EGS-AGU-EUG Joint Assembly, Nice, France, 2002
R. De Matteis, T. Vanorio, B. Ciulli, E. Spinelli, A. Fiordelisi, A. Zollo
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 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. In the explored area, about 400 km2 wide, two steam-dominated reservoirs were tapped by wells at different depth. The shallowest one is hosted in very permeable carbonate formations at depth of about 1 km having pressure between 0.2 and 1.5 MPa and temperatures ranging between 150C and 260C. 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 and 350C. 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. Main goals of this study have been both to determine reliable earthquakes locations in a detailed 3- D velocity model and to define deep structural features of the geothermal field. A high-resolution 3D-velocity model of subsurface structures was lacking for this area. Therefore, this research has been addressed to image the seismic velocity structures by 3D microearthquakes tomographic inversion. Enel Green Power continuously surveys the seismic activity in the Larderello area since 1977 by using a digital local network consisting of 26 seismic stations, three of which are three components. The analyzed data set consists of approximately 500 microearthquakes occurring from January 1994 through September 2000. The good quality of recorded waveforms allowed us for high precision readings of P- and S- wave first arrivals providing a total of about 5,000 P- and 2,500 S- arrival time readings. Results of 3D velocity tomographic inversion have shown both vertical and lateral variations in term of Vp and Vp/Vs ratio that have been correlated to the main geological features of the geothermal system. In particular a low P- velocity structure lies in the middle west of the investigated area from the surface up to 3km depth. At 3 km depth, the existence of a well constrained high velocity zone delineates the morphology of the metamorphic basement which is consistent with the emplacement of the deeper reservoir. Cross sections show that the top of the metamorphic basement uplifts up to 3-4 km in the central part of the area and dips towards SE up to 10 km. By correlation with gravity data, the recovered high velocity anomalies match quite well residual Bouguer gravity highs. Furthermore, earthquake locations in the 3-D velocity model are distributed within peculiar areas clustering by linear features along the thermo-metamorphic basement top up to 13 km. This might indicate either a regional stress zone or preferential fluid propagation paths into the fracture systems.