Geophysical Approach
Research Studies by T. Lebourg and B. Fresia, Geoazur, UNS, Nice, France.

Thursday 10 December 2009 by Thomas Lebourg

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In order to better constrain the observations achieved on the site, several electrical tomography surveys were carried out on the landslide area. The electrical tomography principle fits well to the description of landslides and to the study of hydraulic subsurface circulations. More precisely this approach does not enable to locate accurately interfaces and formations but define the geometry and power with a metric resolution.

A first electrical tomography survey (4 profiles dipole-dipole and pole-dipole), performed by the research laboratory Geoazur, allowed to put forward a structural variation corresponding to the landslide area between 10 to 15 m depth, which matches the results of the inclinometry study.

Profiles performed at the first electrical tomography survey

The inverted data show an upper bed characterized by low resistivities corresponding to clay sands and a lower bed more resistant corresponding to the underlying limestone. Moreover the study reveals the existence of fault zones at the bottom of the landslide that are pointed out by low resistivity strips in the resistant limestone bed, homogeneous in the other profiles.

A second survey, coupled to field mapping, thus was conducted in order to confirm the previous results (powers and interfaces of formations) and to constrain faults in depth, not well defined during the first survey. We used the dipole-dipole (dd), pole-dipole (pd) and pole-pole (pp) methods for each of the profiles. The advantage to work with the three methods is to be able to change the depth scale during the data processing despite the loss of accuracy on the pp and pd profiles.

The profiles give the following informations and interpolations:

  • the geological horizons are discretized: a resistant mass (superior to 60 Ω m) associated to limestone, a conductive mass on the surface (inferior to 30 Ω m) related to the mass mobilized by the landslide and another mass not very resistant in depth representing the Cretaceous marlstone,
  • the rupture surface is situated at a depth of around 15 to 20 m in the middle of the landslide and goes up on the edges to 5 to 10 m depths, which validates the inclinometric data of the study achieved by Simecsol,
  • the landslide is edged on the left side by a draining major fault which sets off the limestone bed, thicker outside the landslide,
  • the same outline is retrieved on the opposite side where a similar fault edges the eastern part of the slide,
  • other irregularities divide up the limestone mass by setting the beds a few meters off,
  • a resistant zone is found at the top in the middle of the landslide. The resistant block can be seen on all profiles and seems to constitute a more adequate bed (Eocene sandstone limestone) in concordance over the sands. For that matter we can notice that no significative movement has been mentioned concerning this part of the landslide.