MONITORING MODERATE SLOPE MOVEMENTS (LANDSLIDES) IN THE SOUTHERN FRENCH ALPS USING DIFFERENTIAL SAR INTERFEROMETRY. Jan Vietmeier and Wolfgang Wagner

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1 MONITORING MODERATE SLOPE MOVEMENTS (LANDSLIDES) IN THE SOUTHERN FRENCH ALPS USING DIFFERENTIAL SAR INTERFEROMETRY Jan Vietmeier and Wolfgang Wagner Institut für Hochfrequenztechnik, Deutsches Zentrum für Luft- und Raumfahrt, D Weßling Tel: , Fax: , Richard Dikau Geographische Institute der Universität Bonn, Meckenheimer Allee 166, D Bonn Tel: , ABSTRACT In this paper differential SAR-Interferometry (D- InSAR) is applied to study a landslide close to La Valette which is situated in the southern French Alps. The general capability of D-InSAR to determine the field of movement of a moderately moving landslide is shown, by carrying out the phase unwrapping and projecting the measured displacement to the direction of the movement. The data analysed in this study comprise three ERS-1/2-Tandem pairs from 1995/96 which allow to measure movements that took place in the time span of 24 hours. Field measurements were available to assess the accuracy of the interferometrically extracted displacement values. Despite of the different acquisition intervals of the satellite and the ground measurements an agreement in the order of 1.2 cm/day could be assigned. Barcelonette in the southern French Alps. ERS-1/2- Tandem data from 1995/96 are evaluated. These data allow to measure movements that took place in the time span of 24 hours. 1. INTRODUCTION In the last years differential SAR-Interferometry (D- InSAR) has been proven to be a useful tool for geophysical investigations (Massonnet et al., 1998). Especially since the launch of the European Remote Sensing Satellites ERS-1 in 1991 and ERS-2 in 1995 the number of D-InSAR applications has increased. The observed processes range from seismic deformation processes (Massonnet et al., 1993), movement of inland ice (Kwok et al., 1996) to land subsidence (Strozzi and Wegmüller, 1999; Fruneau et al., 1999). Fruneau et al. (1996) were the first to demonstrate the capability of SAR-Interferometry to determine the movement field of a moderately moving landslide with displacement rates of a few centimetres per day on a landslide in the Mediterranean French Alps. If environmental conditions preserve coherence over very long time spans then also slow slope movements in the order of centimetres per year can be observed (Rott et al., 1999). The objectives of this paper are to demonstrate the general capability of differential SAR-Interferometry to determine, in absolute values, the field of movement for a moderately moving landslide and to give some ideas of the accuracy of this method. The investigated landslide is close to La Valette situated near Figure 1: Location of the Barcelonnette basin in the south-east of France (top) (modified from Antoine, 1995) and sketch of the La Valette landslide (bottom) (modified from Service R.T.M., 1990).

2 Figure 2: Rates of movement of the La Valette landslide at a survey line in the middle part of the landslide for the period from July 1995 to January Acquisition dates Orbit-frame- Quadrant Table 1: ERS tandem pairs and movement rates of the La Valette landslide. Effective Baseline Rates of Movement 49 m 2.5 cm/day -109 m 1.3 cm/day 17 m 5.0 cm/day 2. STUDY AREA AND DATA BASE The investigated landslide La Valette is situated in the basin of Barcelonnette in the southern French Alps (Figure 1). This landslide developed in 1982 on a slope north of the basin of Barcelonnette at an altitude between 1700 and 1950 m at the transition of permeable flysch and impermeable Jurassic black marls ( Terres Noires ). The initial rotational slide was the trigger for the mobilisation of morainic deposits below. In 1988 the landslide comprised of 6,000,000 cubic meters moving material and extended over 730 m in altitude. The horizontal extend of the slide is about 2,000 m in length and 450 m in width. The movement of La Valette has been monitored by the "Restauration des Terrains en Montagne" across a survey line since Later, another 30 measurement points were added. Figure 2 shows the measured movement rates at the survey line in the time period from July 1995 to January The movement rates are varying from about 0.7 cm/day to about 7 cm/day. Over most parts of the landslide the direction of the slope is south-west (Figure 1). In the middle part the slope angles are varying from 8 to 14 degrees while the angles in the upper parts are much higher. Due to the imaging geometry of radar systems an observed slope should be averted from the sensor. For that reason ERS SAR images from the descending satellite mode were chosen. Three tandem pairs that were acquired during different periods of activity of the landslide were selected. Table 1 shows the acquisition dates and the corresponding movement measurements at the survey line. As the influence of the displacement was very weak for the interferogram from 22/ it was decided to use it as reference to remove topographic effects from the other two interferograms. 3. MEASUREMENT TECHNIQUE Interferometric processing was carried out with the "genesis" processor that has been developed by DFD (Deutsches Fernerkundungsdatenzentrum). For all three image pairs DEMs were generated, and the DEM produced from the 22./ image pair was subtracted from the other two DEMs. The remaining height values h disp can be related to the displacements of the Earth's surface in the line of sight of the sensor r disp through λ r disp = Φ (1) 4π

