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Mapping Mexico city subsidence by SAR correlation techniques : effect of SAR image resolution enhancement

Stephane Guillaso(1), Penelope Lopez Quiroz(1) and Marie-Pierre Doin(1)

(1) Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France

Abstract

Our study focuses on the spatial monitoring of Mexico city subsidence, one of the most populated cities in the world. Due to the extreme water consumption, subsidence around 40 cm per year is observed in some parts of the city (Quiroz et. al., 2007). The Differential Interferometry (DInSAR) technique has been applied in order to map wide-scale subsidence deformations in Mexico city during the time span 2003-2007, during which the subsidence process was proven to be mostly linear in time (Quiroz et al, 2007). The main limitation of the latter study concerns the high fringe rate gradients for interferograms with time span larger than 3 months, making links between acquisitions separated by more than 1 year badly constrained. In particular, interferometry cannot constrain the subsidence between 2000 and 2003, ie., between the end of ERS acquisitions and the beginning of Envisat acquisitions. Furthermore, even the large ERS acquisition gaps occuring in the 1995-1997 period will be difficult to fill by radar interferometry without making strong a-priori on the temporal behavior of the subsidence.

To extend the time span of the study, one possibility consists in analyzing the Mexico city subsidence rate by means of SAR amplitude correlation image. This approach is performed by calculating the spatial correlation of two amplitude SAR images with large time span, making possible to map surface subsidence e.g. between an ERS 1995 image and an Envisat 2006 image. Although less accurate than interferometric techniques, this approach provides here an integrated 1995-2006 displacement field in line of sight plus complementary information on the horizontal displacement along azimuth. To improve the accuracy of the measured displacement field, one possibility is to enhance the resolution of SAR data. Indeed, thanks to the wavenumber shift principle (Gatelli et al, 1994, Guillaso et al 2006), it is possible to improve the resolution of SAR data up to a factor two, by combining two SLCs having a large perpendicular baseline less than the critical one.

We propose to discuss in this paper the potential and limitations of the range resolution improvement technique in order to monitor at longer time span subsidence in Mexico city, by means of theoretical and experimental results. Using two datasets (ERS 1995 and Envisat 2006), two enhanced resolution images are created, making possible to generate a new correlation map. The map created is compared to the former correlation map based on images with initial resolution and to the deformation map generated using the interferometric approach from the time span 2003-2007.

 

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