THE USE OF DIFFERENT REMOTE SENSING TECHNIQUES FOR LANDSLIDE CHARACTERIZATION Ph.D. candidate: ANNA DE AGOSTINI Tutor: Dr. M. FLORIS Cycle: XXVI Abstract The aim of my second PhD year was to understand the potential for satellite InSAR (Interferometric Synthetic Aperture RADAR) techniques in landslide identification and characterization. I conducted a small scale analysis of about 200 km 2 area (Agno basin, Vicenza Province) and a large scale study of two test areas of 4 and 2 km 2 (Rovegliana-Cappellazzi, Vicenza Province and Prezzo, Trento Province). The results show that morphological characteristics, such as slope and aspect, are the main limiting factors of INSAR techniques application at both small and large scale investigation. I evaluated the spatialtemporal evolution of instability processes in the large scale areas, using displacement datasets from 1992 to 2010 provided by advanced interferometric processing (Permanent Scatters and Small Baseline Subset algorithms) of ERS and ENVISAT satellite images. The results allowed improvement and refinement in landslide identification, characterization and hazard evaluation. Further development regards the processing of COSMO-SkyMED high-resolution satellite images and the comparison with previous results. Full Report During the second year of my PhD project I worked mainly on applicability and interpretation of advanced DInSAR (Differential Synthetic Aperture RADAR Interferometry) techniques at basin (Agno Valley) and slope scale (Rovegliana-Cappellazzi and Prezzo landslides). I conducted also a photogrammetric study on vertical outcrop (Valstagna) to investigate the capability of terrestrialphotogrammetric method applied to rock mass geomechanical characterization. The advanced DInSAR methods includes Small Baseline Subset (SBAS) (Berardino et al., 2002) and Persistent Scatterers (PSInSAR) (Ferretti et al., 2001) algorithms. These approaches involve the use of multiple acquisitions stacks (large SAR temporal series). The concept of combining InSAR information from a large number of SAR images (thanks to repeated satellite passes on the same area), allowing the solution of a deformation time series, has been introduced by Ferretti et al. (2000), Berardino et al. (2002), Lanari et al. (2004) and Lauknes (2004). Using a multi temporal technique allows not only the estimation of discrete deformation events, but also the evolution of displacements. Interferometric phase (phase difference between two SAR images) is the sum of many contribute: the topographic one is associated to the relationship between phase and topography, the displacement one is due to the land movements (subsidence, landslides, earthquake), the atmospheric one is due to the variation of atmosphere characteristics and the noise one due to instrumental properties. Comparing two SAR images is possible exploit the phase contribute due to land movements, knowing the other phase components and subtracting them from the total phase difference. The main limits of InSAR techniques are temporal decorrelation and geometrical distortions. Temporal decorrelation is due to changes of electro-magnetic response of objects with time caused by atmospheric phenomena or anthropic changes or vegetation growth. Satellite look angle of 23.3 (for ERS and ENVISAT images that I used) and right side-looking acquisitions mode are the responsible for geometric distortions effects. The layover effect is presents where slopes inclination exceed the look angle producing a strong image distortions, that prevent the correct signal interpretation an qualitative analysis. Shadow effect is present in areas that are not illuminated by the radar signal. These effects, in relationship with aspect and inclination of slopes, need to be taken in account before starting an investigation of mountainous area, because slope instability processes could be located in area affected by layover or shadow effects. At basin scale, I performed a feasibility study on the limits of applicability of DInSAR techniques, in landslide phenomena analysis. Layover and shadow maps (LS map) of Agno basin were combined with morphological characteristics (slope and aspect) and land use data. 1
The Agno Valley is located in the NW sector of Vicenza Province. The valley in characterized by gentle relief (only NW part present high relief over 1500 m and slope over 40 ), the mean altitude is 600-700 m. Volcanoclastic and limestones rocks are the widespread lithologies. Rotational/translational slides and slow flows are the common landslide types. Regarding the main morphometric features of study area, easterly aspect is predominant and slope values between 10 and 30 prevail. I created the two LS maps, one for ascending and one for descending acquisitions mode. These maps show the areas affected by geometric distortions, which are devoid of RADAR information and are unusable for instability processes investigations (17.8% of total Agno Valley area for descending track and 11% for ascending one). Combining ascending and descending LS maps, only 1.3% of Agno Valley is simultaneously affected by LS distortions, therefore these areas can t be investigated through interferometric techniques using ERS-1/2 and ENVISAT images (with incidence angle of 23.3 )(Fig. 2a, 2b). Regarding Italian Landslide Inventory (IFFI) dataset, the 18.2% of mapped landslides is not visible to track descending, whereas the 10.2% is not visible to track ascending and only 1.4% of known instable phenomena is totally invisible to both descending and ascending track (Fig.2c). Focusing on relationship between aspect/slope factors and visible area, I analysed slope and aspect index (normalized invisible areas divided by normalized visible areas for each class: RATIO_TOT_165 and 172 in Fig. 2d) for both ascending and descending acquisition mode. When the index is greater than 1.5, the number of invisible pixels is markedly greater than the visible ones. The results show that slope values greater than 30 are the main morphometric limit for the application of InSAR techniques, whereas the aspect class that hindered the use of interferometric methods on instability characterisation is the East one for descending orbit and West ones for descending track. Comparing land use data and LS maps, the results show that 44% of visible area fall into woody area, the 11% is part of urban zone and 22% fall within grass land class. Therefore a big percentage of visible pixel fall into problematic land use classes for interferometric processing: woody and grass land areas have low density of PS and SBAS data, due to the sparse presence of scatters. Finally results show that areas affected by distortions are very limited and the presence of steep slope is the main factor limiting of the applicability of DInSAR interferometric techniques. Figure 2 - Layover and shadow map of Agno Basin for descending (a) and ascending (b) track. IFFI landslides invisible to track ascending and descending (c). Slope Index Map and table of slope ad aspect index value for each class. At slope scale, time-series displacement datasets derived from SBAS and PS interferometric processing of ERS and ENVISAT images (ground pixel resolution of 25x25m) were analysed to evaluate the real contribute of these two innovative techniques and to determine the state of activity of landslide phenomena that occur in Rovegliana-Cappellazzi area (North sector of Agno Valley, Vicenza) and Prezzo landslide (Giudicarie Valley, Trento). To this end, several time-consuming interferometric processing were conducted in order to find the best interferometric-procedure parameters to process mountainous and vegetated areas such as the test areas. Rovegliana-Cappellazzi slope is mainly affected by rotational, translational and shallow movements that assumed a relevant role after Novembre 2010 extreme rainfall event. These instabilities involve mainly 2
the quaternary deposits and fractured rocks. Due to quite large extension of total area affected by landslides (4 Km 2 ), and to high density of vegetation (Fig. 3c), field detection methods (eg. GPS, laser scanning, optical photo, LIDAR) are expensive and time-consuming. PS and SBAS data, obtained from intererometric processing, were very helpful in this morphologic context. In fact, the presence of scattering houses on the entire slope, facilitated the unwrapping step of interferometric processing, although the presence of vegetated areas. In the study area, geological and geomorphological investigation coupled with the study of historical aerial optical images were conducted to identify instable areas. During these investigations, PS data allowed a better identification and delimitation of landslide unstable area (Fig. 3a). I calculated the return time (state of activity) of landslides dividing the 20 years of ERS-ENVISAT images database by the number of marked displacements that affect the instable area (Fig. 3b). Furthermore, I compared PS displacement data with monthly rainfalls database to investigated landslide triggering factors. Results show that period of marked displacement correspond to period of intense rainfalls (Fig. 3d and 3e). a) b) c) d) e) Figure 3 - Classification of landslides based on types of movements (upper left) and on the return time calculated with PS displacement data (upper right). On the lower left an aerial view of the slope scale study area is showed. On the lower right plots of PS displacement data and monthly rainfalls for landslide n.9 are reported as an example of the identification of landslide activity and its relationship with rainfall regime. Concerning Prezzo landslides, field investigations were carry out in order to better define the geologic and geomorphologic settings of the instable area. DEM derived and LIDAR data are also used. I conducted PS and SBAS processing on ERS 1-2 and ENVISAT RADAR images. Firstly I created layover and shadow maps: in descending acquisition mode, the Prezzo area is affected by layover effect, so the capability of advanced DInSAR techniques to gain data is very low (Fig.4). GPS displacement data were compared with the interferometric ones and they show a good agreement. PS and SBAS data interpretation jointly with geologic and geomorphologic analyses and with field investigations (eg. GPS displacement data), are helpful tools in landslide identification and 3
Scuola di Dottorato in Scienze della Terra, characterization. Where vegetation is strongly present, when geologic or morphologic evidence are lacking, where the extension of instable area is too large for classical methods (GPS or laser scan) or when a displacement dataset is absent or deficient, the interferometric displacement data improve the definitions of instable area and the history of its displacements. Figure 4 - Layover and shadow maps for ascending (left) and descending (right) orbit on Prezzo area. On the left image PS velocity data obtained by the interferometric processing of ERS-ENVISAT images are shown. During this second year of PhD, I also investigated the contribute of photogrammetric techniques in landslide characterizations. This methods is a useful approach despite of its long and hard-working data collection way because it allows to investigate vertical rock face, which is bad detected by InSAR technique. Terrestrial remote sensing techniques (laser scanning, Ground-Based-InSAR, photogrammetry) permit large scale investigations, seeking out details of the instability phenomena; they represent a useful complement to conventional field mapping and rock mass discontinuity characterization (discontinuity sets, trace intensity and block size/shape) (Sturzenegger, 2010). I examined Valstagna vertical outcrop (Vicenza Province) in order to determine the discontinuity geometry of rock mass. I used a Canon EOS 40D and the software 3DM Analyst to obtain the high resolution digital terrain model of the outcrop which allowed the detection of discontinuity sets. The results show that data of joint orientation and spacing are comparable with the data collected with terrestrial laser scanner and abseiling methods. Figure 5 - Valstagna outcrop: DTM and discontinuity sets. Future activities Activities of third PhD year envisage the processing of high resolution COSMO Sky-Med satellite images (3x3m) and the interpretation of PS and SBAS derived data of Rovegliana-Cappellazzi area and of another two instable area located in Vicenza Province (Cumerlati and Cischele landslides). A LIDAR acquisition will be conducting on Rovegliana-Cappellazzi and I am examining the advantages and disadvantages of this airborne remote sensing method. Furthermore I will comparing PS or SBAS velocity data with field displacement ones (mainly GPS data) to test once more the contribute of interferometric techniques in instability characterization. 4
References BERARDINO, P., FORNARO, G., LANARI, R. and SANSOSTI, E. 2002. A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms. IEEE Transactions on Geoscience and Remote Sensing, 40(11), 2375-2383. FERRETTI, A., PRATI, C., ROCCA, F. 2000. Nonlinear subsidence rate estimation using permanent scatterers in differential SAR interferometry. IEEE Transactions on Geoscience and Remote Sensing, 38(5), 2202-2212. FERRETTI, A., PRATI, C., ROCCA, F. 2001. Permanent scatterers in SAR interferometry. IEEE Transactions on Geoscience and Remote Sensing, 39(1), 8-20. HANSSEN, R.F. 2002. Radar Interferometry. Data Interpretation and error analysis. Kluwer Academic Publishers, 327 pp. LANARI, R., O. MORA, M. MANUTA, J. J. MALLORQUI, P. BERARDINO AND E. SANSOSTI., 2004 A smallbaseline approach for investigating deformations on full-resolution differential SAR interferograms. IEEE Transactions on Geoscience and Remote Sensing, 42(7). 1377-1386 LAUKNES, T. R. 2004. Long-Term Surface Deformation Mapping using Small-Baseline Differential SAR Interferograms. Thesis for the Degree of Candidatus Scientiarum, University of Tromsø, Norway. 100 pp. STURZENEGGER, M.,2010. Multi-scale characterization of rock mass discontinuities and rock slope geometry using terrestrial remote sensing techniques. PhD Thesis, Simon Fraser Universtiy, Canada. 367 pp. SUMMARY LAST YEAR S ACTIVITY Courses: R.J. ANGEL: Scientif Communication Course, Dipartimento di Geoscienze, Università degli Studi di Padova Communications DE AGOSTINI, A., PISTELLATO, D. and STEVAN, G. - Rock mass analysis using terrestrial digital photogrammetry. Congresso Nazionale Associazione Italiana di Geologia Applicata, AIGA (Perugia, February 6-7, 2012) DE AGOSTINI, A., CANTONE, A., DEFILIPPI, M., FLORIS, M., GENEVOIS, R., PASQUALI, P., RICCARDI, P., STEVAN, G. and TESSARI, G. 2012. Il contributo dell'interferometria radar satellitare per l'identificazione e caratterizzazione dei fenomeni franosi a differenti scale d'indagine. Congresso Nazionale Federazione Italiana delle Associazioni Scientifica per le Informazioni Territoriali ed Ambientali, ASITA (Vicenza, November 6-9, 2012) Posters: DE AGOSTINI A., FLORIS, M., PASQUALI, P., BARBIERI, M., CANTONE, A., RICCARDI, P., DEFILIPPI, M., STEVAN, G. and GENEVOIS, R. 2012. The contribute of DInSAR techniques to landslide hazard evaluation in mountain and hilly regions: a case study from Agno Valley (North-Eastern Italian Alps). Natural Hazard Sessions EGU General Assembly, Wien (Austria), April 22-27, 2012. S. ADAMI, M. BRESOLIN, M. CARRARETTO, P. CASTELLETTI, S. FIASCHI, L. GANDOLFO, L. MAZZALAI, F. SARTORI, A. VIGANÒ, A. ZULIAN, DE AGOSTINI, A., M. PAJOLA, AND M. FLORIS 2012. An unconventional GIS-based method to assess landslide susceptibility using point data features. EGU General Assembly - Natural Hazard Sessions, Wien (Austria), April 22-27, 2012. 5
TESSARI, G., DE AGOSTINI, A., FLORIS, M., STEVAN, G. and GENEVOIS R. 2012. Stabilizzazione di una infrastruttura viaria nell'area franosa della Val Maso (Valli del Pasubio, VI) tramite la realizzazione di strutture deformabili. Congresso Nazionale Associazione Italiana di Geologia Applicata, AIGA Perugia, February 6-7, 2012. Publications: M. FLORIS, A. D ALPAOS, A. DE AGOSTINI, G. STEVAN, G. TESSARI, AND GENEVOIS R. 2012. A process-based model for the definition of hydrological alert systems in landslide risk mitigation Nat. Hazards Earth Syst. Sci., 12, 1-15. DE AGOSTINI, A., CANTONE, A., DEFILIPPI, M., FLORIS, M., GENEVOIS, R., PASQUALI, P., RICCARDI, P., STEVAN, G. and TESSARI, G. 2012. Il contributo dell'interferometria radar satellitare per l'identificazione e caratterizzazione dei fenomeni franosi a differenti scale d'indagine. Atti 16 Conferenza Nazionale ASITA, 2012, 6 pp. DE AGOSTINI, A., PISTELLATO, D. and STEVAN, G. - Rock mass analysis using terrestrial digital photogrammetry. Engineering Hydro Environmental Geology, 14 (B), 79-80. TESSARI, G., DE AGOSTINI, A., FLORIS, M., STEVAN, G. and GENEVOIS R. 2012. Stabilizzazione di una infrastruttura viaria nell'area franosa della Val Maso (Valli del Pasubio, VI) tramite la realizzazione di strutture deformabili. Engineering Hydro Environmental Geology, 14 (B), 239-240. M. FLORIS, A. D ALPAOS, A. DE AGOSTINI, G. STEVAN, G. TESSARI, AND GENEVOIS R. 2012. Uno studio previsionale dei fenomeni franosi durante eventi alluvionali: l evento del novembre 2010 in provincia di Vicenza. Engineering Hydro Environmental Geology, 14 (B), 119-120. Field works: - Jan - Aug 2012: Field trips at Rovegliana-Cappellazzi, Cumerlati, Pianegonda, Pason, Val Maso and Lusiana instable areas (all located in Vicenza Province) Foreign Mission - March 2012 - Three weeks at Sarmap base office, to improve InSAR processing of ERS-ENVISAT images of Agno basin area, using SARscape (version 4) software. - October 2012 - Three weeks at Sarmap base office, to improve InSAR processing of ERS-ENVISAT images of Rovegliana- Cappellazzi and Prezzo landslide areas using SARscape (version 5) software. 6