Induced polarization applied to landslides. Part 2: Anatomy and water content tomography of a mudflow

Abstract

Induced polarization is a geophysical method that can be applied to determine the water content and CEC (Cation Exchange Capacity) of sediments and rocks. We apply here this technique to image the Harmalière landslide in Isère (East of France). This landslide includes a mudflow of glacio-lacustrine silty clays overlying the Toarcian clayrock formation with local occurrences of compact pebbles from a paleochannel of the Drac river. A petrophysical study is used to characterize the properties of the four main lithofacies occurring in the area with a total of 22 samples. We performed complex conductivity measurements in the frequency range 10 mHz-45 kHz at different salinities (NaCl brines and in situ pore waters). We also measured the porosity and CEC of the samples. We calibrate the relationships between surface conductivity, quadrature conductivity and normalized chargeability with both the porosity and CEC. The relationship between the formation factor and the porosity conforms to Archie's law with a cementation exponent close to 2.0 ± 0.2. In the field, we performed a time-domain induced polarization survey using a 1.26 km long cable (including roll along of the electrodes) with an electrode spacing of 20 m. The landslide is imaged down to a depth of 220 m. The inversion of the data (788 electrodes, 15.7 km of profiling, 13 012 apparent resistivity and 5539 apparent chargeability data) is done with the least-square technique penalizing the roughness of the tomograms using Occam inversion. The resulting 3-D electrical conductivity and normalized chargeability tomograms are analysed in conjunction with the petrophysical data to image the extension of the lithofacies at the field scale. Furthermore, the water content and CEC of the formations are imaged. We demonstrate that the compacted pebbles of the Drac paleochannel form both a mechanical and hydraulic barrier that is locally breached by the mudflow before entering the Monteynard-Avignonet Lake. This study demonstrates the ability of induced polarization to finely characterize the anatomy of such landslide and image its water content.