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This study aims to characterize subsurface heat sources and density variations. It involves identifying and mapping heat sources, quantifying heat flow, and determining density distributions within the Earth's subsurface. The research likely utilizes geophysical methods and/or geological data to understand the interplay between heat and density variations and their potential implications for various geological processes.
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Bachelors of sciences in geophysics and mineralogy Ps29/00040/ A research project report submitted to the department of physics school of pure and applied sciences for partially fulfillment of the requirement for award of the degree of bachelors of science in geophysics and mineralogy Kisii University. March, 2024
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I certify that this is my original work and has not been presented for degree award or any other award in any university or any institution. Name………………………………………. signature………………. SEPERVISORSDECLARATION Name……………………………………. Signature…………………………...
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I dedicate this research project to my family, whose unconditional love and support have been the bedrock of my academic pursuits. Their encouragement and sacrifices have inspired me to pursue knowledge with passion and perseverance. Also, I would like to dedicate to my esteemed colleagues, whose unwavering collaboration, insights, and shared passion have been the bedrock of this endeavor.
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The Olkaria domes area, located within the Kenyan Rift Valley, is a prominent geothermal field renowned for its significant geothermal resources. This study was focused on gravity surveys in the Olkaria domes to assess subsurface structures, identify potential geothermal reservoirs, and detect environmental impacts such as land subsidence. The area was characterized by volcanic Olkaria domes, faults, and geothermal activity, making it a prime location for geothermal energy development. In gravity measurements of the Dome’s geothermal potential, a detailed gravity survey was conducted utilizing a CG5 gravimeter across a network of 43 stations. The survey aimed to characterize the subsurface density anomalies associated with geothermal resources. Precise navigation to each gravity station was achieved using a GARMIN ETREX 32x GPS, ensuring accurate location data under challenging field conditions. The gravity measurements underwent rigorous post-processing to correct for temporal drift and local terrain effects, which are crucial for reducing noise and enhancing the quality of the geophysical data. A Bouguer Anomaly (CBA) map was generated using Oasis Montaj software, employing the Euler deconvolution method. The map reveals gravitational anomalies, measured in milligals (mgals), indicating variations in subsurface density. Contour lines outline areas of equal bouguer anomaly (BA) values, aiding in spatial interpretation and geological analysis. This map serves as a valuable tool for identifying potential geological structures based on gravity anomalies and density contrasts. This study highlights the integration of advanced gravimetric techniques and GPS technology as a reliable methodology for geothermal resource assessment, providing valuable data for energy exploration and geoscientific research in the region.
viii List of figures and symbols Figure 1 location Olkaria domes within the kenyan Rift Valley .................................................. 2 Figure 2 geological map of the study area ...................................................................................... 3 Figure 3 GARMIN ETREX 32x GPS ............................................................................................. 7 Figure 4 CG-5 Autograv gravimeter ............................................................................................... 8 Figure 5 a leveled CG5 Autograv gravimeter ................................................................................. 9 Figure 6 area topography for Olkaria domes ............................................................................... 10 Figure 7 drift correction graph for the first day ............................. Error! Bookmark not defined. Figure 8 Gridded Regional Map for Dome ................................... Error! Bookmark not defined. Figure 9 BA Residual map of Olkaria domes ............................... Error! Bookmark not defined. List of Abbreviation and Acronyms KENGEN…………………………………. Kenya electricity generating company OW…………………………………………Olkaria Well NNE – SSW………………………………. North north east – south south west BA……………………………..………… bouguer anomaly Mgals……………………………………… milli gals
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Geothermal energy is derived from heat contained in earth. The heat energy is a result of radioactive decay of subsurface materials. This geothermal field is renowned for its significant geothermal resources and serves as a crucial source of renewable energy for the region. (Zhu, 2015). It is considered to be cost effective, reliable and sustainable source of energy. geothermal area is located in the belt of rift valley. The geothermal resource in the area has been under development and exploitation since 1981. The Olkaria domes are a prominent geological feature located within the geothermal field in Kenya's Rift Valley. This geothermal field is renowned for its significant geothermal resources and serves as a crucial source of renewable energy for the region. The Olkaria domes represent a series of volcanic Olkaria domes and