1.1 Introduction
Electrical resistivity imaging (ERI) is a geophysical technique used to investigate the subsurface by measuring the resistance of soil and rock materials to the flow of electrical current. The method works on the principle that different earth materials possess varying electrical resistivity values depending on their composition, porosity, moisture content, and the presence of conductive or resistive materials. By injecting electrical current into the ground through electrodes and measuring the resulting potential differences, ERI is used to generate detailed images of subsurface structures and anomalies (Reynolds, 2011; Loke, 2010). Electrical resistivity imaging has gained increasing attention in the field of environmental geophysics due to its ability to produce two-dimensional and three-dimensional images of subsurface structures with relatively high accuracy (Kearey, Brooks, & Hill, 2013).
This chapter will address the background information that motivated this study, the challenges that prompted it, its aim, and its objectives as a preface to subsequent sections of the study. Additional factors include the study's significance, scope, limitations, research questions and hypotheses, and the definition of technical terms.
1.2 Background of Study
The storage and distribution of petroleum products are critical components of the energy supply chain in many countries of the world. Petroleum depots and storage facilities depend largely on underground storage tanks for the safe containment of fuels such as gasoline, diesel, and kerosene. Underground storage tanks are typically buried beneath the ground surface to reduce fire hazards, prevent external damage, and optimize land use within petroleum facilities. According to Reynolds (2011), underground storage tanks are widely used in petroleum depots because their subsurface installation helps to maintain temperature stability and protect stored products from environmental exposure. However, the underground nature of these tanks also presents significant challenges in terms of monitoring, maintenance, and detection when structural problems occur.
Geophysical methods have increasingly been adopted as reliable tools for investigating subsurface conditions and identifying buried objects. According to Kearey, Brooks, and Hill (2013), geophysical exploration techniques are used to study the physical properties of the subsurface without the need for excavation. These techniques are widely applied in environmental studies, engineering investigations, mineral exploration, and groundwater studies. Among the various geophysical techniques available, electrical resistivity imaging has gained prominence because of its ability to map variations in the electrical properties of subsurface materials and provide detailed images of underground structures.
Electrical resistivity imaging is based on the principle that different earth materials exhibit varying electrical resistivity depending on their composition, porosity, water content, and the presence of conductive materials. Loke (2010) reported that electrical resistivity imaging is used to produce two-dimensional and three-dimensional models of subsurface resistivity distribution, which assist in identifying buried objects, geological structures, and zones of contamination. Through the use of electrodes inserted into the ground, electrical current is transmitted into the subsurface, and the resulting potential differences are measured to determine resistivity variations within the ground.
The application of electrical resistivity imaging has become particularly important in environmental and engineering investigations. Telford, Geldart, and Sheriff (1990) asserted that electrical resistivity methods are highly effective in detecting underground anomalies because objects with different electrical properties create measurable contrasts within the subsurface. These contrasts are useful in identifying buried metallic structures such as pipelines, storage tanks, and other infrastructure associated with petroleum storage and distribution systems. As a result, electrical resistivity imaging has become a valuable non-invasive technique for locating underground installations and assessing subsurface conditions.
The increasing global concern over environmental pollution has further strengthened the need for effective monitoring of underground storage facilities. Fetter (2001) stated that leaking underground storage tanks represent a major source of soil and groundwater contamination in many parts of the world. Petroleum products released from damaged tanks may infiltrate the surrounding soil layers and migrate through groundwater systems, thereby posing serious environmental and public health risks. Such contamination may affect drinking water sources, agricultural land, and aquatic ecosystems if not properly detected and controlled.
In developing countries, the challenge of monitoring underground storage tanks is often compounded by poor documentation and aging infrastructure. Reynolds (2011) affirmed that many underground storage tanks installed several decades ago are approaching the end of their operational lifespan, making them more susceptible to corrosion and structural failure. In many cases, the precise location, depth, and condition of these tanks are not well documented, making it difficult for facility managers to effectively monitor and maintain them.
This study is set against the backdrop of the need to apply electrical resistivity imaging as a reliable geophysical technique for detecting underground storage tanks and assessing subsurface conditions at the Total Nigeria Plc Enugu Depot.
1.3 Statement of Problems
Investigation revealed that many petroleum depots in Nigeria, traditional methods of locating underground structures often rely on historical records, manual probing, or excavation. In addition, such methods do not provide adequate information about the subsurface characteristics surrounding the tanks, which is important for assessing potential leakages or structural risks. Geophysical techniques, particularly electrical resistivity methods, have increasingly been used in environmental and engineering investigations because they provide a non-invasive approach to imaging subsurface structures and detecting anomalies associated with buried objects (Reynolds, 2011).
