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Julien Travelletti, School and Observatory of Earth Sciences (France)
Taha-Hocine Debieche, EMMAH (France)
Emilie Garrel, EMMAH (France)
Gilles Grandjean, BRGM (France)
Francis Matthieu, BRGM (France)
Jean Philippe Malet, School and Observatory of Earth Sciences (France)
Julien Ponton, School and Observatory of Earth Sciences (France)
Vincent Allegre, School and Observatory of Earth Sciences (France)
Stephane Garambois, Laboratory of Internal Geophysics and Tectonophysics (France)
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Geophysical methods, such as seismic surveying or electrical resistivity imaging, allow direct and non-invasive measurements of acoustic (P) waves velocity and electrical resistivity, two physical parameters considered as essential to investigate landslide mechanisms. In order to better understand the factors controlling water flows on clay-shale landslides, two controlled rainfall experiments were performed during several days on plots of about 120 m2 (7 14 m) on two landslides on the French South Alps: the Super-Sauze mudslide, and the Laval landslide. Water enriched in bromide and chloride has been infiltrated on the plot, and the flows were monitored in continuous by several water level sensors in piezometers (from 1 m to 4 m deep), water content sensors, distributed shallow soil temperature measurements, displacements measurements, water quality characterization, and geophysical measurements (performed every 3 hours). The geophysical acquisition works involved 48 channels seismic equipment featured by 40 Hz geophones and a handy-hammer source. Electrical equipment was constituted by 48 electrodes designed according to a Wenner-Schlumberger configuration.
The objective of this work is to present the analysis of the geophysical measurements and to correlate the changes in geophysical parameters to changes in hydrological parameters. These results can be considered in two complementary objectives. Firstly, the structure and the mechanical state of the landslides can be described by using seismic first-breaks tomography. The distribution of P-wave velocities along a 2D cross-section shows with a relatively good resolution the topography of the bedrock (high velocity values) in depth. Second, no traveltime differences induced by the increasing saturation of the sites were observed, indicating that seismic wave propagation cannot be used alone to monitor water flows at the site scale. Inversion of resisitivity data in a time-slice approach shows a time decrease of resisitivity in the rainfall area. Globally, the variations are up to 20 - 30 Ohm.m and can be well correlated with water level and soil moisture changes.
The seismic and electrical experiments carried out in the Super-Sauze mudslide have shown potentialities for studying both structural and water circulation aspects. Further works will be done to complete these interpretations with geomorphological observations and displacement measurements to determine the consequences of this input of water on the mudslide behaviour. This analysis may finally contribute to the identification of geophysical thresholds for forecasting the behaviour of the slides, and to the development of robust geophysical warning systems.
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