Multi-dimensional Inversion of Electromagnetic Data from Alalobeda, Tendaho Geothermal field in NE-Ethiopia and its Geothermal Significance

Abstract

Measuring the electrical resistivity of rocks is one of the main geothermal prospecting technique commonly used today. A resistivity survey was carried out in Alalobeda geothermal prospect, Ethiopia through the combined use of MT and TEM soundings. The study area is around 250 km2. In this study, 1D joint inversion of 108 MT and TEM sounding pairs and a 3D inversion of the off-diagonal static shift corrected impedance tensor elements of 107 MT soundings were done. The static shift correction of the MT data was made by jointly inverting the MT and TEM data from the same site. Shift correction was done for the two polarizations before the 3D inversion was performed. The WSINV3DMT code was used to carry out 3D inversion of the MT data. The robustness of the final 3D inversion model was tested by using two different initial models. The first initial model was compiled from the 1D joint inversion of MT and TEM soundings which gave a Root Mean Square (RMS) of 1.7; the second model was a homogeneous Earth of resistivity 10 Ωm, which gave an RMS of 1.2. The final models show similar resistivity structures at shallow depths (the uppermost few hundred m) but the 10 Ωm initial model could not resolve the deep structures.

The main objective of the survey was to come up with a detailed resistivity model and image the deep resistivity structure, detect and characterize a possible geothermal reservoir of the Alalobeda geothermal prospect compare different interpretational techniques and propose drilling sites. The results of the 1D joint inversion of MT/TEM data and 3D inversion of MT data gave comparable results at shallow depths However, at deeper levels 3D inversion reveals much more consistent details confirming that the resistivity structure in the area is highly three dimensional. The resistivity models resulting from the 1D and 3D inversions are presented in the form of depth-slice maps and cross-sections. The results of the inversion show three main resistivity structures. The first one is layer of very low resistivity (<10 Ωm) at shallow depth down to 300 m b.s.l., which is correlated with conductive sedimentary formation and/or smectite alteration. The second layer has high resistivity between the depths of 1000 m to around 4000 m b.s.l., which correlates with the resistive Afar Stratoid basalt Series and/or chloride-epidote alteration. The high resistivity layer is cut by vertical low resistivity columns that follow the main faults in the area and most likely reflect the up flow of geothermal fluid from depth into the sediments/surface.

Beneath the high resistivity at a depth of 5000 m b.s.l. a deep conductor has been imaged that could be associated with a heat source. From the 1D and 3D inversions lithological contacts and lineaments were identified. Sharp resistivity contacts or fault lines with an orientation of NE-SW transverse faults and NW-SE fault were observed. These identified faults and lineaments are in good agreement with gravimetric and micro-seismic results. From this study, the up flow zone of the survey area are mapped and locations of exploratory well sites are proposed based on the resistivity results. Here, three well sites are proposed in the study area, (1) to the southwest of the survey area into one of the up flow zones along the Tendaho graben shoulder; (2) to the east of the survey area into the up flow zone; (3) to the northeast of the surface manifestations of the survey area into the up flow zone.