School of Physical Sciences (Phys-Sciences) Collections

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Browsing School of Physical Sciences (Phys-Sciences) Collections by Subject "Albertine Graben"
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    Estimation of Ground Motion Levels Due To Earthquakes of the Albertine Graben In Uganda.
    (Makerere University, 2021-02-17) Ikara, James
    Present probabilistic seismic hazard maps for the Albertine Graben, for peak ground acceleration and the spectral ordinate at periods of 0.2seconds and 1.0seconds;with 10 % probability of being exceeded on average once in 50 years were determined using data derived from different sources.The catalogue was prepared by collecting datafrom International agencies, review of a catalogue by Twesigomwe,(1997). The magnitudes on the catalogue were homogenized to moment magnitude (𝑀𝑤). This was done by converting both the body wave magnitude (𝑚𝑏) and surface wave magnitudes (𝑀𝑠) into moment magnitude using the regression equations derived for the Albertine Graben. Based on seismicity and the tectonics of the Albertine Graben, the region was divided into Four (04)seismic source areas, each of which contributes to the seismic hazard in the Albertine Graben.The maximum peak ground acceleration(PGA)values 26% g to 30% g for return periods of 50 and 100years on rock site conditions and 28%to 32% g on soil site conditions of the Albertine Graben. These correspond to earthquakes of intensity (I -VIII) which are perceived to cause from no damage to moderate or heavy damage. The ground shaking will vary from not being felt to severe (Table 2.1). These results are in agreement with Twesigomwe,(1997), whose result for the East African Rift System are in the range (10 -22) % g, Midzi V. et al.,(1999) for EARS, for 50years return period which are in the range of (10 -26.0)% g.The spectral accelerations observed on rock and soil site conditions when T=1.0seconds is 27%g to 32% g while for T=0.2seconds, 29%g and 32%g on rock and soil site conditions the Albetine Graben respectively. This shows that at shorter periods of vibration, rock sites register higher spectral acceleration values while at longer periods they register lower spectral acceleration values compared to soil site conditions of the Albertine Graben.The suggests that at shorter periods(higher frequencies) of vibration on rock site conditions, the seismic energy moves with high amplitudes but dies away when the period of vibration is prolonged (lower frequencies) while for soil site conditions, the amplitude of the seismic wave is reduced at shorter periods and is amplified as the time of vibration is prolonged as shown in figures (5. 5 to 5. 12).
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    Gravity and magnetic data analysis to detect the origin of carbon dioxide in Semliki Basin, Albertine Graben
    (Makerere University, 2011-12) Ojangole, George Francis
    Semliki basin is one of the most prospecting basins for Petroleum exploration in Uganda. An exploratory well (Turaco-3) drilled to the depth of 2960 m by Heritage Oil and Gas Limited in the Semliki Basin encountered a hydrocarbon-bearing formation, which was found to be heavily contaminated by carbon dioxide (CO2), however, the origin of CO2 is unclear. In order to investigate the origin of CO2, gravity and magnetic data was used in this study to: (a) define the geometry/shape of the gravity and magnetic source bodies by delineating their edges. (b) estimate the depths to the gravity and/or magnetic source bodies in the study area and (c) to integrate the model results with the qualitative analysis of the potential field signals from the subsurface sources in order to establish the origin of the CO2. Regional- residual separation to enhance the local changes in the potential field was done by applying Band pass filter, analytical signal, tilt derivative and horizontal gradient. Euler Deconvolution was then used to define the basement faults and fractures. The results provided new insights into the gravity and magnetic anomalies of the southern part of Lake Albert and the Semliki basin. Some prominent magnetic anomaly trends were observed, namely NE-SW, E-W and NW-SE. The positive correlation between these anomalies and fault trends is an indication that the observed magnetic anomalies and their associated subsurface sources are fault controlled. The faults consequently would play a significant role in the movement of subsurface fluids across the region. The presence of very high amplitude magnetic anomalies and the values of their respective estimated magnetic susceptibilities are considered to support the possibility of magmatic intrusions existing within the Semliki basin, these could be the source of CO2 in the Basin, and migrates through faults. The Rwebisengo anomaly in particular has very high significant association with subsurface structuring and the high amplitudes associated with these anomalies are a good indication of their correlation with magmatic sources in the subsurface. The models further indicate that the sources could be within 3 km depth and shallower.
