Resistivity Structure of Shalla-Abayata Geothermal Field, Southwest Ethiopia, Using Magnetotelluric Data Inversion
Yosef K. KETO and Hakim SAIBI
[Geological Survey of Ethiopia, Ethiopia]
The Main Ethiopian Rift (MER) constitutes an area characterized by active extensional tectonics with the two main fault systems in the MER, NE-SW trending and NNE-SSW trending faults, which define the Wonji fault belt (WFB). In this study, 15 magnetotelluric (MT) stations were collected with period range from 0.003s to 1000s. The MT stations where located south of Lake Abayata and Langano and northeast of Lake Shalla, set close to the eastern escarpment of the rift along the WFB. The main objective of this study is to understand the nature of the MT data and to apply inversion to detect and delineate the geothermal resource and locate exploitable reservoir. The dimensionality analysis, ellipticity and skew, of the MT soundings has 2D/1D character. Dimensionality increases at depth. 2D and 3D inversions were done to interpret the MT data as a result of the dimensionality analysis. For the inversions, the MT data were rotated to 350 using the tipper strike direction result and fault directions to fit the data in the polarization. The phase tensor pseudo-section shows low resistivity beneath a thin layer of high resistivity at the surface and the ellipses look circular showing 2D/1D character. The resistivity increases with depth and the ellipses indicate 3D character. The Phase tensor pseudo-section result correlated with the resistivity distribution of the inversion results. The 2D and 3D inversion result have low resistivity (1-10 ohm-meter) and 1.2 km depth benth the top surface thin layer high resistive (tens to hundreds ohm-meter) which is fresh volcanic overburden unaffected by alteration. Most probably, the low resistive layer is the clay cap rock and shallow depth reservoir in the sedimentary formation which is conductive hydrothermal mineral alteration smectite and illite. Below the interpreted clay cap and shallow depth reservoir, resistivity values tend to increase at interval 20-100-ohm meter which is interpreted as zone of reservoir due to the formation of high temperature mineral chlorite-epidote starting from 2 km depth. The result corresponds very well with the conceptual model of a high enthalpy geothermal system comprising a resistive up-flow region at depth overlain by a conductive clay cap.
|        Topic: Geophysics||Paper Number: 13047|