| Title | 1D Joint Inversion of MT and TEM Data from Ngozi Geothermal Prospect, Southwest Tanzania. an Integrated Interpretation of Geoscientific Results |
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| Authors | Makoye Mabula DIDAS, Gylfi Pall HERSIR |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | Ngozi, Magnetotellurics, Transient Electromagnetics, 1D joint inversion, Integrated Interpretation |
| Abstract | Ngozi geothermal prospect is the flagship prospect of the Tanzania Geothermal Development Company Limited (TGDC) located in Mbeya region within the Rungwe Volcanic Province southwest of Tanzania. The prospect is within the southern part of the East Africa rift triple junction of Rukwa, Usangu and Karonga basins where the eastern and western branches of the East African Rift System converge and form an important volcanic zone, the Rungwe volcanic Province. The thermal manifestations are mostly located northwest of the focal area. The Ngozi caldera located at an elevation of 2,620 m a.s.l forms the highest elevation of the focal area. It forms an elliptical structure with dimensions of 2.5 km long, 1.6 km wide and 74 m deep caldera forming a lake filled with chloride water on top of the mountain. Geothermometry studies of the hot springs with temperatures of 89°C, 78°C and 66°C discharging at the bottom of the lake indicated a NaCl reservoir with temperature of more than 230°C locally recharged by meteoric water from the Ngozi highlands, thus, making the highest temperature geothermal system so far investigated in Tanzania. This study summarizes the integrated findings of resistivity models based on 1D joint inversion of Transient Electromagnetic (TEM) and Magnetotellurics (MT) sounding data and the previous geological and structural work of the area together with geochemical results. MT and TEM data acquired in the focal area were evaluated, reprocessed and 1D jointly inverted. The results are presented and integrated with geological and geochemical results. Based on the integrated findings from this study three drilling sites are proposed into the reservoir, which is assessed to be 4 km wide and 7 km long. The low resistivity within the fault zone at a depth between 4 and 5 km below ground level (bgl) south of Ngozi Caldera Lake is an evidence of the existence of the localized relatively young, shallow plutonic bodies which may constitute a suitable heat source for the geothermal system. The eastern and western limits of extension of the reservoir correspond to the fault zone observed in the MT cross-sections south and north of Ngozi Caldera Lake. The eastern limit of the geothermal reservoir also corresponds quite well with the fault mapped during the surface geological survey. It has been observed that the shear zones, most of the lineaments and inferred faults previously mapped in the area have remarkable displacement of the conductive anomalies in MT data. Based on these findings, the geothermal conceptual model has been revised and the first priority zone of 28 km2 defined as the maximum probability of hosting a geothermal system of 232°C reservoir temperature. The resource capacity of 49.4 MWe with probability P90 has been evaluated through a volumetric assessment using Monte Carlo approach. |