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Analysis of Permanent Scatterers in a seismic area.

Marc Salmon(1) and André Ozer(1)

(1) Université de Liège, Allée du 6 Août, 2 - B11, B-4000 Liège, Belgium

Abstract

Before getting quality measurements by DinSAR processing on land movements, it is essential to identify the sites with long term stability, i.e. for which phase coherence between the radar images is preserved. Movement detection is only possible on this mesh of elements that preserve coherence. This study is performed in the frame of the first ENVISAT call for proposals. Before analysing ENVISAT-ASAR data, the processing was applied to four ERS-1/2-SAR images: a pair tandem dating from June 24 and 25, 1995 and two independent images of May 25, 1997 and August 3, 1997. All ERS images have been taken in descending mode; therefore, it has been decided that it would be the same for all images studied during this project. Short and long term coherence images were generated.Then, the permanent scatterers – i.e. the stable sites - were highlighted by extracting the pixels with a coherence higher than 0.8. The Centre Spatial de Liege (Belgium) carried out all DinSAR processings and the geocoding of the results. After the generation of coherence images and the extraction of the stable sites, the data were integrated within a Geographical Information System (GIS) to facilitate the comparison with the other cartographic, satellite or field data. The records include geological data (lithology and faulting), ground occupation information and ASTER satellite images. Moreover, a field mission has been carried out in August 2002 to analyze the identified sites. The studied site - the valley of Cosenza (Calabria - Italy) – is subject to a strong seismic activity. Frequent marks of ground instability like landslides were located. The great majority of the Permanent Scatterers are roads, buildings and carparks. In this area,the scarcity in stable sites is mainly due to the type of vegetation coverAll highlands are covered by forests and only present scarce and small-size rock faces. In the same way, the plain of the valley of Cosenza is mainly occupied by agricultural vegetation with two levels: cereal agriculture on the one hand and a relatively dense and regular grid of olive-trees on the other hand. Lastly, the various landslides are also covered with vegetation and seldom with infrastructures. In conclusion, due to the vegetation cover, it is difficult to obtain coherent results in this area. In order to promote the use of DinSAR techniques in similar areas, it is very important to pursue the research concerning the exact nature of the stable sites. Improvements can be made, either by increasing the number of images or by studying the seasonal cycles in order to limit the acquisition periods to the seasons with less vegetation. These observations will be taken into account while ENVISAT-ASAR data will be analyzed.

 

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