SPACE-BORNE AND GROUND-BASED SAR INTERFEROMETRY FOR LANDSLIDE ACTIVITY ANALYSIS AND MONITORING IN THE APPENNINES OF EMILIA ROMAGNA (ITALY): REVIEW OF METHODS AND PRELIMINARY RESULTS

Massimo Barbieri(1), Alessandro Corsini(1*), Nicola Casagli(2), Paolo Farina(2), Franco Coren(3), Paolo Sterzai(3), Davide Leva(4), Dario Tarchi(4) (1) Università di Modena e Reggio E., Dipartimento di Scienze della Terra. L.go S. Eufemia 19, I-41100 Modena (Italy),

Email: corsini.alessandro@unimore.it (*corresponding author) (2) Università di Firenze, Dipartimento di Scienze della Terra. Via La Pira 4, I-50121 Firenze (Italy),

Email: paolo.farina@geo.unifi.it (3) Istituto di Oceanografia e Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/c, I-34010 Trieste (Italy),

Email: fcoren@ogs.trieste.it (4) Joint Research Center (JRC), European Commission, Via E. Fermi 1, I-21020 Ispra (Varese), Italy,

Email: dario.tarchi@jrc.it ABSTRACT/RESUME This work concerns the application of SAR interferometry for the assessment of the long-term analysis of the state of activity of deep seated mass movements affecting some urban areas in the northern Appennines of Emilia Romagna region (from 1994-2001 space-borne ERS data) and for the real-time monitoring of active flow-like landslides in year 2002 (from ground-based system). These activities are part of an ongoing research project supported by civil protection authorities of the Emilia-Romagna region that involves several research institutes with diverse expertises. A set of test sites characterized by a high landslide risk have been selected mainly taking into account phenomena characteristics such as deformation rates and vegetation coverage, with respect to the employed techniques. After a preliminary detailed geomorphologic characterization of the sites, the interferometric analyses, still in progress, have been implemented. In particular space-borne DInSAR has been applied for 10 unstable areas using a set of ERS1/ERS2 data acquired in the last 7 years. From 9 interferograms, the line-of-sight displacement maps have been calculated and, later on, post processed in GIS environment in order to have on-slope-direction displacement maps that could fully be integrated with geomorphologic and ancillary data and that could semi-quantitatively be compared with other traditional monitoring data. The results obtained have been rather satisfactory, especially in some test sites where entire villages are settled on the mass movement, as in the case of Berceto (Parma) presented in the paper, and post-processed products have proved a significant amelioration of basic interferometric ones. Moreover, in order to measure terrain displacements induced by landslide characterized by high deformation rates and little urbanisation, ground-based SAR interferometry (GBInSAR) has been used for the monitoring of a test site located in the province of Bologna. This application proved that the LISA system adopted performs well even in clayey wet material, where signal attenuation is generally very strong. 1 INTRODUCTION A full landslide inventory carried out throughout the 1980’s and 90’s has shown that more that 33,000 landslides affect the Appennines mountain chain in the Emilia Romagna region (Fig. 1). Many of these landslides are large-scale rotational rock-earth slides or earth-mud flows that, originated during the last 15,000 years, now alternate periods of dormancy up to centuries long to periods of movement lasting for months up to some years. For the fact that these landslides stay apparently dormant for long enough to allow reconstruction, and for the fact that the gentle topography that is generally associated to this type of mass movement is highly valuable in a rough mountain environment, many urban areas in the Appennines have been settled, during the centuries, directly on the landslide body or nearby down slope. A preliminary inventory of very high landslide risk zones carried out after national Law 267/98, has pointed out around 100 situations of such kind. This number is believed largely underestimated, since it is generally represented by areas in which landslides have actually moved in the last decades, but it does not account for the many villages located over landslides for which historical chronicles report of reactivations.

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Proc. of FRINGE 2003 Workshop, Frascati, Italy,1 – 5 December 2003 (ESA SP-550, June 2004) 104_corsin

When landslide reactivation occurs the velocity of the phenomena vary in relation to the characteristics of the mass movement: rock-earth slides keep generally slow (< metre/month) while flows can be fast (up to > metres/hour). This has been clearly observed during the most serious hydro-meteorological crises of the 1990’s (i.e. 1994 and 1996) and that of year 2000, during which many landslides resumed activity requiring civil protection intervention [1]. Moreover, before and after periods of reactivation, and even during parts of the periods of so called dormancy, some of the rock-earth slides keep undergoing very slow movements (mm/year) that, while they do not necessary damage structures or cannot even be perceived without instrumentation, are to be seriously considered as markers of a state of efficiency of the hydrogeological mechanisms that in the future can eventually lead to, or even shortly precede or follow, the full reactivation of the landslide during periods of intense and prolonged precipitation such as these experienced during the above mentioned recent hydro-meteorological crises.

ERS coverage

Fig. 1. Location of Emilia Romagna (left), physical setting - landslide inventory (center), ERS-coverage-test site location (right) This paper is divided into two main sections. The first section presents a summary review of the procedures adopted to process and to integrate into GIS environment the D-InSAR dataset obtained from the processing, by means of conventional differential interferometry technique [2], of SAR scenes acquired during the 1994-2001 period by satellites ERS1-2, with the aim to detect and analyse, on a long-term basis, the state of activity of deep seated mass movements affecting a set of 10 test areas in the northern Appennines of Emilia Romagna (Fig. 1). The second section shows the use of differential interferometry using SAR data from a ground based LISA platform [3, 4]) to monitor, in real time, faster movements taking place during the reactivation of a faster flow like phenomena. The research presented has been conducted for the end-use of the Civil Protection Service and Geologic Service of Emilia-Romagna Region. It involved two operative units of the National Research Council Group for Hydrogeological Disasters Defence working on Research line 2 “prevention and forecast of high risk landslide events” (O.U. 2.9 and O.U. 2.14) that have been supported on the technical level by OGS Trieste (for space-borne interferometry) and JRC Ispra (for ground-based interferometry). 2 SPACE-BORNE SAR INTERFEROMETRY (DInSAR) 2.1 DInSAR processing The 10 selected test areas, whose extension varies from 11.1 to 81.2 km2, include zones that are classified as at high landslide risk according to National Law 267/98 or that were affected by landslide emergencies during the 1990’s or of the year 2000. The advantage of the selected test sites on respect to the possible successful application of the conventional differential interferometry technique, was that the movements occurred during the analysed period were expected to have been mostly very slow, and that the presence of villages settled on the landslide body would have provided for large enough coherent pixels patterns, two factors that proved crucial in the success of previous similar studies [5, 6, 7, 8]. Up to now, 20 SAR Raw scenes acquired in the last 7 years from the European satellites ERS1 and ERS2, including also 3 tandem acquisitions, have been selected on the basis of land coverage and geometrical and orbital characteristics (Fig. 1; Tab.1) The following is the interferometric processing chain adopted to detect, quantify and analyse slope movements in the 10 selected test areas.

i.

ii.

Data focalisation. Each of the 20 raw scenes acquired has undergone initial correction based on missing-lines suppression and acquisition parameters errors identification, such as these due to PRF (Pulse Repetition Frequency) and Doppler ambiguity. Scenes have been focalised using the ω-k domain (frequency – wave number) which is phase preserving, a necessary condition for SAR interferometry.

Interferograms generation. From the 20 raw scenes focalised, a total of 9 scenes couples have been processed in a conventional way adopting differential interferometric techniques by means of G-ISP software [9]. The characteristics

of spatial and temporal baseline of the processed interferograms are presented in Tab. 1. Initially, each scene couple has been co-registered and the associated coherence, phase and intensity matrix images have been generated for over the entire ERS scenes frame. Attention has been paid to flat-earth phase removal that can generate phase residuals that make erroneous phase values in the single interferograms.

Tab. 1. List of Interferograms (Int. ID) with orbital characteristics

Int. ID Orbit1 Orbit2 Date1 Date2 ║B ┴ B D-days Technique T-437 F-2709 descending

1 21467 1794 19950823 19950824 -37 -83 1 tandem 3 22469 24974 19951101 19960424 12 -62 175 phase scaling 4 24974 10311 19960424 19970410 -49 -80 351 phase scaling 5 10311 20832 19970410 19990415 -9 -63 735 phase scaling 9 42509 22836 19990901 19990902 -86 -218 1 tandem

11 24840 28848 20000120 20001026 -10 -20 280 phase scaling 12 24840 29349 20000120 20001130 64 213 280 phase scaling 14 25341 29850 20000224 20010104 -122 -41 315 phase scaling

T-165 F-2709 descending 6 41235 21562 19990604 19990605 -50 -94 1 tandem 8 28075 29578 20000902 20001216 -119 -1 105 phase scaling

T-215 F-0891 ascending 15 42287 22614 19990816 19990817 81 197 1 tandem

16 22614 28626 19990817 20001010 56 207 420 phase scaling

iii.

iv.

Calculations of phase contribute due to movement (plus atmosphere bias). This operation was carried out, for each interferogram, on several sub-frames covering each a specific study area. An accepted coherence factor has been assessed (> 0.40) and an acceptance matrix, were applied to the interferograms so to generate accepted phase and intensity images of each sub frame. The technique adopted to remove the topographic phase component was that of phase scaling [2]. This technique does not necessarily require phase unwrapping to be performed, that is particularly difficult over scattered data such as these from the considered study areas. The topographic component was removed by using DTMs obtained either from processing tandem acquisition, in the cases of favourable baseline value, or from Borgefors’s interpolation [10] of 25 m contour lines digitised from existing 1:5000 topographic maps. In this latter case, the DTM was then projected and co-registered to SAR acquisition geometry.

Atmosphere bias correction. This has been carried out using “scissor” software, a custom application developed by OGS. This operates on a selection of phase values in pixels of maximum coherence -where it is quite sure that phase values have not been varied by changes in roughness or dielectric property- and on the calculation of the variance of such phase values into pixels windows large enough to assure that the dispersion of phase values around the average value (that is due to noise and non-linearity of movement and atmosphere component) is minimum. Afterwards the average phase value of pixels contained in the analysed window is computed; this value is assumed as the linear component of interferometric phase due to movement and atmosphere. The atmosphere contribute to this linear component is equal to the average value assumed into estimate windows whose dimensions must be defined on the basis of the temporal baseline of interferograms: larger in case of an interferograms generated from two scenes acquired at months-years time interval, smaller if the time lag is in the order of days or few months. The result of this procedure is a matrix of the atmosphere contribute that can be subtracted to the interferometric phase matrix in order to obtain phase residuals that are proportional to displacement values. The outputs of scissor software are maps of the displacement and yearly movement rate in the line-of-sight of the satellite within the > 0.40 coherence pixels; these products have been Geocoded Terrain Corrected and have been referenced to UTM coordinate system in use at the cartographic archive of Emilia Romagna region by editing the sub-frames corner coordinates. 2.2 Post-processing and GIS integration of interferometric data The displacement maps in LoS produced for each sub-frame (i.e. study area) were imported in GIS environment as geo-referenced ascii files. GIS software adopted was Ilwis 2.1 [11] and Arc-view 8.1 [12]. Compiling and running specific scripts in Ilwis enabled post-processing of raster maps to be performed efficiently over large datasets. Creating a

geodatabase for each study area in Arc-view enabled interferometric data to be integrated with ancillary data such as geologic and geomorphologic maps, and with other traditional monitoring data (inclinometric, GPS, etc.). In particular, the following work-steps were undertaken.

i. Calculation of on-slope-direction displacement maps. Projection of the displacement values detected on the line-of-sight to the slope direction and then to the cartographic plane, has been done, following the geometrical setting in Fig. 3a, with the support of a custom made script (named “slant-pro”), compiled and run into GIS software Ilwis. The script performs using as input maps these of slope angle, slope aspect and movement in the line-of-sight, plus the heading and look-angle parameter. Another factor accounted for by slant-pro during projection, is the so-called foreshortening angle range. This represents a set of angular conditions in which the line-of-sight and the slope angle-direction are nearly perpendicular, a situation for which projection can lead to large estimate errors, especially when small movements in the line-of-sight are processed (Fig. 3b). The output ascii files generated by slant-pro script for each interferogram processed are two: a “binary” on-slope-direction displacement map differentiating pixels having a movement for which projection is possible from pixels having a movement for which projection is not possible because falling in the foreshortening angle range, and a “projected” on-slope-direction displacement map representing the on slope direction displacement value for the pixels where projection took place out of the foreshortening range. Examples of “binary” and “projected” on-slope-direction displacement maps for Berceto (Province of Parma) are in Fig. 4.

Fig. 3. Projection geometry of line-of-sight movement in “slant-pro” script

Fig. 4. Examples of “binary” (left) and “projected” (right) on-slope-direction displacement maps for the study area of Berceto

ii.

iii.

Filtering of on-slope-direction displacement maps. In order to make the projected displacement values more spatially robust, filtering has been performed adopting neighbourhood analysis operators. In practice, another custom-made script, “avg-neight”, has been compiled and run into GIS software Ilwis. This script averages the displacement value in each pixel with that of the neighbour 3x3 pixels matrix, and returns the averaged values only if at least 3 neighbour pixels have a defined displacement value themselves. If this is not the case, has it occurs in an isolated pixel or pixels couple, then the displacement value is not returned. The output ascii file of the avg-neight script, for each interferogram processed, is an “averaged” on-slope-direction displacement map, in which too scattered information is no longer represented while dense information patterns are made more spatially and numerically consistent.

Integration of on-slope-direction displacement maps with ancillary, geomorphic, monitoring data. This work-step has been carried out in order to carry out a qualitative and quantitative validation of interferometric data and, at the same, to favour the full exploitation, for landslide activity and intensity assessment purposes, of the information contained in the on-slope-direction displacement maps and other intermediate interferometric products. Detailed geomorphological information has been produced in the frame of the research project in order to better understand and evaluate the significance of displacement values calculated with DinSAR technique. Data regarding each study area has

been managed in separate projects linked to the a spatial geo-database set up in ArcGIS 8.2 software environment with reference to the UTM (ED 1950) coordinate system (with a false northening of -4,000,000 metres necessary to project and manage the basic documents provided by the cartographic office of Emilia Romagna regional administration such as topographic maps at various scale, DTMs, aerial photo coverage, landslide inventory, geologic layers). The information on the spatial extent and activity characteristics of geomorphological features associated to the different mass movements types, has been disaggregated into several layer containing geomorphic elements represented as lines or polygons. Monitoring data from GPS networks and from inclinometers and piezometers, collected at the public institutions have been managed in attribute tables, jointed to point maps of the location of benchmarks or instrumented boreholes. Two examples of “averaged” on-slope-direction displacement maps for the study area of Berceto, integrated with geomorphological information, damaged houses inventory, GPS monitoring results, are in Fig. 5.

Fig. 5. “Averaged” on-slope-direction displacement maps of Berceto study area, integrated with geomorphologic, damages and GPS

monitoring data (above: INT_11: 20/01/2000 – 26/10/2000; below: INT_03: 01/11/1995 – 24/04/1996)

3 GROUND-BASED SAR INTERFEROMETRY (GB-DInSAR) 3.1 Characteristics of device and test site At the beginning of December 2002, a monitoring campaign of Roccapitigliana landslide (Province of Bologna) has been carried out using a ground-based portable version of the SAR device known as LISA, developed by the Joint Research Centre, HSU Unit. The mechanical parts consist of a 2.8 m long straight track with a motorised sled hosting the antennas and other microwave components (Fig. 6a). The frequency scanner scatterometer is based on a Network Analyser, which includes the signal source between 30 kHz to 6 GHz. A coherent conversion module of about 17 GHz is employed for measures at higher frequencies. LISA technique allows two-dimensional deformation maps to be derived, which offer a complete picture of the movement mechanism, instead of few point-like measurements as for traditional instrumentations (GPS, extensometers, etc.). It is not necessary to install instrumentations in the target area and this is an invaluable advantage in the case of dangerous movements to which access is not possible. The Rocca Pitigliana landslide is a complex movement characterized by an earth slide in the upper part, which in the bottom part turns in an earth flow. This type of landslide is quite typical for the Appennine foothills, and is generally triggered by intense rainfalls. The site has been chosen for the availability of different types of geotechnical and geodetic instrumentation, such as differential GPS surveys, inclinometric and pore pressure measurements. These data are useful for the comparison and the better assessment of the interferometric-derived results. The radar campaign, implemented by using the LISA system, in the 3 m rail configuration, has been performed in a rainy period in order to follow the possible superficial displacements induced by the rainfall. Fig. 6b shows the LISA

mounted at an average distance of about 400 m from the unstable area, in the position and with the look direction indicated in Fig. 6c. A power image of the landslide is in Fig. 6d; red colour corresponds to zones characterized by a high level of backscattered signals, while black zones to areas with low value of backscattered energy.

