GNGTS 2013 - Atti del 32° Convegno Nazionale

tic medium; penny-shaped hydrothermal sources in an elastic halfspace; and penny-shaped magma intrusions in a layered elastic half-space. Geo- physical investigations have also excluded the presence of significant melt formations down to about 7.5 km in depth (Vanorio et al. , 2005; Zollo et al. , 2008), although the methods used in these investigations did not allow the defining of a shal- low magma body with a volume less than 1 km 3 . The phases of rapid inflation have been well-ex- plained by different models (Carlino and Somma, 2010 and references therein), while the subsidence phases that follow the uplift, at rates greater than the secular subsidence, cannot be explained in terms of magma migration. Otherwise, this pro- cess is well modeled in terms of the radial out- wards migration of hydrothermal fluids. Hybrid model sources have also been proposed, in which magma intrusion occurs at the beginning of each period of unrest and produces perturbations of the geothermal system (De Natale et al. , 2001; Battaglia et al. , 2006). The main problem related to these fluid-dynamic modeling is the con- strain of a fundamental physical parameter, namely the permeability (k). In recent time, new data related to the permeability of shallow CFc crust, at 500 m of depth, was inferred during the drilling of the pilot hole performed in the framework of the Campi Flegrei Deep Drilling Project (CFDDP). An adapted Leak Off Test allows us to infer the average value of k at a depth of 500 m (bottom hole) (Fig. 1). The depth permeability variation (>500 m) is then extrapolated by using the empirical formula of Manning and Ingebritsen (1999). This new fundamental data has been applied to the previous fluid-dynamic model published by Troiano et al. (2010) in or- der to obtain a more reliable picture of the process generating the unrest. The simulations have been carried out by the numerical code TOUGH2, considering an injection of fluids (H 2 O+- CO 2 ) with inflow rate inferred from literature (Chiodini et al. , 2012) during 1982-84 unrest. The code TOUGH2 allows to compute the mass and heat exchange related to multidimensional flows of multiphase (gas and liquid) mixtures of many components within a porous medium of assigned permeability. It assumes local equilibrium between fluid and rock matrix, through the direct discretization of the balance equations for mass and energy describing the thermo- dynamic conditions of the system in their integral form, in a scheme called integral finite dif- ference method. The comparison between the experimental curve of 1982-84 uplift, including the subsequent phase of subsidence, and the calculated curve is shown in Fig. 2. It is clear that the fluid-injection supplies only a partial contribution (about 0.7 m) to the total uplift for both the solutions, which, besides, are quite equivalents. Furthermore, this partial uplift is totally recovered after about 30 years due to radial fluids migration and inflow rate decrease. Consid- ering the total uplift in the period 1982-1984 amounted to about 1.8 m, we can conclude about 1.0 m of uplift, interestingly corresponding to the remaining uplift after the partial recovering, should be ascribed to the contribution of magma injection in a shallow reservoir. This result is very important in evaluating the potential eruptible magma, to assess the possible energy of future eruptions, which is crucial in such extremely densely populated area. References Acocella V., 2007, Understanding caldera structure and development; an overview of analog models compared to natural calderas, Earth-Science Reviews, v. 85, 125-160. Battaglia M., Troise C., Obrizzo F., Pingue F., De Natale G., 2006, Evidence for fluid migration as the source of deformation at Campi Flegrei caldera (Italy). Geophys Res Lett 33:01307. doi:10.1029/2005GL024904 Fig. 2 – Compartson between experimental (blue) and calculated (green) curves for 1982-84 unrest at Campi Flegrei caldera using code TOUGH2. 244 GNGTS 2013 S essione 1.3