Analysis of gravity data to delineate structural features controlling the Eburru geothermal system in Kenya

Abstract

The gravity modeling of the Eburru was conducted in an attempt to delineate geological structures controlling the geothermal system and estimate the geothermal reservoir extent. A total of 375 data points were used and a Bouguer density of 2.27 g/cm3 to generate a complete Bouguer anomaly map of the area. Gravity data were separated into regional and residual components to enhance the structural features from the sedimentary and basement rocks in the study area. The gravity data were analyzed using gradient interpretation techniques for edge detection, such as horizontal derivative and an improved normalized horizontal tilt angle. For carrying out the three-dimensional (3-D) modeling, a volume of 12 × 13 km and 5 km deep was selected. The model was constrained using the estimated densities of cuttings obtained from the drilled wells. This study presents the interpretation results of various gravity anomaly maps and 3-D inversion model. Interpretation of horizontal derivative and improved normalized horizontal tilt angle of gravity data indicate the existence of high gradient anomalies. The anomaly maps were used to identify several faults that compared well with the mapped faults. The 3-D model revealed a dense body interpreted as the geothermal reservoir with a volume of about 3.0 km3 and an average block density value of 2.45 g/cm3. The dense body which is a fractured zone overlies a high density body likely to be the heat source responsible for heating the reservoir. There appears to be a close relationship between the faults system and the geothermal reservoir. These faults serve as fluid pathways from deeper parts to shallow regions. The results obtained from this study will lead to an improved understanding of the geothermal system in the study area and aid the future geothermal exploration of the field.