A B C D

Fig. 6. a: sketch of LISA system. b: picture of LISA. c: picture of Rocca Pitigliana landslide and monitored part. d: power image

3.2 Monitoring results The system has worked continuously for about a week acquiring a SAR image every 10 minutes. The quantitative comparison between consecutive images has allowed the temporal evolution of the landslide deformation field to be followed. Consecutive displacement maps with a temporal interval of 12 hours for each map are shown in Fig. 7. The ground-based interferometric analysis has allowed the presence of different sectors in the upper part of the slope interested by localized movement of few millimetres per day to be recognized. As shown on the last map (on the left), which displays the displacement field relative to 72 hours, such a movement, is located on the landslide crown, corresponding to small scarps, triggered by the rainfall occurred during the monitoring campaign.

Fig.8. Consecutive displacement maps (temporal interval of 12 hours, from left to right and up to down)

4 CONCLUSION The procedure presented regarding space-borne interferometry has been applied to a total of 10 study areas in the Apennine of Emilia Romagna. For many areas, the results obtained still need validation through the integration with detailed field survey and comparison with other monitoring data. In the areas where this integration has been carried out already, space-borne interferometry data seem to point out quite convincingly that small movements have been affecting villages located on ancient large scale landslides that have been generally mapped as dormant in current inventories. Therefore, these phenomena might better be considered active-suspended and should be paid particular attention for civil protection purposes. Despite the current limitation of space-borne interferometry (due to low coherence and capability to detect extremely slow movements only) it is believed that the application of DInSAR to situations such has these considered in this study (where mass movements are very slow and on the large scale affect urbanised areas) can lead to the production of useful results that, despite the possible errors preventing them from be used as real monitoring systems, can be valuable on assessing the overall state of activity of the mass movement and on zoning it over the slope with better precision and knowledge than with traditional methods. On the other hand, ground-based interferometry applied in Rocca Pitigliana has produced results that have been proved consistent with the trend evidenced by independent sources. The same success has by now been obtained with GB-InSAR in many other test sites. Unlike space-borne SAR, ground based systems do not suffer of coherence problems and are capable to monitor in real time movements that are up to m/hour fast. Still in the frame of this research project, the system it will soon be used on another site. It can be concluded that space-borne and ground-based interferometry are complementary tools in landslide study and monitoring. Space-borne interferometry covers larger areas in a retrospective mode, so to support long term assessment, but suffers from loss of coherence and phase ambiguity making it a useful tool for hazard and risk forecast rather than for emergency management. Ground-based interferometry is a real time monitoring system of great potentiality during emergency phases but, due to the necessary limited duration of monitoring campaign, can hardly support long-term evaluations of hazard. ACKNOWLEDGEMENTS The research is part of 2002-04 Emilia Romagna Region contract “SAR-RER: Monitoring inhabited centres at high landslide risk in the Emilia-Romagna Region”, conducted as part of the activities of CNR-GNDCI (National Research Council’s Group for Hydrogeological Catastrophes Defence) Operative Unit n. 2.9 (leader Prof. M. Pellegrini) and Operative Unit n. 2.14 (leader Prof. P. Canuti). Distinct collaboration: researchers from OGS Trieste have carried out DInSAR processing of ERS data; researchers from University of Modena and Reggio Emilia have taken care of post processing and GIS integration of space-borne interferometric data; researchers from University of Florence and JRC Ispra have applied ground-based interferometry with the LISA system. The corresponding author has coordinated the laying out of this manuscript. REFERENCES 1. Bertolini G. and Pellegrini M., The landslides of the Emilia Apennines (northern Italy) which resumed activity in

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