Gravity Survey For Geothermal Exploration In Menengai Area, Kenya

ABSTRACT

Kenya has prominent geographical features such as the Rift valley which gives rise to geothermal reservoirs which can be exploited to produce geothermal energy for power generation and heating; Lake Magadi, Suswa, Eburu, Menengai, Lake Bogoria as well as other areas are considered to have potential for power geothermal energy because of the geothermal manifestations. For above mentioned regions it is known that positive gravity anomalies within the Rift valley proper are superposed on the generally negative gravity anomaly typically for rift structures. Research indicates that these local gravity highs can be explained as a result of high density intrusions in the upper crust. The aim of the project was to study the gravity highs in Menengai and determine whether they could be possible sources of geothermal energy. The area of study comprises a part of the Rift valley, which has a geothermal potential. The Menengai area is located in the Great Rift Valley, Kenya, 10 km North of Nakuru town. The area lies on a massive shield volcano at an elevation of 2278m above sea-level, 0.200S 36.070 E. The field survey involved measurement of the differences in gravity for the various gravity stations established, which were later reduced and used to plot a Bouguer anomaly map for qualitative interpretation. The Bouguer anomaly map was taken with contour interval of +28 to -26 mGals. From the contour map it is noted that the gravity anomaly high runs through the central part of the caldera and trends NW-SE which coincides with the presence of fumaroles in the area suggesting presence of potential geothermal reservoirs. The gravity anomaly can be explained to be due to high density intrusions in the upper crust within the caldera. The gravity cross sections through the caldera indicate presence of high density body peaks beneath this structure. Since the volcano is relatively young this bodies could still be hot, the heat being conducted to near surface regions by dykes, sills, lapoliths or laccoliths. This has shown that Menengai area has a dense body that lies less than 1km below the surface. This body does not extend to the deeper crust implying that it may be representing shallow magma chamber. The high heat flow observed in combination with recent occurrence of volcanic activity and seismic activity confined to within 4km imply that a magma body exists at shallow depth and may be forming the heat source of the area. This area therefore has potential, from the heat source point of view, to heat the underground water. This superheated water could then be tapped to generate electricity. Because interpretations are subject to ambiguity, more geophysical and geological information over the gravity anomalies are needed to determine the overall potential of the geothermal activity to fully harness it.