GNGTS 2014 - Atti del 33° Convegno Nazionale

Terme Caronte are close to intersection of two faults: a NS trending fault, along the Bagni River, and a WNW-ESE-trending regional strike-slip fault system (Ghisetti and Vezzani, 1981; Tansi et al. , 2005; Tansi et al. , 2007). The low heat flow measured both in the Caronte area (50 mW/m 2 , Cataldi et al. , 1995), and in the whole Calabria rules out magma intrusions at shallow depth in the crust. This implies that the thermal character of the investigated waters is a consequence of the deepening of the hydrological circuit in fractured areas marked by a normal geothermal gradient (~30°C/km) allowing the existence of low to medium- enthalpy aquifers at a depth of about 1-3 km. For some of the thermal waters, the temperatures calculated by different geothermometers show a limited cooling as a result of a fast uprise through highly permeable discontinuities (namely the local faults). Classical geothermometric relations applied on thermal waters of Caronte (Na/K and K/Mg in Fig. 2, from Italiano et al. (2010) to test their suitability to be used for the estimation of deep temperature conditions provided contrasting and unreal values. This is mainly due to the particular geological and hydrochemical settings of the area; as a result, chemistry of thermal waters is due to isochemical dissolution of country rock-forming minerals rather than a real equilibrium with them. The only reliable geothermometer for the Caronte system (Marini et al. , 1986) indicates a deep reservoir temperature in the range 55-65°C, confirming the low to medium enthalpy of the geothermal system. Moreover, in the Terme Caronte area new deep geophysical investigations were carried out and old seismic lines were re-elaborated (Fig. 1). The main target zones for geothermal utilization is the Carbonate basement rocks, made of limestone/dolomite of Mesozoic age. This kind of layers is thought to have the highest permeability, and its thickness and extent are also significant. The mentioned targeted reservoir is below the Calabrian Terranes. The seismic available VIDEPI data was used to trace the lateral extent of these rock types and to locate fault zones within them. The seismic lines all came with Two-Way-Time (TWT) in ms as a depth reference. Seismic interpretation was focused on to identify the following seismostratigraphic horizons: slate and metapelite (bottom of Calabrian Terranes) horizon and Mesozoic carbonate horizon. Easy identification is reflected by the observed continuity, of Slate and metapelite layers and of tectonic features. Mesozoic carbonate seismic unit consists of low continuity, moderate amplitude and low frequency reflectors with chaotic configuration, poor quality is attributed to seismic lines showing this layers and their tectonic features controlling the permeability and flow patterns. The E-W seismic profile shows low-angle overthrust contacts with basement units (Mesozoic Carbonate and bottom of Calabrian Terranes) obliterated by subsequent overprinting of high-angle faults. Seismic profiles covering the Terme Caronte area demonstrate the presence of a fractured carbonate reservoir, which is deformed to shape a partly buried positive flower structure. One Deep Electrical Resistivity Tomography (DERT) and one magnetotelluric (MT) profile were carried out in the investigated area. The Deep Electrical Resistivity Tomography carried out in the Terme Caronte did not allow to reach an investigation depth of 1000 m. On the contrary, the Magnetotelluric (MT) investigation allowed to retrieve information on the distribution of the electrical properties from hundred meters down to five kilometers. Under this point of view, the MT method can be considered a useful and non-invasive tool to investigate the Earth interior. In the framework of the project, DERT and MT data show clearly the contact between different electrical layers which should be associated to the geological formations. Fig. 2 – A picture of the drill machine. 188 GNGTS 2014 S essione 3.2