THE 1998 SARNO (ITALY) LANDSLIDE FROM SAR INTERFEROMETRY

Andrea Arturi(1), Fabio Del Frate(1), Emanuele Lategano(1), Giovanni Schiavon(1), Salvatore Stramondo(2)

(1) Università Tor Vergata, DISP, Via del Politecnico 1, I-00133 Rome, Italy, schiavon@disp.uniroma2.it (2) Istituto Nazionale Geofisica e Vulcanologia, Via di Vigna Murata 605, I-00143 Rome, Italy, stramon@ingv.it

Keywords: InSAR, pyroclastic flow, map-derived DEM, phase unwrapping

ABSTRACT

The SAR Interferometry is a tecnique for measuring the topography of surface and its change through the phase component of a coherent radar signal [1]. This paper reports on the results of the SAR Interferometry application to the study to the Sarno landslide. Two ERS-1/2 tandem pairs acquired over the Sarno area after the 1998 landslide have been processed to generate a SAR DEM. Due to the strong topographic relief of the study area, to avoid or limit layover and shadowing, we used either an ascending or a descending tandem pair. A DEM generated by means of two ERS1/2 tandem pairs has been compared with a topographic map derived DEM dated 1997 allowing the identification of the areas mostly affected by the landslide. The external DEM has been used because no ERS tandem pairs acquired before the disaster were available to produce a synthetic one. 1 INTRODUCTION

SAR interferometry (InSAR) technique has been successfully applied to study and follow the temporal evolution of landslide events ([2] [3] [4] [5] [6]). On May 4th - 5th , 1998 about 150 landslide movements succeeding in 10 hours, from 2 to 12 p.m. hit an area of 75 km2 near Naples (Italy), in the cities of Sarno, Siano and Bracigliano (in the district of Salerno) and in the city of Quindici (Avellino) (Fig.1). This event caused several victims and shocked the entire nation for its alarming proportions. Actually a considerable part of Italian territory is affected by hydrogeological hazard and the use of remote sensed data like SAR offers an important tool for monitoring and studying these kind of events. The mountainous belt of Campania Region surrounding the Vesuvio are interested by characteristic landslide phenomena due to the pyroclastic deposits produced by the explosive and effusive eruptive activity and left on the limestone relief. May 4th-5th 1998 the landslide was favoured by a strong atmospheric perturbation that reached the highest values in the area comprised in Sarno, Quindici, Siano, Bracigliano and Lauro territories. Pluviometers, 40 km far from the landslide area, did not recorded particularly intense rains to confirm the very spatially limited extension of the perturbation (Fig.2). Meteoric waters (rain) saturated the pyroclastic low-coherent soil, and reached the carbonatic layer where they generated subsurface water streams. Moreover, the abandoned anthropic terracing could not develop its role of collecting meteoric waters. These produced the separation of the pyroclastic terrain from the underneath carbonatic layer, and generated the pyroclastic flow. Following meteoric events the full pyroclastic thickness has been rapidly moved on the slope, involving also the vegetation cover. In November 1997 the same area had still been struck by less dramatic landslide events.

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Proc. of FRINGE 2003 Workshop, Frascati, Italy,1 – 5 December 2003 (ESA SP-550, June 2004) 11_stramon

Fig.1: SPOT image of the hit area, with the cities of Sarno and Lauro.

Fig.2: (a) precipitation preceeding the landslide event; (b) The mountaneous belt of Campania Region surrounding the Vesuvio is covered by pyroclastic deposits due to the explosive and effusive eruptive activity. 2 SAR DATA SELECTION CRITERIA AND PROCESSING Due to the nature of the Sarno event, the only reliable approach to retrieve the landslide by applying SAR interferometry is to perform a DEM comparison. To this aim a 20m pixel-size map-derived DEM is available as pre-event data, while InSAR technique has been applied to produce a post-event DEM. To reduce decorrelation and noise, where possible, and obtain a reliable DEM, ERS-SAR data have been selected according to the following criteria:

− Short temporal baseline (two ERS1/2 tandem pairs). It reduces coherence loss due to temporal decorrelation [7]. − Appropriate spatial baseline (geometric separation of the two satellite positions) selection, to improve the DEM

coverage and accuracy. We have to follow a compromise between two requirements: the sensitivity to the topography and the limitation due the critical baseline.

− Good atmospheric conditions to reduce artifacts [8] − Ascending and descending images. Due to the strong topographic relief of the study area, to avoid or limit

layover and shadowing, we used two different satellite points of view and solve phase indeterminations.

