SURFACE VELOCITY OF SWISS ALPINE GLACIERS FROM ERS SAR INTERFEROMETRY

Tazio Strozzi, Urs Wegmüller, Charles Werner and Andreas Wiesmann

GAMMA Remote Sensing, Worbstrasse 225, 3073 Gümligen, Switzerland,

{strozzi,wegmuller,cw,wiesmann}@gamma-rs.ch

ABSTRACT Differential SAR interferograms were computed over the Swiss Alps with ERS-1/2 Tandem data acquired between 1995 and 2000 from ascending and descending orbits and use of an external Digital Elevation Model (DEM). Comparison with the Swiss glacier inventory 2000 shows that winter interferograms with one day time interval have a generally high degree of coherence and are straightforward to identify the line-of-sight movement of major and minor glaciers visible in the satellite SAR acquisitions (excluding therefore layover and shadow). Signal decorrelation is mainly observed for areas with high crevasses and displacement larger than some tens of centimeters. For areas with sufficient coherence to allow reliable phase unwrapping maps of the satellite line-of-sight displacement were computed and transformed, in certain cases, to surface velocity maps by assuming flowing along the terrain gradient. 1. INTRODUCTION Many studies on Arctic and Antarctic glaciers and ice streams have demonstrated the invaluable potential of satellite Synthetic Aperture Radar (SAR) interferometry to map ice surface displacement at cm resolution [1-6]. Together with the vertical change, the horizontal displacement of a glacier is important to determine its mass balance. In addition, changes in the velocity field can suggest dynamic instabilities (surging glaciers) or lubrication effects by meltwater. Despite the availability of a large ERS-1/2 SAR data archive, only minor studies have been performed in the European Alps [7-9]. Here we report on an ongoing extensive SAR interferometric analysis in the Swiss Alps aimed to estimate the surface velocity of valley glaciers using ERS-1/2 SAR Tandem acquisitions. 2. ERS SAR DATA ANALYSIS In SAR interferometry two complex SAR images acquired from slightly different orbit configurations and at different times are combined to exploit the phase difference of the signals [10,11]. The interferometric phase is sensitive to both surface topography and coherent displacement along the look vector occurring

between the acquisitions of the interferometric image pair. In the so-called 4-pass differential interferometric approach the differential use of two interferograms with similar displacement allows the removal of the topographic-related phase from the interferogram to derive a displacement map [12]. Alternatively, in the so-called 2-pass differential interferometric approach, the topographic-related interferometric phase is simulated from an external DEM and removed from the interferogram to isolate the displacement related phase [13]. In this case, the critical issue is the accuracy of the DEM, which also depends in our case on the date because of substantial glaciers retreat in recent years. For the Swiss Alps the contour lines for deriving the national DEM (DHM25) with a spatial resolution of 25 m were updated in the nineties of the last century and the vertical accuracy is estimated to be 3 m. For short baselines (< ~100 m) no significant artifacts were found on differential ERS SAR interferograms. In addition, the surface displacements measured using the 2-pass approach over Unteraargletscher were found to be very similar to those retrieved with the 4-pass approach [8], with variations in displacement smaller than 1 cm. Therefore the more straightforward 2-pass approach, which has the advantage of avoiding phase unwrapping of the topographic phase and assumption of similar displacement in two interferograms, is considered. Because of its nearly global coverage and 30 arc sec (~90 m) resolution, the C-band SRTM DEM [14,15] is also tested for topographic phase removal for additional possible studies in countries where high resolution DEM’s are not available. The main limiting factors of SAR interferometry are signal decorellation and atmospheric artifacts [16]. For winter SAR interferograms acquired with one day time interval, however, the degree of coherence is generally high. Signal decorrelation is mainly observed for areas with high crevasses and displacement larger than some tens of centimeters. In the winter ERS-1/2 Tandem interferograms considered in this study we did not observed over non-glaciated areas significant atmospheric disturbances at local scale (i.e. a few km’s). On the other hand, a height dependent phase signal has been often detected and partly compensated using a linear regression.

