Integrated geophysical imaging of the Aluto-Langano geothermal field (Ethiopia)

The Aluto-Langano geothermal system is located in the central part of the Main Ethiopian Rift, one of the world\u2019s most tectonically active areas, where continental rifting has been occurring since several Ma and has yielded widespread volcanism and enhanced geothermal gradient. The geothermal system is associated to the Mt Aluto Volcanic Complex, located along the eastern margin of the rift and related to the Wonji Fault Belt, constituted by Quaternary NNE-SSW en-echelon faults. These structures are younger than the NE-SW border faults of the central Main Ethiopian Rift and were originated by a stress field oblique to the rift direction. This peculiar tectonism yielded local intense rock fracturing that may favour the development of geothermal reservoirs. In this paper, we present the results of an integrated geophysical survey carried out in 2015 over an area of about 200 km2 covering the Mt Aluto Volcanic Complex. The geophysical campaign included 162 coincident magnetotelluric and time domain electromagnetic soundings, and 207 gravity stations, partially located in the sedimentary plain surrounding the volcanic complex. Three-dimensional inversion of the full MT static-corrected tensor and geomagnetic tipper was performed in the 338-0.001 Hz band. Gravity data processing comprised digital enhancement of the residual Bouguer anomaly and 2D-3D inverse modelling. The geophysical results were compared to direct observations of stratigraphy, rock alteration and temperature available from the several deep wells drilled in the area. The magnetotelluric results imaged a low-resistivity layer which appears well correlated with the mixed alteration layer found in the wells and can be interpreted as a low-temperature clay cap. The clay-cap bottom depth is well corresponds to a change of thermal gradient. The clay cap is discontinuous, and in the central area of the volcanic complex is characterised by a dome-shape structure likely related to isotherm rising. The propilitic alteration layer, pinpointed as the 80-Ohm-m isosurface, shows two dome-shape highs. The first is NNE-trending, and may be interpreted as an upflow zone along a fault of the Wonji belt. Two productive wells are located along the borders of this area, as well as the alignements of fumaroles and altered grounds. The second is linked to a wide resistive area, located at shallow depth, where no clay cap was detected. It could be interpreted as a fossil high-temperature alteration zone reaching shallow depths, and it is associated to several fumaroles. Modeling of 2D/3D gravity data shows that the anomalies are due to shallow density variations likely related to lithology. The deep lateral variations due to structural lineaments inferred from well stratigraphy have no detectable signature. However, the trend analysis performed on the residual Bouguer anomaly (via horizontal and tilt derivative computations), allowed to identify five lineaments. Three of them exhibit NNE-SSW strike, corresponding to the Wonji Fault Belt Trend, whereas two have NNW-SSE strike, corresponding to the Red Sea Rift trend, which in this area is of minor evidence. The signature of shallow structures is then indicative of major regional structures. One of the lineaments marks the presence of a major fumarolic zone

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

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.The Geothermal Training Programme of the United Nations University (UNU-GTP

The geophysical recognition of a vapor-cored geothermal system in divergent plate tectonics: The Alalobeda (Alalobad) field, Ethiopia

We show the results of a geophysical survey carried out in the Alalobeda (Alalobad) geothermal field (Afar, Ethiopia). The site is located on the western margin of the NW-SW Tendaho Graben, at the intersection with the NNE-SSW Main Ethiopian Rift. The survey included 121 Magnetotelluric (MT) and Time Domain Electromagnetics (TDEM) soundings, and 300 gravity records. We applied 3D MT and gravity inversion to target a previously inferred high-T (220 \ub0C) near neutral pH liquid-dominated chloride reservoir. Geophysical modelling identified the graben basin structure, with normal fault stepping in the Afar Stratoid basalts (2485 kg/m3), underlying a low-density (2035 kg/m3) and conductive (1 Ohm m) sedimentary cover. Along the graben shoulder, a density anomaly (+200 kg/m3) suggests a propylitization zone corresponding to the reservoir, underlying the typical conductive clay cap updomings and downdomings of high-T geothermal systems. A major conductive plume, without an appreciable gravity signature, occurs in the graben shoulder. Integration with the geochemical results allows to interpret it as a vapor-cored system. The hot spring water CO2, H2S and Cl content indicates the contamination of magmatic gases into the reservoir that produce fluid acidification while ascending from a deep source. The local bulk resistivity (10 Ohm m) is explainable by assuming conductive permeating acid fluids (10 S/m), an enhanced porosity (20%) and a significant gas saturation (20%). The absence of an appreciable density increase is also justified by the hypothesised porosity and the gas fraction. Since the acidic fluids are neutralised by interaction with the host rock, the sampled waters are nearly pH-neutral. Our results suggest the first occurrence of a vapor-cored system in divergent plate tectonics, such as the Tendaho Graben sector of the Afar rift zone