Ground subsidence phenomena in the Delta Municipality region (Northern Greece): geotechnical modelling and validation with Persistent Scatterer Interferometry

Land subsidence is a common phenomenon occurring in several regions worldwide. Persistent subsidence causes strong consequences on the affected areas and related problems include environmental, economic and social aspects. A set of forty-two Synthetic Aperture Radar (SAR) images, acquired in 1995–2001 by the European Space Agency (ESA) satellites ERS1 & ERS2, were processed with Persistent Scatterer Interferometry (PSI) technique to investigate spatial and temporal patterns of deformation in the Delta municipality (Thessaloniki plain, Northern Greece), a deltaic area with a long history of land subsidence related to aquifer system compaction. Exploitation of output products of a PSI analysis, both average LOS (Line of Sight) deformation rates and displacement time series, revealed a large subsidence area due to intense groundwater withdrawal. Higher displacement velocities have been observed south-west of Kalochori and south of Sindos, from both sides of the Gallikos River. In those areas deformation rates of roughly 4.5 cm/yr have been recorded, during the period from 1995 to 2001. Increasing subsidence rates are measured moving towards the mouth of the Gallikos River, where the thickest sequence of compressible Quaternary sediments is observed. Displacement time series retrieved by PSI technique have been compared with the temporal evolution of the deformation as measured by pre-existing leveling surveys, showing a great agreement. A 2-D finite element model has been run along two representative cross sections in the Kalochori area, in order to simulate the observed temporal evolution of subsidence, coupling the geotechnical behavior of the formations and the piezometric surface level. Finally, results obtained by the subsidence model have been positively compared with the PSI-based information on displacement, providing accurate and perfectly verified results. Outcomes of this work demonstrated the potential of repeat-pass satellite SAR interferometry (InSAR) as suitable technique for increasing knowledge about the extent and the rate of the deformations in case of subsidence events. Moreover, InSAR turned out to be a valuable tool to validate subsidence models and represents a cost-efficiency method, alternative to ground-based measurements for investigating surface deformation phenomena.