GNGTS 2013 - Atti del 32° Convegno Nazionale

Geophysical and geochemical evidence of the Solfatara-Pisciarelli shallow hydrothermal system A. Troiano 1 , M.G. Di Giuseppe 1 , D. Patella 2 , A. Fedele 1 , R. Somma 1 , C. Troise 1 , G. De Natale 1 1 Istituto Nazionale di Geofisica e Vulcanologia - Osservatorio Vesuviano, Napoli, Italy 1 Dipartimento di Fisica - Università degli Studi di Napoli ‘Federico II’, Italy Solfatara and Pisciarelli settings. Campi Flegrei caldera (CFc) has been formed by huge eruptions, occurred 39000 and 15000 years ago, which have been the largest ones occurred in the Mediterranean since the beginning of mankind (Rosi and Sbrana, 1987). Up and down ground movements with rates from centimetres to meters per year characterize the dynamics of this area also during quiescent periods (Dvorak and Mastrolorenzo, 1991) Since 1969, the area started a new phase of uplift after several centuries of subsidence dating back to 1538 A.D., when the last eruption occurred in the area (Di Vito et al ., 1987). Recent studies on the interpretation of such uplift episodes point out the important role played by the the geothermal system, which is characterized by hydrothermal manifestations such as distributed degassing zones and fumaroles (De Natale et al. , 2001; Chiodini et al. , 2003; De Natale et al. , 2006). The Solfatara-Pisciarelli area represents the most active zone within the CFc in terms of hydrothermal manifestations and nowdays local seismicity. The Solfatara volcano is located inside the CFc, about 2 km east-NE of the town of Pozzuoli. It is a tuff cone formed about 3,7- 3,9 ky ago, which generated in 1198 AD a low-magnitude hydromagmatic explosive eruption that ejected tephra over a small area (<1 km 2 ) (Di Vito et al. , 1999). The crater has a roughly elliptical shape with the two axes of 580 and 770 m, and a maximum elevation of 199 m asl. The Solfatara crater is located very close to the area of maximum ground uplift, the benchmark 25A at Pozzuoli, during the last unrest crises. It hosts large and spectacular fumarole vents, with maximum temperatures in the range 150-160°C at the Bocca Grande (BG) and Bocca Nuova (BN) and about 100°C at Le Stufe (LS) and La Fangaia (LF) ones (Chiodini et al. , 2001). Systematic measurements of the gas fluxes from the soil evidenced up Conclusion. Our procedure rely on a very affordable basis, being a very good reconstruc- tion of induced seismicity already been obtained. The new step that we are calibrating involve an estimate of the permeability enhancement correlated to stimulation process of geothermal boreholes. The proposed procedure lead to promising results, being the permeability enhance- ment estimated distributed in the space in a coherent way. The magnitude of this enhancement too, result coherent with the experimental data, once the wellbore overpressure and fluid flow has been imposed. SP anomalies generated during the stimulation process has been recon- structed in order to evaluate the effectiveness of SP monitoring to mitigate the induced seis- micity risk. References Bai M, Elsworth D. 1994: Modeling of subsidence and stress-dependent hydraulic conductivity for intact and fractured porous media . Rock Mech Rock Eng , 27 , 209-234. Majer, E., Baria, R., Strak, M., Oates, S., Bommer, J., Smith, B. & Asanuma, H. 2007: Induced seismicity associated with enhanced geothermal sys-tems , Geothermics , 36 , 185-222. Massachussetts Institute of Technology 2006: The Future of Geothermal Energy: Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century: An Assessment , MIT Press, MA, USA. Peng, S., Zhang, J. 2007: Engineering Geology for Underground Rocks , 207-211. Springer. Pruess, K., 1991: TOUGH2-A General Purpose Numerical Simulator for Multiphase Fluid and Heat Flow, L.B.L. Report, Berkeley, CA. Troiano, A., Di Giuseppe, M.G., Petrillo, Z., Troise, C. and De Natale, G. 2011: Ground deformation at calderas driven by fluid injection: modelling unrest episodes at Campi Flegrei (Italy). Geophys. J., 187 , 833-847. Troiano, A., Di Giuseppe, M.G., Troise, C., Tramelli, A., and De Natale, G. 2013: A Coulomb stress model for induced seismicity distribution due to fluid injection and withdrawal in deep boreholes . Geophys. J. Int., 195 , 504-512. 288 GNGTS 2013 S essione 1.3