Browsing by Author "Alemu Abera (PhD)"

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    Applications of Gravity and Magnetic Methods to Map Subsurface Structures, Contributing to Ground Water Flow in The Bulbula-Arsinegele Rift Corridor, Central Main Ethiopian Rift.
    (2018-06-01) Woyema Duresa; Alemu Abera (PhD)
    Analysis of newly acquired magnetic and existing gravity data constrained by the existing geologic information is made in order to delineate and map the subsurface geologic structures contributing to groundwater flow within the Bulbula-Arsi Negele rift corridor. Data enhancement (filtering) techniques have been applied both to the observed gravity and magnetic data in order to highlight the anomaly components resulting from deeper and shallower sources. The compiled residual anomaly map and its derivatives (regional, residual maps) reveal that relatively positive values which coincide with the locations of the Aluto volcanic center, Munesa ridge and high density basalt, rhyolite intrusion and extrusion of the Shala caldera. Correspondingly, the compiled magnetic anomaly maps including its derivatives (regional and residual maps) revealed that the volcanic centers residing the Aluto and Shala volcanic center and the Munesa basalt, rhyolite intrusion and extrusion are associated with relatively high magnetic anomalies. Relatively low magnetic anomalies are associated with the sediment dominated around the shore of Lake Langano, Lake Abijata, Lake Shala and the surrounding of Bulbula and O’itu towns. The horizontal gradient gravity map and tilt derivative magnetic map compiled for the study area have determined the existence of NE-SW and N-S lineated subsurface geologic structures (faults and fractures) that favor the flow of groundwater from Lake Langano towards Lake Abijita and Lake Shala. This M.Sc. thesis research has shown that there are no east west lineated structures that favor the flow of groundwater from Lake Langano towards Munesa ridge. Hence, the direction of groundwater flow in the study area takes place from Lake Langano towards Lake Abijita and Lake Shala being controlled by the N-S and NE-SW oriented faults and fractures mapped in the study area.
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    Gravity Field Studies In fuel Main Ethiopian Rift and Southern Afar
    (Addis Abeba university, 1997-06) ZemIchael Lijam; Alemu Abera (PhD)
    All available data, over 3000 stations, obtained from the EIGS and the Geophysical observatory (Dr. Abera's Ph.D work) were reduced to sea level with a uniform crustal density of 2.67 gmicm3 • Effects of Bouguer masses were calculated applying the simple Bouguer cOITection. Terrain correction was not applied in which case its effect was treated as systematic error in computing the over all mean square elTor of the simple Bouguer anomalies at each station. Theoretical gra\·ity field was computed by means of the intemational gravity fonnula of 1967 (GRS67) and tied to the IGS};71. The accuracy of the Bouguer anomaly at each station is calculated to about ± 2.5 mga!. The regional and residual gravity field were calculated from the Bouguer anomaly map by mea~ of a computer software. The compiled Bouguer anomaly map shows a strong con'elation between the regional pattern and topographic feature of the survey area. Owing to higher elevations, the adjacent plateaus are marked by negative Bou"ouer anomalies. the minimum of magnitude -270 mgals being located around Debreberhan for the study area. The maximu'm gravity values occur in the Afar de]Jression conesponding to the relatively lower elevations. TIle main Ethiopian rift including Afar is characterized by a relatively positive anomaly as compared to the plateaus. The high on the rift accounts for the excess mass at depth on which the crust lies relative to the plateaus where a mass deficiency is assumed. Gravity values on the main Ethiopian rift are generally less than those in the Afar. TIils accounts for the th. inning of crust material towards the .-\far along the rift, or it can equall.y be explained as by a thickening of a denser material beneath the crust (probably the anomalous mantle). The margins separating the rift £i'om the plateaus are marked by steep ~ gra\ity gradients with their magnitude' representing the slope of the escarpment. Steep gradients characterize the transition of gra\.ity fidd from the plateaus to the rift floor