Geophysical Studies in the Aluto Geothermal Area

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Addis Ababa University


This geophysical investigation comprises gravity and electrical resistivity methods that add new information to the geology and tectonic setting of the study area. The results show major structures, major lithologic layers, intrusive bodies and thermally affected zones fi'om the mantle-crust boundary to the surface. The radially averagedlYower-spectrurrrresult of-the Bouguer gravity marks the-crust-mantleboundary at an average depth of 27 km. The crust 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. Geologicall y, the first deepseated gravi-stratigraphic unit of relatively high-density is about IS to 17 km thickness and is interpreted as the Basement Complex (?) It is expected to be highly perforated, partially melted and assimilated with magmatic and basaltic intrusions. On top of this Precambrian Basement a 5 to 7km thick Tertiary 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 mgaJ 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 townand at north of lake Langano bay. Major fractures and faulted zones (?) at about 2 to 3 km depth are mapped. The presence of these structures is crucial if at all there is ground water flow and hydrothermal fluid circulation at this depth. The mapped intrusions are considered to be the geothermal heat sources for the study area. In the electrical resistivity interpretation, the apparent resistivity maps, the psuedosections and tile_ geoelectric sections could delineate thermally affected regions, hydrothennaly-altered zones and structurally 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 fractured and very 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 di stribution 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 confirm the outcome of this study. I Crystallographic layer is coined here to mean various layers of the Earth's crust with respect to their density contrast.



Geophysical Studies