Application Of Geoelectrical Resistivity Imaging To Investigate Groundwater Potential
Geoelectrical resistivity imaging using both 2D and Vertical Electrical Sounding (1D) method was carried out in Atan , Ado/Odo Ota Local Government, Ogun State ,Nigeria using the PAS earth resistivity meter.Three profiles was carried out using both Wenner and Schlumberger array configuration. The data was interpreted using RES2DINV for 2D and computer iteration method (WinResist) for VES. Results showed that there are five major layers which described the geological structure of the study area. The resistivity values ranges between 1200â„¦m to 4000â„¦m,
500â„¦m to 1000â„¦m, 400â„¦m to 800â„¦m , 200â„¦m to 400â„¦m and 60.7 â„¦m to approximately 100â„¦m respectively according to the structures. VES1,VES 2, VES 3 and VES 4 apparent resistivity curve shows the different layers with their corresponding thicknesses. The study area revealed that the depth to the aquifer ranges between 130m t0 140m.
Table Of Contents
- List of Tables
- List of Figures
- 1.1 Introduction
- 1.2 Geological Description of Underground Water
- 1.2.1 Occurrence of Groundwater
- 1.2.2 The movement of Groundwater
- 1.2.3 Transmission of Groundwater
- 1.2.4 Recharge
- 1.2.5 Leakage
- 1.2.6 Withdrawal
- 1.2.7 Aquifers
- 1.3 Location of the study area
- 1.4 Aim and Objectives
- 1.5 Scope of the Study
- 1.6 Justification of the study
- 2.1 Literature Review and Geological Setting
- 2.2 Geological Setting of the study area.
- 3.1 Electrical Resistivity Theory and Methodology
- 3.1.1 Single Current Electrode at Depth 2
- 3.1.2 Single Current Electrode at Surface
- 3.1.3 Two Current Electrode at the Surface
- 3.2 Apparent Resistivity and Electrode Configuration
- 3.3 Resistivity Survey
- 3.4 Field Measurement Procedure
- 3.5 The Field Work Procedure
- 4.1 Results and Interpretation
- 4.2 Data Collected for Profile 1
- 4.3 Data Collected for Profile 2
- 4.4 Data Collected for Profile 3
- 5.1 Result, Discussion, Conclusion and Recommendation
- 5.1.1 Profile 1 Discussion
- 5.1.2 Profile 2 Discussion
- 5.1.3 Profile 3 Discussion
- 5.2 Conclusion
- 5.3 Recommendation
- 5.4 Contribution to Knowledge
List Of Tables
- Table 4.1; Raw Data for 2D in Profile 1
- Table 4.2; Raw Data for 2D in Profile 1
- Table 4.3; Raw Data for 2D in Profile 1
- Table 4.4; Raw Data for 2D in Profile 1
- Table 4.5; Raw Data for 2D in Profilr 1
- Table 4.6; Raw Data for 2D in Profile 1
- Table 4.7; Raw Data for 2D in Profile 1
- Table 4.8; Raw Data for 2D in Profile 1
- Table 4.9; Raw Data for 2D in Profile 1
- Table 4.10; Raw Data for 2D in Profile 1
- Table 4.11; Raw Data for 2D in Profile 2
- Table 4.12; Raw Data for 2D in Profile 2
- Table 4.13; Raw Data for 2D in Profile 2
- Table 4.14; Raw Data for 2D in Profile 2
- Table 4.15; Raw Data for 2D in Profile 2
- Table 4.16; RawData for 2D in Profile 2
- Table 4.17; Raw Data for VES in Profile 3
List Of Figures
- Figure 1.1; Approximation of time varying infiltration rate
- Figure 1.2; Sketch diagram of (a) homogeneous (b) Inhomogeneous (C) Anistropy aquifers
- Figure 1.3; Hydrological cycle and aquifers type
- Figure 1.4; Base map of the study area
- Figure 1.5; Geological map of Ogun State showing the study area
- Figure 3.1; A conventional four electrode array to measure the subsurface Resistivity
- Figure 3.2; Common arrays used in resistivity surveys and their geometric factors (Loke, M.H 1997; 1999)
- Figure 3.3; 2D measurement Using Wenner Arrangement Configuration (Loke, 1999)
- Figure 3.4; Schlumberger Array Configuration
- Figure 4.1; Showing the PAS Earth Resistivity Meter used for Data Aquistion
- Figure 5.1:; Showing the measured data of the study area for profile 1
- Figure 5.2; Showing the measured apparent resistivity, calculated apparent resistivity and inverse model resistivity
- section for profile 1 of the study area
- Figure 5.3; Showing the field work survey in progress
- Figure 5.4; Showing the measured data of the study area for profile 2
- Figure 5.5; Showing the measured apparent resistivity, calculated apparent resistivity and inverse model resistivity section for profile 2 of the study area
- Figure 5.6; Showing typical curve for VES 1
- Figure 5.7; Showing typical curve for VES 2
- Figure 5.8; Showing typical curve for VES 3
- Figure 5.9; Showing typical curve for VES 4
Groundwater has an excellent microbiological quality and generally adequate chemical quality for most uses. Nine major chemical constituents (Na, Ca, Mg, KHCO3, Cl, SO4, NO3 and Si) make up 99% of the solute content of natural groundwater. The proportion of these constituents reflects the geology and history of the groundwater. Minor and trace constituents make up the remaining 1% of the total, and their presence (or absence) can occasionally give rise to health problems or make them unacceptable for human or animal use (British Geological Survey to Africa) .
