GNGTS 2015 - Atti del 34° Convegno Nazionale

144 GNGTS 2015 S essione 1.3 areas of Italy (Dimitrov, 2002; Carveni et al. , 2012; Adrian et al. , 2015). In Sicily, in addition to Paternò (Catania), mud volcanoes can be observed in the areas of Agrigento and Caltanissetta, where they are also known with the name Macalube (Carveni et al. , 2001; Etiope et al. , 2002; Cangemi and Madonia, 2014). The results of some studies conducted on mud volcanoes represent the basis for the geothermal energy utilization, which presence is inferred by the composition of the gas ascending from depth (Mburu, 2014). The present study through microtremor survey aims to provide useful information to increase the knowledge on the geo-thermal activity concerning the Salinelle of Paternò (Catania, Italy), whose activity is also well recognizable at a macroscopic level. Different studies throughout the world were performed to monitor the activity of mud volcanoes in terms of gas outflow (Albarello et al. , 2012) and to study the presence of spectral anomalies in the passive seismic wavefield over different hydrocarbon reservoirs by using passive seismic surveys (Dangel et al. , 2003; Holzner et al. , 2005). The Salinelle mud volcanoes features. Etna volcano edifice overlies the sedimentary basement made of flysch and clayey deposits that belong to the limestone of the Maghrebian- Appenninic chain (Lentini, 1982). The whole volcanic sequence can be considered as a highly porous medium, with a permeability coefficient that varies as a function of both the lithology and the volcano-tectonic structures. Inparticular, the volcanic sequence is characterizedby alternating porous and fractured highly permeable lava layers and scarcely permeable pyroclastics. Then, the limit between lava and clay represents the base of the main aquifers of Etna. An exception is represented by the aquifer feeding the Salinelle mud volcanoes (Aiuppa et al. , 2007). In this area the emitted waters are characterized by an abundant free gaseous phase and show typical features of waters linked to hydrocarbon reservoirs. Geothermometric estimates carried out on both the liquid and the gaseous phases emitted at the Salinelle , gave temperatures, at depth, that range 100-150 C (Chiodini et al. , 1996). The fluids emitted generally consist of hydrocarbons (mainly CH 4 ) and hypersaline water (Giammanco et al. , 1998; Amici et al. , 2013). The mud and water mixtures are highly variable, and in some cases mud is the only fluid erupted with gas that builds cones up to a few meters high having a base diameter that can reach ten meters (Amici et al. , 2013). These findings seem to suggest the presence of hydrocarbon reservoirs trapped in the shallow sedimentary rocks and characterized by the presence of thermal water enriched and heated by gas coming from magma of the Etna conduits. The Salinelle , therefore, represent a very interesting geological-natural area, located at the NW boundary of Paternò, covering around 30,000 m 2 (Savasta, 1905). The main activities take place in two sites of Paternò area: Salinelle del Fiume and Salinelle dei Cappuccini . We focused our investigations in the latter site, in an area located close to the public football stadium (Fig. 1b), which is characterized by the most active vents. For practical reasons, we will refer to the general name Salinelle to indicate the study area. How thermal water reaches the surface is not fully clear, but some authors believe that at the Salinelle the mud rises through an old lava conduit (Carveni et al. , 2001). To support this hypothesis there are data concerning a mechanical drilling performed during 1958 for hydrocarbons research (Carveni et al. , 2001). A thick vacuolar lava rich in pyrite up to 400 m was found, in contrast with the average thin thickness of the surrounding lava in the area. The temperature of the muddy waters varies from 10 and 20 C (Giammanco et al. , 2007), but during some paroxysmal phases (1866, 1879 and 1954) the temperatures reach values between 46 and 49 C (Etiope et al. , 2002). In the latter case, columns of muddy water as high as 1.5 m (Cumin, 1954) were observed. Silvestri (1867, 1879) reports of intense eruptive events occurred in early 1866 and late 1878. This phenomena included fountains of muddy water up to 3 m high and water temperature increases up to 46 C, that the author associated with local seismic events that occurred some days/weeks prior to the gas eruptions. In addition, previous studies revealed a strong correlation between specific earthquakes in eastern Sicily, the paroxysmal phases of Salinelle and significant variation of the concentration of the main gases emitted.