Delaloye, R.

Lambiel, C.

Lugon, R.

Raetzo, H.

Strozzi, T.

Serveur académique lausannois

Grazer Schriften der Geographie und Raumforschung  Band 43 / 2007113ERS InSAR for Detecting Slope Movement in a Periglacial Mountain Environment (Western Valais Alps, Switzerland)Reynald Delaloye (1), Christophe Lambiel (2), Ralph Lugon (3), Hugo Raetzo (4),and Tazio Strozzi (5)(1) Department of Geosciences, University of Fribourg, Switzerland(2) Institute of Geography, University of Lausanne, Switzerland(3) Kurt Bösch University Institute, Switzerland(4) Federal Offi ce for the Environment, Switzerland(5) Gamma Remote Sensing, Switzerlandpp. 113 - 120AbstractThe potential of ERS InSAR for detecting slope motion in a periglacial mountain environment has been tested in the western part of the Swiss Alps. An inventory has been built from the analysis of the InSAR data. It contains a large number of areas that can be possibly attributed to various types of mass movement (glacier, debris-covered glacier, rock glacier, landslide, sagging, gelifl uction) occurring with various rates of activity above the tree line. The applied methodology is briefl y described and several typical examples of detected slope movement are illustrated.KEY WORDS: ERS InSAR, slope movement, rock glacier, push-moraine, landslide114geo9th International Symposium on High Mountain Remote Sensing Cartography3m. The InSAR products were also computed at 25 m spa-tial resolution.Main limitations of InSAR are atmospheric effects, i.e. phase delay caused by changing tropospheric water va-pour conditions at the time of the two SAR acquisitions, and signal decorrelation. Over alpine areas, high coher-ence of ERS interferogram data is regularly obtained dur-ing the snow free period between early summer and mid fall above the tree line. The favourable period becomes reduced at higher elevation because of the more frequent occurrence of old or fresh snow. The presence of late-lay-ing snow avalanche deposits may also disturb the SAR signal. Coherent data are also observed for daily time lapse in winter time where the snow surface is cold (see Fig. 3b). Further limitations of InSAR arise from layover and shadowing, that prevent to cover steep rock wall and deep valley, and from the sensitivity of the interferomet-ric phase to the deformation component along the radar look direction (23° from nadir). Therefore, north and south slopes are not favorably illuminated by the ERS SAR, what decreases the capacity for detecting mass movements in these orientations.3. Procedure for slope movement detectionThe procedure applied in this study for the detection and qualifi cation of moving areas comprised fi ve succes-sive steps.(1) The major obstacle limiting the potential of InSAR is the presence of wet snow which strongly disturbs the radar signal. To prevent any misinterpretation of the SAR interferograms, estimating the (old and fresh) snow con-ditions at SAR image dates has proved to be a necessary fi rst step. In this study, a rough assessment of the eleva-tion of the snow line depending on aspect was performed on the basis of the daily reports on weather conditions by meteoswiss.(2) A combined visualization of valid interferograms with various time lags (1 day, 1-2 months, 1-3 years) was applied over smaller areas (about 5 x 5 km) in order to de-tect the polygons potentially related to slope instabilities. This systematic method permitted to identify not only the location and spatial extent of unstable zones but also the magnitude order of the displacement velocities in the ra-dar line-of-sight (three categories were defi ned, namely cm/day, cm/month, cm/year). The identifi cation and de-limitation of the polygons was subjective and obviously might differ depending on user.