Sensing the Land Subsidence in the Venice Lagoon by Interferometric Point Target Analysis

Land subsidence is a severe geologic hazard threading the lowlying coastal areas worldwide. Monitoring land subsidence has been significantly improved over the last few years by space borne earth observation techniques based on SAR (Synthetic Aperture Radar) Interferometry. Within the INLET Project, funded by Magistrato alle Acque di Venezia – Venice Water Authority (VWA) and Consorzio Venezia Nuova (CVN), we have used the Interferometric Point Target Analysis (IPTA) to characterize the ground displacements within the Venice Lagoon. IPTA measures the movement of backscattering objects (point targets, PT) at the ground surface which persistently reflect radar signal emitted by the SAR antenna. For this study 80 ERS-1/2 and 44 ENVISAT scenes recorded from 1992 to 2005 and from 2003 to 2007, respectively, have been processed by IPTA. High reliable land subsidence data have been detected for thousands of PT located on the lagoon margins, along the littorals, in major and small islands, and on single anthropogenic structures scattered within the lagoon. On the average, land subsidence ranges from less than 1 mm/year to 5 mm/year, with some PT that exhibit values also larger than 10 mm/year depending on both the local geologic conditions and anthropogenic activities. A network of few tens of artificial square trihedral corner reflectors (TCR) has been established before summer 2007 to monitor land subsidence in the inner lagoon areas where natural reflectors completely lack (e.g., on the salt marshes). The first interferometric results on the TCR appear very promising.