1 INTRODUCTION French Alps are affected by numerous active land- slides, particularly in clay-shale deposits (Malet, 2003). These unstable areas are characterized either by movements occurring along discrete shearing sur- face (Brunsden & Ibsen, 1993; Hungr et al., 2001) or by continuous deformations depending on local con- straints, slope gradient and subsurface material properties (Baum et al., 1998). They are generally composed of heterogeneous clay-rich clastic materi- als, nearly saturated in water during the wet seasons. The objective of this work is to propose an origi- nal method for characterizing the geometry of sub- surface clay-rich materials susceptible to be eroded by analyzing and combining several geophysical pa- rameters by the mean of the Fuzzy Set theory. Non- intrusive geophysical methods such as seismic re- fraction and electrical resistivity imaging are well adapted for studying the mechanical behavior of landslides. The combined interpretations of seismic travel times and electrical apparent resitivities can reveal the mechanical and hydrological properties of a terrain sub-surface. Three physicals quantities are here considered: the acoustic P-wave velocity (Vp) providing information on the fissuration state of the soil matrix; the electrical resistivity (ρ) giving indi- cations on the water saturation of the porous media and S-wave velocity (Vs) providing information on soil stiffness when Spectral analysis of Surface Waves (SASW) is used. According to previous studies (Reynolds et al., 2002; Cutlac, 2005; Grandjean et al., 2007; Godio et al., 2003) a multi-method approach is proposed to increase the level of information. Grandjean and al. (2006) have studied the correlation between Vp and ρ on the active Super-Sauze mudslide. This work showed that a cross-plot between inverted Vp and ρ values help distinguishing between several soil char- acteristics. Referring to field observations, Grand- jean and al. (2007) demonstrated also that one type of behavior was related to the amount of soil poros- Geophysical data fusion applied to the characterization of the La Valette landslide G. Grandjean, C. Hibert & A. Bitri BRGM, Planning service and natural hazards, Orléans, France J. Travelletti & J.-P. Malet CNRS UMR 7516, School and Observatory of Earth Sciences, University of Strasbourg, Strasbourg, France ABSTRACT: Geophysical methods such as seismic surveying or electrical resistivity imaging appear to be well-adapted to investigate landslide’s structures and understand related mechanisms. These methods allow direct and non-intrusive measurements of acoustic (P), shear (S) wave velocity and electrical resistivity, three physical parameters considered as essential to estimate mechanical properties of reworked moving materials. Both of these methods were applied on the La Valette landslide, in the French South Alps, where a typical ex- ample of intra-material mudslide can be observed. Measurements were taken simultaneously along 2 profiles of 400 m and 300 m in length, respectively perpendicularly to and along the axis of the mudslide. The P and S-wave velocity fields, as well as the electrical resistivity field, were inverted from recorded data according to suitable algorithms. P and S-wave velocity images as well as resistivity tomographies are presented as results and discussed in term of reliability. Preliminary interpreted results show a correlation between the seismic ve- locities and electrical resistivity data, confirming that the simultaneous use of both methods gives comple- mentary information on geomechanical behaviour of the landslide. Seismic data provide information on fis- sure density variations and the presence of shear-bent material, whereas the electrical resistivity ones provide information on water content variations. In order to go deeper into petrophysical interpretation, a data fusion strategy based on fuzzy subsets theory is developed and applied to the geophysical dataset. The resulting cross-sections show the possibility of geomechanical hypotheses to be realized in specific areas of the tomo- graphic cross-section, highlighting the places where sediment mobilization could occur. -119-