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Development of a procedure for correcting and reducing unwrapping artefacts using a set of ERS SAR interferograms. Case of the September 9, 1999 Athens Earthquake

Panagiotis Elias(1) , Demitris Paradissis(2) , and Charalabos Kontoes(1)

(1) National Observatory of Athens, Metaxa & Vas. Pavlou Str., 15236 Palaia Penteli, Athens, Greece
(2) National Technical University of Athens, 9 Heroon Polytechniou Str., 15773 Zographos, Athens, Greece

Abstract

On September 9, 1999 a magnitude Ms=5.9 earthquake occurred in the Athens area (Greece) producing a subsidence of about 6cm detected by radar interferometry. Using seventeen ERS SAR images acquired between December 1997 and January 2001, ninety-five interferograms of the area have been calculated using the CNES DIAPASON software. Sixteen co-seismic, out of the ninety-five interferograms, were finally selected on the basis of criteria concerning the quality of the calculated deformation patterns and the corresponding coherence values. In addition, twenty-eight post-seismic interferograms were retained. The fourty-four interferograms in total are unwrapped using the SNAPHU software. A coherence image is derived from the entire set of interferograms. Orbital effects are reduced by tilting and shifting the unwrapped interferograms, making the assumption that no deformation has occurred in zones located at the edges of the scene, that is far away of the area affected by the earthquake. This procedure was applied in a way constraining the value of deformation of the most coherent pixels in the selected zones at the edges of the image to a zeroed value. A specific algorithm has then been developed and applied, using lowpass filtering and procedure of detecting and minimising high frequency signal values keeping the original phase intact. The set of the sixteen corrected | co-seismic interferograms, is then stacked, keeping only the "confident" pixels, and used | to produce a smoothed image of the average deformation field. The resulted deformation field is used as input in order to recalculate the deformation source. The synthetic interferogram issued from the model is then substracted from each corrected interferogram. The residuals of the co-seismic interferograms are examined, along with the corrected post-seismic interferograms, in order to detect any temporal evolution of the occurred deformation after the main event.

 

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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