Seismic Velocity Structures of Larderello Geothermal System, Italy: Preliminary Results

The steam-dominated geothermal system of Larderello is located in Tuscany and is the largest Italian area of electricity generation from geothermal resources. Enel Green Power, the main company of the ENEL Group involved in the renewable resources development, has drilled several wells down to maximum depth of about 4.0 km below see level in order to exploit deep and hot steam reservoirs. The explored area is about 400 km² with an installed running capacity of about 530 MW. Two steam-dominated reservoirs were found at different depth. The shallowest one at depth of about 1 km, with pressure between 0.2 and 1.5 MPa and temperatures ranging between 150°C and 260°C, is hosted in very permeable carbonate formations (limestone and anhydrite). The deepest reservoir is located in the metamorphic basement up to depth of 3-4 km b.s.l. and is characterized by pressure of about 7.0 MPa and temperature ranging between 300°C and 350°C. Water reinjection is operating in the shallow reservoir of the geothermal area with the aim of both sustaining and increasing reservoir pressures as well as steam production. A network of 26 seismic stations, three of which are three components, permanently records the seismic activity of the Larderello area. Data analysis showed that epicenters span over the whole exploited region even though clusters are visible in particular areas; hypocentral depths are mainly distributed up to 10 km. More detailed hypocenter re-localization might indicate linear features due to regional stress field regime and to the fluid propagation paths into the fracture systems that previously might have been obscured within the seismic clouds. However, precise hypocenter localization calls for high-resolution 3D-velocity model of subsurface structures that is lacking for this area. This study has been addressed to purse this goal and, as a consequence, images of the seismic velocity structures from earthquakes tomographic inversion have been computed. This area was chosen as a suitable test site since the availability of well data can provide a more constrained a priori velocity model. The analyzed data set consists of approximately 500 microearthquakes occurring from January 1994 through September 2000. The estimate duration magnitude ranges between 0-3. The good quality of recorded waveforms allowed us for high precision readings of P- and S- wave first arrivals. Results of a 3D velocity tomographic inversion contributed for a high-quality imaging of subsurface structures in term of Vp and Vp/Vs ratio that may be correlated to the main geological features of the geothermal system.