Small displacements detected by SAR interferometry on the city of Paris (France)

The feasibility of SAR interferometry for the detection of slow deformations on urban areas with standard atmospheric conditions is shown. The authors focus on the city of Paris (France), on which ten interferograms are derived from tandem images acquired during the period of July 28 1993, August 10 1996. The main limitation for this kind of measurements is due to tropospheric inhomogeneities, which lead to significant phase variations as high as one fringe. The best solution to compensate for the artifacts introduced by these inhomogeneities in the propagating medium appears to be the addition of interferograms. Then, a halo of subsidence, whose extension is about 600 m by 700 m is evidenced. It is precisely straight above an important working site: the construction of an underground station for the new "Eole" subway. This subsidence is produced by the lowering of the piezometric level, due to the pumping of the phreatic water.