Application of the Electrical Resistivity Method in Groundwater Exploration
CHAPTER ONE: INTRODUCTION
Water is one of the essential necessities of nature, indispensible to all organic life and a necessary resource for livelihood; so therefore, its importance cannot be overemphasized. It is however alarming that this requisite resource is becoming scarce everyday due to booming industrialization and increasing population. According to WHO/UNICEF (2000), 1.1 billion people in the world do not have access to potable water. This is more intense in the rural parts of developing and under-developed countries where reports show that over 70% of the people have no access to safe water for domestic use, so they depend mostly on surface water (from lakes, ponds, streams, rainfall, rivers) for sustenance. Surface sources however, are not dependable or reliable because they are associated with high evaporation rates, often susceptible to pollution and waterborne diseases. In other for surface water to be safe for use, especially domestic use, it needs to be treated and these rural communities cannot afford these treatments, hence, they use these water for their day to day activities which in the long run result to outbreaks of waterborne diseases such as typhoid, cholera, hepatitis etc.
Groundwater which exists in the pores of subsurface sedimentary rocks and fissures/fractures of basement rocks are protected and covered from pollution and evaporation, as such, are potable for domestic, agricultural and industrial purposes (Alisiobi and Ako, 2012). Groundwater has so many advantages over surface water, from its potability, availability all year long (even in areas affected by drought) to the fact that it can be found almost at any spot of its need beneath the earth, ready to be exploited. These benefits have made groundwater most sort out for.
Because the subsurface depth which groundwater exists cannot be easily ascertained, a number of scientific techniques are employed to gather information regarding its depth of occurrence in the subsurface. The use of hydro-geophysical methods help answer these questions, they give us information regarding the subsurface geology of the area, act as tools for groundwater resource mapping and help in water quality evaluations. Several methods employed in groundwater exploration include electrical resistivity, magnetics, gravity, electromagnetics, seismic, remote sensing methods (Chinyem F. I., 2013). These geophysical techniques have been applied in groundwater exploration with some showing more success than others. Methods like gravity and magnetics have been used fortunately to map regional aquifers and large scale basin features. Reynolds (1997) explains how seismic methods have been used to delineate subsurface aquifers and fractured rock systems.
The electrical resistivity method is the most widely used and it has proved particularly effective in groundwater exploration because many geological formation properties such as porosity and permeability that are critical to hydrogeology can be correlated with electrical conductivity signatures (Eke and Igboekwe, 2011). This method is also widely appreciated because of its portability in equipment, ease of operation, usefulness and efficiency in drilling operations (Chinyem F.I., 2013).
All these and more are why this project attempts to use this method in the search for uncontaminated groundwater potential zones in Alode Community.
1.2 PROBLEM STATEMENT
Water is among the many God given resources obtainable in Rivers State. Due to the increased oil and gas exploitation, influx of migrants from all parts of the country (in search of green pastures), and poor environmental hygiene in the area, the quality of water has been tremendously tampered with and therefore rendered unfit for human consumption.
In the study area, Alode Community in Eleme Local Government Area of Rivers State, some of the boreholes drilled have the problem of low yield, objectionable taste and they show dark coloration when fetched into transparent containers.
All these must have happened as a result of the pollutions caused by the increasing population. Also, since most of the boreholes are drilled haphazardly by non-professionals (quacks),no geological or hydro-geophysical investigations are carried out prior to these drillings, so these boreholes are either sunk into perched aquifers or aquifers contaminated due to careless waste disposal. The problems may also have happened due to the indiscriminate location of petroleum storage tanks, septic tanks of houses and the leakage of these facilities into the aquifer.
Unfortunately, the water table in these areas are very close to the surface and all these have led to its contamination, therefore, there is need to carry out these resistivity surveys before drillings so as to identify suitable sites for tapping into good uncontaminated aquifers.
1.3 AIM/OBJECTIVES OF THE STUDY
The project aims at carrying out geophysical investigations in the study area using the electrical resistivity method.
– To determine uncontaminated groundwater potential zones in Alode Community.
– To determine the geo-electrical and hydrogeological characteristics of the aquifer(s) present in the study area.
– To have an insight into the subsurface geology of the study area, to detect the thickness of the subsurface layers and their resistivity.
