Browsing by Author "Alemu, Abera (PhD)"
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Item Application of Gravity and Magnetic Methods to Characterize Subsurface Structures Contributing to Ground Water Flow in the Meki-Bulbula Rift Corridor, Cmer(Addis Ababa University, 2018-05-05) Wakgari, Tolu; Alemu, Abera (PhD)The objective of this MSc thesis is to characterize subsurface structural influences on groundwater flow in the Meki-Bulbula rift Corridor located in the central main Ethiopian rift using integrated geophysical methods namely, gravity and magnetic methods. The surface geology of the study area ranges in composition from volcanic rocks of rhyolite, pyroclastic, obsidian, and ignimbrite rocks, to lacustrine sediment and alluvial deposits. It is determined that the tectonic setting and rock types of an area are important in establishing the distribution of the groundwater flow. Existence and orientation of subsurface linear structures, which may include faults, fractures, and veins have been investigated in the Meki-Bulbula rift Corridor using horizontal gradient gravity map and tilt derivative magnetic map compiled for the study area. These geophysical investigation results have identified subsurface geologic structures, which are responsible for the flow of groundwater from Meki to Bulbula. The depth, distribution and location of the subsurface geologic structures have been shown by Euler deconvolution gravity and magnetic maps. Additionally, field observations made during the magnetic survey have confirmed the flow of ground water from Lake Ziway towards Lake Langano. This conclusion is also confirmed by the existence of hot springs at the northern shore of Lake Langano, which is missing at the southern shore of Lake Ziway. This work has shown that there are no east west structures that favor the flow of groundwater. Hence, the direction of groundwater flow in the study area takes place from Lake Ziway towards Lake Langano being controlled by the N-S and NE-SW oriented faults and fractures mapped in the Meki – Bulbula corridor.Item Application of Gravity, Magnetic and Electrical Resistivity Methods for Geothermal Investigation at the Corbetti Caldera, Main Ethiopian Rift, Ethiopia(Addis Ababa University, 2006-05-30) Kebede, Bisrat; Alemu, Abera (PhD)In this thesis work integrated geophysical techniques, involving electrical resistivity, gravity and magnetic surveys have been carried out over the Corbetti Caldera in the central Main Ethiopian Rift (CMER) to verify the geothermal potential of the caldera. Corbetti geothermal prospect is located about 250 km south of Addis Ababa and 20kms from Awassa and the area is bounded by Lake Awassa to the south and Lake Shalla to the north with geographic location between latitude 7.170N-7.250N and longitudes 38.300E-38.470E. The Corbetti Caldera is characterized by Quaternary volcano tectonic activity which is mainly silicic volcanism and a resurgent caldera structural system. The Quaternary volcanism is associated with a wide spread of steaming ground and fumarolic activity which evidenced the existence of a heat source at depth. 56 VES points, 200 gravity and about 200 magnetic data have been used and analyzed and results are presented as magnetic, gravity and electrical counter map for qualitative interpretation and also gravity and magnetic 2-D model constrained with Geoelectric section for quantitative interpretation. The complete Bouguer gravity anomaly in conjugation with total magnetic field anomaly map of the area indicates the existence of intrusion beneath the caldera, i.e. the highest Bouguer gravity anomaly resulting from the higher density of the intrusion correspondingly the shallower heat source caused by this intrusion is characterized by the lowest magnetic anomaly response. Results from vertical electrical soundings along profile-2 indicates the presence of middle thicker conductive zone which is associated with the increase in temperature and alteration of rocks and apparent resistivity map for AB/2=1810m and AB/2=2700m shows low resistivity anomaly follows the eastern and northern caldera rim, stretching north of the caldera towards Lake Shalla. Based on the joint analysis of data from these geophysical methods and thermal manifestations in the area, the area has the potential for large scale electrical power development. Besides, additional geophysical methods are proposed to further reinforce the outcome of the study.Item Application of Integrated Geophysical Methods for the Landslide Studies of the Bonga Town(Addis Ababa Universty, 2000-03) Mekuria, Senay; Alemu, Abera (PhD)Integrated geophysical methods, particularly refraction seismic, resistivity sounding and profiling and magnetic were conducted on the landslide affected part of the