The Alid Volcanic District (Eritrea): Remote sensing analysis and preliminary conceptual model of the geothermal reservoir.

The principal aim of this work was to improve the characterization of Mt. Alid geothermal system (eastern Eritrea), considered the most favourable geothermal resource of the country. To conduct a detailed remote sensing analysis, a high resolution digital elevation model has been provided, the new ALOS DEM (courtesy of JAXA). Before its use, it required a validation process, using GCPs and other already validated data, such as SRTM. With the aid of different DEMs, Bing and Google high resolution images and processed Landsat 8 acquisitions, a remote sensing analysis has been conducted, in order to characterise geological, geomorphological and geostructural features of the Alid area and surrounding. Two main structural lineaments have been found: a NNW-SSE normal fault system, strictly related to the regional rift tectonic context, and a NE-SW trend with a dextral strike-slip component, related to the presence of the underlying shallow magma intrusion. The results obtained from the remote sensing approach have revealed that the crossing of the two fault systems caused the ascent of geothermal steam to the surface, since the position of fumarolic vents seems to be closely related. The mapping of geological units has brought the realization of and an updated and more accurate geological map of the Alid geothermal district. A preliminary fieldwork, aimed at validating the remote sensing analysis, was carried out in February 2015 and confirmed, to a first approximation, the validity of the mapping. An analysis on ASTER TIR images has also been conducted, to extract heat anomalies from processed thermal bands and measure the heat contribution due to the presence of surface geothermal manifestations. Normalized temperature has confirmed that thermal anomalies are related with fumaroles and has given us promising results for further detailed studies. An attempt to quantify the potential of the geothermal resource has been made. In cooperation with the IGG-CNR research institute of Florence, a first numerical model of the geothermal reservoir has been realized. Simulation results indicate a possible reservoir temperature of 260-270 °C, a pressure of about 50 bar and a vapour fraction of 0.7. These values are coherent with a dominant steam geothermal reservoir.