– To establish the usefulness of the electrical resistivity method as a potential tool in solving geo-hydrological problems associated with groundwater occurrence.
1.4 SCOPE OF STUDY
This work centers on determining possible subsurface uncontaminated water bearing zones in Alode Community. Groundwater exploration will be executed in selected parts of the community, using the electrical resistivity method to measure variations in resistivity of the subsurface formations with depth. Vertical electrical soundings (VES) based on the Schlumberger configuration will be conducted at these selected sites in the community.
1.5 SIGNIFICANCE OF THE STUDY
Most of the hand dug wells and boreholes sited in Alode Community were done by quacks (people with no knowledge of hydrogeology and people who just want to make good money), as such, these wells have not been sunk properly.
The importance of this work is that the electrical resistivity method will be used to explore for aquifers with good yield and excellent water quality.
Hopefully, the depth to aquifer and other information gotten from this work will serve as a guide or basis for future borehole drillers, and will in the long run help to reduce death caused by waterborne illnesses in the community.
1.6 RESEARCH METHODOLOGY
The first step carried out in this research work is the desk study. Reconnaissance survey of the various selected areas in the community was then carried out afterwards to gather useful firsthand information about the study area.
Thirdly, the electrical resistivity method using the vertical electrical sounding (VES) technique was carried out to for data acquisition. The last step was processing, analyzing and interpretation of the various field data collected.
1.7 PROJECT LAYOUT
This project work consists of five chapters, each of which discusses important topics.
Chapter one introduces the research topic, outlining the background of the dissertation, the problem statement, aim/objectives of the study, the scope, the project significance, research methodology and finally talks about the study area.
Chapter two presents a literature review about the research topic;it goes further to talk about groundwater, the conditions suitable for groundwater accumulation, porosity and permeability, and properties affecting groundwater. It further talks deeply about the electrical resistivity method.
Chapter three houses the approaches and procedures used in conducting the research – how data was collected, software used for data analysis and processing.
Chapter four presents the various graphs obtained during the soundings at the selected sites in the community. The chapter also gives interpretations based on the graphs, showing depths most suitable for abstracting potable water.
Chapter five ends the project work with conclusions made from the findings obtained from the analysis of the graphs in chapter 4. Also, recommendation to future scholars for improving groundwater exploration using the electrical resistivity method is contained in the chapter.
1.8 THE STUDY AREA
Alode is one of the major communities among the ten communities in Eleme Local Government Area, located south-east of Rivers State and 30 minutes away from the State’s capital, Port Harcourt. It is bordered by Nchia-Eleme central market in the North-West, Alesa in the West, Ogale in the East and Onne in the South-East. It falls within the Niger Delta sedimentary basin of Nigeria, with a population of approximately 20 000 people. They rely majorly on groundwater as a source of water for their domestic and agricultural activities.
1.8.1 LOCATION AND ACCESSIBILITY
The study area is located on the coast of the Gulf of Guinea within latitude N4⁰46’57.1’’, longitude E7⁰7’27.5’’ and latitude N4⁰46’47.2’’, longitude E7⁰7’28.8’’ of the Greenwich Meridian. It has a major road that runs through it, from the Nchia main market, the road cuts the East-West road perpendicularly and further terminates at the boundary between Alode and Onne. The Community is connected to other parts of Rivers State by the East-West road.
Prior to the advent of industries in the area, the Alode people depended on farming and hunting as a source of livelihood
1.8.2 CLIMATE AND RAINFALL (HYDROGEOLOGY)
Alode has a tropical wet climate with seasonal lengthy and heavy rainy seasons (which runs from April to October) and a short dry season (November to March) (Ayibawari and Reward, 2015). Sometimes from February to April, the weather alternates, giving a rainy atmosphere and other times sunny. Heaviest precipitation occurs during June/July with an average rainfall of approximately 367mm (en.m.wikipedia.org, 2016). December and early January are the driest months of the year, with average rainfall of about 20mm.
Temperatures are almost constant with little variation throughout the year. Average temperatures are between 25⁰C to 28⁰C, humidity is high with mean value of about 75⁰C (onlinenigeria.com, 2016).
