MT Resistivity Structure at Abijata and Shalla Geothermal Field, Southern Main Ethiopian Rift Geothermal Prospect, in 2017
[Geological Survey of Ethiopia, Ethiopia]
The Abijata and Shalla Lake is one of the geothermal prospects in the Lakes districts of the main Ethiopian rift (MER), which about 220 km south west of Addis Ababa. The Abijata and Shalla prospect area lies between 7º25'N and 38º30'E, and on the elevation of 1570m above sea level. The lake has no surface outlet and receives the discharge from Lake Ziway, Langano and Abijata. Many hot and warm springs emerge along its eastern, southern and western shores due to the temperature survey. The most important and boiling thermal springs located at the eastern part of the shore. Due to those to all in the above information electromagnetic methods (EM) are frequently used in the exploration of Geothermal resources for determining the spatial distribution of electrical resistivity (conductivity). Magnetotellurics (MT) and time domain electromagnetic (TDEM) methods are especially used for geothermal exploration when using EM methods. Geothermal resources are ideal targets for EM methods since they produce strong variations in underground electrical resistivity (conductivity). Electrical resistivity is directly related to parameters that characterize geothermal systems. In this report, the application of TEM and MT methods and interpretation of data from the Abijata and Shalla geothermal field in Ethiopia are discussed. MT data were analyzed and modeled using 2-D Occam inversion of the rotationally invariant impedance tensor by rotating the MT data N 350 E using the strike direction. The Bahr skew, swift skew and elliptical representation between 0.003 s to 1 s and 100 s to 1000 s shows the MT data has 2-D/1-D character. The 2-D inversion result shows a low resistivity at shallow depth is interpreted as a sedimentary formation, lateral flow of geothermal fluids or a fracture zone. The existence of clay minerals, especially smectite makes the region below the resistive unaltered layer very conductive. In the region, the temperature is getting high and the most common alteration products are smectite and illite. The unaltered zone is a region where the hot geothermal fluid does not affect the rocks near the surface and the temperature is low. The high resistivity below the low resistivity can be associated with less permeable. The up-flow permeable region below the conductive clay zone is considered the geothermal reservoir in such high-enthalpy geothermal systems and is characterized by high resistivity due to the formation of secondary minerals like chlorite and epidote. Generally, from the 2-D inversion model three main resistivity layers are recognized: a shallow depth relatively thin high-resistivity layer around the stations (100 Ωm), a conductive layer probably the cap rock with resistivity below 10 Ωm, the third resistive zone with high resistivity value ( more than 100 Ωm).
|        Topic: Geophysics||Paper Number: 13042|