Furthermore, the presence of undetected or poorly monitored underground storage tanks poses serious environmental risks. Leakages from corroded or damaged tanks may lead to contamination of soil and groundwater, which may affect nearby ecosystems and human health. In areas where petroleum depots operate in close proximity to residential or commercial zones, such contamination may result in long-term environmental degradation and economic losses (Fetter, 2001). It is against this backdrop that this study seeks to investigate the use of electrical resistivity imaging for detecting underground storage tanks at the Total Nigeria Plc Enugu Depot.
1.4 Aim and Objectives of Study
The aim of the study is to apply electrical resistivity imaging to accurately detect and evaluate the condition of underground storage tanks at the Total Nigeria Plc Enugu Depot. In achieving this aim, the following specific objectives were laid out as follows:
- To assess the current state of underground storage tanks within the depot.
- To identify the exact location and depth of underground storage tanks using electrical resistivity imaging.
- To detect potential leakage or contamination zones surrounding the tanks.
- To evaluate the effectiveness of electrical resistivity imaging compared to the existing detection methods.
- To provide recommendations for improved subsurface monitoring and maintenance of underground storage tanks.
1.5 Research Questions
The study came up with research questions so as to be able to ascertain the above stated objectives. The specific research questions for the study are stated below as follows:
- What is the current state and condition of underground storage tanks at the Total Nigeria Plc Enugu Depot?
- How effective is electrical resistivity imaging in detecting the location and depth of underground storage tanks?
- What are the potential leakage or contamination zones surrounding the underground storage tanks?
- How does electrical resistivity imaging compare to traditional monitoring methods in accuracy and efficiency?
- What measures can be implemented to improve subsurface monitoring and maintenance of underground storage tanks?
1.6 Research Hypothesis
In order to pursue the objective of this study, the following generalized statements have been designed to guide and aids in obtaining the result for the experiment to be conducted. For this work, the null hypothesis will be represented with H0 while the alternative hypothesis will be represented with hypothesis H1.
- H0: Electrical resistivity imaging does not significantly improve the detection and mapping of underground storage tanks compared to existing monitoring methods.
- H1: Electrical resistivity imaging significantly improves the detection and mapping of underground storage tanks compared to existing monitoring methods.
1.7 Significance of Study
The outcome of this research will provide accurate identification and mapping of underground storage tanks at the Total Nigeria Plc Enugu Depot, enabling facility managers to locate buried tanks with precision. The study will also help detect potential leakage zones, reducing the risk of soil and groundwater contamination in the surrounding environment.
Furthermore, the findings will support compliance with environmental regulations and safety standards, ensuring that the depot operates within legal and industry requirements.
Lastly, the study will contribute to better operational planning, risk management, and environmental protection within petroleum storage facilities.
1.8 Scope of Study
The scope of this research is focused on the application of electrical resistivity imaging to detect underground storage tanks at the Total Nigeria Plc Enugu Depot in Enugu State, Nigeria. It covers subsurface investigations within the depot premises, including assessment of tank location, depth, structural condition, and surrounding soil resistivity variations. The study does not extend to other depots or petroleum companies outside Enugu State.
1.9 Limitations of the Study
A study of this nature is bound to experience certain problems as such the constraints imposed on the research include:
- Time Constraints: A study of this nature needs relatively long time during which information for accurate or at least near accurate inference could be drawn. The period of the study was short, time posed as constraints to the research.
- Financial Constraints: The research would have extended the survey to other area at the empirical level, but limitation as included cost of transportation to the source of material and the cost of time setting of the already completed work.
1.10 Definition of Terms
Electrical Resistivity Imaging (ERI): A geophysical method used to investigate subsurface structures by measuring variations in electrical resistivity. ERI is used to detect underground objects, cavities, and contamination zones without excavation (Reynolds, 2011).
Underground Storage Tanks (USTs): Containers buried below the ground surface for storing petroleum products or hazardous liquids. USTs require monitoring to prevent leakages that may contaminate soil and groundwater (Fetter, 2001).
Subsurface Anomalies: Variations in soil or rock properties detected through geophysical methods that may indicate the presence of buried objects, voids, or contamination (Kearey, Brooks, & Hill, 2013).
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