1.8.3 GEOLOGY OF THE AREA
Since the Alode Community is located in Rivers State which is part of the Niger Delta basin, it is made up of both fluvial and marine sediments gathered and accumulated since the Cretaceous period (UNEP, 2011). The geological layers in the study area consist of the Benin Formation, Agbada Formation and Akata Formation. The Benin Formation which is mainly where aquifers are located is made up of many layers of mud/clay, sand, silt conglomerate and peat/lignite all of different thicknesses and texture.
Abbildung in dieser Leseprobe nicht enthalten
Figure 1: Cross Section of the Niger Delta Sedimentary Basin (Thomas, 1995)
CHAPTER TWO: LITERATURE RCUEW
In the quest to provide solutions to the yearning groundwater problems, a great number of papers have been written in the field of groundwater exploration.
In one of such papers, a Kenyan based company “EARTH SCOPE GEO-HYDRO SERVICES” in 2012 carried out groundwater prospecting in Kisumu North District of Nyanza Province, Kenya. The electrical resistivity method was used and three vertical electrical soundings (VES) were carried out to probe the condition of the subsurface. The result indicated a shallow superficial layer to a depth of less than 5mbgl. According to this paper, the resistivity of the layer ranged between 38 and 7041Ωm interpreted to be sandy soils and dry clays. The paper further states that the superficial layer is underlain by a 139 to 495Ωm resistivity layer to a depth of 19mbgl, interpreted to be slightly weathered volcanic rocks at VES 1 and VES 2. At VES 3, the superficial layer is said to be underlain by a 14 to 56Ωm resistivity layer to a depth greater than 50mbgl interpreted to be weathered volcanics which are aquiferous and so groundwater is expected in the layer.
Also, Oseji Julius Otutu and O. Ujuanbi (2009) both of The Department of Physics in Delta State University, Abraka and Ambrose Alli University, Ekpoma respectively, carried out geo-electrical surveys using the electrical resistivity method in Emu Kingdom of Delta State, Nigeria. Ten vertical electrical sounding at different parts of Emu Kingdom were executed using the Schlumberger array in an attempt to gather useful information on the aquifer distribution within the study area and hence, delineate possible sites that boreholes can be drilled for potable and sustainable water supply. Result showed that within the depths penetrated, the litho-successions seen where sandy clay, clayey sand, clay, fine grained sand, medium grained sand and some silt. In some parts of the Kingdom, the aquifer was seen to be semi-confined and unconfined at other parts. They concluded that the fourth geo-electric layer which is the second aquifer in Emu Kingdom is not an encouraging prospect for groundwater development due to its small thickness.
Chinyem F.I. (2013) also applied electrical resistivity method in groundwater exploration at Oshimili South LGA, Asaba, Delta State, Nigeria. Seven vertical electrical soundings (VES) were applied to locate good yielding aquifers for the community. The inferred lithologies include sand, clay and medium to coarse grained sand, spanning large depth and thickness. The aquifers were confined to the coarse grained sandstone formation.
Alile et al (2008) carried out geophysical exploration involving the use of vertical electrical sounding in a sedimentary terrain 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.
Badmus and Olatinsu (2010) carried out a research work on aquifer characteristics and groundwater recharge pattern in a typical basement complex: A case study of Federal College of Education, Osiele, Abeokuta, South-west of Nigeria. In their work, they used vertical electrical sounding (VES) using the 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 also was discovered that the weathered and fractured basement constitute the main aquiferous units in the area. They were able to discover that the reasons for borehole failure and poor recharge in the area were due 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.
Egwebe et al (2004) estimated the aquifer potential at Ivbiaro, Ebesse Edo State using the geo-electrical direct current resistivity technique. The interpretation of the data indicated a depth of 96-147m to the aquifer (sand) within the sand/shale sequence of the Mamu Formation.
Geophysical investigation using combined electrical profiling with Wenner array configuration plus the ABEM SAS 300 Terrameter with all other accessories were used in some selected areas at the Federal Polytechnique Ado-Ekiti by Isife et al. (2000) for the purpose of locating sites for productive water boreholes. They revealed that the subsurface geo-electric section and iso-resistivity maps showed that a thick overburden overlaid a fractured zone in the south-eastern, south-western and north-eastern parts of the area which are diagnostic of a reliable and sustainable groundwater source suitable for borehole development at the Polytechnic.