Mt. Vesuvio Sarno

Lauro

(a) (b)

The two selected tandem pairs are characterized by a perpendicular baseline of 339 m (such to have an ambiguity height of about 26 m) and 245 m (ambiguity height of 36 m). The branch cut phase unwrapping algorithm has been applied to the whole area [9]. In particular, the very short ambiguity height and the high slope topographic relief of Mt. D’Avello needed a recursive use of the unwrapping procedure. The coherence threshold is reduced for each step. The resulting SAR DEM has been obtained by merging the two single pair SAR DEMs, also considering a weighting function based on the coherence of each interferometric pair.

Fig.3: Phase unwrapping block diagram. The branch cut algorithm has been used recursively. Each cycle starts from the results of the previous step changing the coherence threshold

Fig.4: Results from unwrapping the interferometric phase. Due to the topography of Mt. D’Avello and the short ambiguity height, the algorithm allows to unwrap the hit area only starting from .3 coherence threshold. 3 MAP-DERIVED DEM AND SAR-DEM COMPARISON

After the DEM coregistration, the map of the differences between the two DEM’s has been generated. The ERS-SAR DEM (post-event) has been subtracted from the pre-event map-derived DEM. For this reason positive differences were were expected to identify landslide features. Except for the portions of the SAR interferogram characterized by low coherence and layover, where processing do not produce reliable results, the areas affected by the landslide have been identified. More precisely as the map-derived DEM accuracy is about 10m and the standard deviation of residual values is about 14 m, a positive threshold of 20m seems to be satisfactory to identify the landslide.

Fig.5: Residual values obtained by subtracting map-derived DEM and ERS-SAR DEM. The blue sector points out the part of the difference distribution corresponding to the landslide values.

Coh. thresh. 0.8 Coh. thresh. 0.6 Coh. thresh. 0.5

Coh. thresh. 0.3

Coh. thresh. 0.2

Fig.6: Shaded relief of Mt. D’Avello with the landslide features.

Fig.7: DEM comparison results placed on top of the landslide features. The zooms points out the good fit of the main branches.

4 CONCLUSIONS

InSAR technique allowed to retrieve the Sarno landslide features by comparing a SAR DEM with the available map-derived DEM. Due to the topography the study area is quite difficult for InSAR application. In particular some narrow “V shape” valleculas are the favoured ways of the landslide movements. Interpolation artifacts in the 20m pixel size map-derived DEM probably reduce the capability of the proposed approach and show strong differences also where the landslide did not produce evaluable effects. Notwithstanding this a qualitative comparison between DEMs differences and landslide features has shown a good fit. Further analysis of this area are in progress. In particular a more accurate map DEM is available. Besides also a pre-event SAR DEM should be produced. A multidisciplinary approach also involving ground based techniques seems to be the key point 5 REFERENCES 1. D. Massonnet and K.L. Feigl, “Radar interferometry and its application to changes in the Earth’s surface,” Rev. of Geophys., 36 (4), pp. 441-500, 1998. 2. B. Fruneau, J. Achache, and C. Delacourt, “Observations and modelling of the Saint-Etienne-de-Tinee landslide using SAR interferometry,” Tectonophys. 265, pp. 181-190, 1996. 3. H. Rott and A. Siegel, “Analysis of mass movements in alpine terrain by means of SAR interferometry,” Proceedings of IGARSS’99, Hamburg, Germany, 28 June - 2 July 1999 V. Singhroy, K.E. Mattar, and A.L. Gray, “Landslide characteristics in Canada using interferometric SAR and combined SAR and TM images,” Adv. Space Res., Vol. 3, pp. 465-476, 1998. 4. H. Kimura and Y Yamaguchi, “Detection of landslide areas using radar interferometry,” Photogram Eng. and Rem. Sens., Vol. 66, No. 3, pp. 337-344, 200 5. H. Rott and B. Scheuchl, A. Siegel, and B. Grasemann, “Monitoring very slow slope movements by means of SAR interferometry: A case study from a mass waste above a reservoir in the Ötztal Alps, Austria,” Geophysical Res. Letters, 26, pp. 1629-1632, 1999. 6. T. Strozzi, U. Wegmüller, C. Werner, and A. Wiesmann, “Alpine Landslide Periodical Survey,” Proc. of IGARSS 2002, Toronto, Ontario, Canada, June 2002 7. Zebker, H., And Villasenor, J., Decorrelation in interferometric radar echoes. IEEE Trans. on Geoscience and Remote Sensing 30, 5, 950-959, 1992 8. Zebker, H., A., Rosen, P., A. and S. Hensley, Atmospheric effects in interferometric synthetic aperture radar surface deformation and topographic maps. Journal of Geophysical Research, 102(B4), 7547-7563, 1997 9. R. M. Goldstein, H. A. Zebker, C. L. Werner, “Satellite radar interferometry: two-dimensional phase unwrapping,” Radio Science, vol.23, no.4, pp. 713-20, July-Aug. 1988.