(3) The detected polygons were then compared to the geomorphic landforms with the help of topographic maps and orthophotos. This step permitted on the one hand to evaluate the reliability of the detected InSAR signal and, 1. IntroductionA proactive management of natural hazard related to the creep of mountain permafrost (debris fl ow, landslides) requires regional overviews (mapping) of slope instabili-ties to be produced. Available data on slope motion in re-mote mountain periglacial areas is however often lacking or restricted to a few sites. InSAR (space-borne Synthetic Aperture Radar interferometry) appears to be a potential tool to fi ll in this gap by providing, at least partially, such large scale overviews on landslide and permafrost creep (rock glacier) activity in high mountain areas. The tech-nique is well established for the mapping of topography changes at mm to cm resolution (Bamler and Hartl 1999, Rosen et al. 2000, Strozzi et al. 2001). Over alpine areas, where dense vegetation is no longer present, observa-tions of mass movements in the satellite line-of-sight di-rection are feasible during the snow free period between early summer and mid fall in spite of incomplete coverage caused by the very rugged topography (Rott et al. 1999, Kenyi and Kaufmann 2003, Strozzi et al. 2004, Kääb et al. 2005).With the support of ESA (European Space Agency) in the framework of the SLAM (Service for Landslide Monitoring) project and of the Swiss Federal Offi ce for the Environ-ment (FOEN), the capability of InSAR for detecting and in-ventorying both location and magnitude of slope instabili-ties in a mountain periglacial environment was tested in the western part of the Valais Alps (50 x 30 km) with ERS (European Remote Sensing Satellites) SAR data (Delaloye et al. 2005a). The reliability of the results (location of “in-stable” zones and movement rate) was evaluated by com-parison with existing data such as rock glacier inventories (Delaloye and Morand 1997, Lambiel 1999), GPS measure-ments (e.g. Lambiel and Delaloye 2004, Perruchoud and Delaloye 2007) and air-borne photogrammetry (e.g. Kääb 2005). They were also compared to geomorphic features observed on orthoimages and to new GPS surveys and fi eld observations that were performed. The paper sum-marizes and illustrates some of the most important as-pects and results of the project.2. InSAR data A series of 34 interferograms from the ERS-1 and ERS-2 satellites (wavelength of 5.6 cm) was used. Interferograms were computed from SAR images acquired along ascend-ing and descending orbits, dating from 1995 to 2000, and with baselines shorter than 100-150 m. Time lapse ranges between 1 and 1085 days (only multiples of 35 days +/- 1 day, time necessary for the satellites to accomplish an orbit). The topographic reference was determined from an external Digital Elevation Model (DEM) with a spatial resolution of 25 m and an estimated vertical accuracy of 115raum Reynald Delaloye et al.raines (Fig. 2c), that are, according to Haeberli (1979), gla-cio-tectonically deformed frozen sediments. The spatial pattern of alpine push-moraine systems mostly dates back to the advance of the glacier during the Little Ice Age (LIA). In many cases, the former frozen materials were pushed up to lateral slopes. Movements that are currently suscep-tible to occur in such landforms may be the back-creep-ing of still frozen sediments towards the glacier talweg and/or the surface subsidence due to melting of massive ice at depth (in some cases melting of buried glacier ice) (Reynard et al. 2003, Delaloye 2004). At Penna, most of the whole complex of push-moraines is distinguishable on a 35-days lag SAR interferogram in front of the debris-cov-ered glacier (Fig. 2b). The later is already well delimited at a daily time lapse in summer (Fig. 2a) with a decorrelated signal that can be caused by the exposure of the melting ice itself (thinner debris coverage), by the rapid settlement of the glacier surface and/or by a signifi cant glacier mo-tion.on the other hand, to attribute a possible/probable cause to the slope movement.