associated structures that have played a pivotal role in the formation and evolution of the geothermal reservoir beneath the surface. These Olkaria domes are believed to have originated from volcanic activity associated with the East African Rift System, which has created a complex geological setting conducive to the accumulation of geothermal fluids and the development of high-temperature reservoirs. The presence of these Olkaria domes influences subsurface fluid flow patterns, reservoir permeability, and heat distribution, making them key considerations in the exploration and development of geothermal resources within the field. Understanding the geological characteristics and behavior of the Okaria domes is essential for optimizing geothermal production strategies, identifying potential drilling targets, and mitigating geological hazards associated with geothermal operations in the region (Darnet, 2002). Ongoing research and exploration efforts continue to enhance our understanding of the Olkaria domes and their significance in the context of geothermal energy development in Kenya. 1.2 Background information The olkaria domes geothermal area, situated within the Kenyan Rift Valley, stands as a pivotal hub for geothermal resources, playing a vital role in Kenya's energy sector. Through continuous of gravity surveys, various objectives are achieved. These surveys aid in assessing subsurface structures and identifying potential geothermal reservoirs by analyzing gravity anomalies, thereby revealing critical underground features like faults, fractures, and magma chambers
2 essential for reservoir formation. The ongoing gravity surveys to determine structural subsurface densities provides insights into reservoir conditions over time, facilitating effective reservoir management and optimization of geothermal production strategies. surveys aid in detecting changes in mass distribution indicative of environmental impacts such as land subsidence or groundwater contamination. 1.3 Regional setting of study area The study will be carried out in Olkaria domes area. It is located within the Great Rift Valley of Kenya, approximately 120 kilometers northwest of Nairobi. The Olkaria domes are situated northwest of Olkaria, which is known for its geothermal power plants and southwest of Lake Naivasha, one of the freshwater lakes in the Kenyan Rift Valley. This region is characterized by a diverse geological and geographical landscape, including volcanic features, and freshwater lakes. The Olkaria domes area is known for its volcanic Olkaria domes, which are composed of basaltic lava flows and volcanic breccias formed by past volcanic activity. The region experiences active geothermal activity, with hot springs, fumaroles, and geysers indicating the presence of high temperature geothermal reservoirs beneath the surface. It has an estimated potential of 2,000 MW, (Sack 2006) Figure 1 location Olkaria domes within the kenyan Rift Valley
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2.1 Literature review Gravity surveys play a crucial role in characterizing subsurface heat sources and density variations in geothermal fields like Olkaria. These surveys measure variations in the Earth's gravitational field caused by differences in the density of subsurface materials (Monasterial, 2015). By mapping these gravitational accelerations, researchers can gain insights into the potential size, shape, and characteristics of heat sources driving geothermal systems (Mickus and Jalel, 2018). Gravity data analysis can reveal high-density anomalies associated with intrusive igneous bodies or uplifted basement rocks, which often act as heat sources for geothermal reservoirs (Horta et al., 2021). Additionally, low-density anomalies may indicate the presence of hydrothermal alteration zones, sedimentary basins, or fault zones, which can influence fluid flow and heat transfer within the reservoir (Golla et al., 2019). In the Olkaria region, previous studies have highlighted the importance of fault systems in controlling hydrothermal fluid flow. Mwania (2015) reported that the interaction of post-caldera NNE-SSW and old NW-SE trending faults has contributed to the permeability of fluids in the area, serving as pathways for mass distribution within the field. Faults with high permeability can act as preferential pathways for fluid migration, influencing the distribution of heat sources and density variations (Simiyu and Keller, 2001). Gravity surveys have been conducted in the Olkaria geothermal field to determine structural densities and support reservoir management strategies. Early gravity measurements from 1984 to 2000 were carried out in the Northeast, central, and east fields using the Lacoste and Romberg analogue gravimeter. However, benchmarks were not constructed in the Olkaria domes area, limiting the coverage of these surveys. To address this gap, new benchmarks were established in the Olkaria domes region, enabling comprehensive gravity surveys to be conducted in 2018. These surveys aimed to detect small changes in mass variation, which are crucial for characterizing subsurface heat sources and density variations across the entire Olkaria geothermal field (Mariita, 2020). Ongoing gravity surveys and integration with other geophysical techniques, such as seismic surveys and magneto telluric studies, can provide a more detailed understanding of the subsurface dynamics, including fault structures, fluid pathways, and the interplay between heat sources and reservoir conditions (Simiyu, 2010). This integrated approach is essential for optimizing well targeting, field development strategies, and sustainable resource management in the Olkaria geothermal field.
