Application of Geoelectrical Resistivity Imaging to Investigate Groundwater Potential in Atan, Ogun State Southwestern Nigeria
CHAPTER ONE
Aim and Objectives
The aim of this research work is to determine the groundwater potential in Atan.
The objectives of the study are:
- To determine the availability of groundwater resources in Atan using geoelectrical resistivity method.
- To determine the depth of groundwater of the study area using vertical electrical sounding.
- To determine the geological structure of the study area.
CHAPTER TWO
LITERATURE REVIEW AND GEOLOGICAL SETTING.
Alile et al; (2008) carried out geophysical exploration involving the use of vertical electrical sounding in a sedimentary environment to determine the suitability of the method for underground water study. In their work, the Schlumberger electrode array configuration and the Schlumberger automatic analysis method of interpretation was adopted. Alile and Ujunabi, (2009) carried out a research work on the application of Dar Zarrouk parameters to evaluate aquifer transmissivity where Schlumberger array configuration in electrical resistivity survey was adopted. Alile and Amadasun, (2008), carried out direct current probing of the subsurface earth for water bearing layer in Oredo Local Government Area of Edo. They engaged electrical resistivity survey method. The result of the work ascertains the depth to the bearing formation. Badmus and Olatinsu, (2010), carried out research work on aquifer characteristics and groundwater recharge pattern in a typical basement complex: A case study of Federal College of Education, Osiele, Abeokuta, Southwestern Nigeria. In their work, they used vertical electrical sounding (VES) using Schlumberger electrode array. It was revealed that Abeokuta has seven major geological formations which are topsoil, shale or clay, sandy clay, clayey sand, sandstone, fractured basement and fresh basement. It was also discovered that the weathered and fractured basement constitutes the main aquiferous units in the area. They were able to discover that the reasons for borehole failure and poor recharge in the area is attributed to inadequate geophysical investigation, the depth at which drilling was terminated and the geological formation of the aquifers. They also engaged 3-D view to show the overburden thickness.
Aizebeokhai et al; (2010) engaged 2D and 3D geoelectrical resistivity imaging for an engineering site investigation. This was used in a crystalline basement terrain southwestern Nigeria. In their work, they make use of orthogonal set of 2D geoelectrical resistivity field data that consist of six parallel and five perpendicular profiles which they collected in an investigation site using the conventional wenner array. Also, seven Schlumberger sounding were conducted in the site to discover the 1D information to supplement the orthogonal 2D profiles. In their work, it was observed that there is distortion in 2D images from the inversion of the 2D profiles but it was not so when extract 2D images from the 3D inversion. Aizebeokhai, (2011), carried out research work on potential impacts of climate change and variability on groundwater resources in Nigeria. In his work, he ascertain that there is need to evaluate groundwater resources, quality and vulnerability to climate change and variability, therefore concerted effort should be made in addressing those changes that relates to the potential impacts of climate changes and vulnerability. Aizebeokhai (2010), carried out research work on basic theory and field design on application and importance of 2D and 3D resistivity Imaging.
Magnusson (2008) applied 2D geoelctrical imaging to determine the subsurface geology of rock quarry industry in Sweden and it was revealed that using 2D parallel data sampling 3D inversion of the dataset is possible which enhances the visualization of the subsurface of quarry rock site in Sweden. It was revealed that the use of geoelectrical resistivity techniques was very efficient in detecting fracture frequency, identification of major fracture zones and variations of rock-mass quality which affect the aggregate quality. In the quest to see the usefulness of electrical resistivity method for surface and groundwater exploration, Ogilvy et al; (2009) developed an automated time-Lapse Electrical Resistivity Tomography called ALERT. This equipment was used to investigate the detection and tracking of mine-water pollution from abandoned mines. This ALERT automated machine has been used manually at the central Wales Orefield in the UK. From the result, it was confirmed that the use of electrical tomography proof reliable. Wisen et al; (2005), combined 1D laterally constrained inversion and 2D smooth inversion of resistivity data with a priori data from boreholes. In their work, the result shows that 2D smooth inversion resolve lateral changes well while 1D – LCI results in well defined horizontal layer interfaces. Meju (2006) carried out research on landfill sites by using geoelectrical resistivity method and the work is majorly on characterization of covered landfill.
Campbell and Fitterman (1999), engaged geoelectrical resistivity methods for investigating Mine Dumps and in their work, the result reflects lithology, pore water saturation and dissolved solids in the pore water. In the research work, it was observed that if the pore water has a pH less than 5, conductivity maps indicate acid generating potential; the measurement of IP can help distinguish mineralogy in mine dumps and EM and DC help in locating acidic or high total dissolved solids groundwater associated with mine dumps. Al- zoubi et al; (2007) used 2D multi- electrodes resistivity imaging for assessing sinkholes hazard along the eastern part of Dead Sea, Jordan. In their research work, with application of 2D geoelectrical resistivity imaging, it was revealed that soil of the study area contains alluvial, conglomerate and silty clay and the interpretation of resistivity data along the profiles show different model of the resistivity variation in active sinkhole zones compared with inactive zones in the study area.
CHAPTER THREE
ELECTRICAL RESISTIVITY THEORY AND
METHODOLOGY.
All resistivity methods employ an artificial source of current which is introduced into the ground through point electrodes or long line contacts. The procedure is to measure potentials at other electrodes in the neighbourhood of the current flow. Electrical methods of geophysical investigations are based on the resistivity or its inverse (i.e.) conductivity of materials. The electrical resistance, R of a material is related to its physical dimension, cross sectional area, A, length, L, through the Low frequency alternating current is employed as source signals in the DC resistivity surveys in determining subsurface resistivity distribution. Usually a complete homogeneous and isotropic is considered. For homogeneous medium, the current density J and electric field E are related through Ohm’s law. Where E is in volts per meter and σ is the conductivity of the medium in Siemens per meter (m).