(4) Where available, the comparison of the InSAR data with “terrestrial” slope motion measurements was per-formed. This analysis also permitted to evaluate the reli-ability of the InSAR technique. (5) A few additional fi eld observations (geomorphic mapping and GPS survey) were performed locally to pre-cise or to confi rm the InSAR results.Steps (3) to (5) have shown an encouraging and often excellent fi tting of both the magnitude order and spatial pattern of slope movements between the signals detected on interferograms and the fi eld data.More than 600 polygons were thus identifi ed as ERS InSAR detected slope instabilities within the tested area above the tree line (about 2000-2200 m a.s.l.) (Fig. 1). The inventory was not an exhaustive list of all the slope insta-bilities in the area, but an inventory of all polygons show-ing any kind of signal on ERS SAR interferograms that can be interpreted as a possible slope movement. It was also possible to defi ne typical ERS InSAR signatures depending on the activity and/or type of the geomorphic landform (glaciers, debris-covered glaciers, push-moraines, rock gla-ciers and landslides with various rate of activity, gelifl uc-tion areas). Three examples are illustrated hereafter.4. Push-moraine and debris-covered glacierThe fi rst case illustrates the typical ERS InSAR signals detected on many historical forefi elds of alpine cirque glaciers located in the belt of discontinuous permafrost for both 1-day and 35-days time lapses in summer time (Fig. 2a-2b). The Penna area (Val d’Entremont, 47°56’30 N, 7°13’00 E) comprises a debris-covered glacier (La Trouye Glacier) surrounding by an impressive edifi ce of push-mo-Figure 1: Extract of the “ERS InSAR detected slope movement” in-ventory in the Valais Alps (upper Val d’Hérens, 46°07’ N, 7°30’ E). Figure 2a-b: Daily and monthly ERS InSAR signal on push-moraines and debris-covered glacier at Penna. Orthoimage by swisstopo.116geo9th International Symposium on High Mountain Remote Sensing Cartographycurred in the upper surging part, lowering the surface of about 5 to more than 10 m. 5. Active and “surging” rock glaciersThe velocity range of active rock glaciers is large. For most active features, an obvious signal is detectable at monthly interval (see Fig. 4b). For the most active, a low signal may be already detected at 1-day interval in sum-mer and decorrelation is widespread over the landform at monthly interval. For the slowest, any signal can be dif-fi cult to be evidenced at monthly time lapse but becomes evident, sometimes decorrelated at yearly interval.A peculiar case is the so-called “surging” rock glacier, that is a (part of) rock glacier moving very fast and show-ing crevasses and/or collapse features (Delaloye et al. 2005b). The Petit-Vélan rock glacier (45°55’ N, 7°14’ E) is such a surging rock glacier whose 200 m long terminal section is collapsing (Fig. 3a). The superfi cial velocity has been measured by GPS in summer 2005 up to 7 m/a (Dela-loye et al. 2005a). Its dramatic rate of activity was fi rst dis-covered by InSAR : obvious signals were observed at daily interval both in winter and summer 1997 (Fig. 3b-3c). The GPS data has confi rmed that the magnitude order of the collapsing rock glacier movement is to much large for any coherent detection on 35-days ERS interferograms (Fig. 3a, up to more than 40 cm of horizontal displacement in 35 days), but fi ts with the detection range of 1-day interfero-grams. The comparison of several terrestrial and aerial photo-graphs dating back to 1988 shows that the “surge” began between 1989 and 1995: the rock glacier terminal section was still intact in autumn 1988, but large crevasses al-ready occurred in 1995 (Delaloye et al. 2005b). The scarce at the top of the collapsing