Stimac, Armadillo, Kebede, Zemedkun, Kebede, Teclu, Rizzello, Mandeno Integration and Modeling of Geoscience Data from the Tendaho Geothermal Area, Afar Rift, Ethiopia Jim Stimac 1 , Egidio Armadillo 2 , Solomon Kebede 3 , Meseret Zemedkun 4 , Yiheyis Kebede 3 , Asfaw Teclu 3 , Daniele Rizzello 2 , Petronila E. Mandeno 5 1 – Stimac Geothermal Consulting, 4210 Chaparral Rd, Santa Rosa, CA 95409 USA 2 – DISTAV, Università di Genova, Genova, Italy 3 – Geological Survey of Ethiopia, Geothermal Division, Addis Ababa, Ethiopia 4 – ARGeo, Regional Office for Africa (ROA), United Nations Environment Programme, Nairobi, Kenya 5 – Digitial Mapping Solutions, Santa Rosa, CA, USA Keywords: Tendaho, geothermal assessment, 3D model, conceptual model, exploration, areas of interest, well targets, Leapfrog© ABSTRACT The 2014 ARGeo Program included gathering of new data and update of the conceptual model for the Tendaho geothermal area, Ethiopia. The geothermal area is located within the NW-SE trending Tendaho Graben (TG), which includes the southern part of the younger and active Manda Hararo Rift (MHR) in the Afar region. Rifting and volcanic activity within TG occurred mostly between 1.8 and 0.6 Ma but extended to at least 0.2 Ma. Extension gave rise to about 1600 m of vertical displacement verified by drilling of the Afar Stratoid sequence, over a crust with a mean thickness of ~23 km. The main emphasis of our study was on the Dubti and Ayrobera geothermal areas, near the central axis of the TG, although we also reviewed data for Allalobeda, located near its western bounding faults. New data from these areas include magnetotelluric (MT) and time domain (TDEM) surveys completed in 2013-2014, as well as radon and shallow temperature surveys. The digital enhancement and modeling of existing potential field data revealed previously unrecognized structural lineaments trending mainly NW, NE, and WNW. New MT and TDEM data have been integrated with the existing electromagnetic data set, obtaining >200 MT stations covering the study area. 1D and 2D modeling of the data set has revealed local updoming of a widespread shallow conductor associated with thermal areas at Dubti and Ayrobera, and a possible relation to intrusive heat sources. Updoming is attributed to conversion of smectite clay to higher-temperature mixed-layer clays and chlorite due to hydrothermal fluid circulation. A review of the linkages between faulting and volcanism was also undertaken. Mapping the main axis of young volcanism and the base of the shallow electrical conductor provides input to a 3D temperature model constructed from well data (6 geothermal, 9 shallow gradient and multiple shallow water), near surface temperature measurements, liquid and gas geothermometry, and assumptions regarding conductive heat flow from young intrusion. We have compiled the most relevant data for Tendaho and organized it in a way that it can be viewed and interpreted in 3D using Leapfrog© Geothermal Software. We integrated all the key data to visualize patterns and develop internally consistent subsurface temperature and conceptual models. The model for Dubti indicates deep upflow to the SE of the drilled wells that migrates up NW- trending structures and then spreads laterally in shallow permeable horizons. At Ayrobera, deep upflow is likely localized by NW- trending faults on the eastern flank of the main axis of youngest volcanism. The temperature model, along with conceptual considerations, was used to define risked areas of interest for further exploration activities including targeting and drilling of deep wells. A key uncertainty is to what extent large regions of deep (>2 km) low resistivity are real, and how to model them in terms of temperature and permeability. 1. INTRODUCTION The United Nations Environment Program (UNEP) selected a proposal on “Geothermal Resource Assessment of Tendaho Geothermal Field in Ethiopia” from the Geological Survey of Ethiopia (GSE) as part of its 2014 ARGeo program. A team of experts were engaged in Feb. 2014 to provide advisory services, training and mentoring, and guidance on data gathering activities. The project team reviewed all aspects of the existing geological, geochemical, and geophysical data; established linkages between geological structures and the morphology of volcanic deposits; provided input to new geophysical and geochemical data collection and quality control; and integrated all data to develop revised geothermal conceptual models for Dubti and Ayrobera with a view to siting exploration wells. A temperature model of the Tendaho area was developed in Leapfrog©, a 3D visualization software for geothermal systems. The Leapfrog model is based on the main data sets available for Tendaho and provides a means to compare and visualize the data in three dimensions for the first time. This makes a wide variety of information accessible to all those involved in the project through use of the free Leapfrog Viewer© software and facilitates development of one or more conceptual models that are consistent with the various interpretations made by individuals that focused on individual data types. The 3D temperature model was constructed using available temperature data and interpretations of the likely thermal and resistivity structure of the crust in the Tendaho Basin (Mandeno and Stimac, 2014; Stimac et al., 2014). This model can be easily refined and revised by input of new data as it becomes available. The model was used to define regions deemed to have the highest geothermal potential based on the available data. Example wells were also planned in the final Leapfrog model.