Passive Seismological Inspection of a Geothermal Field: The GAPSS Experiment.

Authors: Piccinini, Saccorotti, Mazzarini, Zupo, Capello, Musumeci, Cauchie, Chiarabba
Keywords: Larderello, Seismicity,
Conference: European Geothermal Conference Session: 6 Hydrothermal Systems – Applications (HS2-4)
Year: 2013 Language: English
Abstract: Passive seismological imaging techniques based on either transient (earthquakes) or sustained (background noise) signals can provide detailed descriptions of subsurface attributes as seismic velocity, attenuation, and anisotropy. However, the correspondence between these parameters and the physical properties of crustal fluids is still ambiguous. Moreover, the resolving capabilities and condition of applicability of emerging techniques such as the Ambient Noise Tomography are still to be investigated thoroughly. Following these arguments, a specific project (GAPSS-Geothermal Area Passive Seismic Sources) was planned, in order to test passive exploration methods on a well-known geothermal area, namely the Larderello-Travale Geothermal Field (LTGF). This geothermal area is located in the western part of Tuscany (Italy), and it is the most ancient geothermal power field of the world. Heat flow in this area can reach local peaks of 1000 mW/m3. The deep explorations in this area showed a deeper reservoir (3000 to 4000 m depth) located within the metamorphic rocks in the contact aureole of the Pliocene-Quaternary granites [Bertini et al., 2006]; it is characterized by a wide negative gravimetric anomaly, interpreted as partially molten granite at temperatures of 800°C [Bottinga and Weill, 1970]. From seismic surveys the marker K (pressurized horizons) was found at depths between 3 and 7 km [Batini and Nicolich, 1984; Bertini et al., 2006]. The structural grain of the geothermal field is characterized by N-W trending and N-E dipping normal faults whose activity lasts since the Pliocene [Brogi et al., 2003]. GAPSS is ongoing since early May, 2012, and it consists of 12 temporary seismic stations, complemented by two permanent stations from the National Seismic Network of Italy. The resulting array has an aperture of about 50 Km, with average station spacing of 10 Km. Stations are equipped with either broadband (40s and 120s) or intermediate-period (5s), 3-components seismometers. LTGF is seismically active. During the first 2 months of measurements, we located about 250 earthquakes, with a peak rate of up to 40 shocks/day. Preliminary results from analysis of these signals include: (I) a study of local micro-earthquakes remotely triggered by the surface waves from the Po Plain main-shock (May 20,2012). Results suggest a triggering process most likely related to the Coulomb failure of faults kept close to rupture by elevated fluid pore pressure. (ii) The detailed location of clustered microearthquakes from inversion of differential times, thus obtaining a detailed picture of fracture geometry. (ii) Seismic noise analysis, thus far mostly aimed at elucidating the directional properties of the noise wavefield over the microseismic (0.1-0.5 Hz) frequency band.
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