part is well observable on the orthoimage in 1999 (Fig. 3a). In 2005, the front of the rock glacier was laying at least 20-30 m downward of its posi-tion in 1995. In compensation, a strong subsidence has oc-Figure 2c: Daily and monthly ERS InSAR signal on push-moraines and debris-covered glacier at Penna. Orthoimage by swisstopo.Figure 3a-c:The “surging” Petit-Vélan rock glacier : GPS data (sum-mer 2005) and daily ERS SAR interferograms in winter and sum-mer time. Orthoimage by swisstopo.117raum Reynald Delaloye et al.7. Conclusions and perspectivesThe resulting inventory of ERS InSAR-detected instabili-ties can be seen as a preliminary tool compiled at regional scale which can be useful for further investigations to be carried out (if required) at local scale. It appears to be an attractive perspective both for early natural hazard man-agement and process understanding of slope movement in permafrost areas. As mentioned earlier, the inventory of slope motion resulting from an ERS InSAR analysis is not exhaustive : in particular, steep rock walls and unfavourable northern and southern oriented slopes are not or only partially illu-minated by ERS InSAR. Moreover, the detection of surface motion does not provide information on the deepness of the mass movement. The type of movement is thus some-times diffi cult to evaluate, what requires some additional fi eld geomorphic observation to be carried out. The topog-raphy of the test region, mainly consisting of north-south directed valleys has proved to be optimal for an applica-tion of the ERS InSAR technique. It is certain that another confi guration of the valley system can strongly limit its potential.Data used in this study is however already about 10 year old and more recent data would be appreciated. The Euro-pean Environmental Satellite ENVISAT, launched in 2003, operates similarly to ERS. We can thus expect with these new SAR data, the possibility to update the inventory of slope movement in a near future.ERS SAR data courtesy of C1P.2338 (© ESA, processing GAMMA). DHM25 © 2003 swisstopo.6. LandslidesSuperfi cial landslides and deep-seated saggings are slope movements that are not related to glacier or perma-frost occurrences. They are nevertheless frequent in the alpine periglacial zone and can be detected by ERS InSAR with various rate of activity. A good illustration is provided by the wide and steep south-western fl ank of Bec-des-Rosses (46°04’ N, 7°17’ E) dominating the village of Lourtier (Bagnes Valley). It comprises several active landslides that have been evidenced by InSAR. A large sagging (BaR-201) (Fig. 4c) is well observed at monthly time lapse (Fig. 4a), what reveals a subsidence rate of about 10 cm/y. Several other slower saggings or landslides can be detected on a 1-year interferogram (Fig. 4b). Most landslides correspond to the starting zone of frequent debris fl ows (BORLE, in prep.) that are crossing the village in a deep channel and reach the valley fl oor partially damming the main river.Figure 4c: Sagging, landslide and active rock glacier detection at Bec-des-Rosses on monthly and yearly ERS SAR interferograms. Orthoimage by swisstopo.Figure 4a-b: Sagging, landslide and active rock glacier detection at Bec-des-Rosses on monthly and yearly ERS SAR interferograms. Orthoimage by swisstopo.118geo9th International Symposium on High Mountain Remote Sensing Cartographychanges in surface velocities on the Becs-de-Bosson rock glacier (western Swiss Alps). In: HMRSC-9 Proceedings, this issue.REYNARD, E., LAMBIEL, C., DELALOYE, R., DEVAUD, G., BARON, L., CHAPELLIER, D., MARESCOT, L. and MONNET, R., 2003: Glacier/permafrost relationships in forefi elds of small glaciers (Swiss Alps). In: Proceed. 