Electrical resistivity monitoring of an earthslide with electrodes located outside the unstable zone (Pont-Bourquin landslide, Swiss Alps)

Electrical Restivity Tomography (ERT) is one of the most employed geophysical technique to monitor landslide evolution. The measured variations of resistivity can be related to changes in underground moisture, porosity, water chemistry, etc. With electrodes installed on the moving mass, resistivity variations may also be related to changes in electrode location relative to each other (the so-called geometric factor K). As such, ERT monitoring should also require the monitoring of electrode location. Wilkinson et al. (2010, 2015) were able to track movements of electrodes by measuring variations of resistivity. However, this approach needs the strong assumption that resistivity variations are caused by changes in the geometric factor without any underground change. For example, Gance et al. (2015) showed the significant effect of surface fissures on ERT measurements.

In this work we tested ERT monitoring of an earthslide (the Pont-Bourquin Landslide in the Swiss Alps) with electrodes located immediately outside the unstable zone. The setup was composed of 36 electrodes (24 on the right bank and 12 on the left bank) acquiring 1654 measurements per day in a dipole-dipole configuration (half direct and half reciprocal measurements). 235 daily sequences were acquired between February and November 2015. Data were filtered and then processed with the BERT package (Günther et al., 2006). Several time-lapse approaches were tested with different starting models originating from the 3D inversion of 4, 2D profiles and the results were analyzed in terms of resistivity and sensitivity variations. The resulting 3D models were then split in distinct zones (transport and accumulation zones) and the ERT time-series were then correlated with environmental time-series (e.g. rainfall).

Results indicate that, despite a lack of sensitivity in the unstable zone because of the monitoring set-up, ERT is sensitive to environmental variations but no distinct behaviour could be observed within the zones. However, correlations provide informations in agreement with passive seismic monitoring (Bièvre et al., 2018) and suggest that resistivity (along with shear wave velocity) is strongly affected by rainfall with an effect that does not last more than 2 to 3 days. These results confirm that the superficial layers (first metres) have a major influence on resistivity measurements. More generally these results, along with many published works, question the added value of ERT to monitor landslides for depths greater than the superficial phreatic water table.

References

Bièvre G et al. (2018) Eng. Geol. 245, 248 - 257. doi:10.1016/j.enggeo.2018.08.01

Gance J et al. (2015) Geophy. J. Int. 200, 1118-1135. doi:10.1093/gji/ggu453

Günther T et al. (2006) Geophy. J. Int. 166, 506-517. doi:10.1111/j.1365-246X.2006.03011.x

Wilkinson P B et al. (2010) Geophy. J. Int. 183, 543-556. doi:10.1111/j.1365-246X.2010.04760.x

Wilkinson P B et al. (2015) Geophy. J. Int. 200, 1566-1581. doi:10.1093/gji/ggu483