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Ipta Analysis and Geothecnical Investigation: Anomalous Subsidence in the city of Rome

Salvatore Stramondo(1), Urs Wegmuller(2), Francesca Bozzano(3), Michele Saroli(4), Fabrizio Marra(1), Marco Moro(1) and Francesca Romana Cinti(1)

(1) Istituto Nazionale di Geofisica e Vulcanologia, via di Vigna Murata 605, 00143 Rome, Italy
(2) GAMMA Remote Sensing, Worbstr. 225, CH-3073 G├╝mligen, Switzerland
(3) University La Sapienza, P.le Aldo Moro, 5, 00185 Rome, Italy
(4) University of Cassino, Via Di Biasio, 43, 03043 Cassino, Italy

Abstract

The Interferometric Point Target Analysis (IPTA) is a method to exploit the temporal and spatial characteristics of interferometric signatures collected from point targets to accurately map surface deformation histories, terrain heights, and relative atmospheric path delays. The use of targets with point like scatter characteristics has the advantage that there is much less geometric decorrelation. This permits phase interpretation even for large baselines above the critical one. Consequently, more image pairs may be included in the analysis. Important advantages are the potential to find scatterers in low-coherence areas and that interferometric image pairs with large baselines may be included in the analysis. Finding usable points in low-coherence regions fills spatial gaps in the deformation maps while the ability to use large baselines improves the temporal sampling. We applied the IPTA technique to study the city of Rome (Italy) aiming to detect and measure the surface movements of buildings and urban structures. The available SAR dataset has been delivered by ESA CAT1 3258 and ranges the period 1992-2005. In particular ERS1-ERS2 data covers 1992-2000, while Envisat ASAR 2002-2005. The point target velocity map shows a general stability except for some very local areas affected by subsidence rate larger than 10 mm/year. The analysis of the time-series, compared to a detailed investigation of the lithostratigraphy of the alluvial sediments of the Tiber River, and combined with a temporal reconstruction of the expansion of the city over the alluvial valleys, allowed us to depict the main factors controlling the observed subsidence. These are: the in situ effective stress conditions, the related compressibility and viscous characteristics of the loaded soils, the thickness of the compressible stratum, the time since loading instant, and the entity of loading. We mainly focused on the Grottaperfetta stream valley that is characterized by an anomalous high and time-lasting subsidence. We also compared remote sensing data with in situ investigation addressed to explain the phenomenon. Original data on the lithopstratigraphic setting of this alluvial valley allowed us to identify the possible causes of its peculiar behaviour.

 

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