Geothermal energy in southern Tuscany has been used for heating and electrical power supply for many decades. Electrical power generation is still increasing, raising the need for new exploration in areas adjacent to the Larderello-Travale and Monte Amiata areas. Such an area (some 10 km southwest of Monte Amiata; see Fig. 1 ) with dimensions 23 × 16 km is the focus of the present study. Numerical simulations are used to evaluate the geothermal potential of the area, i.e. to predict the

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... temperatures and flow rates. Numerical modelling of the Larderello geothermal field has previously been accomplished by Della Vedova et al. (2008) and Romagnoli et al. (2010) to forecast the future evolution and the sustainability of the field. In the same vein, Fulignati et al. (2014) performed hydrothermal simulations of the Monte Amiata area. The present study is similar to these in that it aims at finding a valid numerical representation of the subsurface both with respect to geological structure and physical property distributions, thereby deriving the fluid flow and temperature fields. Moreover, the primary heat source-and hence controlling structure for heat transport-of the geothermal reservoir is studied by quantifying the uncertainties in its temperature, depth, and shape and their influence on the heat transport processes. Abstract Granitoid intrusions are the primary heat source of many deep geothermal reservoirs in Tuscany. The depth and shape of these plutons, characterised in this study by a prominent seismic reflector (the K horizon), may vary significantly within the spatial scale of interest. In an exploration field, simulations reveal the mechanisms by which such a heat source influences temperature distribution. A simple analysis quantifies the sensitivity of potentially measurable indicators (i.e. vertical temperature profiles and surface heat flow) to variations in depth, temperature, and shape of the heat source within given ranges of uncertainty.