Oluma, BefekaduAyele, Tibebu2018-07-112023-11-092018-07-112023-11-092001-06http://10.90.10.223:4000/handle/123456789/7960This geophysical investigation comprises :gravity and electrical resistivily mdhods that: add new infonnation to the geology and tectonic setting of the study area. The results show major structures, major lithologic layers, intrusive bodies and thermally affected zones from the mantle-crust boundary to the surface. The radially averaged power spectrum result of the Bouguer gravity marks the crust-mantle boundaty at an average depth of 27 km. The Clust above this contact is classified in to three major gravi-stratigraphic 'units, each unit consisting of a statistical ensemble of gravity anomaly sources generating comparable gravity field intensities. Geologically, the first deepseated gravi-stratigraphic unit of relatively high-density is about 15 to 17 km thickness and is interpreted as the Basement Complex (?) It is expected to be highly perforated, paliially melted and assimilated with magmatic and basaltic intrusions. On top of this Precambrian Basement a 5 to 7km thick Tertiaty basaltic sequence is found and this is overlaid by recent siliceous volcanic products and recent rift sediments. The regional residual maps show the most tectonically active spreading zones at a depth greater than 10 km. The band pass filter applied to the Bouguer map shows intrusions that make a gravity contrast of 18 to 20 mgal with the host rock. These intrusions are situated under Shala caldera and under Aluto volcanic complex. They are interpreted as magmatic intrusions or magma chambers. The tops of these magmatic intrusions are below a depth of 7 km. The major intrusion beneath Aluto is bifurcates at this depth and show twin positive gravity anomalies at SSW of Adami-Tulu town and at nOlth of lake Langano bay. Major fl'actures and faulted zones (?) at about 2 to 3 km depth are mapped. The presence of these stlUctures is clUcial if at all there is ground water flow and hydrothermal fluid circulation at this depth. The mapped intlUsions are considered to be the geothelmal heat sources for the study area. In the electrical resistivity interpretation, the apparent resistivity maps, the psuedosections and the geoelectric sections could delineate thermally affected regions, hydrothermaly-altered zones and stlUcturally weak zones. The Aluto volcanic rocks up to a depth 200 m show high resistivity values reaching to 3000 Ohmmeter. A very conductive region shows itself below this resistivity zone. The YES interpretations confirm the existence of a fi'actured and velY conductive zone at the location of La-3 and La-6 geothermal wells. This zone is considered as the major up flow zone (TlMariam, 1996; ELC, 1986). The apparent resistivity maps also show the increase in the conductivity of the subsurface with depth and the relative lateral resistivity distribution of the subsurface. Based on the joint analysis of the resistivity and the gravi-stratigraphic units at an estimated depth of 0.5 to 2.5 km about six geothermal wells are anticipated. Besides, additional geophysical methods are proposed to further reinforce the anticipated wells and further confilm the outcome of this study.enAluto Geothermal AreaThesis