The LASUGEO project:

monitoring LAnd SUbsidence caused by Groundwater exploitation through gEOdetic measurements

In the last decades, rapid urbanization, global climate change and uncontrolled anthropogenic transformations of the territory caused a relevant increase in geo-hazards events with huge economic and social consequences (Galloway et al., 1999). The dramatic increase of these events with environmental degradation highlights the importance of improving ground monitoring and natural resources management with a continuous exchange of knowledge between the scientific community and authorities in charge of environmental risk management. Since the late 1990s, SAR (Synthetic Aperture Radar) data allow measuring slow-moving ground deformations. In the last decades, the use of spaceborne InSAR (Interferometric SAR) has increased significantly thanks to the availability of large-area coverage, millimetre precision, high spatial/temporal data resolution and good cost-benefit ratio with respect to other conventional topographic techniques (i.e. subsidence in Tomas et al., 2014; Karila et al., 2013). The development of Multi Temporal Interferometric SAR techniques (MT-InSAR), commonly grouped into PSI-like (Persistent Scatterers Interferometry) and SBAS-like (Small BAseline Subset) algorithms, has changed the way in which radar images can be exploited for geohazard monitoring (like natural gas extraction, mining activities, groundwater overexploitation, karst or landslides processes, etc.). In Belgium, most of the subsidence bowls already identified and mapped by the PSI technique are related to strong fluctuations of an aquifer implying at the surface ground deformations that are monitored (Declercq et al., 2017a, b). The problem of land subsidence induced by anthropogenically changed groundwater conditions can be significant (Poland, 1984; Galloway and Burbey, 2011). The most affected regions correspond to areas located where compressible loose sediments are found. Due to the highly heterogeneous nature of the underground, all geological settings are always partly compressible to some extent. Any change in pore pressure (and thus also in the piezometric heads) may induce consolidation if the geological formations are compressible. Effective stress changes with water pressure variations at depth in confined and unconfined conditions, inducing consolidation. Geomechanical aspects are fully coupled to groundwater flow equations. When and where groundwater levels and water pressures are restored, a partial rebound (uplift) corresponding to the elastic part of the geological formations behaviour is observed. Consolidation and elastic rebound processes occur in confined and unconfined conditions. The most sensitive parts of the concerned aquifers contain clay, loam or peat lenses but consolidation occur mostly in the underlying and overlying layers that are often less permeable and more compressible than the aquifer itself. In this case, it is largely a delayed process occurring as far as the pore pressure variation can propagate slowly in the low permeability (aquitard) layers. Comparing successive InSAR measurements provide a kind of cumulative total value of the subsidence or uplift over large-scale areas They are cumulative values as they do not provide any information about the relative importance of the settlement/swelling in each geological formation. It is a priority now to reconcile the results of the PSinSAR technique data with the knowledge, understanding and modelling of aquifers and more generally of the spatial and temporal distribution of groundwater pressures in several targeted region of interest (ROI) covering specific areas in Flanders and Wallonia. The fourth ROI located outside Belgium in the neighbouring area of Jülich (Germany) will be a perfect test case where all the different techniques/data (i.e. GNSS, hydrogeology, gravity and PSInSAR) could be compared altogether in an active land subsidence bowl associated to brown coal open quarries activities and groundwater withdrawal.

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The scientific partners

Royal Belgium Institute of Natural Sciences (RBINS) / Geological Survey of Belgium (GSB) is the coordinator of the project. The team is composed by Dr Xavier Devleeschouwer (coordinator), Pierre-Yves Declercq (GIS-SAR geologist).
Royal Observatory of Belgium (ROB). Michel Van Camp (head of the Directorate Seismology/Gravimetry at ROB).
University of Liège / Hydrogeology and Environmental Geology (HGE). Prof. Alain Dassargues & Philippe Orban
University of Gent – Laboratory for Applied Geology and Hydrogeology of Gent University (UGent-LTGH). Prof Kristine Walraevens & Dr Marc Van Camp

The project is funded by Belspo under the BRAIN-be 2.0 – Belgian research action through Interdisciplinary Networks (2018 -2023) in the Pillar 1 Thematic priority