Contribution of Persistent Scatterer Interferometry (PSI) to map surface displacement in the Travale – Radicondoli Geothermal area (Tuscany, Italy)

Synthetic Aperture Radar Interferometry (InSAR) has been successfully used to map the deformation of the earth surface. Multi-interferogram techniques, known as Persistent Scatterer Interferometry (PSInSAR), are a powerful tools to monitoring surface deformation connected with seismic and volcanic activity, landslides, and subsidence due to fluid extraction. The availability of many data acquired by space agencies, as well as European Space Agency (ESA), and the high spatial resolution of PSI methodology, allow to reconstruct the temporal evolution of the ground surface deformations, measuring relative displacements of individual points (Permanent Scatterers, or PS) and estimating the velocity of deformation recorded in the period covered by satellites acquisitions. The possibility to detect the continuous ground surface displacement can provide an important information about reservoir behavior during production, helping to improve the development of a geothermal field (e.g. Hole et al. 2007; JVGR). The present study aims to test PSInSAR techniques over Travale-Radicondoli area, in order to assess the surface deformation connected with the exploitation of this geothermal field. The Travale-Radicondoli area is located about 15 km E-SE of the well-known Larderello-geothermal filed - southwestern Tuscany, Italy-, extending at the south-western margin of the Anqua-Radicondoli Basin. In this area two different reservoir have been identified: a shallow steam dominated reservoir, consisting of carbonate and evaporitic units, and a deep superheated steam reservoir, within metamorphic basement units and thermometamorphic rocks. Industrial exploitation of geothermal resources in the Travale-Radicondoli area began in 1950 and concerned only a small zone on the southern margin of the area, known as the "old field", characterized by a water dominated system. Since 1973, an intensive exploitation started in a more productive area located north-east of the "old field", where the first well exploiting a deeper and hotter steam dominated reservoir was drilled, this "Horst" reservoir, as well as defined in the literature, is characterized by a depth of about 600-800 m b.g.l., In the early 1980s the exploration was extended northward and to a deeper level Finally, a deep exploration phase began after 1990 with the aim to investigate a deeper reservoir placed below the shallow one at a depth of about 3-4 Km. The high structural complexity and long exploitation history make this site an ideal candidate to study the ground deformation related to exploitation of geothermal reservoirs and to test the applicability of this methodology also in structurally complex geothermal systems. We have analyzed PS information, from radar satellites ERS (European Remote Sensing Satellite) for the period 1993-2000, and ENVISAT (Environmental Satellite) satellite acquired between 2003-2010. For ERS satellites only descending geometry data set was available. Unfortunately there are no satellite data covering periods prior than 1993, nevertheless previous studies concerning the monitoring of ground vertical movements performed with precise topographic leveling (from 1973 to 2003), allow us to reconstruct the subsidence trends of the area and to make a comparison between the two different methodologies for the 1993-2003 time span.