Integrated TLS and GBInSAR System for Monitoring Structural Instabilities over Urbanized Areas: the Case of Volterra (Tuscany, Italy)

An integrated TLS (Terrestrial Laser Scanner) and GBInSAR (Ground Based Synthetic Aperture Radar Interferometry) remote sensing system have been developed for monitoring the cultural heritage built environment of Volterra (Tuscany, Italy) hit on 31st January 2014 by a sudden collapse of a 35 m wide section of city walls. In this case GBInSAR is used to detect with high acquisition rate and sub millimetre accuracy surface displacements of structures affected by structural and geological criticalities. The output of this radar system consist in a 2-D plane so that a detailed identification of observed elements is not immediate. To overcome such limitation, a TLS survey of the scenario observed by the radar system has been undertaken and the registered 3D point cloud has been coupled with displacements measured by the GBInSAR to obtain a 3D displacement map. This product guarantees an accurate and focused real-time monitoring of the south-west side of the city including walls, buildings and monuments thus allowing to discover ongoing deterioration processes and alert population in case of imminent failures. Effectiveness of this alert technique became evident when precursors of a second impressive wall collapse were individuated two days in advance. In this case an acceleration of a 15 m wide wall section movement was revealed and effectively localized accordingly to the 3D displacement map. Then alarm was promptly delivered and consequently local authorities started consolidation works and interdicted area to citizens. Results achieved in Volterra highlights effectiveness of a monitoring system taking advantage from high accuracy of GBInSAR displacement measures coupled with imaging capability of TLS.