5 2.2 Statement of the problems The Olkaria Geothermal Field, located in the Kenyan Rift Valley, was a significant source of geothermal energy for the region. However, despite its vast potential, several wells drilled in the field had proven to be unproductive or had experienced a decline in productivity over time. Due to its location along the rift axis, it is associated with a high geothermal gradient with the heat source occurring below the volcanoes (Georgina, 2020). Wells such as OW-720, OW-905, and OW-914 had been identified as underperformers, producing substantially lower outputs than anticipated. These underperforming wells posed a significant challenge, as they represented significant investment costs without yielding the expected returns. To address this issue, a comprehensive investigation was necessary to identify the root causes of the low productivity and develop strategies to optimize the field's performance. In the project, the use of gravity methods, specifically gravity surveys, was used to gain insights into the subsurface structure and the distribution of heat sources within the Olkaria Geothermal Field. Detailed Gravity measurements helps in evaluating the natural state of the reservoir, considering human induced perturbations from reinjection, injection or withdrawal during exploitation (hinderer., 2015; sofyan et al., 2012). By integrating gravity data with other geophysical and geological information, the aim was to delineate the extent and geometry of the heat sources, as well as the distribution of hot fluids and altered rock formations associated with the geothermal system. This approach had the potential to identify areas with favorable conditions for productive wells, as well as to shed light on the factors contributing to the underperformance of existing wells like OW-720, OW-905, and OW-914, ultimately informing future exploration and development efforts in the field. 2.3 Justifications Geothermal energy represents a valuable and renewable source of energy, making the optimization of existing geothermal fields crucial for meeting sustainable energy demands. The underperforming wells in the Olkaria Geothermal Field, such as OW-720, OW-905, and OW-914, pose a significant challenge as they represent substantial investment costs without yielding the expected returns. This warrants a thorough scientific investigation to identify and address the underlying causes of low productivity. Gravity surveys emerge as a well-established and effective
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3.1 Instrumentation Instruments used were CG-5 Autograv gravimeter, GARMIN ETREX 32x GPS and a watch GARMIN ETREX 32x GPS is handheld device allowed me to mark waypoint, tracks and routes. Making me easy to navigate back to a specific location or plan a new route. Surveyors can use the eTrex 32x to record waypoints corresponding to gravity measurement locations. These waypoints are logged with latitude, longitude, and altitude data, providing precise location information for each measurement point. Figure 3 GARMIN ETREX 32x GPS CG-5 Autograv gravimeter was used to record gravity changes due to the deformation of earth from tidal. The primary function of the CG-5 Autograv was to measure the local gravitational acceleration with extremely high precision. It can detect even subtle variations in gravity caused by variations in the density and composition of the subsurface geology.
8 Figure 4 CG-5 Autograv gravimeter The watch was used to determine the time at which the measurement at was measurements were taken. 3.2 Operation in the field The coordinates of the points were feed to the GARMIN ETREX 32x GPS so as to ease locate the benchmarks. The constructed Benchmarks were compacted well ensure the stability, consistency, and accuracy of the measurements. (Melhorato, 2016) Properly stabilized benchmarks provide a reliable reference point for gravity surveys, enabling more precise and meaningful interpretations of the data. the tripod serves an essential function for ensuring accurate and stable measurements. The tripod's primary purpose is to provide a stable and level platform for the gravimeter during data acquisition. The tripod provides a sturdy and stable base for the gravimeter, minimizing any potential movement or vibrations that could affect the sensitive measurements. (Gettings, 2009). Gravity measurements require extreme stability, and even small movements can introduce errors. Leveling: Most tripods used for gravity surveys are equipped with adjustable legs and a built-in bubble level. This allows the operator to precisely level the tripod, ensuring that the gravimeter is perfectly horizontal and aligned with the gravitational field. Proper leveling is crucial for accurate gravity readings. 3.2.1 Levelling of the gravimeter The CG-5 Autograv gravimeter features a digital display screen that provides visual aids to assist the operator in achieving precise leveling. One of the key visual indicators is a crossing or crosshair symbol accompanied by an emoji or icon. The crossing or crosshair symbol represents
10 3.3 Field area topography The Surfer software was utilized to create an intricate contour map that meticulously illustrated the undulating terrain and elevational variance of the geographic area Figure 6 area topography for Olkaria domes The heights above sea level of the study area ranges from approximately highest 2170m and lowest 1870m. Figure 6, reveals that the elevation is higher around the southern parts of the Olkaria domes, and slightly lowers towards the northern parts. The volcanic activity in the area probably resulted in increased elevation to the north due to mantle materials pushing within the Earth’s crust. High gravity values in this area could be due to hot denser materials from the mantle that penetrated towards the crust.
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4.1 Introduction Prior to conducting interpretations of the gravity survey results, it is essential to first perform corrections on the gravity data to account for various factors such as latitude, elevation, and instrumental errors. Standard gravity data processing usually consists of latitude correction, free- air correction, and Bouguer correction to compensate for the gravity reduction due to the variations in topography and position (Boddice et al., 2018; Hackney, 2020; Long & Kaufmann, 2013; Seigel, 1995). This step ensures that the data accurately reflects the true gravitational field, providing a reliable foundation for subsequent analysis and conclusions. 4.2 Data corrections performing gravity data corrections was essential for maintaining the quality and integrity of gravity measurements. This enables accurate geological interpretations and informed decision making in various geoscience applications. Several types of corrections ware typically applied to raw collected gravity data to ensure accuracy and remove biases: Drift Correction corrects for any instrumental drift or long-term variations in the gravity meter's sensitivity. Typically performed by comparing repeated measurements at a base station or applying calibration data. Δgdrift = Δg base - Δg_base_mean Drift corrections for the first day was displayed with the following graph