Bonga town. Based on the data obtained in the survey refractor velocity and depth maps, total field magnetic and derivative maps, resistivity maps at three depth levels (AB/2=10, 45, and 100 m) and resistivity and seismic sections on selected profiles are prepared. Tertiary volcanic, namely basalt and tuff, are the main rocks covering the Bonga town landslide area. These rocks were affected by high degree of weathering that is responsible for the formation of different grade, from weak to relatively strong basalt and fine to coarse-grained tuff. The quatematy colluvial and residual soils are found in the area covering the volcanic on the gentle to moderately sloping topography, whereas the alluvial soil is occupying the flood plains. The results of these surveys give that the saturation of unconsolidated colluvial and alluvial soil and the highly weathered volcanic formations of basaltic origin from recharge of groundwater system and the high intensity and long term rainfall in the area are determined to have a major contribution to landslide processes of the Bonga town. This high intensity and long duration rainfall by wetting and increasing the volume of the loose colluvial formation to introduce imbalance between the shearing and resisting forces that generate instability. Geophysicaly inferred structural features were delineated that have strong correlation with the occurrences and orientation of the geologically mapped active landslide scarps in the area. The landslide area is classified into five zones based on the measured physical responses, such as resistivity, P-wave velocity and total field magnetic intensity, together with the geological observations of which Zone five (Z-5) is determined to be the most unstable one that favour landslide occurrences. Finally, some preliminary remarks and recommendations are forwarded which would help to better understand the overall landslide process in the area. The recommendations are thought to lead towards taking mitigating and controlling measures by minimizing the effect of landslide occurrences on the wellbeing of the dwellers in particular and infrastructure development of the area in general.Item Application of Integrated Geophysical Methods to Map Geological Stractures and Assess the Ground Water Potential of the Legedadi-Legetafo-Ayat Prospective Area(Addis Ababa University, 2015-05-05) Biazen, Mequanent; Alemu, Abera (PhD)This MSc thesis work entitled “Application of integrated geophysical methods to map geological structures and assess the groundwater potential of the Legedadi-Legetafo-Ayat prospective area" located in Central Ethiopia, Oromia National Regional State, Finfine Area Special Zone of North Shewa Zone, and Berek Woreda. The objectives of the study are to explore deep high potential groundwater saturated zones and map geologic structural elements which act as a hydraulic barrier that impedes or enhance the movement of the groundwater. The work is being undertaken by Water Works Design and Supervision Enterprise (WWDSE). Mainly the study was studied to solve the shortage of water for the Addis Ababa City. Further, the work is aimed at determining the depth to water table and identification of drilling point through interpretation of the geophysical, geological and borehole data. Geophysical data have been measured, processed and interpreted in one and two dimensions using special software. The results of interpretation indicate that the study area composed of two aquifers, the first is the Upper Basalt aquifer has wide distribution and forms confined and unconfined aquifer system. The thickness of this formation is highly variable from more than 400 meters at Legetafo area to less than 50 meters in Becho plains and the second is Lower Basalt aquifer (LBA) which is composed of tertiary Tarmaber basalt composed of dominantly scoraceous basalt and Amba Aiba basalt. These lineaments can be group into four main systems based on their orientation .These are: NE-SW, NW-SE, N-S/NNE-SSW and E-W trending fracture systems.Item Applications of Integrated Geophysical Techniques to Map Subsurface Structures, Contributing to Ground Water Flow in the Ziway – Langano Corridor, Central Main Ethiopian Rift(Addis Ababa Universty, 2017-06) Tadesse, Mesfin; Alemu, Abera (PhD)The aim of this MSc thesis is to evaluate subsurface structural influences on groundwater flow in the Ziway - Langano Corridor area located in the central main Ethiopian rift using integrated geophysical methods which include gravity and magnetic method. The study is deemed to be an essential resource in evaluating the subsurface geologic structures. The surface geology of the study area ranges in composition from volcanic rocks of rhyolite, pyroclastic, obsidian, and ignimbrite rocks, to sedimentary