Geophysical Investigation of Geothermal Potential of the Gilgil Area, Nakuru County, Kenya Using Gravity Method
Nyakundi, Erick Rayora
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In this study, gravity survey method was used to investigate the geothermal potential field in Gilgil area, Nakuru County, Kenya. No previous geophysical study has been done in Gilgil area. Gilgil area is volcanic and gravity survey in volcanic regions has shown that it gives useful information about the density changes within the Earth’s subsurface associated with the heat source. Gravity highs are related to centres of volcanism, faults and geothermal activity. The discovery of geothermal reservoir in Gilgil area will provide an alternative source of clean energy in Kenya, increase the amount of electricity supplied to the national grid thus lowering the cost of electricity and ensuring it is enough to meet the market demand of electricity in Kenya. The specific objectives of this study were to conduct ground gravity measurements of Gilgil area, determine gravity anomalies and figure out the size, depth and form of the underground body that produces the gravity anomaly. The ground based CG-5 Autograv gravimeter was used to accurately measure gravity at each field station. A total of 147 gravity stations were established over an area of about 68 km2 and gravity corrections done. The complete bouguer anomaly was computed and a contour map for the study area plotted using surfer 8.0 software. Qualitative interpretation of the map shows gravity highs in the study area which were interpreted as dense intruding bodies within the subsurface. Five profiles along the gravity highs were drawn and positioned in the southwest-northeast, northwest-southeast and almost north-south directions for analysis. The regional trend of the profiles was subtracted from the observed data yielding the residual anomaly. 2D Euler deconvolution was done on the profile data and revealed subsurface faults and bodies at a depth range of 790 m – 4331 m. It was discovered that the deep faults transport thermal fluids from deep parts of the Earth to the subsurface. The shallow faults in the Earth’s subsurface direct the flow of thermal fluids on the upper part of the basement. The top faults direct the flow of water from rift scarps to the hot masses underground. This faults were concluded to be responsible for underground movement of thermal fluids. Forward modelling of selected profiles using Grav 2DC software revealed presence of dense intrusive bodies on the northern and southern parts of the study area with the density contrast range of 0.25 − 0.28. These bodies were interpreted as intrusive dykes that have higher density than surrounding rocks. Thus the high heat flow observed in the area as evidenced by hot springs could be due to these intruding bodies within the rift floor faults. This intrusive in form of dykes was concluded to be tapping heat from large magma bodies at few kilometers from the surface. Such intrusive dykes may be geothermal heat sources.