Three-Dimensional Inversion of Magnetotelluric Data: Geological/Geothermal Interpretation of Asal Geothermal Field, Djibouti

Sakindi, Gaetan. Three-Dimensional Inversion of Magnetotelluric Data: Geological/Geothermal Interpretation of Asal Geothermal Field, Djibouti. University of Iceland, 2015.

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  • Sub-type Master's thesis
    Author Sakindi, Gaetan
    Title Three-Dimensional Inversion of Magnetotelluric Data: Geological/Geothermal Interpretation of Asal Geothermal Field, Djibouti
    Year 2015
    University University of Iceland
    Department Faculty of Earth Sciences
    Place of Publication Reykjavik
    Publisher United Nations University Geothermal Training Programme
    Pages 81
    Language eng
    Abstract This research study is aiming at becoming acquainted with the resistivity method and different ways of performing interpretation of MT data for deep lying subsurface investigations. The MT data collected from Asal geothermal area were used; the comparison between the results of 1-D inversion carried out previously and 3-D inversion is performed. A total of 105 MT soundings were considered in this research project and the same number of corresponding TEM soundings collected at nearby sites. To allow the static shift correction in the 1-D inversion, the MT data were jointly inverted with TEM data. Shift correction was then applied to the two polarizations for 3-D inversion. The modern computing systems have made 3-D modelling of MT data achievable and it is now becoming common and useful for detailed subsurface surveys in geothermal industries as well as in other fields including ground water, oil and natural gas and mineral exploration. The WSINV3DMT code was used to perform 3-D inversion of the static shift corrected off-diagonal impedance tensor elements. Three different initial models were considered in order to test the inversion robustness and 31 periods evenly distributed on logarithmic scale from 0.003 to 300 s were used. The first initial model was compiled from the joint 1-D inversion of TEM and MT soundings; the second was a homogeneous earth of resistivity 10 Ωm and the third initial model was a homogeneous earth with a resistivity 50 Ωm. The RMS in all three different initial models was not of big difference with values of 1.44, 1.48 and 1.87, respectively. The final models gave similar resistivity structures underneath Asal rift and are presented here as iso-depth resistivity maps and cross-sections. The result of the interpretation shows four main resistivity structures below the geothermal area: A shallow lying thin high resistivity layer followed by low resistivity (conductive cap). Below there is a high resistivity layer (resistive core) underlain by a deep lying conductor. Lithology based on well data shows that the shallow thin high resistivity layer corresponds to dry basaltic rocks covering the surface, the conductive layer reflects saline fluids but correlates also with low temperature alteration (smectite and zeolites), the deep resistive core correlates with the high temperature alteration minerals (chlorite and epidote) whereas the deep seated conductive body is most likely connected to the heat source of the Asal geothermal system. At sea level, high resistivity dominates the northeast part of Asal rift towards Lake Asal in the vicinity of Ardoukoba volcano and the southeast part of the rift around Baddikoma region. An updoming conductive cap intersects the high resistivity, running NE-SW and reflects presumably alteration within the geothermal system. It covers the central part of the Asal rift including the Fiale explosion crater (Lava Lake). This is the same area where the fumaroles and hot springs in the Asal rift are located.
    Keyword 3D Inversion
    Magnetotelluric data
    Asal geothermal field
    Transient electromagnetic method
    Copyright Holder United Nations University Geothermal Training Programme
    Copyright Year 2015
    Copyright type Fair use permitted
    ISBN 9789979683759
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    Created: Wed, 24 Feb 2016, 00:42:24 JST by Malfridur Omarsdottir on behalf of UNU Iceland