rocks of lacustrine sediment and alluvial deposits. It is determined that the tectonic setting and rock types of an area are important in establishing the distribution of the groundwater flow. Existence of subsurface structures which include faults, fractures, and veins have been investigated in the Ziway – Langano Corridor using horizontal gradient gravity map and tilt derivative magnetic map compiled for the study area. These geophysical investigation results have identified subsurface geologic structures which are responsible for the flow of groundwater from Lake Ziway towards Lake Langano. In addition to this, field observations made during the magnetic survey have confirmed the flow of ground water from Lake Ziway towards Lake langano. This conclusion is also confirmed by the existence of hot springs at the northern shore of Lake Langano which is missing at the southern shore of Lake Ziway. This work has shown that there are no east west structures that favor the flow of groundwater. Hence, the direction of groundwater flow in the study area takes place from Lake Ziway towards Lake Langano being controlled by the N-S and NE-SW oriented faults and fractures mapped in the Ziway – Langano corridor.Item Assessment of Geothermal Sources Using Integrated Geophysical Methods a Case Study of the Aluto Langano Geothermal Field and Its Environs Central Main Ethiopian Rift Ethiopia(Addis Ababa University, 2021-06-05) Demissie, Gemeda; Alemu, Abera (PhD)The Main Ethiopian Rift is one of the world’s most tectonically active areas with widespread volcanisms and enhanced geothermal gradient. However, the areal extent, nature and depth of the geothermal resources remain imprecise. The study area which mainly comprises the Aluto-Langano geothermal field, the Munesa escarpment and the Gedemota ridge is located in the central part of the Main Ethiopian Rift. The Aluto-Langano geothermal field is bounded by Lake Ziway to the north, Lake Langano to the southeast and Lake Abiyata to the southwest. The hydraulic gradient of Lake Ziway dips towards the south and is thought to feed water to the Aluto-Langano geothermal. The Aluto-Langano geothermal system encompasses younger NNE-SSW oriented en-echelon faults of the WFB and NE-SW oriented boarder faults. The fault systems belonging to the WFB are generally thought to generate fracturing of rocks that could favor the development of geothermal reservoirs beneath the study area. Gravity and magnetic studies constrained by the available geological, borehole and geophysical data are conducted over an area consisting of the Aluto-Langano geothermal field and its environs. Data enhancement techniques (upward continuation, analytical signal, tilt derivative, Euler deconvolution and reduction to pole) have been applied to the observed gravity and magnetic data in order to isolate the effects of deep seated and shallow origin geologic sources. Interpretations of the gravity and magnetic data in combination with the available data (seismic, geological and well-log data) have provided important clue about the geothermal sources of the study area. 2D gravity models computed for the study area and running across and along the rift axis reveal six density layers (silicic products, lacustrine sediments, basalts, Tertiary ignimbrite, Mesozoic sediments and crystalline basements). The computed 2D gravity models show occurrence of a magma chamber/intrusion occurring at depths between 2km and 3km beneath the Aluto volcanic complex. A detailed investigation of the shallow subsurface using 2D gravity models, volcano-tectonic evidences, surface thermal manifestations, various gravity and magnetic maps has revealed the existence of a potential heat source/magma chamber occurring beneath the Aluto volcanic complex. Quaternary faults of the Wonji Fault Belt are inferred to play a vital role in transporting melt from a deeper intrusion to the magma chamber, in acting as conduits for migration of surface and/or groundwater to the magma chamber and in transporting thermal fluid to the surface (detected as surface thermal manifestations) beneath the Aluto-Langano geothermal system.Item The Gravity Field of the Southern Ethiopian Rift Systems Between 3.30 To 7.150 Latitude and 360 To 420 Longitude(Addis Ababa University, 1997-05) Asrat, Estianos; Alemu, Abera (PhD)1800 gravity data covering the research a rca are reduce to Bouguer and free air values and interpreted based on the Bouguer anomaly map and a pair of short wave length (SWL, residual) and long wave length (LWL, regional) anomaly maps. All data are reduced to sea level wth uniform density of 2.67 glcm'- Effect of Bouguer masses were calcu lated applying the simple Bouguer corrections. Terrain correction "vas not applied. The Bouguer anomly map of the rift