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    Potential Hydrogeological Contaminants derived from petroleum activities in the Albertine Graben
    (Makerere University, 2021-02) Guma, Brian Emmanuel
    Current understanding of the hydrogeological characteristics of rift-margin environments is limited and not well reconciled to large- scale conceptual models of rift evolution. In Uganda with substantial dependence on groundwater for rural water supplies, it is unclear how global change including localised development of petroleum impacts the quality and quantity of available groundwater resources. Here, synthesis of hydrogeological and hydrogeochemical properties of the rift margin environments of the Albertine Graben, Uganda is used to understand the influence of geology and petroleum development on groundwater quality. Datasets include surface resistivity surveys and hydraulic tests, supported by evidence from lithology, structures, drainage, and water chemistry from both the current study and archived/historical sources. There are two aquifer environments, (1) transmissive (mean T of 45 m2.d-1), unconsolidated sediments within the trough below the escarpment and, (2) deeply weathered Precambrian crystalline basement rocks of lower transmissivity (mean T of 6 m2.d-1 - 28 m2.d-1) within the Lake Albert basin above the escarpment. WHO (2012) guideline values exceedance included physico-chemical (pH, EC, TDS), and major chemical parameters (Ca, Na, K, and SO4) and heavy metals (Pb, Fe, As, Mn, and Hg), ranging from 2 to 100%, during both wet and dry seasons. Key processes influencing groundwater chemistry are: weathering of felsic granites and reverse cation exchange in both wet and dry seasons. Groundwater is predominantly earth alkaline in both Bunyoro and Karuma Groups and alkalis in the Albertine Rift, with bicarbonate being the major anion. Multivariate statistical analyses attribute the presence of heavy metals (Fe, Hg, Cd, Cu, Zn, and Mn) to anthropogenic sources associated with petroleum development. Geogenic sources are suggested for all the major cations and anions and some heavy metals (Pb, As, Cr, and Co). Environmental isotope data suggested recharge by direct precipitation to the Albertine Rift, Karuma, and Bunyoro Groups as well as through the rift margin environment.
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    Source rock maturation and hydrocarbon generation evaluation using 3D Basin modelling
    (Makerere University, 2019-11) Nakisita, Betty Mpongo
    The Semliki basin is located in the Albertine Graben which is part of the western arm of the East African Rift System (EARS). The basin demonstrated existence of a Petroleum System (Kiconco, 2005 and PEPD, 2012). Although an oil seep has been found in Semliki basin, the major hydrocarbon discovery in the wells is natural gas. This therefore created a need for an explanation for the hydrocarbon nature observed. The evaluation of the source rock maturity and hydrocarbon generation of the Semliki basin was done using 3D basin modelling approach. The research utilized the existing 2D Seismic data and well completion reports from Semliki basin provided by the Directorate of Petroleum and geochemical data abstracted from literature. The methods involved interpretation of faults and horizons using Schlumberger Petrel software to acquire surfaces which in turn were used as input data for PetroMod software to generate the final hydrocarbon maturation and generation model. The results showed that the basin has generally experienced several episodes of subsidence at both slow and rapid rates. The deposition of the source rock commenced in Early Miocene (12 Ma) and continued at a gentle subsidence until Mid-Miocene (9 Ma). This was followed by a series of rapid subsidence (9-7.5 Ma; 5-4.5 Ma; 4.5-3 Ma; 2-1Ma) and slow subsidence episodes (7.5-5 Ma; 3-2 Ma; 1 Ma –present). A total burial depth of 2500 m was reached at Pleistocene. The maximum temperature reached by this interod is 140oC and the kerogen is currently at the Early to Late Oil window maturity. The transformation ratio shows that source rock in the deeper parts of the basin has completely transformed to hydrocarbons, while those in the shallower parts of the basin are immature or have only attained the early oil window. The Kasande source rock in the Semliki basin started generating hydrocarbons during Pleistocene. Almost half of the source rock has generated but at different maturity levels, whereas, the other half has kerogen which has not yet expelled any hydrocarbons. Therefore, the source rock is still generating to date. This research has provided a better understanding of the key controls to the hydrocarbon generation in the Semlki basin.