Study of periodic dependencies among ground deformation, rainfall and sea level in the city of Naples (Italy)

Abstract

Since early 1990s, the advent of satellite-based techniques, such as the Differential Interferometry Synthetic Aperture (DInSAR) (e.g. Curlander & McDonough, 1991) and Global Navigation Satellite System (GNSS), has revolutionized the monitoring of ground level displacements thanks to satellite-based techniques. These observation systems enabled us to control with unprecedented accuracy and resolution deformation phenomena caused by transient events, such as earthquakes, volcanic eruptions and sinkholes, or long-term processes, like subsidence and landslides, thus revealing useful applications in the geological, geotechnical and structural analysis fields (e.g. Lanari et al., 1998; Herrera et al., 2009). Today, the Advanced DInSAR (A-DInSAR) technique is based on high resolution sensors, which monitor the evolution of instability phenomena in any atmospheric and lighting condition, and in both historical centers and archaeological sites. Within this framework, we propose the application of an exhaustive approach for studying both permanent and periodic components of vertical land deformation in the urban area of Naples (southern Italy). Such methodology, successfully applied in the Po Delta area (Vitagliano et al., 2020), is proposed to study the geological risks in urban areas and to evaluate the effects of the anthropic context on the periodic processes related to climate variability. Firstly, a spatial analysis has been carried out to calculate the interferometric velocities of Sentinel-1 and COSMO-SkyMed data in the time intervals January 2016 – December 2019 and March 2017 – December 2020, respectively. The obtained ground displacement and velocity have been used for mapping the stable and unstable urban areas and for evaluating the dependencies between the distribution of the velocity zones and some geological features (e.g. ancient alluvial deposits and paleo-coast lines). Afterwards, a spatial-temporal analysis of continuous GPS data available for a site located in the city center has been performed, which provided a reliable identification of the seasonal trends of the ground displacement. Finally, an automatic cross-correlation procedure developed in Matlab environment was applied to systematically correlate the interferometric trends of each reflection point with the rainfall and sea level oscillations, measured in the same time interval. The correlation process allowed the identification of the most significant dependencies and the triggering mechanisms of the slow soil oscillations, and gave also interesting insights into urban instability processes, such as sinkholes.