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Combining InSAR and optical data to detect earthquake damages

Salvatore Stramondo(1) and Christian Bignami(2)

(1) Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143, Italy
(2) Universita degli Studi di Roma La Sapienza, Universita degli Studi di Roma La Sapienza, Na, Italy

Abstract

The prompt detection, mapping and amount evaluation of urban damages due to earthquakes is a key point, particularly in remote areas or where the infrastructures are not well developed to ensure the necessary communication exchanges or where their operability has strongly decreased as a consequence of the event. The optical images presently available from space at resolution in the order of meter or better are certainly a suitable tool to perform damage assessment. However, radar remote sensing can give also a strong contribution in this application, especially for its capability to make observations almost independently on the meteorological conditions and cloud coverage. Both remote sensing techniques are increasing their capability to provide measurements as soon as they are required thanks to pointing flexibility of the last generation of spaceborne systems and the future availability of constellations of satellite. The Synthetic Aperture Radar (SAR) collects maps of the backscattering coefficient and has the capability to detect urban extension and urban changes. The Interferometric SAR (InSAR) technique increases this change detection capability by exploiting another quantity measured by the radar, that is the coherence between signal echoed by the same portion of the surface as observed at different times. The urban settlements do not change their geometric and dielectric structures and therefore generally produce coherent signals if they are observed from comparable directions. Conversely, any change in the structure and thus in the scattering mechanisms producing the backscattered signal causes random variations of echo intensity and phase. In this paper, we made a preliminary experimental evaluation of the capability to detect the urban changes due to an earthquake by InSAR. We have considered the earthquake that occurred in Izmit (Turkey) on August 17, 1999. This event was studied by InSAR technique to assess surface movements using two pairs of tandem ERS images collected on 12-13/08/1999 and 16-17/09/1999. Two panchromatic images collected by IRS before and after the earthquake (8 August 1999 and 27 September 1999), with a spatial resolution of about 5 meters, have been also acquired for this study. In the first stage of the work, that is described in this paper, we have used the optical images as a reference map to detect the earthquake damages. A number of well identified changes have been detected from IRS data by simple image processing algorithms, confirmed by a visual inspection of the images. Unfortunately, we have not yet ground survey available to identify with enough reliability the nature of the observed changes. Once a number of changes were detected, an analysis of the radar images have been performed. Standard interferometric techniques have been used to coregister the SAR data (four images) each other. The image collected on 13/8/1999 has been considered as master for all the coregistration tasks. A multichannel/multitemporal dataset has been created including the intensity image of the pre event and the post event ERS acquisitions (we have used a combination of the two tandem images), the complex coherence and the intensity correlation coefficient between the two pre event images (the tandem couple) and between the master and the one of the post event images. Note that the evaluation of the intensity information content with respect to the phase coherence was one of the objective of this study. Using this SAR data set and the change detection provided by IRS as a "ground truth", we have done a number of statistical tests to understand at what extent the InSAR is able to help detecting urban damages and what particular features extracted by SAR better contribute to this objective.

 

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