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[Observatoire Gravitaire Géoazur] Déplacement de surface
Déplacement de surface

- Use of the simultaneous seismic, GPS and meteorological monitoring for the characterization of a large unstable mountain slope in the southern French Alps
S. Gaffet, Y. Guglielmi, F. Cappa, C. Pambrun, T. Monfret and D. Amitrano

Abstract
The presence of unstable rock slopes in the mountainous regions makes these areas particularly prone to the development of sudden dramatic events, which may cause damage and injuries. The efficient disaster management requires a good understanding of the main causes of these mass movements, such as, increased fluid pressure and seismic shaking. In order to improve our understanding of the mechanics and dynamics of the mountain slope instabilities, we developed a monitoring system based on microseismic, geodetic and meteorological measurements. The system operated in the La Clapière rockslide in the southern French Alps. It was composed of four three-component seismological stations, four Global Positioning Systems (GPS) and three meteorological stations deployed together for a period of 4 months. This system was installed over the territory of ∼1.5 km² of the fractured gneissic terrain that moves at a mean velocity of about 70 cm yr–1. This passive measurement method may represent an interesting approach to the characterization of the landslides difficult to access, since it allows the use of the lighter instrumentation and easier processing tools. Our investigations were based on the H/V method to image shear wave contrasts that we correlated with slip surfaces. The analysis of the seismograms allowed us to point out a correlation between the thickness of the surface layers and the measured resonance frequencies. The results indicated that the H/V ratios are heterogeneous in the rockslide. Comparison between seismic and GPS data proved our monitoring approach to be promising for landslides characterization.

Keywords : Spatial Analysis ; Geomechanics ; Seismicity and tectonics ; Surface waves and free oscillations ; Site effects
Geophysical Journal International, Volume 182, Issue 3, September 2010, Pages 1395-1410, doi:10.1111/j.1365-246X.2010.04683.x


- Control of slope deformations in high seismic area : Results from the Gulf of Corinth observatory site (Greece)
T. Lebourg, S. El Bedoui and M. Hernandez

Abstract
The northern coast of the Peloponnesus (Greece) is characterized by high seismic activity related to the Gulf of Corinth opening with an extension rate of 16 mm y− 1. Studies presented in this paper focus on the characterization of links between tectonic and slope deformations on the Panagopoula slope, located on the southern coast. The approach is centred on qualitative and quantitative data acquisition based on geological and geomorphological investigations, geophysical imagery by electrical resistivity tomography and slope displacement monitoring.

Firstly, we highlight two different types of slope deformation on Panagopoula : a superficial landslide affecting weathered limestone, and a large-scale deformation without global failure expressed in the field. Tectonic features play a major role in these two dynamic processes, taking into account the strong geometrical link between the inherited fractures and gravitational scarps mapped in the field. Secondly, the displacements survey network, distributed on both sides of a significant fault crossing the slope, allows the quantification of slope displacements underlying two components : (i) a gravitational sliding (N010) along the slope, and (ii) a supposed tectonic component (N240).

Keywords : Gulf of Corinth ; Tectonic ; Landslide ; Slope deformation ; Electrical resistivity tomography
Engineering Geology, Volume 108, Issues 3-4, 8 October 2009, Pages 295-303, doi:10.1016/j.enggeo.2009.04.004


- Deep-seated failure propagation in a fractured rock slope over 10,000 years : The La Clapière slope, the south-eastern French Alps
S. El Bedoui, Y. Guglielmi, T. Lebourg and J.L. Pérez

Abstract
The “La Clapière” area (Tinée valley, Alpes Maritimes, France) is a typical large, complex, unstable rock slope affected by Deep Seated Gravitational Slope Deformations (DGSD) with tension cracks, scarps, and a 60 × 106 m3 rock slide at the slope foot that is currently active. The slope surface displacements since 10 ka were estimated from 10Be ages of slope gravitational features and from morpho-structural analyses. It appears that tensile cracks with a strike perpendicular to the main orientation of the slope were first triggered by the gravitational reactivation of pre-existing tectonic faults in the slope. A progressive shearing of the cracks then occurred until the failure of a large rock mass at the foot of the slope. By comparing apertures, variations and changes in direction between cracks of different ages, three phases of slope surface displacement were identified : 1) an initial slow slope deformation, spreading from the foot to the top, characterized by an average displacement rate of 4 mm yr− 1, from 10–5.6 ka BP ; 2) an increase in the average displacement rate from 13 to 30 mm yr− 1 from the foot to the middle of the slope, until 3.6 ka BP ; and 3) development of a large failure at the foot of the slope with fast displacement rates exceeding 80 mm yr− 1 for the last 50 years. The main finding of this study is that such a large fractured slope destabilization had a very slow displacement rate for thousands of years but was followed by a recent acceleration. The results obtained agree with several previous studies, indicating that in-situ monitoring of creep of a fractured rock slope may be useful for predicting the time and place of a rapid failure.

Keywords : Progressive failure ; Surface displacements ; La Clapière ; DGSD
Geomorphology, Volume 105, Issues 3-4, 15 April 2009, Pages 232-238, doi:10.1016/j.geomorph.2008.09.025


- Remote-sensing techniques for analysing landslide kinematics : a review
C. Delacourt, P. Allemand, E. Berthier, D. Raucoules, B. Casson, P. Grandjean, C. Pambrun and E. Varel

Abstract
Surface displacement field of landslides is a key parameter to access to their geometries and mechanical properties. Surface displacements can be calculated using remote-sensing methods such as interferometry for radar data and image correlation for optical data. These methods have been elaborated this last decade and successfully applied on sensors (radar, cameras, terrestrial 3D laser scanner imaging) either attached to space or aerial platforms such as satellites, planes, and unmanned radio-controlled platforms (drones and helicopters) or settled at fixed positions emplaced in the front of landslides. This paper reviews the techniques of image analysis (interferometry and optical data correlation) to measure displacements and examines the performance of each type of platforms. Examples of applications of these techniques in French South Alps are shown. Depending on the landslide characteristics (exposure conditions, size, velocity) as well as the goal of the study (operational or scientific purpose), one or a combination of several techniques and data (characterized by several resolution, accuracy, covered surface, revisiting time) have to be used. Radar satellite data processed with differential interferometric or PS methods are mainly suitable for scientific purposes due to various application limitations in mountainous area. Optical satellite and aerial images can be used for scientific studies at fairly high resolution but are strongly dependant on atmospheric conditions. Platforms and sensors such as drone, fixed camera, fixed radar and Lidar have the advantage of high adaptability. They can be used to obtain very high resolution and precise 3D data (of centimetric accuracy) suitable for both scientific and operational purposes.

Keywords : Remote sensing ; Landslide ; Surface displacement ; InSAR ; Image correlation
Bulletin de la Société Géologique de France, Volume 178, Issue 2, 1 March 2007, Pages 89-100, doi:10.2113/gssgfbull.178.2.89