Gravity Survey of the Shalla Caldera and its Environs for the Exploration of Geothermal Energy
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Date
2001-02
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Addis Ababa Universty
Abstract
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.
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Keywords
Shalla Caldera and its Environs