8th Int. Conf. Per-mafrost, Zurich 2003 : 947-952.ROSEN P., HENSLEY, S., JOUGHIN, I., LI, F., MADSEN, S., RO-DRIGUEZ E. and GOLDSTEIN, R., 2000: Synthetic aperture radar interferometry. In: Proceedings of the IEEE, 88(3): 333-382.ROTT, H., SCHEUCHL, B., SIEGEL, A. and GRASEMANN, B., 1999: 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. In: Geophysi-cal Research Letters, 26(11): 1629-1632.STROZZI, T., WEGMÜLLER, U., TOSI, L., BITELLI, G. and SPRECKELS, V., 2001:  Land Subsidence Monitoring with Differential SAR Interferometry. In: Photogrammetric En-gineering & Remote Sensing (PE&RS), 67(11): 1261-1270.STROZZI, T., KÄÄB, A. and FRAUENFELDER, R., 2004: De-tecting and quantifying mountain permafrost creep from in situ inventory, space-borne radar interferometry and airborne digital photogrammetry. International Jour-nal of Remote Sensing, 25(15): 2919-2931.ReferencesBAMLER, R. and HARTL, P., 1998: Synthetic aperture radar interferometry. Inverse Problems, 14: R1-R54.BORLE, Y., in prep.: Etablissement d’un bilan sédimentaire à l’aide d’une cartographie de terrain détaillée. Méthod-ologie et application sur le bassin versant de Lourtier (Valais). Master thesis, Fac. Geosc. Env., Univ. Lausanne, Switzerland.DELALOYE, R., 2004: Contribution à l’étude du pergélisol de montagne en zone marginale. PhD thesis. Fac. Sc., Univ. Fribourg, Geofocus10.DELALOYE, R., LAMBIEL, C. and LUGON, R., 2005a: Valida-tion of InSAR data in permafrost zone, Bas-Valais. ESA SLAM project, phase 2, fi nal report, Swiss Federal Offi ce of Water and Geology (unpublished).DELALOYE, R., LAMBIEL, C., LUGON, R., RAETZO, H. and STROZZI, T., 2005b: Surging rock glaciers: observations in the Valais Alps. In: 4D-earth, 3rd Swiss Geosciences Meet-ing, 18-19 Nov. 2005, Zurich, 187.DELALOYE, R. and MORAND, S., 1997: Du Grand-Combin au Val Ferret (Alpes Valaisannes): inventaire des glaciers rocheux et analyse spatiale numérique à l’aide d’un sys-tème d’information géographique. Diploma thesis, Fac. Sci., Univ. Fribourg, Switzerland.HAEBERLI, W., 1979: Holocene push-moraines in alpine permafrost. In: Geogr. Ann., 61A (1 – 2), 43-48.KÄÄB, A., 2005: Remote sensing of mountain perma-frost and permafrost creep. In: Schriftenreihe Physische Geographie, Glaziologie and Geomorphodynamik, Univ. Zurich, 48.KÄÄB, A., HUGGEL, C., FISCHER, L., GUEX, S., PAUL, F., ROER, I., SALZMANN, N., SCHLAEFLI, S., SCHMUTZ, K., SCHNEI-DER, D., STROZZI, T. and WEIDMANN, Y., 2005: Remote sensing of glacier- and permafrost-related hazards in high mountains: an overview. In: Natural Hazards and Earth System Science, 5, 527-554.KENYI, L. and KAUFMANN, V., 2003: Estimation of rock gla-cier surface deformation using SAR interferometry data. In: IEEE Trans. Geosci. Remote Sensing, 41(6): 1512- 1515.LAMBIEL, C., 1999: Inventaire des glaciers rocheux entre le Val de Bagnes et le Val d’Hérémence (Valais). Mémoire de licence, Fac. Lettres, Inst. Géogr., Univ. Lausanne.LAMBIEL, C. and DELALOYE, R., 2004: Contribution of RTK GPS in the study of creeping mountain permafrost, cases studies in the Western Swiss Alps. Permafrost and per-iglacial processes. 15: 229-241.PERRUCHOUD, E. and DELALOYE, R., 2007: Short-term 119raum Reynald Delaloye et al.Correspondence to:REYNALD DELALOYE  Dept. of Geosciences, GeographyUniversity of Fribourg, Switzerlande-mail: reynald.delaloye@unifr.chCHRISTOPHE LAMBIELInstitute of GeographyUniversity of Lausanne, Switzerlande-mail: christophe.lambiel@unil.chRALPH LUGONKurt Bösch University InstituteSion, Switzerlande-mail: ralph.lugon@iukb.ch  HUGO RAETZOFederal Office for the Environment, Switzerlande-mail: hugo.raetzo@bafu.admin.chTAZIO STROZZIGamma Remote SensingGümligen, Switzerlande-mail: strozzi@gamma-rs.ch

ERS InSAR for detecting slope movement in a periglacial mountain environment (western Valais Alps, Switzerland)

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ERS InSAR for detecting slope movement in a periglacial mountain environment (western Valais Alps, Switzerland)

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