Geophysical Characterization of the Eburru Geothermal Field, Kenya from the Analysis and Interpretation of Magnetotelluric and Gravity Techniques

Abstract

Magnetotelluric (MT) and gravity surveys were carried out in the area to delineate the heat source, estimate the geothermal reservoir, and assess the extent of geothermal system. Three-dimensional magnetotelluric interpretation of geothermal systems is a powerful tool for characterization of the deep electrical resistivity structure of geothermal reservoirs with high resolution and accuracy. Three- dimensional magnetotelluric (MT) inversion of Eburru datasets using ModEM program was performed to obtain the resistivity structure of the field. The preferred electrical resistivity model shows a conductive zone ( less than 10 Ωm) interpreted as the cap rock within 1 km from the surface overlaying an intermediate resistive layer (~ 35 Ωm) and a deep conductor ( less than 10 Ωm) extending from sea level to a depth of 2.5 km below sea level. Additionally, the 3-D resistivity model shows pronounced low resistivity regions which trend in the north-south direction, consistent with the local fault strike. A total of 375 gravity data points were used in creating the Bouguer anomaly, using a Bouguer density of 2.27 g/cm3. Gravity survey results indicate Bouguer anomaly having both high and low amplitude anomalies. Three-dimensional MT inversion based on full impedance tensor and three- dimensional gravity model obtained by inverting Bouguer anomaly imaged similar features. The gravity model shows that the high-density anomaly bodies occur along the fault zones. A relatively low resistivity and a high density body located approximately 1.5 km below sea level may be related to the geothermal reservoir which is likely heated by magmatic fluid emanating from deeper melt sources.