Gravity Survey of the Shalla Caldera and its Environs for the Exploration of Geothermal Energy

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Addis Ababa Universty


For the possible assessment of geothermal resources it is nccessaty to obtain additional geophysical infollnation fium the gravity survey regarding the Shalla caldera and its environs. All available data, over 1041 stations, obtained from the EGS were reduced to sea level with a uniform cl1lstal density of 2.67 gmicm3 Effects Bouguer masses were calculated applying the simple Bouguer correction. TelTain 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 gravity field was computed by means of the international gravity fOllnula of 1967 (GRS67) and tied to the IGSN71. The accuracy of the Bouguer anomaly at each station is calculated to about ± 2.82 mgal. The data shows that the study area is characterised by short wavelength positive Bouguer gravity anomaly of the Ethiopian Rift. The observations define a line of positive residual gravity anomalies due to the mass excesses below the rift floor associated with strong hydrothellnal activities in the rift. There is strong correlation between the positive residual gravity anomalies in the rift following the strike of the WFB, the geothermal activity and the location of Quaternaty siliceous domes and recent fissural basalt flows. The root of the denser fissural basalt's may be still hot and the siliceous domes of the Shalla caldera geothermal field with strong hyrdrothellnal manifestations more likely have hot basaltic dykes underneath associated with their magma chambers that feed their thernlal features. In general all the geothelmal activity north of Shalla caldera may be related with the Alutu-Langano geothelmal field through the recent complex volcanic stl1lctures and the active trending fault system. Similarly the geothelmal activity south of Shalla caldera may be ascribed to the Corbetti caldera through the Shalla-Corbetti segment of the Wonji Fault Belt.



Shalla Caldera and its Environs