Groundwater and other mineral resources such as hydrocarbons and solid minerals are of great abundance in Nigeria. The true riches of any country depend on its ability to provide for its dwellers. Potable water is one of the major resources that a citizen of any nation can benefit from (Alile, 2008). This is so because water is a free course of nature. It is a gift of God to mankind. This wonderful resources have transcends so many generation because of its value to human life. Water is one of the major determinants of economical development. Water has found its usefulness in every human endeavour such as manufacturing industry, agricultural industry, transportation industry, construction industry, home usage and so on. Because of the importance of this resources new technology have been developed in search of this resources. Water search has extended from surface to ground exploration. There are parameters that characterized groundwater such as conductivity, porosity, permeability and transmissivity. All these parameters are determined using any geophysical methods such as magnetic methods, gravity methods etc. In this research work, electrical resistivity method was employed for groundwater exploration (Alile, 2008). Geoelectrical resistivity surveys are often used to search for groundwater in both porous and fissured media. Clean sands and gravels which have porosities always make good aquifers when saturated with fresh water which can easily be differentiated from lower- resistivity impermeable clays and marls and also from bedrock which is mainly of much higher resistivity (Sharma, 1997). Areas where the groundwater is significantly saline, the aquifers resistivity is reduced greatly and resistivity surveying can delineate the boundaries of the body of saline water (Sharma, 1997).
Geoelectrical resistivity method has developed greatly and has become an important instrument in hydrological studies, mineral prospecting and mining as well as in environmental and engineering applications. (Griffiths et al; 1990; Alile et al; 2010; Griffiths and Barker, 1993; Dahlin and Loke; Aizebeokhai et al; 2010). This underlying principle of measuring subsurface variation using electrical resistivity within the earth was developed by Schlumberger who conducted the first experiment in 1912 in the field of Normandy and the same idea was also developed by frank Wenner in the united State of America. (Kunetz, 1966). This geoelectrical resistivity method has been found useful in locating groundwater in fissured rock, mapping of plumes, mapping of boundaries of saline groundwater and exploration of geothermal fluids.
Due to the successful application of geoelectrical resistivity over the years in groundwater exploration, this propels me to adopt geoelectrical resistivity method to investigate groundwater potential and geological structure of the study area However; geoelectrical resistivity surveys have undergone significant changes in the last three decades. The traditional horizontal layering technique for investigating geoelectical resistivity data are rapidly being replaced with 2- dimensional and 3-dimensional models of interpretation especially in complex and heterogeneous subsurface media.
Field techniques have advanced from manual measurements made at separate and independent points to the use of automated machine called terrameter with multi-electrode array along the measurement profile. Till 1980s, data acquisition was more or less carried out manually and this is demanding and slow and the quality of the measured data is poor. Therefore a range of fast automated multi-electrode and multichannel data acquisition system now exists that follows flexibility in the acquisition of geoelectrical resistivity data. (Barker, 1981; Stummer and Maurer, 2001; Auken et al; 2006).
1.2 Geological Description of Underground Water
Groundwater can be found almost everywhere. The water table may be deep or shallow and may rise or fall depending on many factors.
