GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.3 171 thermal conductivity of terrains that generally shows high variability, ranging from about 1 up to 6 W/mK. The variegated lithologies of the island of Salina are characterized by rocks variable in their nature, different structures and textures of deposits and therefore porosity, presence of organic particles, degree of water saturation, which all together strongly influence their thermal characteristics. In support of a preliminary feasibility studies for the direct use of low enthalpy geothermal energy, it was therefore necessary to fix relationships between rock types and their thermal conductivity. According to the characteristics of rocks investigated during the geological field work, values of thermal conductivity for the identified lithotypes have been obtained. The empirical method we have taken into account in this work considers thicknesses of the lithostratigraphic units and the thermal conductivity for each type of lithology investigated (Jorand et al. , 2015). Evaluations on these rock types have led us to fix an average thermal conductivity value for each analysed lithology. The recent alluvial deposits come from dismantling of upstream deposits and occupy a large part of the investigated area. They are characterized by heterogeneous and altered material. The low degree of cementation, high porosity and absence of pore water have led to the estimation of their thermal conductivity between 0.7 and 1 W/mK. The deposit underlying the epiclastic material is referred to the Grey Porri Tuff unit. It is characterized by a fine matrix (mm-sized) with some coarse elements (cm- sized) with a general low degree of cementation. Values of thermal conductivity have been estimated considering the degree of water saturation of volcanic tuffs, which ranges from very low values (0.5-1 W/mK) for dry tuffs up to 2.3 W/mK for wet tuffs. In our case, values of thermal conductivity for tuffs are consistent to those of dry tuffs, being water not present (0.5-1 W/mK). Deposits underlying the Grey Porri Tuff are those referred to the Strombolian activity of the Rivi-Capo complex and the Fossa delle Felci, which have rather basic compositions, grain size and degree of cementation consistent with those of volcanic pyroclastic materials having thermal conductivity ranging from 1.1 W/mK to 1.5 W/mK. The potential presence of water would be relevant in modifying the final thermal conductivity, being this affected positively in porous materials saturated with water. In the investigated area, the presence of water in deposits at the surface is irrelevant, but interaction with sea water for layers beneath the sea level cannot be excluded due to the proximity to the shoreline. Results and discussion. In this study, t he detailed field survey allowed us to get information on lithological/lithostratigraphic features and their spatial distribution, putting into evidence important variations of thermal conductivity. This suggests that small-scale variations can affect significantly the final value of thermal conductivity, indicating that studies aimed at constraining the geothermal micro-zonation are of great impact. To assess the thermal potential from low enthalpy resources of the area, the production of thematic maps concerning the thermal conductivity variation of the analysed lithologies and their 3D development in the subsoil has been realized. The used method to get this type of representation is based on the relationships between the geothermal characteristics of the explored geological units and their relative thickness. According to an empirical model for the calculation of thermal conductivity, based on the intrinsic characteristic of materials, an average thermal conductivity value, which is function of the thickness for each lithology, has been adopted for each sector. Consequently, a detailed mapping of the thermal conductivity fluctuations has been carried out for depths between 50 m and 150 m (Fig. 2). Maps of the thermal conductivity variation put into evidence the significant influence of alluvial deposits for the first 50 m of the succession, reflecting the alluvial fan shape. The thematic cartography shows a remarkable contribution of volcanic terrains for the depth range 100-150 m, which finally imply the increase up to the maximum extractable thermal energy in the investigated area. The most important aspect of this study is the extrapolation of specific thermal conductivity data for a very narrow area, which leads to the innovative concept of geothermal micro-zonation.