D31 Chronological sequence of interferograms and/or aerial images for two landslides – UCBL

Potential and limitation of ERS Differential SAR Interferometry for landslide analysis in the French Alps and Pyrenees has been estimated. Geometrical constraints are a little stronger in the Alps than in the Pyrenees. By combining both ascending and descending ERS orbit, we show that around 5% of the total area cannot be imaged and only 50% can be imaged on both orbits. As most of the areas are below the tree line, state surface variation between acquisitions lead to dramatically loss of coherence. Only images acquired with high temporal repetitivity (1 day or 3 days) are suitable for landslide studies. This study has been presented and published in the framework of the fringe meeting (See Delacourt, C., Allemand, P., Squarzoni, C., Picard, F., Raucoules, D., and C. Carnec)

Then two landslides have been studied in details. La Valette Landslide has been studied using DINSAR and "La Clapière" has been studied using the correlation of aerial and QuickBird images.

“La Valette”, placed in the southern French Alps has been observed using 15 differential interferograms produced from SAR images acquired by ERS 1 and ERS 2 satellites between 1991 and 1999 (Fig 1). Velocity maps of “La Valette” landslide have been established. Four domains characterized by their own velocity field have been detected. Three of them can be distinguished from aerial photographs and field analysis. Slow velocity of a resistive bar located near the top of the landslide has been detected on SAR interferograms. Between 1991 and 1996, evolution of the limits of the landslide has been observed both in the upper and in the lower part, by a back erosion corresponding with the main scarp and a progress of the main body of the landslide. The average velocity of the landslide between 1991 and 1999 decreased from 1cm/day to 0.2cm/day in agreement with ground based measurements. This work has been published in Engineering. Geology (See Squarzoni, C.; Delacourt, C.; and P. Allemand)

In order to study "La Clapiere landslide" from multitemporal aerial photographs, a new method of DEM calculation from aerial photographs has been developed and applied. The method uses new techniques of image correlation and of restitution of camera parameters. The calculated DEMs are used to ortho-rectify three aerial photographs acquired between 1983 and 1999. Quantitative measurements, made from these ortho-rectified images, show that the average velocity of the landslide was of 1.7 cm/day, with the rate of the headscarp retreat of 4.1 cm/day, and the toe of the landslide advancing at 1.7 cm/day, between 1983 and 1999. Within the landslide, velocities of internal movements are relatively homogeneous, although their directions are heterogeneous. This observation is completed by the observation of an heterogeneous development of scree slopes and scarps within the landslide, that shows the governing of the tectonic structures on the movements in this landslide. Because the headscarp moves faster than the front of the landslide, material spreads laterally at the scree slopes, and/or is eroded at the front of the landslide by the river, and/or is accumulated within the landslide, on a low slope area located above a competent bar called Iglière bar. This work has been published in Engineering. Geology (See Casson, B.; Delacourt, C.; Baratoux, D.; and P. Allemand)

Two displacement maps of the "La Clapière" Landslide (France) have been derived over two periods of 4 years (1995-1999 and 1999-2003) by correlation of aerial photographs and QuickBird satellite image. Planimetric motion of the landslide is ranging between 2.5 m and 20 m per year. Those values have been validated over 20 points monitored by conventional tacheometric measurements. Three areas with significant differences in velocity field have been mapped. Those areas are in good agreement with in situ observations. Velocity maps obtained by a correlation technique show the low temporal variability of the landslide movement and its high spatial variability. The optical correlation method using images derived from various sensors (airborne and spatial) is a promising technique for increasing the accuracy of velocity field observation of landslides over several years.

This work is under review in Geophysical Research Letter (See Delacourt, C., Vadon, H., Casson, B., and P. Allemand).

Fig. 1: Chronological sequence of interferograms on "La Valette Landslide"

Fig. 2a: Chronological sequence of interferograms on "La Clapiere Landslide"

Fig. 2b: Chronological sequence of interferograms on "La Clapiere Landslide"

Chronological sequence of orthorectified aerial images on "La Clapiere Landslide"