Geothermal heat sources characterization and mapping using heat flow, gravity, and magnetics in Eburru, Kenya

Abstract

Eburru area is a geothermal heat prospect region in Kenya’s Rift Valley. The area is characterized by faults, fumaroles, geysers, volcanic craters, geothermal grass, and hot springs. Such surface manifestations indicate a possible occurrence of a geothermal heat source. Six exploration wells were drilled in the Southern part of the study area within the Eburru crater. Five wells did not produce steam pressure; hence there was a need to determine the depth of the heat sources and their characterization in the area. Previous studies were mainly carried out within the Eburru crater; however, this study covered a larger area for better imaging of deep heat structures. This study undertook a geophysical survey using heat flow, gravity, and magnetic geophysical methods to map and characterize the structures likely to be possible geothermal heat sources in the area. Spike mechanical tool was used for hole digging, while a thermocouple thermometer was used for temperature measurements during heat flow data collection. Gravity data were collected using a CG-5 gravimeter, and magnetic data were collected using a proton precession magnetometer G-856 model. The data were processed and analysed using Oasis montaj geosoft software. The heat flow data was used to plot thermal contour maps, which identified high and low heat flux areas. The gravity and magnetic data were reduced and processed to obtain anomalies that showed the variation of density and magnetic susceptibility within the Earth’s subsurface. Gravity and magnetic data were forward modelled using Oasis Montaj geosoft software to determine the causative sources' depth, density, and magnetic susceptibility. Heat flux values were estimated to range between 0.02 Wm^(-2) and 92.80 Wm^(-2) with a mean of 10.06 Wm^(-2). Processed gravity and magnetic data revealed positive and negative anomalies. Euler deconvolution of gravity data located five solutions on high gravity amplitudes at a depth range of 433 m-2271 m while ground magnetic data located five solutions either on a positive magnetic anomaly pole or negative magnetic anomaly pole with a depth range of 1090 m to 3973 m. Spectral analysis of ground magnetic data estimated the curie point isotherm depth at 2970 m with a curie temperature range of 130 ℃ to 680 ℃. Joint forward modelling of gravity and magnetic data revealed intrusions within the Earth’s subsurface with depth to the top ranging from the shallowest at 739 m to the deepest at 5811m. It was established that areas with high heat flux on the Earth’s surface have shallow heat sources while those with low or moderate heat flux have deep heat sources. The densities of the anomaly sources were determined to range between 3.0 gcm^(-3) and 3.2 gcm^(-3) while the magnetic susceptibility was zero, implying the intrusions are from the mantle materials or dykes extending from a batholith rock with higher temperatures than their curie temperature. Joint forward modelling of gravity and magnetic data revealed geothermal heat sources with positive gravity anomaly and zero magnetic susceptibility on a high heat flux area. This was located in the western part of the study area along Opuru, Eburru settlement scheme, Oldoinyo Opuru, and towards Lake Elmenteita to the North of the study area. The study imaged a positive gravity anomaly oriented in a North-South direction at the central part of the study area near Eburru forest and Opuru. A Positive gravity anomaly implies that the causative body is an intruding dyke from the mantle or a batholith rock of mantle origin with a higher density than the host crustal rocks. Magnetic susceptibility of zero implies that the intrusions have a higher temperature that affects the magnetic properties of constituent elements or the presence of hot fluids causing hydrothermal demagnetisation. This study shows that the Eburru area has high-temperature structures within the accessible depth. Geothermal wells should possibly be located vertically above the intrusions with high heat flux on the Earth’s surface.