Integration of topographic and InSAR surveys for studying the long-term evolution of the land subsidence process in an urban area

Since the beginning of the 1960s, the urban area of Bologna has experienced land subsidence due to excessive groundwater withdrawals. Ground deformation reached its peak during the 70s of the last century when maximum displacement rates of about 10 cm/year were documented, and significant damage to structures and infrastructures occurred. This process has been intensively monitored over the years, and extensive ground displacement measurements were collected employing increasingly sophisticated techniques, ranging from topographic levelling to GNSS surveys and, since 1992, satellite interferometry. Satellite data, in particular, has given a substantial contribution to the reconstruction of the subsidence process in more recent times, with a progressively higher spatial and temporal resolution towards the newer surveys. The available interferometric data are the results of four consecutive SAR campaigns undertaken by local authorities: 1992 – 2000 (ERS), 2002 – 2006 (ENVISAT), 2006 – 2011 (RADARSAT), 2011 – 2016 (RADARSAT and COSMO-SkyMed), and a fifth survey performed by the UniBo spin-off “Fragile” from the free SENTINEL1 2014 – 2020 data. As long-term data are essential to comprehensively understand the ongoing subsidence process evolution, within this work, a methodology was developed to integrate ground-based and remotely sensed monitoring data collected over the years, and produce continuous cumulative ground displacement time series and maps, depicting the long-term temporal evolution and spatial distribution of the subsidence process, respectively. The results obtained through the adopted processing chain highlight that the long-term ground displacement field well agrees with the 3D geological model of the area and that the cumulative subsidence and displacement rates temporal evolution nicely matches the pluriannual trend of the piezometric and groundwater pumping time series.