Jones and Hockey 1964, Ajayi and Adegoke 1988; Okwueze and Ezeanyim 1991; Ojelabi et al. 2002). 2D imaging surveys have been carried out for purposes such as the detection of leakage of pollutants from landfill sites, areas with undulating limestone bedrock, mapping of the overburden thickness over bedrock (Ritz et al. 1999), leakage of water from dams, saline water intrusion in coastal aquifers, freshwater aquifers (Dahlin and Owen 1998), monitoring of groundwater tracers (Nyquist et al.1999) and mapping of unconsolidated sediments (Christensen and Sorensen 1994). The resistivity imaging method has also been used in underwater surveys in lakes and dams. Even areas of moderately complex geology have been mapped using the 2D surveying (Griffiths and Barker 1993). Since all geological structures are 3D in nature, 3D resistivity survey using a 3D interpretation model would be most appropriate to get the most accurate result (Loke 2004). 3D resistivity survey is expensive and takes time, especially when the area to be surveyed is large. However, the development of multi-channel resistivity survey instruments, faster microcomputers and inversion software now make it possible to take more than one reading at a time thereby reducing survey time, and carry out inversion of very large data sets in reasonable time (Griffiths et al.1990; Loke and Barker 1996b). Three array types are commonly used for 3D resistivity surveying; the pole-pole, pole-dipole and dipole-dipole. The reason for this is that other arrays have poorer data coverage near the edges of the survey grid (Loke 2000). 680

2 THE STUDY AREA This survey was carried out at three different locations within the University of Agriculture, Abeokuta (UNAAB), a basement complex terrain of southwestern Nigeria (figure 1.0). UNAAB is located within longitudes to East and latitudes to North respectively. It has coordinates of m North and m East (Ibikunle 1998; Akinsola 2004). Each location formed a grid as shown in figure 2.0, described by parameters like elevation above sea level, longitudes and latitudes as follows: Location 1 (Opposite Chief Olatunde Abudu Building): 152m, N, E; 153m, N, E; 153m, N, E; 152m, N, E. Location 2 (Behind Extension and Research Building): 151m, N, E; 146m, N, E; 150m, N, E; 149m, N, E. Location 3 (Behind College of atural Sciences): 145m, N, E; 147m, N, E; 145m, N, E; 144m, N, E. METHOD Data acquisition A 49-electrode system with 7 by 7 square grid arrangement of electrodes with 1m, 3m and 5m unit electrode spacing were used in succession at each location (figure 2.0). The pole-pole electrode array was used in this survey with the two remote electrodes placed at distances of 23m, 40m and 65m from the grids of 1m, 3m and 5m unit electrode spacing respectively. The cross-diagonal survey method was adopted for this research work (Loke and Barker, 1996b). To reduce survey time consumption, since a single-channel resistivity meter was used, the crossdiagonal survey method was adopted. The ABEM Terrameter SAS 300B was used for the Survey The data from the survey was inverted and interpreted using the RES3DINV software. RES3DINV ver. 2.16; for Windows 2000/NT/XP/Vista and 3D images were also generated using the Slicer Dicer software. RESULTS A D DISCUSSIO S The study revealed seven geo-electric layers covering total depths of 7.75m, 23.2m and 38.7m for unit electrode spacing of 1.0m, 3.0m and 5.0m respectively. It revealed resistivity ranges of about 7.3Ωm to 224Ωm, 13.8Ωm to 163Ωm and 3.9Ωm to 154Ωm for unit electrode spacings of 1.0m, 3.0m and 5.0m respectively at location 1; showing geo-electric model layers composed of lateritic soil, sand, sandstone, sandyclay and clay. It also revealed resistivity ranges of 0.53Ωm to 81.8Ωm and 0.30Ωm to 68.9Ωm for unit electrode spacing of 3.0m and 5.0m respectively at location 2 with model layers composing of sand, sandstone, sandy clay and clay. It revealed resistivity ranges of 6.1Ωm to about 545Ωm, 12.0Ωm to 513Ωm and 0.85Ωm to 1603Ωm for unit electrode spacings of 1.0m, 3.0m and 5.0m respectively at location 3 with geo-electric model layers composing lateritic soil, sand, sandstone, sandyclay and clay. For 1.0m Unit Electrode Spacing Location 1: From the horizontal sections (figure 3a); the first geo-electric layer has a thickness of 0.7m with resistivity value of 224Ωm. The second layer is between depth 0.7m and 1.5m and has resistivity value of 137Ωm. These layers are suspected to be lateritic soil, sand, sandstone, sandyclay and clay. The third layer has resistivity of Ωm between depths 1.5m and 2.43m. The fourth layer is between depths 2.43m and 3.5m, with resistivity of Ωm. These two geo-electric layers consist of sand, sandstone, sandyclay and clay. The resistivity of the fifth and sixth layers is between 7.3Ωm and 19.4Ωm between the depths of 3.5m and 4.72m and 4.72m and 6.13m. The seventh layer between the depths of 6.13m and 7.75m has resistivity values ranging from 7.3Ωm to 11.9Ωm. The fifth to the seventh go-electric layers revealed the layer to be clays. The vertical sections (figure 3b) show the vertical extents of the layers to a depth of 6.94m. Locations 2: From the horizontal sections (figure 4a); the first geo-electric layer has resistivity value of 545Ωm. The second layer has resistivity of 151Ωm. These layers are suspected to be lateritic soil, sand, sandstone, sandy clay and clay with the third layer having resistivity range of Ωm. The second and third layers have inhomogeneous resistivity distributions as depicted by the resistivity variations. The fourth to the seventh geo-electric layers have resistivity values of Ωm. The geo-electric layers appear to consist of lateritic soil, sand,sandstone, andy clayey and clay. The vertical sections (figure 4b) show the vertical extents of the layers to a depth of 6.94m. For 3.0m Unit Electrode Spacing Location 1: From the horizontal sections (figure 5a); the first geo-electric layer has a thickness of 2.10m has resistivity value of 114Ωm. The second layer is between depth 2.10m and 4.51m and has resistivity value of 163Ωm. The third layer has resistivity of Ωm between depth 4.51m and 7.29m. The second and third layers have highly inhomogeneous resistivity distributions. The fourth layer is between depth 7.29m and 10.5m, with resistivity values of Ωm. The resistivity of the fifth layer is 681

