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Monitoring and identifying small scale deformation sources in the area of Athens (Greece)

Panagiotis Elias(1), Charalabos Kontoes(1) and Ioannis Papoutsis(1)

(1) National Observatory of Athens, I. Metaxa & Vas. Pavlou, GR-15236 Penteli, Greece

Abstract

Classic InSAR technique has offered a great deal of reliable measurements of ground deformation. The accuracy of this method though is limited by components related to spatial and temporal decorrelation, signal delay due to tropospheric and ionospheric disturbances, orbital errors as well as DEM artefacts. These components are dealt with the promising PSInSAR technique. The PSInSAR methodology offers significant improvement in estimating the near vertical displacement rates with accuracy of the order of 1mm/year. Thus, this technique is ideal for measuring small-scale ground deformation due to seismic precursor activity, urban subsidence, creep effects in fault zones as well as displacements in active fault zones and volcanoes. In this study we are implementing the PSInSAR technique, called PerSePHONE (Permanent Scaterrers Project Held by the Observatory, National, of Hellas), which has been developed in the Institute for Space Applications and Remote Sensing of the National Observatory of Athens. Its development has been based on a number of algorithmic adaptations for image focussing, image registration, geo-correction methods as well as for PS candidates (PSC) selection. These processing steps, together with the existing processing capabilities for interferogram formulation and projection to a common cartographic system, are integrated in a single processing chain. This methodology has already been used to assess the deformation pattern present in the Gulf of Corinth. The analysis results showed a LOS deformation trend of 2mm/year occurring in the North of Alkyonides islands. The same procedure is now being implemented for the city of Athens, a densely populated urban Metropolis, covering about 200 Km2. The socio-economic importance of the city is well known. Over the years a high seismic risk has been reported with the most severe event being the Athens earthquake held on September 7th, 1999 with a large number of damages and human casualties. During the last decades an intense construction activity has taken place in the city. In the wide frame of the preparation of the Olympic Games 2004 in Athens, several major infrastructure projects like Eleftherios Venizelos International Airport, Athens sub-way, new tram lines, and highways have been realised. This construction activity together with old mining works and the geophysical phenomena reported (e.g. earthquakes, possible subsidence due to extended water pumping, shrink and swell of geological formations - especially clay-rich), dictate the city monitoring over the years. The scope is to identify any existing slow deformation phenomena, which might be pre-cursors of future abrupt and destructive events. For this, a total number of 85 ERS and Envisat scenes have been acquired, starting from 1992 till today. At this stage a subset of 39 ERS-2 images are being processed, from August 1998 to November 2005, a time span of 6 years. This time period was selected for two reasons: First it coincides with the Olympic Games preparation time period; second it refers to a period during which strong non-linear phenomena have occurred, such as the September 1999 earthquake, therefore testing the robustness of the algorithm under less favourable conditions. Initial results indicate the presence of localized sources of deformation which are being finalized through refinement of the processing products. Additionally further processing of new subsets will be a complementary work which will be validated with the existing studies in the city of Athens (eg. in the frame of GMES project). Moreover this study will offer the potential of continuity of the deformation monitoring. The PSInSAR analysis will produce a diachronic PS distribution map which combined with the corresponding deformation rates will identify, correlate and interpret the possible sources of deformation (construction activity, old mining works and geophysical phenomena).

 

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