A method for the automatic characterization of interferometric fringes free of atmospheric artifacts : application to the study of the subsidences on the city of Paris

Differential SAR interferometry allows for the detection and mapping of ground subsidences, usually attributable to human activities, associated with the extraction of fluids beneath the surface, or underground mining... Limiting factors for monitoring slow subsidences are mainly temporal coherence loss and varying atmospheric conditions between the acquisitions of images. Such variations induce a path difference generating InSAR artifacts that cannot be corrected if only one interferogram is available. On urban areas, the coherence remains often high over long time scales. The main problem then appears to be the atmospheric heterogeneities. This kind of artifacts can be easily detected on the different interferograms we generated on the city of Paris. Significant phase variations are clearly visible, and can not be associated with topographic fringes, nor with displacements. Several techniques exist in order to eliminate or reduce the effect of atmospheric artifacts: a solution consists in summing and averaging interferograms, but requires several interferograms over the same site. The advantages of a novel approach based on complex correlation of interferograms are presented here, in particular robustness when only a few interferograms are available (two interferograms are enough, under given hypotheses). These algorithms were tested in the context of automatic detection of atmospheric artifacts by means of correlation of interferometric triplets. On the city of Paris, this method reveals 2 subsiding zones. One zone has the same location as an important underground working site, which took place from 1995 to 1997. The existence of subsidences in the area was known already from ground truth data. Their spatial extent can now be mapped by interferometry, and the temporal evolution of the subsidences is also examined here. INTRODUCTION The use of SAR interferometry has now become essential for the detection of ground deformations caused by earthquakes, volcano activity, and ice motion, which are usually phenomena of large spatial extension. It has also proven to be a feasible tool for the detection and mapping of ground subsidences, associated with geothermal fields, oil fields, compaction of aquifer system, underground mining...Most of these studied subsidences occur either in non urban context, and are of small spatial extension (Carnec et al., 1996), or concern urban areas, but are very large (Amelung, 1998; Fielding et al., 1998; Wegmüller et al., 1999). Here we examine the feasibility of SAR interferometry for the detection of slow deformations on urban areas with standard atmospheric conditions. We focus on the city of Paris, where displacements of a few hundred meters in extension and a few centimeters in amplitude occur. They are mainly due to the water pumping of working sites, and also to underground quarry.