Estimating Surface Displacements Using SAR Data and Hydro-Geomechanical Models and IGRS Performance Assessment

Interferometric Synthetic Aperture Radar (InSAR) has been a crucial technique for monitoring land subsidence and uplift due to its ability to provide high-resolution, continuous spatial and temporal data. This thesis aims to enhance the understanding of ground deformation mechanisms in regions with complex geological settings by integrating InSAR estimations with hydrogeological and geomechanical models. Two case studies are investigated: Antwerp Harbour, a major industrial area experiencing significant land subsidence, and the Saint-Vaast region, a former coal mining area facing land subsidence and uplift challenges.

In Antwerp Harbour, subsidence is driven by natural and anthropogenic factors. The natural consolidation of Holocene sediments, the additional consolidation due to the backfill overload along harbour docks, the saturated-unsaturated consolidation of backfill materials, and the consolidation of compressible layers induced by groundwater pumping in Cenozoic aquifers are identified as the possible key drivers. Persistent Scatterer InSAR (PS-InSAR) was employed to monitor displacements, providing high-resolution surface deformation data. Coupled hydrogeological and geomechanical modeling was conducted in a local scale model out of the border of backfill to isolate only the effects of groundwater extraction on land subsidence. This approach enabled the exclusion of other factors and focused specifically on the impact of groundwater pumping. By integrating these methods, the study quantified the contribution of groundwater extraction relative to other factors, enhancing the understanding of complex interactions between surface deformations and subsurface processes.

The Saint-Vaast region presents unique geohazards due to presence of an old drainage adit in the historical coal mining region. During the post-mining period, this drainage adit was filled with water, changing subsurface pressure regimes and causing both land subsidence and uplift. This study used three decades of PS-InSAR data to detect and map temporal and spatial dynamics of ground displacements. All relevant spatial data, including an old drainage adit from the coal mining exploitation period, geological layers, and hydrogeological conditions, were mapped to understand the structural context of observed deformations. Thirteen years of piezometric well data provided insights into subsurface hydrodynamics. Breakpoints in the PS-InSAR time series, significant shifts in deformation rates, were identified showing the temporal evolution of ground deformation and its correlation with hydrogeological events. Correlation analysis linked ground deformation with mudflow events in 2009 and 2018, demonstrating a strong relationship between changes in groundwater levels and surface deformation patterns.

This thesis provides a comprehensive methodology for studying surface deformation phenomena by integrating the PS-InSAR technique with hydrogeological and geomechanical analyses and models. The insights gained contribute to a better understanding of the complex interactions between surface and subsurface processes, supporting the development of effective strategies for mitigating deformation-related geohazards. These geohazards are associated respectively with industrial areas under extensive developments and with densely populated urbanized areas. The main idea of the thesis is also to conduct a comparison between SAR data obtained by different satellites vs the estimated ground displacement measurements deduced from the SAR LOS timeseries and other techniques like hydrogeological and geomechanical models.

Additionally, this research assesses the performance of Integrated Geodetic Reference Stations (IGRS) installed in Antwerp, close to the Doel nuclear plant. The performance assessment focused on analyzing the IGRS data for stability and reliability, demonstrating generally high consistency and accuracy in range and azimuth measurements using SAR data. The results, including time series and Signal-to-Clutter Ratio (SCR) analysis, validated the quality of radar measurements. However, for a definitive assessment of their ability to measure and monitor surface displacements accurately, further validation with GPS antenna installed on the station would be necessary. Future studies should aim to seek this purpose. This would involve decomposing the line-of-sight (LOS) deformation into vertical and horizontal components and comparing these with GPS measurements to confirm their accuracy in monitoring surface displacements.