Geo-electrical and hydrogeological investigation techniques with ten Schlumberger VES profiling were conducted across Gombi, Hong and Mubi, a part of Adamawa State, were interpreted by Nur and Afa (2002). they qualitatively and quantitatively revealed three electro-stratigraphic earth model. The topsoil with an average thickness of 8.10m and mean resistivity of 176.447Ωm, the second layer having a thickness of 7.82m and a mean resistivity of 187.124Ωm and finally a third layer of mean resistivity 356 to 718Ωm. Based on these, they inferred that the weathered/fractured basement rocks of the areas with the least resistivity make up the aquifer.
This project therefore seeks to contribute to the efforts made by these able scholars in the pursuit of reducing water crisis in the country.
Groundwater is freshwater (majorly from precipitation, snow or ice) that has infiltrated through the soil and is stored in tiny open spaces (pores), fractures or fissures within rocks or soil particles. Groundwater exists in two zones, the unsaturated zone (also known as the vadose zone or the zone of aeration) and the saturated zone (phreatic zone). The unsaturated zone exists directly below the land surface and it contains air and water in its pores. Some of the water in these pores is held by molecular attraction forces which do not allow the water flow away. The saturated zone lies underneath the unsaturated zone. In this zone, all the pore spaces, fissures or rock fractures are always completely filled with water.
How well a rock body holds water (porosity) depends on the size and shape of the rock particles that it possesses; a rock with loosely arranged particles of uniform size (such as sand) will hold more water than a rock with particles of different sizes. This is because the rock with different particle sizes will contain smaller rock particles that can settle in the pore spaces between the larger particles, thereby decreasing the amount of pore spaces that can hold water. Also, rock particles with round shape will pack more tightly together than particles with angular shaped edges. In other words, rocks with angular shaped particles will retain more water because of its large pore spaces.
For water to move through an underground rock, pores or fractures in the rock must be connected. If a rock has a good pore connection and water can move freely through its pore, the rock is said to be permeable. Permeability refers to the measure of how readily fluids can pass through a rock material (Okafor, 2011). If these pores or fractures are not connected the rock body will not be able to transmit water and therefore will not be regarded as an aquifer.
According to Wikipedia (2016), an aquifer is an underground layer of water bearing permeable rock, rock fracture or unconsolidated reservoir material (gravel, sand or silt) from which large quantity of groundwater can be extracted from using water wells. The amount of water an aquifer can hold depends on the volume of the rock body in the subsurface (the reservoir), and the size and number of pores and fractures that can be filled with water. An aquifer may be a few feet to several thousand feet thick and less than a square mile or hundreds of thousands of square miles in area.
Aquifers can be confined or unconfined but must be both porous and permeable.
Confined aquifers are aquifers that are overlain by a low permeability, confining layer, often made up of clay. Unconfined aquifers, also known as water-table aquifers are aquifers that are close to the land surface and open to the direct influence of climatic conditions (Fetter, 2001). The unconfined aquifers extend from the water-table (the surface or top of the water in the saturated zone) to the bottom of the aquifer. Other types of aquifers are:
– Perched Aquifer: This aquifer occurs above the regional water-table in the vadose zone. Perched aquifers are formed when there is an impermeable layer of rock or sediment (aquiclude) or relatively impermeable layer (aquitard) above the main water-table (aquifer) but below the surface of the land.
– Artesian Aquifer: This is a confined aquifer containing groundwater under positive pressure. This causes the water level in the well to rise to a point where hydrostatic equilibrium has been reached. A well drilled into this type of aquifer is called an artesian well.
Abbildung in dieser Leseprobe nicht enthalten
Figure 2: Diagram Showing Aquifier Types (Modified after Harlan et al., 1989)
2.1.1 HOW ARE AQUIFERS RECHARGED AND DISCHARGED?
According to Fetter (2011), unconfined aquifers are mainly recharged by the downward seepage (or percolation) of water through the unsaturated zone of the excess water over-passing the field capacity of the soil. Recharge can also occur through upward seepage (discharge) from underlying aquifers.
Confined aquifers are recharged directly by precipitation in the area where the aquifer crops out to the surface, having the characteristics as an unconfined aquifer. Another source of recharge is the infiltration or percolation from the unsaturated zone or by leakage or discharge from underlying aquifers.