GNGTS 2013 - Atti del 32° Convegno Nazionale

caldera at depths higher than 3 km (AGIP, 1987). Further consideration arise from the analysis of the seismic P-wave velocity (v P ) and the P-wave/S-wave velocity ratio (v p /v s ) in the same zones (Battaglia et al. 2008). The conductive C-zone almost completely coincides with a low v p /v s area (v p /v s ~1,73). The reason for assuming v p and v p /v s as test parameters resides in the relationship existing between their variations and reservoir fluid phases. In detail, low v p /v s values are related to a decrease of v p in areas with low pore pressure, high heat flow, fracturing and steam/gas saturation in reservoirs, while high v p /v s values are found in liquid-saturated high-pressure fields (e.g., Nakajima et al. , 2001; Simiyu, 2009; Jousset et al. , 2011; Mormone et al. , 2011; Gritto et al. , 2013). It is well established, in fact, that the presence of steam/gas in rocks generally changes the rock compressibility with a v p decrease, whereas waters in rock voids do not sustain shear stress and decrease the v s without any v p variation (e.g., Vanorio et al. , 2002). It has also been ascertained that the v p /v s ratio increases with pressure increase and temperature decrease from vapour-saturated to liquid-saturated conditions (e.g., Ito et al. , 1979), and that v P is affected by the degree of water saturation (e.g., O’Connell and Budiansky, 1974, 1977). The lowest resistivity values that characterize this zone, combined with the seismic evidences, allow us to exclude a water-saturated reservoir, but very likely to admit the presence of a dry and impermeable hydrothermally mineralized, clay-rich body. Geochemical evidence. An important issue for further discussion is the implication that this EM model, correlated with the evidences emerging from geochemical analysis, can have on the understanding of the fluids uplift in the Solfatara-Pisciarelli area. According to Caliro et al . (2007) the peaks of the CO 2 /H 2 O concentration ratio, occurred in 1986, 1991 and 1995-96 at the Solfatara crater a few months later an uplift of the ground (Chiodini et al ., 2010), reflect the increased component of magmatic gases in the composition of the fumaroles, probably due to episodes of intense degassing of magma at depth. The Pisciarelli area is also characterized by emission of gases and fluids through fractures mostly trending N110-120E and mainly NWSE and NE-SW. The main component of the fumaroles is H 2 O followed by CO 2 and H 2 S and with a range of temperature between 100-110 °C (Chiodini, 2009; Fedele, 2013). Fig. 3 – Fluid flows patterns as reconstructed by numerical simulation. Black arrows show the CO 2 fluxes migrating from the injection point, placed below the Solfatara crater, towards the surface, ending in the Pisciarelli area. Colour contours shows the CO 2 mass fraction. 291 GNGTS 2013 S essione 1.3