Title: 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 NWSE
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.