zone between 3.3° N to 7.15° Nand 36° E to 42° E is presented. The geologic and tectonic setting of the area is di scussed based on the compilation of gravity data and synthesis of publi shed papers. A positive Bouguer gravity anomaly of - 165 mGal occur over the rift floor of the Abaya - Chamo rift and is flanked by negative anomalies that res ide on its shoulders. Gravity anomaly of magnitude -135 mGal is observed over the Chew Bahir ri ft floor and flanked by relative positive anomalies on its shoulders, which is different from the MER. This could be due to lacustrine deposits of the rift floor. The free - air anomaly map is si mply relatcd to topographic relief. The uplifted blocks and horsts are characterised by high free - air gravity anomaly and depressions defined by free - air minimum (-40 to - 75 mGal ). The transition from grabens to horsts is marked by sharp free - air gravity gradients (0 to +40 mGal). After removal of the long wave length components of the Bouguer gravity anomaly the residual gravity anomaly is interpreted. The residual gravity field exhibits rapid changes of gravity gradients over short distance appears to show the presence of fa ults which bound major rift basins. Anomalous areas are corelated to their respective geologic in te rpretation. The regional gravity anomaly map produced by trending analysis shows the deep structural features of the study area. It shows the transition from negative to positive gravity fie ld from MER to SER and negative towards the Kenyan rift indicating the distinction between the SER and the other two rift systems (MER and Kenyan rifts).Item The Gravity Field Southern Ethiopian Rift Systems Between 3.30 to 7.150 Nlatitude and 360 to 420 Elongitude(Addis Ababa Universty, 1997-05) Asrat, Estifanos; Alemu, Abera (PhD)1800 gravity data covering the research area are redilrc t·) fItem The Gravity Field, Magnetic Field and Regional Tectonic Setting of the Corlletti Caldera and its Adjacent Areas(Addis Ababa Universty, 1999-06) Getachew Shiferaw; Alemu, Abera (PhD)In this thesis work analysis of about 2220 gi'aVity' alia 75 liuigileli6 data' colieCted ih the 'Lakes District of the Main Ethiopia Rift (MER), between latitudes 6.88° - 7.75° and longitudes 38.00° - 39.00° consisting of the Corbetti caldera is made. A gravity survey covering about an area of 12100 sq. kms with obselvation points of about 2220 stations are discussed (about 50 wavity and 75 magnetic stations, were occupied during field work by the author of tlus thesis and Dr. Abera Alemu, advisor to tlus thesis work, wIllie the rest of Wavity data were obtained from the Ethiopian Institute of Geological SUlveys and the Geophysical Observatory of Addis Ababa University). Both the magnetic and gravity data sets are of senu-regional type, with station separations ranging on the average from about 1-10 kill. The analysis includes compilation in a standard fonnat of the recent and the previous gravity data sets, wluch required homogelusation to IGSN-71 datum from the previous surveys that was found out not available in this datum. The theoretical (nornml) gravity values for each stations were calculated by means of the international gravity fonnula (GRS-67) and all Wavity field stations are tied to the IGNS-71 datum of the Geophysical Observatory of Addis Ababa Uluversity (whose gravity value is 977452. 16 mOaI). Similarly, the magnetic daia an! alr,o d.r,ffi cqll'e4t~:(or tl¥ di\lfiial v~llations: Results of both the gravity and magnetic surveys'll'ave' revealed sig'nwcailf achievetllehtsin mapping several zones of hydrothennal alterations, weak stmctural indications such as faults, lineaments, fi'actures, joints, etc. within the study area. The delineation of weak stmctural features and hydrothennal manifestations i.e. altered grounds with fumarolic activities that are associated with fissures, craters, fanlts and caldera rims, where tectonic stmctures give major access for hydrothennal fluids which in tum are favourable for economic mineral potentials like sulphides, epitennal gold deposits, etc. In addition, to these invaluable natural resources these data are also important in the mapping and location of potential geothennal zones. Both the gravity anomaly maps and the magnetic anomalies are found in agreement in mapping of the geological and stmctural trends. The distinction between the lift and the adjacent plateaus and the regional stmcture features were depicted fi'om the anomaly maps on the basis of the general shape and wavelengths ofthe anomalies.Item The Gravity Filed and Crustal Structure of the Southern end of the Ethiopian Rift System(Addis Ababa University, 1993-07) Lakew, Alemayehu; Alemu, Abera (PhD)The Bouguer anomaly map of the rift zone south of the southern end of the Main Ethiopian rift