Christophe Delacourt

OBSERVATION AND MODELLING OF THE SAINT-ETIENNE-DE-TINEE LANDSLIDE USING SAR INTERFEROMETRY

Abstract Six different interferograms of the “La Clapiere” landslide were derived from ERS-1 SAR images during the period Aug. 20–Sept. 4, 1991. The coherence of the associated images is shown to remain significant over most of the surface of the landslide during the two weeks of the survey. The interferograms are remarkably similar, and indicate steady-state displacements over at least 12 days. The displacement field derived from the interferograms is shown to be characterized by a non-uniform displacement gradient from top-to-bottom and reveals a significantly faster motion of the western part of the landslide. The amplitude of the motion deduced from interferometry is shown to be in good agreement with ground measurements. Finally, SAR interferometry is shown to be able to evidence small-scale instabilities which may not be observed with discrete ground measurements.

Tropospheric corrections of SAR interferograms with strong topography. Application to Etna

The accuracy of spaceborne geodetic techniques, including SAR interferometry, is limited by the time and spatial variation and altitude dependance of the propagation delay of electomagnetic waves in the lower troposphere, particularly in mountainous areas. In this paper, we use a 1D model developed for tropospheric corrections of GPS and DORIS measurements to correct SAR data. The differential tropospheric delay is computed at each pixel of the interferogram from ground temperature, humidity and pressure using two empirical parameters calibrated from several radio-soundings acquired in various latitude and climate conditions. It is shown that with such a model, given the 3300 meters topography of Etna, tropospheric variations can generate up to 4π phase rotations between the top and the bottom of the volcano. In 16 out of the 20 interferograms processed with images acquired between August 1992 and October 1993, correction of the tropospheric effect reduces the number of fringes associated with the 1991–93 eruption from previous estimates. The remaining deformation is consistant with a deforming source located at a depth of 14±1 km. During the second half of the eruption, the subsidence rate at the top of the volcano is roughly stable at 13±3 mm/month. These values are in good agreement with tiltmeter data collected on Etna during the same period and with the estimated volume of erupted material. No significant deformation can be observed during the last month of eruption. Inflation of the volcano seems to resume immediately after the end of the eruption at a rate of 3 mm/month.

Fluvial and lacustrine activity on layered deposits in Melas Chasma, Valles Marineris, Mars

[1] Valley networks on Mars are the most obvious features attesting that different geologic processes and possibly climatic conditions existed in the past. THEMIS images reveal valley networks within Melas Chasma, in Valles Marineris, a Hesperian-age canyon system. The valley networks in Melas Chasma are dense and highly organized, and the heads of the valleys are scattered at different elevations. All these features suggest that the networks were fed by precipitation. The morphological details reveal inner channels on some valley floors, attesting that water flowed within these valleys. On the DEM, the valleys flow into a completely enclosed depression. The edge of this feature follows a MOLA contour line, and the depression shows many sedimentary morphologies suggesting lacustrine environment. These landforms are located on remnants of layered deposits possibly composed of sulfate layers suggesting that fluvial activity could have contributed to the erosion of the layered terrains in Valles Marineris. Collectively, the features in Melas Chasma are a maximum of Hesperian in age. These results suggest that warm, wet environmental conditions on Mars persisted through the Hesperian and were present during the formation of Valles Marineris. The evidence for a paleolake in Melas Chasma attests to adequate environmental conditions for life development through the Hesperian period.

Velocity field of the La Clapière landslide measured by the correlation of aerial and QuickBird satellite images

[1]xa0Two displacement maps of the “La Clapiere” landslide (France) have been derived over two periods of 4 years (1995–1999 and 1999–2003) by correlation of aerial photographs and a QuickBird satellite image. The movement of the landslide ranges from 2.5 m to 20 m per year. Those values have been validated over 13 points monitored by conventional tacheometric measurements. Three areas with significant differences in velocity field have been mapped. Limits of those areas are in good agreement with in situ observations. Velocity maps show the low long term temporal variability of the landslide movement and its spatial variability. The optical correlation method using images derived from various sensors (airborne and spatial) is a promising technique for improving the spatial resolution of velocity field observation of landslides over several years.

Post‐eruptive deformation associated with the 1986–87 and 1989 lava flows of Etna detected by radar interferometry

We analysed 92 interferograms produced using ERS1 SAR images taken on Etna between May 1992 and October 1993. Nineteen show a local subsidence in the eastern flank of the volcano, correlated with the location of the 30 October 1986 – 1 March 1987 and 27 September – 9 October 1989 lava flows. Using fringe unwrapping and data gridding techniques, and assuming that over the sampled time-window, deformation was a linear function of time, we derive a map of along range rate of motion. The correlation between the deformation field and the area covered by recent lavas suggests that compaction of the lava flows continues several years after the eruptions. The area of maximum subsidence (47 mm/yr.) is localised at the narrowing of the 1989 flow, between 1500 and 1700 m a.s.l‥ We observe that subsidence extends outside the lava flows, accounting for at least 12 mm/yr. Assuming a relaxation process of the substrate in response to loading produced by recent lavas, a simple 1D Maxwell visco-elastic model predicts a maximum subsidence rate of 25 mm/yr and a relaxation time of about 3.5 years. The relaxation time agrees with those derived from post-eruptive displacements observed by levelling on Etna and Piton de la Fournaise after several eruptions. We conclude that at the time of our measurements, 25 to 50% of the deformation was related to relaxation of the substrate and the other part due to compaction of the lava flows.

