Gravity modeling of the Paka geothermal prospect in Baringo County, Northern Kenya Rift

Abstract

Paka volcano is situated in active northern Kenya rift. It is considered one of the geothermal prospects. It has been covered by most studies that focused on the regional tectonic evolution of the East Africa Rift, lithospheric and crustal structural analysis. However, regional analysis overlooked local analysis, where key geothermal structural features like thin massive intrusive and shallow faults that are significant have remained relatively poorly constrained. Hence, the key concealed structural features at local analysis have not been mapped at regional analysis. The study is an advancement of the regional analysis in the northern Kenya rift to a detailed local analysis in Paka volcano. This study aimed at developing a Bouguer anomaly map showing different relative density distribution in the subsurface, to generate a 3D inversion models from Bouguer anomaly data, to interpret the 3D density model developed from gravity data inversion, to delineate the subsurface volcanic fault structures with the view to determine their influence to the hydrothermal fluid flow and contributions to the geothermal system. A total of 157 gravity data points were established over the prospect area for gravity modeling and geological structures delineation at local analysis. Raw data were reduced to Bouguer data and band pass and upward filter were used to generate Bouguer maps. For local analysis, residual data was separated from regional data through application of filters. Our gravity analysis method produced the following results: Bouguer anomaly maps revealed regions of positive gravity anomalies and negative gravity anomalies that trend in a northwest direction. An inversion model under positive gravity regions revealed an intrusive dykes. At a depth less than 1000 m from the surface, the intrusive dyke were interpreted as due to lava rocks of magnesium iron reach and at depth greater than 3000 m as due intrusive magma that can serve as a geothermal heat source. The negative gravity zones were due to mean fractured rocks to a lower calculated density range (1.90 – 2.40) g/cm3 that can serve as a good storage for geothermal fluids. 3D density model revealed a high density distribution that is associated with the existence of several intrusive rocks within the study area that is overlain with surficial pyroclastic or Paka Basalt of layer thickness 200 – 300 m across the prospect of calculated density range (2.20 – 2.60) g/cm3 that may serve as a capping layer for geothermal system around Paka volcano. The gravity data also reveals several NW – SE trending of the mapped structural features that are parallel to the major rift fault and pre-existing Proterozoic regional structures in the northern Kenya rift which seems to have played a role in guiding the orientation and trending for this local structural feature in Paka area. The study revealed and delineated both deep and shallow subsurface faults at shallow depth that are essential in enhancing geothermal fluid permeability. Among the many recommendations highlighted, the study emphasizes an exploratory geothermal borehole drilling be undertaken in Paka prospect from the caldera summit towards the eastern flank and northwestern flank along the fault zones.