GNGTS 2019 - Atti del 38° Convegno Nazionale

204 GNGTS 2019 S essione 1.3 accompanying volcanic unrest (Napoli et al. , 2011; Currenti et al. , 2011; Battaglia et al. , 2008). Non-magmatic sources associated to the perturbation of the hydrothermal system may produce observable geophysical signals (Coco et al. , 2016). The hydrothermal activity results in the heating and pressurization of hydrothermal fluids, which in turn induces changes in different geophysical parameters. Temperature and pore-pressure changes necessarily induce stress and strain variations, which alter the density distribution and the magnetization of the porous media and are reflected on the ground surface in observable variations in deformation, gravity and magnetic fields. Monitoring these geophysical observables, which are the surface expressions of processes that are not directly accessible, and developing modeling tools for their interpretation are the keys to open up new perspectives in the exploration and monitoring of hydrothermal areas. In order to investigate how hydrothermal fluid circulation may affect ground deformation, magnetic and gravity changes, a multi-parametric thermo-poroelastic simulator is proposed in a 2D axis symmetric formulation, that is a reasonable approximation for representing many volcanic edifices, which are usually characterized by radial structures. The multi-parametric thermo-poroelastic simulator jointly solves (i) the mass and energy balance equations for a multiphase ground-water flow to evaluate temperature, pressure and density changes, (ii) the elastostatic equation for evaluating the induced deformation field and (iii) the Poisson’s equations for estimating the thermo-magnetic and gravity changes. In particular, pressure, temperature and density changes induced by fluid circulation are computed using the TOUGH2 numerical code, a well-known multi-phase multicomponent software for simulating fluid flow and heat transfer in porous media for a two-phase flow, which incorporates the equations of state in the temperature and pressure range 0–350 °C and 0–100 MPa, respectively. Subsequently, the obtained variations referred to the values achieved at the steady-state, are used to compute the related ground deformation and gravity changes by a specifically thermo-poroelastic solver developed under the software COMSOL Multiphysics (COMSOL, 2012) by using the PDE engine. The two codes, TOUGH2 and COMSOL Multiphysics, are linked by programming new specific subroutines for automatic sequence execution and data transfer. In the thermo-poroelastic model the computational domain is bounded by infinite mapped elements, which use appropriate transformation functions to map the finite domain into an infinite one and, hence, make the computational variables vanish toward infinity (Currenti, 2014). To illustrate the feasibility of the numerical approach, computations are designed to model the geophysical response of the hydrothermal system of Vulcano Island to a generic unrest period. Vulcano Island has been selected as a case study since: (i) its hydrothermal activity is suitable to be described by the presented numerical approach and to show its capability; (ii) geochemical, geological and volcanological investigations have provided the required information to set up the model material parameters (Chiodini et al. , 1996; Granieri et al. , 2006; Federico et al. , 2010; Napoli and Currenti, 2016). Moreover, several geophysical data have been acquired over the last decades at Vulcano Island using different kinds of discrete and continuous measurement techniques, which may be used to assess the simulated results. Using a model-based approach, numerical results demonstrated that detectable geophysical changes may be revealed in association with the resumption of hydrothermal activity at Vulcano Island. Under the model assumptions, a generic unrest of 1-year engenders on the ground surface low amplitude changes in all the geophysical observables, that are, however, above the accuracies of the modern state-of-the-art instruments (in the order of a few mm, μGal and tenths of nT for deformation, gravity and magnetic changes) applied in volcano monitoring. However, the model has a limitation due to the limited range of temperature, up to 350°C, within which TOUGH2 can be applied. This sets limits on the injection depth that will remain constrained in the shallower areas of the volcano edifices and implies that supercritical fluids, that are generally present in hydrothermal systems, are not taken in consideration.