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A methodological approach to reduce the tropospheric effects in the ERS SAR interferograms: application to the Gulf of Corinth (Greece) region

Antonio Avallone(1) and Henri Maitre(2)

(1) Institut de Physique du Globe de Paris, 4, place Jussieu - tour 24 - 4eme etage, 75252 - Paris - cedex 05, France
(2) Ecole Normale Superieure des Telecommunications, 46, Rue Barrault, 75013 - Paris, France

Abstract

Detection of vertical movements provided by differential SAR (Synthetic Aperture Radar) interferometry is still limited by the presence of temporal decorrelation and tropospheric contributions. These effects strongly affect interferograms but reliable deformation measurements can be obtained over a large population of mult-temporal differential interferograms. For that, 81 differential interferograms of the Gulf of Corinth (Greece) were produced on three different tracks, covering variable time spans (1day to 7.42 years) in the period 1992-1999. Both the 1992 Ms=5.9 Galaxidi earthquake and the 1995 Ms=6.2 Aigion event were covered by the data. A methodological approach was developped to reduce at both global and local scales the tropospheric contributions in the interferograms. This processing was applied, for each track, to all the calculated interferograms in six main steps. First, using all available interferograms, a coherence map of the area is produced to identify the permanent scatterers of the region. Second, the interferograms are filtered to reduce the noise and then masked by a collective coherence map where the coherence is poor and therefore the phase information lost. Third, assuming a horizontally homogeneous troposphere model, a linear correlation is estimated between the phase value of the most coherent pixels and their elevation; the obtained models are validated using the Bellman-Ford algorithm which detects any false estimation with respect to the others. Fourth, the interferograms are then unwrapped to remove the phase ambiguity and, fifth, a small-scale correction is performed. Comparing all the interferograms containing a common image, the common local phase anomalies are considered to be associated to that image and subsequently substracted from the interested interferograms. Finally, the corrected interferograms are stacked and averaged to reduce the high-frequency noise still present after the processing and to retrieve a smoothed deformation field related to the 1992 and 1995 earthquakes, expected to be smooth and provided by the offshore locations of the ruptured faults. The cleaned deformation field related to the Aigion earthquake concerns a N-S 16 km wide by E-W 28 km long band, reaching the maximum value of ~250 +/- 12 mm on Psaromita cape, a limestone mesozoic platform located on the northern side of the gulf, in front of the town of Aigion. The comparison between each corrected interferogram and the average deformation map reveals that, within the uncertainty of our displacement map (12 mm), no postseismic deformation is observed in the data in the period 1995-1999.

 

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