3 between 13.8Ωm and 56.5Ωm and its depth is between 10.5m and 14.2m. These geo-electric layers revealed the lithology to consist of sand, sandstone, sandyclay and clay. The sixth and seventh geoelectric layers revealed depths ranging from 14.2m to 18.4m and 18.4m to 23.2m with resistivity values ranging from 13.8Ωm to 39.7Ωm. The sixth and sevenths geo-electric layers are suspected to be clays. The vertical sections (figure 5b) show the vertical extents of the layers to a depth of 20.8m. Location 2: From the horizontal sections (figure 6a); the first geo-electric layer has resistivity value of 81.8Ωm. The second layer has resistivity values ranging from 19.4Ωm to 81.8Ωm. The third layer has resistivity of 39.8Ωm with the second and third layers having highly inhomogeneous resistivity distributions. These layers revealed the Lithology to consist of sand, sandstone, sandyclay and clay. The fourth geo-electric layer has resistivity value of Ωm with the resistivity of the fifth layer between 1.1Ωm and 4.6Ωm. The sixth and seventh layers have resistivity values of Ωm. This layer is suspected to consist of clay. The vertical sections (figure 6b) show the vertical extents of the layers to a depth of 20.8m. Location 3: From the horizontal sections (figure 7a); the first and second geo-electric layers have resistivity values ranging from 56.0Ωm to 513Ωm while the third layer has resistivity value of 6.1Ωm. The fourth and fifth geo-electric layers have resistivity values ranging from 2.9 to 513Ωm. The sixth and seventh layers have resistivity values ranging from 12.0Ωm to 245Ωm. These layers have inhomogeneous resistivity distributions with lithology consisting of lateritic soil, sand, sandstone, sandy clay and clay. The vertical sections (figure 7b) show the vertical extents of the layers to a depth of 20.8m. For 5.0m Unit Electrode Spacing Location 1: From the horizontal sections (figure 8a); the first geo-electric layer has thickness of 3.5m with resistivity value of 51.0Ωm. The second layer is between depths 3.5m and 7.53m and also has resistivity value of 154Ωm. The third geo-electric layer has resistivity value of 18.9Ωm between depths 7.53m and 12.2m. The first to third geo-electric layers have inhomogeneous resistivity distributions. The fourth layer is between depths 12.2m and 17.5m, with resistivity value of 91.1Ωm. These geo-electric layers revealed a formation consisting of lateritic soil, sand, andstone, sandyclay and clay. The resistivity of the fifth layer is 11.2Ωm and its depth is between 7.5m and 23.6m. The sixth layer is between the depths of 23.6m and 30.6m and it has resistivity values of Ωm. These two layers consist of sand, sandstone, sandy clay and clay while the seventh layer is between the depths of 30.6m and 38.7m has resistivity ranging from 3.9Ωm to 32Ωm. The vertical sections (figure 8b) show the vertical extents of the layers to a depth of 34.7m. Location 2: From the horizontal sections (figure 9a); the first and second geo-electric layers have resistivity values ranging from 14.6Ωm to 68.9Ωm. The third layer has resistivity value of 6.7Ωm. The fourth layer has resistivity values of Ωm and these geo-electric layers revealed formations of sand, sandstone and clay. The resistivity of the fifth layer ranges from 1.4Ωm to 14.6Ωm. The sixth layer has resistivity values between 0.65Ωm and 3.1Ωm. These two layers consist of clay. The seventh layer has resistivity value of 0.30Ωm. The vertical sections (figure 9b) show the vertical extents of the layers to a depth of 34.7m. Location 3: From the horizontal sections (figure 10a); the first three geo-electric layers have resistivity ranging from 7.3Ωm to 1603Ωm. These layers have highly inhomogeneous resistivity distributions and revealed a formation consisting of laterite, sandstone, sand, sandyclay and clay. The fourth layer has resistivity value of 63.2Ωm; revealing a composition of sand, sandstone and clay. The resistivity value of the fifth and sixth geoelectric layers are between 7.3Ωm and 1.5Ωm and these layers suspected to be clay formations. The seventh layer has resistivity value of 0.85Ωm indicating a clay formation. The vertical sections (figure 10b) show the vertical extents of the layers to a depth of 34.7m. It is a general observation that the three different unit lectrode spacings gave rise to similar geoelectric models and the wider the spacing, the deeper the depths of investigation. However the RMS errors were smallest in the inversion of the data from the 1.0m spacing and highest in the 5.0m spacing for each location. Figure 11.0 showed the Slicer Dicer images for all the units spacing. CO CLUSIO This study has shown that it is possible to carry out 3D ERT using a many-electrode system with singlechannel 1D equipment, invert and interpret the data using the RES3DINV software and still get the required result. The results obtained from this study showed reasonable similarity with the results from studies for which multi-channel 3D equipments were used. It has also shown that the pole-pole array is better used for small unit electrode spacing and the observed RMS errors in the inverted data increase with increase in the unit electrode spacing. Using a unit electrode spacing of 0.5m and a system of many electrodes will give a better result. REFERE CES Ajayi, O. and Adegoke, C.W. (1988): Groundwater prospects in the basement complex rock of southwestern Nigeria. Journal of African Earth Science, Vol. 7, No 1, pp

