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Conventional and PS differential SAR interferometry for monitoring vertical deformation due to water pumping: the Haussmann-St-Lazare case example (Paris, France)

Bénédicte Fruneau(1) and Anne-Marie LE PARMENTIER(2)

(1) Université de Marne-la-Vallée, 5 bd Descartes, 77454 Marne-la-Vallée cedex 2, France
(2) Inspection générale des carrières, 1 place Denfert Rochereau, 75014 Paris, France, France

Abstract

Natural and anthropogenic hazards in urban areas will be a major concern in the next few years, and require a complete monitoring of cities in order to prevent possible deformation they may encountered, especially during construction works. Comprehension of the mechanisms of deformation is necessary to better prevent such urban risks and at least better protect population and buildings. Combining conventional and PS SAR interferometry technique appears to be an efficient and operational tool for low-cost, large-coverage (compared with levelling) surface deformation monitoring. This study focuses on the construction of the Haussmann-St Lazare-Condorcet underground railway station (French railway company) for the EOLE line. The important water pumping, done to keep the works in dry conditions, began in spring 1994, was stable during the construction work, and lowered end of 1997 to stop definitively in mid 1998. This affected slightly the sus-jacent topography, as it is usually observed with such water pumping. Numerous data in order to analyze those vertical deformations have been precisely studied: piezometric measurements provided on 87 piezometers by SNCF and IGC, precise levelling done on 626 points by SNCF, a large series of SAR images acquired by ERS-1 and ERS-2, as well as a cartography of deformation observed on the buildings. SAR images were used to established conventional differential interferograms, as well as Permanent Scatterers (PS) time series [Ferretti et al., 2000]. Analysis and comparisons of all available data show their good agreement and complementarity. For instance, differential interferograms allow one to build maps of vertical deformation, and then to derive the spatial extension and the amplitude of deformation even starting from a few SAR acquisitions. The new PS technique allows for a full description of the time evolution of single privileged radar targets (PS) with high precision (atmospheric artifacts are compensated for and no spatial filtering is carried out). The PS density can reach values of several hundreds points/km2, even though, in this study, only a limited set of 76 PS time series has been exploited. The PS approach requires a sufficient number of SAR images (e.g. > 20) and in areas affected by time non-uniform deformation (like the selected test site) is rather computationally intensive. With DINSAR data, three different phases are highlighted: (1) a maximum of 1.7 cm subsidence after the beginning of the water pumping (from the end of 1993 to mid 1995), (2) a stability phase during the water pumping (between 1995 and end of 1997), at last (3) an uplift phase (1.6 cm) after the end of the water pumping, showing first a slight increase of the topography, then a major uplift between July and August 1998. Piezometric data, evidencing the fluctuations of the different water tables during the construction works, give quite similar periods of variation, as well as levelling data and PS time series. Different maps of piezometric surface variations are superimposed with maps of deformation obtained with DINSAR and show similar spatial extension. Comparison between the levelling, piezometric data and PS time series confirm which water nappe is responsible of surface deformation. It is then possible to propose some reasons of the mechanisms of the discharge/recharge of the multilayer aquifer of the Paris Bassin in the studied area, which may allow in the future to define new specifications for water pumping in order to reduce surface deformation.

 

Full paper

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry