| Abstract |
Magneto-Telluric (MT) survey was conducted in Tendaho Geothermal prospect around Dubti and Allalobeda geothermal fields between December 29/2012 and April 11, 2013 in an area bounded between the geographic coordinates (40°59'E, 11°47'59.4''N) and (41°14'50.7''E, 11°37'2.91''N﴿. The major objectives of the study were to define the assumed up flow zone of the geothermal fluid south-east of the major hydrothermal manifestations in the Dubti field and refine the proposed conceptual model, to study the subsurface resistivity structure of the geothermal system at the Allalobeda field, its relation to the geothermal field marked by fossil altered ground around Logya town situated north of it and to outline possible geothermal reservoirs at an economical depth level around both fields. Magntotelluric survey in the frequency range between 320Hz and 0.001Hz was conducted over six profiles across the two fields and the data analyzed. As a result the Swift strike analysis indicated the general NW-SE impedance strike direction coincident Magneto-Telluric (MT) survey was conducted in Tendaho Geothermal prospect around Dubti and Allalobeda geothermal fields between December 29/2012 and April 11, 2013 in an area bounded between the geographic coordinates (40°59'E, 11°47'59.4''N) and (41°14'50.7''E, 11°37'2.91''N﴿. The major objectives of the study were to define the assumed up flow zone of the geothermal fluid south-east of the major hydrothermal manifestations in the Dubti field and refine the proposed conceptual model, to study the subsurface resistivity structure of the geothermal system at the Allalobeda field, its relation to the geothermal field marked by fossil altered ground around Logya town situated north of it and to outline possible geothermal reservoirs at an economical depth level around both fields. Magntotelluric survey in the frequency range between 320Hz and 0.001Hz was conducted over six profiles across the two fields and the data analyzed. As a result the Swift strike analysis indicated the general NW-SE impedance strike direction coincident with the N155°E strike of the Tendaho rift axis and the dimensionality analysis revealed 1D conductivity structure up to 10 sec and 5 sec periods around the Dubti and Allaobeda fields and non-1D structure for the longer periods at the respective fields. Combined results using 1D model obtained from the Occam procedure to the invariant mode of the impedance tensor and 2D inversion model using the joint TE+TM modes, revealed zones of conductive layers successively decreasing in magnitude with depth delineating possibly the cap rock, a horizon of low temperature alteration clay minerals in the zeolite-smectite zone and the mixed layer zone followed by a zone of high temperature alteration product in epidote and chlorite clay minerals, adjacent to the structural discontinuities AD1, AD3 and AD4 in Allalobeda field and D0, D2, D3 and D4 in the Dubti field. Possible geothermal reservoirs inferred at both fields related to these resistivity structures. A well-defined low conductivity zone ﴾≈ 10 Ohm-m﴿ in the south-eastern Dubti field delineates the up flow zone of the geothermal fluid and circulation of geothermal fluid was deduced to be related to the discontinuities D2/D3 whose northern extension of the inferred structures is the Dubti fault. The trend of high and low resistivity contour lines and the discontinuities (NW-SE alignment) indicated the possibility of the connection of the Allalobeda and Logya geothermal systems. Additional MT soundings towards Logya at Allalobeda field and south of TDO8 in the Dubti field and reinterpretation of result using TEM data were recommended for a better definition of the anomalies and achieve the stated objectives. |