4 2 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (4): (ISSN: ) Akinsola, O. K. (2004): Forced eviction and uncertain future: The travails of the evacuees on UNAAB land. Dissertation submitted to the Department of Urban and Regional Planning, faculty of Social Sciences, University of Ibadan. Christensen, N.B. and Sorensen, K.I. (1994): Integrated use of electromagnetic methods for hydrogeological investigations. Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems, March 1994, Boston, Massachusetts, Dahlin, T. and Owen, R. (1998): Geophysical investigations of alluvial aquifers in Zimbabwe. Proceedings of the IV Meeting of the Environmental and Engineering Geophysical Society (European Section), Sept. 1998, Barcelona, Spain, Griffiths, D.H., Turnbull, J. and Olayinka A.I. (1990): Two-dimensional resistivity mapping with a computer-controlled array. First Break 8, Griffiths, D.H. and Barker, R.D. (1993): Twodimensional resistivity imaging and modeling in areas of complex geology. Journal of Applied Geophysics, 29, Ibikunle, O. K. (1998): Planning and development of tertiary institutions in Nigeria: A case study of University of Agriculture, Abeokuta, UNAAB. Professional Diploma in Town and Regional Planning of The Polytechnic, Ibadan, Nigeria. Jones, H.A. and Hockey, R.D. (1964): The Geology of the part of Southwestern Nigeria. Geological Survey Nigeria Bulletin, No 31, Pp Loke, M.H and Barker, R.D. (1996b): Practical techniques for 3D resistivity surveys and data inversion, Geophysical Prospecting, 44, Loke, M.H. (2000): Electrical imaging surveys for environmental and engineering studies. Loke, M.H. (2004): Tutorial: 2-D and 3-D electrical imaging surveys. Nyquist, J.E., Bradley, J.C. and Davis, R.K. (1999): DC resistivity monitoring of potassium permanganate injected to oxidize TCE in situ. Journal of Environmental & Engineering Geophysics, 4, Ojelabi, E.A., Badmus, B.S. and Salau, A.A. (2002): Comparative analysis of Wenner and Schlumberger Methods of Geoelectric Sounding in subsurface Delineation and groundwater exploration - A case study. Journal Geological Society of India, Vol. 60, Dec. 2002, Pp Okwueze, E.E. and V.I. Ezeanyim (1991): Geophysical exploration for fresh groundwater sources in a saline shale area. AJST Series B. Vol.5, No.2, July Ritz, M., Parisot, J.-C., iouf, S., Beauvais, A. and Dione, F. (1999): Electrical imaging of lateritic weathering mantles ver granitic and metamorphic basement of eastern Senegal, West Africa. Journal of Applied Geophysics, 41, APPE DIX 3 1 Figure 1: Data Acquisition Map 683

5 x-direction y-direction X X X X X X 1.0m unit electrode spacing - Potential electrode + - current electrode Figure 2: Arrangement of electrodes for the survey 684

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7 Slicer Dicer 3D models Fig. 3c Fig. 4c Fig. 5c Fig. 6c Opposite Chief Olatunde Abudu Building 1.0m unit electrode spacing Opposite Chief Olatunde Abudu Building 3.0m unit electrode spacing Opposite Chief Olatunde Abudu Building 5.0m unit electrode spacing Behind Research and Extension Building 3.0m unit electrode spacing Fig. 7c Fig. 8c Fig. 9c Fig. 10c Behind Research and Extension Building 5.0m unit electrode spacing Behind Gollege of Natural Sciences 1.0m unit electrode spacing Fig. 11: 3D images generated by using Slicer Dicer software Behind Gollege of Natural Sciences 3.0m unit electrode spacing Behind Gollege of Natural Sciences 5.0m unit electrode spacing 686