– Scientific coordinator : Stéphane Garambois (ISTerre, UGA)

Introduction

The mechanisms at the origin of huge landslides and the external factors modifying their internal dynamics are various and complex, although it is widely acknowledged that earthquakes and rainfall represent the main triggers. Séchilienne is an emblematic rocky landslide which still gathers prickly questions raised for 25 years after administrative, scientific and political managements. This landslide is heavily monitored both for surveillance purposes (surface displacements) and for scientific purposes within the OMIV national observatory (hydrology, seismology, rainfall). Despite this monitoring, 2D geophysical measurements and the drilling of deep boreholes and galleries (Meric et al., 2005), there is still a need of multi-physics 3D imaging of the internal deformation and fluid content, in particular to confirm the presence of a deep aquifer, which may control the landslide dynamics (Cappa et al., 2014, Fig. 2). In addition, the derivation of a 3D seismic velocity model will strongly help the micro-earthquake localization, which is performed within the OMIV observatory.

Objectives/Methodology

The deployment of the multi-physics array on a 1 km2 area will allow us to characterize in 3D the mechanical and hydrological properties from different attributes (seismic velocity, attenuation, resistivity and induced polarization) using active sources, down to a depth of 250 m. These data will be complemented by other available geophysical techniques, notably the NMR method, highly sensitive to water content and diffusivity of the rocks. These combined attributes will be quantitatively interpreted in terms of deformation and water content. The one-month-long dense monitoring will help identifying active fractures as well as calibrating the micro-seism localizations obtained by the low-resolution permanent OMIV seismic network. Any changes over the period of monitoring will be detected and interpret in terms of hydrological changes, with a particular focus on the main aquifer whose variations are still unknown.

Expected results

Expected results are a 3D map of geophysical attributes and particularly (i) the depth and spatial geometry of the deep aquifer and its monitoring, (ii) fine localization of seismic internal activity, and (iii) calibration of the seismic permanent network. This will open new perspectives in landslide monitoring for surveillance purposes.

Updated on 15 octobre 2020