Structural monitoring of Cultural Heritage assets at urban and local scale through MT-InSAR
In recent years, Multi-Temporal Interferometric Synthetic Aperture Radar (MT-InSAR) has become an increasingly popular technique for Structural Health Monitoring (SHM) purposes. The technique allows for the measurement of ground deformation with high accuracy and spatial resolution by utilizing Synthetic Aperture Radar (SAR) imagery taken from multiple time periods. MT-InSAR has been proven to be particularly effective in urban contexts due to the high reflectivity provided by structures, which makes them visible in SAR imagery. Several interferometric algorithms have been developed that are specifically tailored to the urban environment, making it possible to extract detailed information about buildings and infrastructure. Moreover, the growing availability of high-resolution SAR satellite constellations, such as the Italian COSMO-SkyMed, has also contributed to the increased use of MT-InSAR for SHM purposes. These constellations provide high-quality SAR imagery in which a high density of Measurement Points (MP) can be detected, allowing for the recording of detailed information on individual structures. With MT-InSAR, it is possible to collect information about the deformations at both global scale, detecting the most critical areas within the urban context, and local scale, focusing on individual structures such as buildings or bridges. Despite its many advantages, MT-InSAR has some drawbacks that must be taken into consideration. The technique requires complex post-processing and expert interpretation of results to avoid data misinterpretation, and technical difficulties such as geocoding errors and noisiness in the time series can be encountered during the analysis. Furthermore, the technique is sensitive to changes in the environment, such as changes in vegetation cover or weather conditions, which can affect the quality of the SAR imagery. Overall, MT-InSAR, despite its limitations, is a cost-effective and highly efficient tool for monitoring structures. It offers significant benefits in identifying potential problems and detecting deformations, providing valuable insights into the stability and health of structures. With the increasing availability of high-quality SAR imagery, MT-InSAR is predicted to have even more widespread usage for SHM purposes in the future.
In this work, the MT-InSAR technique is applied in the urban center of Verona (northern Italy), a city full of Cultural Heritage assets. The study examined images captured by the COSMO-SkyMed constellation in Stripmap mode for both ascending and descending orbits during the period 2011-2022, to detect deformations at both global and local scales. Initially, spatial interpolation algorithms were utilized to gauge the overall deformations at the urban level, identifying the most critical areas. The results show that the area of Verona presents an overall stability: the velocities of deformation of the historic center lie within the so-called range of stability (-1.5 – +1.5 mm/year), whereas the most critical areas can be identified in the northern part of the city in correspondence of the northern portion of the town beltway. Later, attention was directed towards some of the main cultural assets in the city, namely the Roman Arena, the Lamberti Tower, and the Roman Theater. For each asset several MPs were detected, distributed along the structures' height. The information contained in each MP, in terms of displacement velocity and displacement time series, allow for an understanding of the structural stability and of the evolution of the deformations during the monitoring period. As the urban analysis suggested, the investigated structures appear to be quite stable and no evident criticalities could be detected. However, despite the low magnitude of the deformations measured in the city of Verona, this research demonstrates the potential of MT-InSAR in the field of structural monitoring of Cultural Heritage.