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Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

3. RESULTS 3.1. Differential SAR interferograms Differential interferograms were computed over the Swiss Alps with ERS-1/2 SAR Tandem data acquired between 1995 and 2000 with one day time interval and the DHM25. The mosaics of winter SAR interferograms from descending and ascending orbits are presented in Figures 1 and 2, respectively. Acquisition dates and perpendicular baselines of the available image pairs are indicated. These two figures give an impressive view of the differential SAR interferograms over the Swiss Alps with a generally high degree of coherence. However, the signals related to the glaciers can be hardly recognized because of their generally small size. For comparison and interpretation, the interferograms were therefore analyzed together with the Swiss glacier inventory 2000 [17,18]. An extract for the area around the Gornergletscher, Findelgletscher and Allalingletscher (VS) is presented in Figures 3 and 4 for ERS SAR data of descending and ascending orbits, respectively. In Figures 3 and 4 the line-of-sight component of the movements of major and minor glaciers visible in the satellite SAR acquisitions (excluding layover and shadow) can be often identified. 3.2. C-band SRTM DEM Because of its nearly global coverage and 30 arc sec (~90 m) resolution, the C-band SRTM DEM [14,15] is also tested for topographic phase removal for additional possible studies in countries where high resolution DEM’s are not available. The differential interferograms for the pair March 7 to 8 1996 using the DHM25 and the SRTM topographic references are shown in Figures 5 and 6, respectively, for the area around the Grosser Aletschgletscher (VS). Over the glacier no significant difference is found between the two results for an interferogram characterized by a small perpendicular baseline of 34 m. However, the SRTM DEM shows large areas without data, as a consequence of shadow, layover and insufficient interferometric coherence. A similar analysis performed with the image pair of 14 and 15 February 1996 with a perpendicular baseline of 138, on the other hand, showed many artifacts on the interferometric phase with topographic contribution removed by use of the SRTM DEM. 3.3. Displacement maps For areas with sufficient coherence to allow reliable phase unwrapping displacement maps of the satellite line-of-sight displacement were computed. Selected glaciers carefully analyzed until now include Unteraargletscher (BE) and Grubengletscher (VS) [8], Ghiacciao del Basodino (TI) and Vadrecc di Bresciana

(TI) [9], Ghiacciao del Belvedere in Italy and Gornergletscher (VS). Areas outside the glaciers on the slope of the valleys were selected as stable reference. For Ghiacciao del Belvedere a series of ERS-1/2 Tandem SAR data of ascending orbit was analyzed in addition to the pair of 11 and 12 March 1997. Four displacement maps are presented in Figure 7. Over Ghiacciao del Belvedere decorrelation is observed also in winter, because of melting snow and ice at low elevation, high strain rates close to the confluence zone of the glacier cirque, and very steep topography by the Monte Rosa wall. During the melting season coherence is generally very low. During summer and early autumn, some of the Tandem pairs show coherence over parts of the glaciers depending on the meteorological conditions, which we attribute to the debris cover of the glacier. The highest velocities along Ghiacciao del Belvedere are in August 1995. Next highest occur in December 1995 / January 1996, then March 1996 and finally October 1996. The maximum velocity of more than 6 cm in one day in summer is measured in the centre of the glacier and in the upper part characterized by the steeper topography. As expected, the displacement velocity decreases towards the glacier terminus. The effects of the line-of-sight direction are very clear: where look and flow directions are approximately perpendicular, SAR interferometry is unable to record the displacement. For alpine glaciers that are not strictly confined in a valley it can be reasonably assumed that ice is flowing along the surface slope. Therefore, the line-of-sight displacement maps of Ghiacciao del Belvedere were projected along the slope of the DEM, filtered with an averaged distance of about 600 m in order to reduce local effects. Areas where look direction and assumed flow direction were close to perpendicular were removed because of the very high uncertainty under this geometric configuration. Over the central part of the glacier maximum surface velocities in one day in winter are on the order of 6 to 9 cm (about 20-30 m/year), in some days in summer at least double (about 50 m/year). Mean values of the four different periods are consistent with a horizontal surface velocity field of around 38 m/year from annual aerial photographs in October 1995 and September 1999 [19]. The differences in velocity at different periods are most likely due to increased meltwater which causes more basal sliding and not to different glacier regimes, because between 1995 and 1999 front position measurements show a small constant retreat and the ice thickness of the tongue of the glacier below about 2160 m a.s.l. was approximately stable. The extraordinary change in flow, geometry and surface appearance of Ghiacciao del Belvedere only developed between summer 2000 and summer 2001 [19].

Figure 1. Mosaic of ERS-1/2 differential Tandem SAR interferograms of descending orbits over the Swiss Alps.

Acquisition dates and perpendicular baselines of the various Tracks are indicated.

Figure 2. Mosaic of ERS-1/2 differential Tandem SAR interferograms of ascending orbit over the Swiss Alps.

Acquisition dates and perpendicular baselines of the various Tracks are indicated.

