Dense 3D electrical resistivity tomography to understand complex deep landslide structures

Large scale slope instabilities are complex objects controlled by multiple parameters. The underground and superficial structure of the slope plays a major role as it often controls water circulations, potentially causing weathering and damaging processes, and permits the local storage of water masses, causing temporary overload. In addition, the structure of the subsurface often delineates rock-volumes with variable mechanical properties, whose spatial distribution greatly influences the behavior of the slope. This work illustrates how Dense 3D Electrical Resistivity Tomography can provide relevant constraints on these parameters.

The village of Viella, in France (Hautes-Pyrénées), is affected by strong slope movement since 2018, when a massive rockslide above the village modified the stress conditions of the entire slope and, potentially, the hydrogeological context. As a consequence, some houses and infrastructures are progressively damaged, leading to heavy measures (houses evacuation). This complex, deep-seated (> 80 m), slope instability covers an area of ca. 650 000 m², is primarily composed of altered shists, colluviums, and non-consolidated alluvial deposits, forming several kinematic units with surface velocities in the range [0.5 – 5] mm.month^-1.

 

A 3D dense electrical resistivity tomography was realized using the FullWaver system, to characterize the structure and the forcing factors of this unstable slope. 55 V-FullWavers receivers (3 -electrodes, 2 channels sensors) were quasi-evenly distributed over a surface area of 400 x 500 m² with an interval of 90 m, apart from the village area, where no electrode could be grounded. Each V-FullWaver recorded signals through two orthogonal dipoles of 25 m length. Current injections were realized with a high-power transmitter (6 kW, 16 A, 3000 V). 235 injection dipoles were used. The system injected current between a fixed remote electrode (more than 1 km away from the site to increase the investigation depth) and a local mobile electrode, moved all over the investigated area in between the V-Fullwaver receivers, with an interval of approximately 40 m, except in the village area.

 

The resulting 3D resistivity model presents a high spatial variability until 100 to 150 m depth approximately, that highly relates to the complex strain dynamics of the slope and the hydrogeological observations. It highlights the relation between the most active kinematic compartments and the large-scale structure of the slope.

It provides a first understanding of the role of local compacted rocks in the buildup of surface deformation but also on the localization of heterogeneities (fissures, scarps) which may relate to water circulation paths.

. This 3D image of the slope is the first structural reference model for future hydrogeological and geomechanical studies aiming at deducing the possible evolution of the slope.