Electrical tomography and TDEM prospection in the Chianciano thermal basin (Siena, Italy)

Abstract

Chianciano thermal basin in Tuscany belongs to a great structure that extends, from San Casciano Bagni to Rapolano, orientated SSE-NNW. Several springs are located in this thermal field: Acqua Santa, Macerina, Casuccini and Sillene well. They are located close to the contact between the lower complex of the «Tuscan series» (Triassic limestones and dolomites) and Pliocene formations (sands and clayey sands, clays and silty clays). According to the groundwater flow scheme, the former formations represent the geothermal reservoir and, where they outcrop, the recharge areas. Thermalized fluids rise through direct faults, having an apennine orientation. In order to locate a new production well, electrical tomography and TDEM prospection were performed. Geophysical interpretation led to the construction of 1D imaged and 2D sections showing the main tectonic features. The different electrical behaviour of the hydrogeological units disclosed a fault located near the old Sillene well; this tectonic feature caused the uplifting of Triassic formations towards the north-east. A new production well was located close to the fault in the uplifted sector with excellent results as it captured the thermal aquifer at a depth of 30 m with a discharge up to 70 l/s.