between Lat. 4° OO'N .;; 7° OO'N and Long. 36° 00 I E - 39° 45 ' E is presented. The computed Bouguer anomaly map shows a complex pattern of positive and negative Bouguer gravity anomalies. A positive Bouguer gravity anomaly of -165 mGal occurs over the floor of the Abaya-Chamo rift and is flanked by negative anomalies that reside on its shoulders. The anomaly pattern associated with the Chew Bahir rift is completely different from the other regions. The floor has a negative gravity anomaly of magnitude -135 mGal and is flanked by positive gravity anomalies residing on its shoulders. This may be attributed to the swamp and volcano-Iacostrine nature of the rift floor. Further south in Kenya, the Bouguer anomaly map shows that there is a relative increase in magnitude of the regional gravity field with a complex pattern of alternating positive and negative anomalies which are generally oriented a NNW-SSE and E-W direction. Two profiles that run through the major tectonic systems (the Abaya-chamo rift and the Chew Bahir rift) of interest in the area were extracted from the Bouguer anomaly map. 2%-0 gravity models were constructed along these profiles. The results of the model calculations are that: Along Profile AA, the crust beneath the plateaus is 35 to 40 km thick. within the axis of the Abaya-Chamo rift the crust thins to 29 km. Along profile BB, the crust beneath the high lands in the vicinity of the Amaro horst is 35 to 40 km thick. Beneath the rift zone, i.e., the floor of the Chew Bahir rift and the flanking uplifted regions the crust thins to 27 km. The observed crustal thinning beneath_the rift zones along both profiles is thought to be caused by an upward progression of low density mantle material (anomalous mantle) that intrudes itself into the lower crustItem Ground Velocity Prediction Based on Shot Distance, Shot Size and Shot Site Environment Across the Main Ethiopian Rift(Addis Ababa Universty, 2005-06) Abebe, Demisachew; Alemu, Abera (PhD)Accurate Prediction of grand velocity is essential in determining seismic shot size which minimizes risk to nearby manmade structures yet still ensures acceptable signal-noise ratio at large distances. In our study ground velocity data from the 2003 EAGLE conducted across the Main Ethiopian Rift were compared with the semi-empirical ORIARD formula on either rare of the MER. The ORIARD formula predicts velocities for scaled distance greater than 1.85 km per kgl/2, but over predicts for velocities less than 1.85 km per kgl/2. However, within MER, the ORIARD formula predicts velocities between 2 and 8 scaled distances, but over predicts velocities of a scaled distance less than 2 km per kgl/2 while under predicts for a scaled distance greater than 8 km per kgl/2.Item Integrated Analysis and Interpretation of Helicopter Borne Aeromagnetic and Gamma Ray Spectrometric Data of the South Meleka (Adola Greenstone Belt, Southern Ethiopia)(Addis Ababa University, 2015-07-07) Mammo, Behailu; Alemu, Abera (PhD)This work uses part of an airborne survey data of Adola belt generated by Geological Survey of Ethiopia in 1994. The belt has been studied by different researchers to explain the structural pattern and mineralization. Here, the different enhancing filters are applied on the aeromagnetic and gamma ray spectroscopic data and the geophysical signatures gleaned are used to explain the rock units and structural features associated to mineralization. In addition, the airborne coverage data of Ethiopia is updated with new coverage data gathered for petroleum exploration. The Upward Continuation (UWC) filter at different height shows the effects of deeper regional source by removing the shallower effects and possible noises. Upward continued TMI (up to 500m) above the flight height of the magnetometer showed a WNW-ESE trending linear structure at the southern parts of the area. Further enhancement using tilt derivative reveals some more geological features. The Gamma ray spectrometric data is regridded with a cell size of 15m and a bi directional line gridding is applied using Oasis Montaj, and filtered and shaded maps of the individual radio elements (U, TH, K, and TC) and a ternary image are produced. The ternary map demarcated major geological units such as basalts, gneisses and central volcano sedimentary terrain. These are peculiar signatures well marked at the eastern, western and central part of the area. The ratio map of Th/K is prepared to select the alteration zones favourable for mineralization. The Total Count map mapped the central volcano sedimentary belt with a low total count from northern end up to the southern covering a wide part of the study area. It also portrayed area around Kilenso Babicho, Meleka and north Sakaro with a high total count. The analysis and