3 where λ is the radar wavelength and Φ is the phase difference: hdisp Φ = 2π (2) H a H a is the specific height of ambiguity for the DEM, or other words, the elevation difference on the Earth s surface which is equivalent to a phase shift of 2π. The displacements in the line of sight have been projected to the direction of the movement of the landslide assuming slope parallel motion (Rott et al., 1999). Our reasons for calculating r disp in this way were Existing interferometric processing software could be fully utilised; The generation of the DEMs using ground control points partly corrects for offsets in the unwrapped phase between images. interferogram can cause large values. Additionally, the error generally increases with increasing distance from the landslide because the ground control points for the phase to height conversion were all chosen to be close to the landslide. However, all the mentioned error sources are restricted to areas outside the landslide and therefore have no influence on the analysis. Only at the flanks of the landslide important errors may occur due to phase unwrapping errors. For example, at the right flank of the landslide north of point eight high movement rates can be seen in the 1995 image, while in 1996 the landslide is much narrower at this position. We believe that this effect is caused by a phase unwrapping error. High phase gradients at the flanks of a landslide can easily lead to errors in the phase unwrapping (PU) process. To prevent this kind of error it is necessary to prevent the PU algorithm to integrate across the landslide borders. Therefore a main criteria for the suitability of a PU method for investigating landslides is the ability to steer the direction of the integration process by human intervention. Three different PU algorithms were tested: a weighted least square method (Adam, 1995), a fusion algorithm (Reigber and Moreira, 1997), and a minimum cost flow algorithm (Constantini, 1997). Although still not flexible enough, the best results were achieved with the minimum cost flow algorithm. 4. RESULTS Displacement images of the La Valette landslide for the two dates 13./14. August 1995 and 10./11. March 1996 can be seen in Figure 3. The images are geocoded to a perpendicular coordinate system (UTM) with a pixelsize of 15 x 15 m 2. The movement scale spans from -1.0 cm/day to 5.5 cm /day. Low rates of movement are represented by green colours, medium rates by yellow and high rates by red colours. The numbered points in the images indicate the locations of the field measurements carried out by the "Restauration des Terrains en Montagne". In both images the landslide stands out clearly against the surrounding areas. The direction of the movement is south-west. But also outside the visible body of the landslide (mainly above the top of the landslide and, for the image from 1995, at the lower right corner of the image) areas with seemingly high movement rates can be seen. These high values are not caused by real displacements, but rather are erroneous values related to layover areas or to areas where the direction of the slope is almost perpendicular to the look-direction of the sensor. In the near perpendicular case the projection of the measured displacement to the slope direction converges to infinite and then even noise in the Figure 3: Geocoded displacement images of the landslide of La Valette from 13./ and 10./ The numbered points representing the location of the field measurements. The size of the visible area is about 1.8 x 1.9 km 2. The direction of the movement is from the upper right to the lower left. The displacement images reveal some interesting features of the landslide. For example, a division of the movement field in two parts above measurement point three is visible. Between this point and point one is an

4 area which is completely surrounded by the landslide, but remains unaffected (Figure 1). Around points five, six and seven a local maximum of displacement rates is visible. The small cross-section of the landslide at this position may be the reason for this phenomenon. Further above is a larger area of activity which is limited in the upwards direction by a sudden drop of the movement rates to zero above point nine. This area is the main body of the landslide whereas the inactive area above it represents a part of the main scarp. The main scarp is the visible part of the surface of rupture at the upper edge of a landslide (Dikau et al. 1996). Besides of the spatial characterisation of the landslide D-InSAR is in principle capable of monitoring dynamic features of the landscape. The comparison of the 1995 and 1996 images shows that there is a general increase of the activity of the upper and lower parts of the landslide between 13./ and 10./ This trend is confirmed by the ground measurements. The interferogram used for the elimination of the topographic dependent phase contribution (22/ ) also contains displacement information. Therefore, for a correct comparison of the D-InSAR and the in-situ measurements, the displacement rates observed at the acquisition date of the reference interferogram was subtracted from the other ground measurements. The maximum difference between the ground and the D-InSAR measurements is -1.2 cm/day for the 1995 image and -1.7 cm/day for the 1996 image. The mean of the difference of 13 or 12 points is 0.1 cm/day or 0.15 cm/day while the standard deviations are 0.58 cm/day and 0.6 cm/day. 5. CONCLUSIONS This study showed the general capability of differential SAR-Interferometry to determine the movement field of a moderately moving landslide in absolute values. Despite of the different acquisition intervals of the satellite measurements and the ground measurements an agreement in the order of 1.2 cm/day (two times the r.m.s. error) could be assigned. This value is comparable with the results of earlier investigations (Carnec et al., 1996; Massonet et al., 1998; Rott et al., 1999). Additionally, the change of the activity of the La Valette landslide between the two acquisitions could be detected. With small alterations of standard interferometric processing systems it is possible to generate displacement images of landslides. But one has to consider the likely errors of phase unwrapping due to the generally sharp transition of the interferometric phase at the flanks of the landslide and due to the small areal extent of landslides (relative to the resolution of the ERS SAR). Until now no adequate solutions to these problems exist. ACKNOWLEDGEMENTS The authors wish to thank Dr. Marcus Schwäbisch from Aerosensing Radar-systems for the support and fruitful discussions. We also would like to acknowledge Mr. Charles Bosshard from the "Restauration des Terrains en Montagne" for providing ground measurement data of La Valette. REFERENCES Adam, N. (1995). Das Phase Unwrapping in SAR- Interferogrammen. Diplamarbeit, Universität Rostock. Antione, P., A. Giraud, M. Meunier and T. van Asch (1995). Geological and Geotechnical Properties of the Terres Noires in Southeastern France: Weathering, Erosion, solid Transport and Instability. Engineering Geology, 40: Carnec, C., D. Massonet and C. King (1996). 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