SAR Interferometric Point Target Analysis of Slope Instabilities in the Community of Biasca, Switzerland
Tazio Strozzi(1) , Lorenza Re(2)
, Giorgio Valenti(2)
, Hugo Raetzo(3)
, Kaspar Graf(4)
, Urs Wegmüller(1)
, Andreas Wiesmann(1)
, and Charles Werner(1)
Gamma Remote Sensing,
(2) Ufficio pericoli naturali, incendi e progetti, Viale S. Franscini 17, 6500 Bellinzona , Switzerland
(3) Federal Office for Water and Geology, Ländtestrasse 20, 2503 Biel / Bienne , Switzerland
(4) GEOTEST, Birkenstrasse 15, 3052 Zollikofen , Switzerland
Repeat-pass Interferometric Synthetic Aperture Radar (InSAR) is a powerful technique for mapping land surface deformation at fine spatial resolution over large areas. The application of repeat-pass InSAR is however limited due to temporal and geometric decorrelation and inhomogeneities in the tropospheric path delay. Interferometric Point Target Analysis (IPTA) utilizes the unique characteristics of the tropospheric path delay and SAR backscattering of certain ground targets to achieve higher accuracy deformation measurements than attainable with conventional InSAR. Over urban areas with numerous man-made structures or in areas where single persistent structures (e.g. houses, exposed rocks, powerline masts) scattered outside the cities and villages are visible, it is possible to identify targets with a point-like scatter characteristic that remain phase coherent over time. These targets can be used to estimate the progressive deformation of the terrain at millimeter accuracy. Building upon conventional InSAR techniques, IPTA overcomes atmospheric delay anomalies and temporal and geometric decorrelation using multi-temporal interferograms by exploring the temporal and spatial characteristics of radar interferometric signatures collected from point targets.
In this contribution the application of IPTA to the analysis of slope instabilities in the Community of Biasca (Switzerland) is described. Numerous unstable phenomena, including sagging of various dimensions and rock slides, can be found in this mountainous region, with an elevation range from approximately 300 m a.s.l. to almost 3000 m a.s.l. Geological and geomorphological studies, including in-situ and aerophoto interpretation, are available and in two areas of particular interest high resolution ground motion data are regularly performed. The results achieved with IPTA by using all the available snow-free ERS SAR images between 1992 and 2000 are extremely interesting due to the high target spatial coverage in sparsely vegetated areas with numerous exposed rocks. On the other hand, over forested areas IPTA failed to retrieve displacement information. The validation of the ITPA results with the high quality geodetic data demonstrated a very good accuracy of the satellite SAR-based observations for low displacement rates of 1 to 2 cm/year. For higher displacement rates and in particular in the case of accelerations, the analysis demonstrated that there are errors on the determination of the correct phase ambiguity. However, IPTA has the potential to retrieve important information also for large areas not covered by the geodetic survey during more stable periods.