interpretation from the ground magnetic and radiometric surveys are compared with that of the aeromagnetic and gamma ray spectrometric data on selected areas and are found to be well corroborated. From the result of all the enhanced maps the trends and dispositions of the inferred faults at the southern part are modified. Power spectrum technique is applied to estimate the depth to source of the magnetic body. The average depth for both the shallower source and deeper source anomalies using the FFT analysis method are estimated to be 11.98m and 43.54m respectively. From all the interpreted maps composite map is produced which show the magnetic lineaments, altered zone, and selected target area for further study and follow-ups.Item Integrated Geophysical Investigations at Sodere Thermal Springs, Main Ethiopian Rift(Addis Ababa Universty, 2008-07) Solomon, Eyasu; Alemu, Abera (PhD)In this thesis work two integrated geophysical surveys namely magnetic and electrical resistivity sounding have been carried out over the Sodere thermal springs area. The area is located within the Main Ethiopian Rift (MER), close to Nazareth town and is accessible on an all weather asphalt road on a detour from Nazareth-Assela main road. The Sodere thermal waters are one of the famous and relatively well-utilized centers in the country, which is being used for both recreational and healing purposes. Total magnetic field surveys at station spacing of 25 m well distributed and covering the thermal area were carried out. About 300 magnetic data points were obtained. Further, a total of ten VES stations, distributed to lie on three convenient traverses were measured using the Schlumberger symmetrical array. Maximum current electrode separation (AB/2) of up to 500m was used for the sounding survey. The results of the magnetic survey were presented in terms of profile plots and also contour maps which were later continued upward to remove near surface effects. The resistivity survey data were used to construct apparent resistivity pseudosections and the data were further interpreted and plotted to obtain geoelectric sections. From the magnetic anomaly map of the magnetic survey, the heat source is characterized by the lowest magnetic anomaly response interpreted as resulting from rock units that are forced to lose their magnetization due high temperature. Such areas with low magnetic anomaly and weak zones have been outlined. From the electrical survey, areas of low resistivity (typically < 20Wm) observed on the pseudo- and geoelectric sections along Profiles-1and -2 are interpreted as hydro thermally altered zone and the overall low resistivity observed along Profile-3 is interpreted as highly water saturated region of the area because of the close proximity of the profile to the Awash river and the absence of structures that act as a barriers for ground water movement. Based on the analysis of the two geophysical methods and thermal manifestation, the area has geothermal potential for further development from central to northwest of the study area. Considering the limitations in depth of the electrical methods employed, further geophysical investigations are proposed to reinforce the outcome of the IV study.Item Integrated Geophysical Investigations of the Central Main Ethiopian Rift and Adjacent Plateaus: An Implication to Crustal Structure and Moho Depth Determinations(Addis Ababa Universty, 2016-06) Kelemework, Yemane; Alemu, Abera (PhD)Compilation of newly acquired magnetic and existing gravity, magnetic and seismic data were carried out to examine nature of the crustal structure, probable depth to Moho and density distributions of the crust in the central Main Ethiopian Rift and adjacent plateaus. Data enhancement (filtering) techniques have been applied to the observed gravity and magnetic data in order to highlight the effects of deep seated and shallow origin geologic sources. The compiled Bouguer gravity anomaly map and its derivatives (regional, residual maps) and gravity profiles constructed along selected lines reveal minimum anomalies over the south east and south west plateaus, superimposed with relatively high values over the major volcanic centers and high density rift floor. Correspondingly, the compiled magnetic anomaly map including its derivatives (regional and residual maps) and magnetic profiles constructed along selected lines reveal that the volcanic centers residing the rift floor are associated with relatively high magnetic anomalies as compared to the relatively low magnetic anomalies associated with the sediment dominate rift lakes and parts of the highlands. Close examination of the regional and residual gravity and magnetic anomaly maps and enhancement techniques reveal that the Quaternary volcano-tectonically active