Heavy rains or snow may cause the water table to rise or fall. Groundwater is stored in and moves slowly through layers of soil, sand and rocks called aquifers. The speed of groundwater flows depends on the size of the spaces in the soil or rock and how well the space is connected. Groundwater is brought to the surface naturally through a spring or can be discharged into lakes and streams. This water can also be extracted through a well drilled into the aquifers. A well is a pipe in the ground that is filled with water. This water can then be brought to the surface by a pump. Shallow wells may go dry if the water table falls below the bottom of the well. Some wells called artesian well do not need a pump because of natural pressures that force the water up and out of the well.
Groundwater supplies are replenished or recharged by rain and snow melt. In some areas of the world, people face serious water shortages because groundwater is used faster than it is naturally replenished. In other area groundwater is polluted by human activities. Groundwater is a natural resource that is use for drinking, recreation, industry and agriculture. In areas where material above the aquifer is permeable pollutants can sink into the groundwater. Groundwater can be polluted by landfills, septic tanks, leakages of underground gas tanks and from overuse of fertilizers and pesticides.
1.2.1 The Occurrence of Groundwater.
The groundwater is a term used for water which occurs beneath the ground surface. It is an important constituent of hydrological cycle and plays a major role in augmenting water supply to meet the major increasing demands in various sectors. Groundwater occurs in the upper layers of the earth’s crust. These layers consist of igneous, sedimentary and metamorphic rocks. During their origin and later evolution, these rocks develop porous and permeable structures containing pore spaces. Within these pores, the water of meteoric, juvenile, connate or metamorphic origin occurs in both liquid and gaseous phases along with other gases and liquid such as hydrocarbons and magma. Meteoric water includes rain water, lake and river waters. Juvenile water is assumed to be derived from the mantle during degassing processes (Bredchoeft and Norton, 1990; Rai, 2004). Formation water is the water trapped during the deposition of sediments and produced during diagenetic reaction. Metamorphic waters are derived from the dehydration of hydroxyl bearing minerals through rising pressure and temperature. These fluids are subjected to a wide variety of stresses such as recharge due to precipitation, return flow from irrigation, seapage from canals, lakes, ponds etc. As a result, a groundwater regime consisting of geological formation and groundwater is established.
1.2.2 The Movement of Groundwater
Groundwater, apart from being a major source of water supply, groundwater is the most important geological agent among all fluids of the earth’s system and plays important role in many geological processes. This is so because of its moving ability and its ability to interact with the surrounding environments. The interaction of groundwater with its surrounding generates various natural process, products and conditions and the moving ability helps in self organizing the effects of interaction within the flow system. Three main types of interactions have been identified; they are chemical, mechanical and kinetic. Accordingly, the processes of these interactions are classified as chemical processes, mechanical processes and kinetic processes. (Toth, 1999; Rai, 2002). Chemical processes include dissolution, hydration, hydrolysis, oxidation, reduction, chemical precipitation and base exchange. Pore – pressure change and lubrication are the two most important physical processes that affect many geological phenomena. Reduction or increases in pore pressures affect the magnitude and direction of groundwater velocity which ultimately affect the type, rate and direction of chemical reactions, the solubility of minerals etc. They also affect the strength and integrity of rocks, leading to their deformation. Lubrication by water of discontinuity boundaries in the rock frame work such as grain surfaces in soils and in unconsolidated sediments or fracture and fault planes in hard rock reduces friction and enhances the effect of shear stresses. As a consequence, shear movements of soil and rocks can be induced along the discontinuities which may lead to the occurrence of land subsidence, landslides and earthquake like geologic phenomena.
A wide variety of matter in many different forms such as aqueous solutions of organic and inorganic ions, matter in colloidal forms or larger-sized suspended grains, gases, molecules of liquid hydrocarbons, viruses and bacteria are transported by groundwater movement. The importance of transport of these matter resulted in the leaching and removing of minerals from soils and rocks, carrying nutrients to surface water bodies, building and emigrating deposits of metallic and non-metallic minerals and hydrocarbons, causing washing and biodegradation of ore deposits and hydrocarbon accumulation and concentrating contaminants at suitable subsurface locations. Heat transport by moving groundwater leads to the formation of hot springs, hydrothermal ore deposits or other types of geothermal anomalies.
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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.
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.
5.4 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.