19970311/12 Track 022, B⊥ = 38 m

19960307/08 Track 251, B⊥ = 34 m

19960304/05 Track 208, B⊥ = 40 m

19970311/12 Track 029, B⊥ = 28 m

19990313/14 Track 487, B⊥ = 16 m

19960129/30 Track 215, B⊥ = 50 m

19960214/15 Track 444, B⊥ = 138 m

Look direction (incidence angle 23º)

Look direction (incidence angle 23º)

Figure 3. Differential interferometric phase for the pair March 7 to 8 1996 (B⊥ = 34 m) of descending orbit with the outlines of the Swiss glacier inventory 2000 [17,18] for the area around the Gornergletscher, Findelgletscher and

Allalingletscher (VS). The look direction is indicated by the white arrow (incidence angle is 23º).

Figure 4. As Figure 3 but for the pair March 11 to 12 1997 (B⊥ = 28 m) of ascending orbit. Image size is ~20 km.

Figure 5. Differential interferometric phase for the pair March 7 to 8 1996 (B⊥ = 34 m) of descending orbit using

the DHM25 topographic reference.

Figure 6. As Figure 5 but using the C-Band SRTM topographic reference. Look direction is from the left

(incidence angle is 23º). Image size is ~15 km.

Figure 7. Line-of-sight surface displacement maps of Ghiacciao del Belvedere (Italy) in map geometry: (a) 19950829 / 19950830, B⊥ = 86 m, (b) 19961231 / 19970101, B⊥ = -338 m, (c) 19970311 / 19970312, B⊥ = 27 m, (d) 19971007 /

19971008, B⊥ = -286 m. Look direction (ascending mode) is indicated by the arrow (incidence angle ~23º). The negative sign indicates movement away from the satellite. Image size is ~10 km. Image size is ~6 km.

For Gornergletscher the line-of-sight displacement map in 1 day for 7 and 8 March 1996 was used to estimate the displacement in a horizontal plane assuming that the glacier is flowing approximately along this direction. Figure 8 shows the result with two color scales to enhance the velocity of slow and fast flowing parts of the glacier. As also visible in Figure 3, the upper and lower parts of Gornergletscher are separated and the lower part of Gornergletscher is feed to the south by Grenzgletscher. Over large areas of Grenzgletscher decorrelation is observed because of rapid and incoherent flow. Dual-azimuth SAR interferometry, i.e. the combination of data in ascending and descending mode with the assumption of flowing parallel to the surface of the glacier, is not applicable over the European Alps, because the orientation angle between observations in ascending and descending mode is only 24º and most valley glaciers are only favorably illuminated by one of the two ERS SAR orbits. 4. DISCUSSION AND CONCLUSIONS We reported on an ongoing extensive SAR interferometric analysis in the Swiss Alps aimed to estimate the surface velocity of glaciers using ERS-1/2 SAR Tandem acquisitions. The 2-pass differential SAR interferometric approach (with use of a high precision external DEM) was considered for motion analysis because it was found more straightforward for steep topography and not sensitive to areas of decorellation in

one of the two Tandem pairs that would be necessary for 4-pass interferometry. Surface displacement was estimated in the line-of-sight direction and transformed to 3-dimensional velocity by assuming flowing along the slope of the filtered DEM. Over Ghiacciao del Belvedere ERS SAR interferometry was successfully applied to derive ice surface displacement in winter and summer. Summer measurements show a higher velocity than in winter. Care is therefore necessary when making interpretation of SAR interferometry results from a single day to calculate annual glacier surface velocities, but there is a potential for studying seasonal variations in velocity. The vast potential of ERS-1/2 SAR Tandem acquisitions together with the nearly global coverage of the SRTM DEM makes it possible to study the surface displacement of alpine glaciers in many mountainous regions around the world. 5. ACKNOWLEDGMENTS ERS SAR data courtesy of AO3.178, CP1.2338, CP1.3349 and CP1.3069. DHM25 2003 swisstopo. Work supported by the FP6 EC INTEGRAL project (Contract No. SST3-CT-2003-502845 with Swiss contribution under BBW Nr. 03.0049), by the FP6 EC GALAHAD project (Contract No. 018409), and by the ESA SLAM and TERRAFIRMA projects.

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(a) (b) (c) (d)

Figure 8. Displacement map in 1 day for 7 and 8 March 1996 over Gornergletscher (VS) in SAR geometry shown using two different scales. The line-of-sight displacement component was used to estimate deformation assumed in the horizontal plane. The negative sign indicates movement away from the satellite. Look direction is from the left

(incidence angle is 23º). Image size is ~12 km.

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