zones of the Wonji Fault Belt (WFB) and Silti DebreZeit Fault Zone (SDZFZ) with a general NNE - SSW to N - S trending anomaly zones are interpreted to underlie by shallow magmatic intrusions and/or magmatic chambers. In addition, our result confirms the presence of deep and regional E-W trending anomaly zone in the south west of the study area which is assumed to be coincident with the Goba-Bonga transverse tectonic lineament and associated Wagebeta caldera complexes. Based on the evidences derived from 2-dimensional gravity modeling and seismic data, the crustal thickness (Moho depth) in the study area varies from 35 – 40 km beneath SW plateau, 38 - 39 km under the rift floor and ranges from 42 - 43 km beneath the SE plateau. Our investigation results generally support the idea which suggests that rift extension and crustal deformation in the central Main Ethiopian Rift are currently concentrated in the Quaternary volcano-tectonically active zones of the Wonji Fault Belt, Silti DebreZeit Fault Zone and the Goba-Bonga tectonic lineaments. Keywords: Central Main Ethiopian Rift; Bouguer gravity anomaly; magnetic anomaly; receiver function; crustal thickness.Item Mapping Geological Structures Using Integrated Geophysical Methods and their Possible Implications, A Case Study of the Ziway-Shala Lakes Basin, Central Main Ethiopian Rift(Addis Ababa University, 2020-10-23) Kebede, Hailemichael; Alemu, Abera (PhD); Nedaw, Desie (PhD); Fisseha, Shimeles (PhD)The Ziway-Shala Lakes Basin in the Central sector of the Main Ethiopian Rift (38000‟ E - 39030‟ E and 7000‟ N -8030‟ N), has been affected by active Cenozoic tectonic, volcanic and sedimentation processes. These processes are mainly responsible for the current geological and structural setting of the basin. Even though there exists numerous works on the surface and deep structure of the region, the shallow to intermediate depth litho-structural set up of the Ziway-Shala Lakes Basin is least studied. For its implications on hydrology, geothermal and hydrocarbon resources, this PhD research focuses on delineating the shallow to intermediate depth litho-structural features of the basin using gravity, magnetic and seismic noise data with application of various mathematical enhancements and numerical algorithms. These mathematical techniques include derivatives filters, upward continuation, Line module algorithm, Source Parameter Imaging, Power Spectral Analysis, 2D Werner De-convolution, joint 2D forward modeling and 3D structural inversion. The orientations of subsurface lineaments obtained from gravity and magnetic data analysis are dominantly oriented in NNW-SSE to NW-SE and E-W directions. These results appear to differ from the general orientation of surface geologic structures (NNE-SSW to NE-SW) in the Central Main Ethiopian Rift which is the major tectonics of the Cenozoic rift. The shallow to intermediate depth structural trends revealed in this study seem to reflect the influence of the pre-existing Mesozoic Ogaden rift. Gravity and magnetic depth estimation along the rift axis shows the northward thickening of the Hailemichael K. low-velocity volcano-sedimentary layer. Furthermore, about 561 m thick horizon, possibly of Mesozoic sediments, has been identified between the Tertiary ignimbrite layer and the crystalline basement for the first time. The information obtained with regard to water balance of the basin, salinity level of the lakes and the conceptual hydrological flow model adopted appear to reveal that the groundwater flow in the study region is controlled by subsurface structures (lineaments and the mapped interface topographies). Interpretation of the compiled magnetic anomaly maps reveal the existence of a structural path, between 1.5 km and 3 km of depth, along a traverse running from the Aluto-Langano geothermal field to the Silti Debre Zeyte Fault zone. The structural path is thought to serve as a conduit connecting the Aluto-Langano geothermal field and the likely heat source located around the locality of the Silti Debre Zeyte Fault Zone. The study also identified magnetic lineaments most of which are oriented in the direction of pre-existing Mesozoic structures and in the direction of thermally altered structures. Mesozoic sediment delineated under a thick volcanic cover (2.5 km) could be taken as a favorable geologic setting for accumulation of hydrocarbon and CO2 degassing in the region. Generally, the identified geological structures of shallow to intermediate depth are useful in understanding the groundwater dynamics, geothermal heat sources and hydrocarbon accumulations associated with the regions.Item Structure of the Crust Beneath the Gedemsa Tectono-Magmatic Segment and Its Environs Perceived from Geologically Constrained Geophysical Studies: an Implication for Geothermal Heat Source Assessment, Central Main Ethiopian Rift, Ethiopia(Addis Ababa University, 2020-08-28) Nigussie, Wubamlak; Alemu, Abera (PhD)The Main Ethiopian Rift is in a stage of tectono-magmatic segmentation where the mechanism of rifting is primarily enhanced by a magmatic addition to the crust. However, the extent, nature and structure of the crust beneath each magmatic segment and occurrence of the associated geothermal resources remain unclear. Geologically constrained gravity and magnetic studies have been conducted in the Gedemsa tectono-magmatic segment and its environs. Interpretation of combined potential field data in this study have provided important contributions regarding the geodynamic setting, crustal structure and geothermal resources of the study area. 2D gravity modelling of the entire crust across and along the Gedemsa tectono-magmatic segment reveals three density layers (upper volcanics/sediments, upper crust with intrusion, and lower crust) overlying the upper mantle with underplating beneath the Ethiopian plateau. The along axis tectono-magmatic segmentation and the bilateral off rift volcanic zones revealed by gravity data show the distribution of deformation beyond the rift axis and the complex geodynamic setting of the central to northern Main Ethiopian Rift transition zone. Gravity modelling of the upper crust show the occurrence of intrusions with an estimated depth range of 10-20 km and 5-10 km beneath the Tulumoye-Gedemsa and Boku Volcanic complexes respectively. This magmatic addition to the crust is attributed to be an ongoing dyke intrusion which induces surface fracturing/faulting. The gravity data reveals that at deeper depths the Gedemsa tectono-magmatic segment (25 km wide and 60 km long) is elongated parallel to the Main Ethiopian Rift axis and changes its orientation oblique to the rift axis at the near surface. A detailed investigation of the structure of the shallow crust based on 2D gravity models, 3D geologic interpretation and gravity/magnetic maps offer the presence of promiceable thermal heat sources (as shallow magma chambers at the Boku, Gedemsa and Tulumoye volcano). Quaternary faults/fractures of the Wonji Fault Belt play a triple fold role regarding the geothermal system of the area: 1) Transporting melt from the deep intrusion to shallow magma chambers. 2) Acting as pathways for the surface/groundwater migrating to the shallow magma chambers. 3) Transporting thermal fluid to the surface to be detected as thermal manifestation.Item The Use of the Resistivity and Magnetic Methods for the Investigation of Ground Water, Intermediate and Shallow Structures in Ajie Western Arsi Zone(Addis Ababa University, 2019-05-02) Hailu, Samson; Alemu, Abera (PhD)In this study a combined geophysical data was used for groundwater controlling factors; shallow and intermediate structures in the central main Ethiopian rift. This study focused on a assessing hydrogeological conditions of the area with the controlling factors subsurface layers and structures. Two Geophysical methods were employed in this study are, Vertical electrical sounding and magnetic survey technique. Electrical sounding the sculmerger method was used to investigate for the potential the groundwater in the study area. On the other hand, magnetic survey was used to delineate geological shallow and intermediate structures which control the groundwater flow system. The geophysical survey was conducted, twe15 VES readings by using Schlumberger array along three survey lines, and above two hundred and fifty magnetic reading at about 50meter interval along 4 profiles. The field data obtained was processed and analyzed by using special computer softwares. The2D model with geological layers and resistivity values was interpreted to enhance the result. The results of the geophysical survey are presented in the form of interpreted VES curves, sliced stacked map, pseudo depth section, geoelectric section, magnetic anomaly plot and 2D model. The study result revealed four to five (6) main geoelectric layers that differ in degree of weathering and fracturing. The highly weathered and fractured ignimbrite with is the main water bearing horizon which has good groundwater potential but it is deeply layed. The aquifer thickness in the area is obtained to range between 100m and 240m. The result also shows there are sub surface structures in the area is dividing the area by several subsurface geological shallow and intermediate structures that are important for the movement and occurrence of groundwater. It seems that, some well sites can be towards the end of profile -1 and along profile -3.Further hydrogeologicaland and geophysical survey and can be recommended along the proposed sites for farther studies.