PSI - Issue 44

Andrea Nettis et al. / Procedia Structural Integrity 44 (2023) 1996–2003 Andrea Nettis et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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structural assessment. While several studies (mentioned in the following section) analyse the potential of this technique for structure-specific applications, this study discusses the use of satellite-based MTInSAR for monitoring large bridge portfolios. An interferometric analysis based on the persistent scatterer (PS) approach is performed on the case-study network in Rome. The displacement time series of the PSs and their spatial distribution with respect to bridges showing ongoing deformations are discussed. The results for several test bridges are shown to allow the reader for an easy understanding of the potential situations to be recognized while applying the methodology. 2. Basics of MTInSAR and applications for bridge monitoring A short description of the theoretical fundamentals of MTInSAR and several state-of-the-art applications for bridge monitoring are presented in this section. SAR sensors on satellite platforms send out a pulse of radio waves and measure the energy scattered back from the targets (identified as scatterers) on the terrestrial surfaces. The phase of the signal recorded by SAR sensors informs about the distance measured along the line of sight (LoS) of the sensors to scatterers. The general differential SAR interferometry technique exploits the phase difference between the backscattered microwave signals of two SAR images displaced in time and received from slightly different positions, to measure the LoS-projected displacements with centimetric to millimetric accuracy of terrestrial scatterers from the sensors. The accuracy related to this displacement measurement is strongly affected by several inaccuracies in the topographic information, the position of the satellite in the orbit, the atmospheric contribution and the noise related to temporal and spatial decorrelations (Crosetto et al., 2016). The MTInSAR is aimed at reducing these errors by processing a large stack of spaceborne SAR images (instead of two) collected over the same areas in large time periods to derive displacement time series of terrestrial scatterers of improved accuracy. The PS interferometry is a subclass of the MTInSAR techniques based on the selection and analysis of “persistent” scatterers on the terrestrial surface which are characterised by a high phase coherence over time, usually corresponding to man-made structures such as buildings, monuments, roads or natural components such as rocks. For each PS, the interferometric analysis reports a time series of the relative displacement along the LoS of each scatterer with respect to a reference point (assumed coherent and stable). This technique is particularly accurate for detecting linear deformation phenomena having a slow evolution in time (e.g. ground subsidence). The PS displacement time series are characterised by an accuracy approximately equal to ±5 mm in terms of absolute displacement and ±1÷2 mm/year in terms of mean deformation velocity (Di Carlo et al., 2021). The sensitivity of MTInSAR measurements to real deformation measurements is affected by the wavelength of the SAR sensor, in particular, it increases moving from C-band to X-band. A value of coherence ranging from 0 to 1 is also associated with each PS r esuming the quality or the “reliability” of the achieved displacement information with respect to the adopted displacement model. The interest in using MTInSAR for structural health monitoring derives from the introduction of X-band SAR sensors (e.g. TerraSAR-X and COSMO-SkyMed) which offer high-resolution spatial data up to 1 m (~3 cm wavelength) at a relatively short revisiting time (4-16 days). In Italy, the ReLUIS ( Rete dei Laboratori Universitari di Ingegneria Sismica e Strutturale ) consortium together with the National Research Council and the Department of Civil Protection recently addressed a research project on the use of MTInSAR for structural assessment purposes. Additionally, several studies discuss the use of MTInSAR for the structural assessment of buildings (Miano et al., 2021; Talledo et al., 2022). Concerning network-scale monitoring of bridges, an MTInSAR can be adopted for different scopes. Several studies aim to adapt conventional MTInSAR algorithms for analysing seasonal thermal deformations of bridges (Cusson et al., 2021). According to these studies, an early-warning system can be developed by monitoring via MTInSAR modifications on the behaviour under seasonal temperature variation of bridges. Other studies aim at decoupling the displacements due to temperature-induced effects and vertical deflections to detect evolving degradation phenomena. Giordano et al., (2022) propose a methodology for detecting structural damages based on PS time series neglecting undesired temperature-induced or environmental deformations with reference to a case-study steel truss bridge subjected to subsidence-induced deformations. Other studies aim to correlate MTInSAR with structural damages or degradation phenomena. For example, Peduto et al., (2018) correlate settlement-induced damages on bridges in Amsterdam detected via in situ surveys with MTInSAR products. Other studies focus on the potential of MTInSAR products as an early warning for avoiding catastrophic bridge collapses. Sousa & Bastos (2013) apply MTInSAR for studying the collapse of the Hintze Ribeiro bridge in the Douro region (Portugal) occurred in