CHAPTER FOUR
RESULTS AND INTERPRETATION
Electrical resistivity data was collected using the programmable PAS earth resistivity meter. This earth resistivity meter has a multicore cable to which electrodes were connected at takeouts moulded on at predetermined equal intervals. A computer-controlled system was then used to select the active electrodes for each electrode set-up automatically. This computer-controlled system was included in the PAS earth resistivity meter which was used in the survey.
CHAPTER FIVE
RESULT, DISCUSSION, RECOMMENDATION AND CONCLUSION
: Profile 1:
Figure 5.1 present illustrative measured data that was directly measured from the field. The pattern shows the point that was measured during the field work. Figure 5.2 shows the measured apparent resistivity pseudosection, calculated apparent resistivity pseudosection and inverse model resistivity section. The figure was generated through the use of robust software called RES2DINV. Due to the geological nature of the study area being a sedimentary area, there are resistivity’s values that shows the structure of the study area which are in layers.
The geological map of Ogun State shows that Atan is mainly dominated by coastal plain sands. From the inverse model section, the first layer in the structure has the highest resistivity value that ranges between 1200 Ω.m to 4000Ω.m. This value of resistivity indicates that the first layer is Sand which naturally reflects the geological nature of the study area. The second layer with resistivity value of 500Ω.m to 1000Ω.m represents shale. The resistivity value between 400Ω.m and 600Ω.m indicates that the third layer is Alluvium. The resistivity’s value ranges between 200Ω.m to 400Ω.m which is fourth layer in the structure represent Limestone. The resistivity ranges between 60.7Ω.m to approximately 100Ω.m is an indication of presence of fresh groundwater and clay which is the fifth layer. Though the aquifer resistivity value of a place varies. This depends on the location where the study is being carried out.
CONCLUSION
In conclusion, the main objective of this study is to determine the potential of groundwater and geological structure of the study area. In this research work, it can be concluded that the geological structure of Atan are as follows based on their layers. Layer – 1 is Sand with resistivity value of approximately 3000Ωm for both profiles, layer – 2 is Shale with approximate value of resistivity values that ranges between 500Ωm and 1000Ωm, layer – 3 is Alluvium with approximate value of resistivity of about 400Ωm to 800Ωm for both profiles. Furthermore, layer – 4 and layer – 5 is for both clay and groundwater respectively except for profile 1 where the resistivity value is 60.7Ωm which can likely be the aquifer resistivity value of the study area. However, the sitting of borehole can only be properly done along the profile 1 instead of profile 2. This is because it will be easier getting to the aquifer. Therefore, it is easier to site borehole along vertical profile than horizontal profile at the depth of 130m to 140m and there is almost correlation between resistivity value for Vertical Electrical sounding and 2D.
RECOMMENDATION
The research has shown that the geological structure of an area can be detected by adopting geoelectrical resistivity imaging. However, there are few recommendations to be made for the improvement of the research work. They are as follows:
- The research work should be made to have more profiles for better resolution.
- The length for electrode spreading should be increased to at least 400m or 500m in order for the injection current to penetrate deep the ground for Wenner array configuration.
- The study area should be increased to 3 or 4 zones with the region. This is to ensure good fit and to reduce the error to the bearest minimum.
- The length of electrode spreading for the Vertical Electrical Sounding (VES) should stop at 350m or 500m for the depth of groundwater resolution instead of 100m used in this research work.
CONTRIBUTION TO KNOWLEDGE
Though so many researchers have worked on groundwater exploration using geoelectrical resistivity imaging. This research work has also proved that the electrical method is very efficient in exploring groundwater and in studying geological structure of any area. Through this work it has been emphasized that electrical method is cost effective compare to other method used for groundwater exploration. This research work emphasized to the upcoming generation in the field of study that geoelectrical resistivity imaging is a useful tool in determine the geological structure any area and for groundwater exploration.
REFERENCES
- Abadallah Al-zoubi, Abdel-Rahman .A, Abueladas and Rami .I, Al-
- Rzouq (2007): Use of 2D Multielectrodes Resistivity Imaging for Sinkholes Hazard Assessment along the Eastern Part of the Dead Sea, Jordan. American Journal of Environmental Sciences 3(4): 230-234.
- Adeoti .L, Alile, O.M and Uchegbulam (2010): Geophysical Investigation of Saline Water Intrusion into Freshwater Aquifers: A Case Study of Oniru Lagos State. Scientific Research and Essays Vol. 5(3), pp. 248-259. ISSN 1992-2248, Academic Journals.
- Aizebeokhai A.P, Olayinka A.I, Singh V.S (2010): Application of 2D and 3D Geoelectrical resistivity imaging for engineering site investigation in a crystaline basement terrain, Southwestern Nigeria.
- Journal Environmental Earth Sci. DOI: 10.1007/s12665.010-0.474, p. 1481.
- Aizebeokhai A.P (2010): Full Length Research Paper on 2D and 3D on
- Geoelectrical resistivity Imaging: Theory and Field Design. Scientific Research Essays Vol. 5(23), Pp. 3592-3605.
- Alile, Owens. M. and Amadasun, C.V.O (2008): Direct Current pobing of the subsurface Earth for Water Bearing Layer in Oredo Local Government Area, Edo State, Nigeria. Nigeria Journal of Applied Science, Vol. 25: 107-116.