Nine years of spatial and temporal evolution of the La Valette landslide observed by SAR interferometry

Abstract The La Valette landslide located in the Ubaye valley (southern French Alps) has been investigated using 15 differential interferograms realized from ERS-1 and ERS-2 satellite radar images acquired between 1991 and 1999, both in 3-day cycle and TANDEM phase. Displacement values of the landslide have been deduced from the Synthetic Apenture Radan (SAR) interferometric products interpretation and compared with ground laser measurements. Four domains characterized by different velocity fields have been detected. Three of them can be distinguished from aerial photographs and field analysis. The slow velocity of a resistant bar located in the eastern side of the landslide has been detected on SAR interferograms. Between 1991 and 1996, changes in the landslide limits have been observed both in the upper and in the lower part; the changes have been caused by a retrogression of the main scarp and a down slope progression of the main body of the landslide, respectively. The average daily velocity of the landslide between 1991 and 1999 derived by interferometric analysis decreased from 1 to 0.4 cm/day, in agreement with ground-based measurements. A peak velocity of around 2 cm/day was observed in 1996.

Seventeen years of the ''La Clapiere'' landslide evolution analysed from ortho-rectified aerial photographs

We present a new method of digital elevation model (DEM) calculation from aerial photographs and apply this method to the La Clapiere landslide in the Mercantour Massif in eastern France. 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. 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, the velocity field is are relatively homogeneous, although their directions are locally heterogeneous. Scree slopes and scarps develop in an heterogeneous way within the 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 Igliere bar. D 2002 Elsevier Science B.V. All rights reserved.

The 1995 Grevena (northern Greece) Earthquake: Fault model constrained with tectonic observations and SAR interferometry

After the 1995 Grevena Ms=6.6 event in northern Greece, we mapped the earthquake fault break in detail. The surface break is small (8–12 km long, 4 cm slip) compared to the moment release of the event. However, the morphologic and tectonic study of the active faults, in the field and using the SPOT satellite imagery, suggests that the earthquake ruptured part of a much larger fault system including interconnecting segments. We used SAR interferometry of the satellite ERS-1 imagery to characterize the coseismic displacement field. This shows a kidney-shaped zone of subsidence reaching 30 cm flanked by an uplift zone reaching 5 cm. We reproduce this field using dislocations in an elastic half-space and our observations of the fault system. This requires 1 m slip from 4 to 15 km depth on a main normal fault segment dipping NNW. Our preliminary model includes significant NE-dipping scissors faulting at the eastern end of the rupture, clearly seen in the interferograms.

Application of a Terrestrial Laser Scanner (TLS) to the study of the Séchilienne landslide (Isère, France).

The active Sechilienne landslide (Isere, France) has been continuously monitored by tacheometry, radar and extensometry devices for 25 years. Indeed, if the 3 mil. m3 of rocks in the active zone named ―Ruines‖ fell down, the debris would dam the Romanche valley. The breaking of the dam by overtopping and rapid erosion would bring a catastrophic flood and other dramatic consequences throughout the valley. Given the rockfall hazard in the most active zone, it is impossible to use targets in this area: Only reflectorless remote sensing techniques can provide information. A time-series of seven Terrestrial Laser Scanner (TLS) point clouds acquired between 2004 and 2007 enable us to monitor the 3D displacements of the whole scanned area, although point coverage is not homogeneous. From this sequential monitoring, the volume of registered collapses can be deduced and the landslide movement along the main geological structures can be inferred. From monitoring associated subsidence and toppling observed on TLS data, it can be deduced that blocks rearrangements are linked to structural settings and that the Sechilienne landslide is complex. To conclude, TLS point clouds enable an accurate monitoring of the evolution of the inaccessible Ruines area and, proven its ability to provide reliable kinematic information, even in areas where on-site instrumentation is infeasible.

Assessing the Accuracy of High Resolution Digital Surface Models Computed by PhotoScan ® and MicMac ® in Sub-Optimal Survey Conditions

For monitoring purposes and in the context of geomorphological research, Unmanned Aerial Vehicles (UAV) appear to be a promising solution to provide multi-temporal Digital Surface Models (DSMs) and orthophotographs. There are a variety of photogrammetric software tools available for UAV-based data. The objective of this study is to investigate the level of accuracy that can be achieved using two of these software tools: Agisoft PhotoScan ® Pro and an open-source alternative, IGN