In France, in the context of regulatory mapping (PPRNs –Prevention Plans of Natural Risk based on French regulatory rules), landslide hazard assessment follows an empirical approach and uses basic available data. Therefore, the results are closely linked to the quality of the expertise and divergent opinions may arise in some specific cases. In recent years, numerical approaches using GIS, the availability of new databases, the development of new acquisition tools in the field and web visualization services have improved the knowledge of phenomena and different landslide-prone areas. Numerical approaches using GIS, that allow the transparency and traceability of results, have various levels of complexity and require different quantities of input data. However, they are often neglected by experts and new data and tools are not currently used to develop regulatory mapping documents. Numerous scientific examples show that these numerical approaches, web services and new tools can be a significant help in improving knowledge and provide a credible alternative to the expert approach, even in a regulatory context such as a PPRN. Thus, through this synthesis carried out as part of the regulatory mapping of landslide hazards and risks in France, a state-of-the-art spatial assessment of current landslide hazards is performed. The new tools and newly available databases to support this type of analysis are then described. Finally, the perspectives and limitations of alternative approaches and new tools and data are discussed, leading to some considerations for the improvement of the current method of producing landslide hazard maps for PPRNs in France.

Improvement of landslide hazard assessments for regulatory zoning in France: STATE–OF–THE-ART perspectives and considerations

SUMMARY The growth of the geothermal industry sector requires innovative methods to reduce explo- ration costs whilst minimizing uncertainty during subsurface exploration. Until now geoelec- trical prospection had to trade between logistically complex cabled technologies reaching a few hundreds meters deep versus shallow-reaching prospecting methods commonly used in hydro-geophysical studies. We present a recent technology for geoelectrical prospection, and show how geoelectrical methods may allow the investigation of medium-enthalpy geothermal resources until about 1 km depth. The use of the new acquisition system, which is made of a distributed set of independent electrical potential recorders, enabled us to tackle logistics and noise data issues typical of urbanized areas. We acquired a 4.5-km-long 2-D geoelectrical survey in an industrial area to investigate the subsurface structure of a sedimentary sequence that was the target of a ∼700 m geothermal exploration well (Geo-01, Satigny) in the Greater Geneva Basin, Western Switzerland. To show the reliability of this new method we compared the acquired resistivity data against reflection seismic and gravimetric data and well logs. The processed resistivity model is consistent with the interpretation of the active-seismic data and density variations computed from the inversion of the residual Bouguer anomaly. The combination of the resistivity and gravity models suggest the presence of a low resistivity and low density body crossing Mesozoic geological units up to Palaeogene–Neogene units that can be used for medium-enthalpy geothermal exploitation. Our work points out how new geoelectrical methods may be used to identify thermal groundwater at depth. This new cost- efficient technology may become an effective and reliable exploration method for the imaging of shallow geothermal resources.

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Deep electrical resistivity tomography for the prospection of low- to medium-enthalpy geothermal resources

3 D Deep Electrical Resistivity Tomography of the Major Basin Related to the 2016 M w 6.5 Central Italy Earthquake Fault

Glaciations Quaternaires dans les Alpes Franco-Suisses et leur piedmont, Excursion 099C, première journée: vallée de l'Aar entre Berne et le lac de Thoune

3D Deep Electrical Resistivity Tomography of the Lusi Eruption Site in East Java

\n This work presents a 3-D resistivity model of the Séchilienne unstable slope acquired with a network of portable resistivimeters in summer 2017. The instrumentation consisted in distributed measuring systems (IRIS Instruments FullWaver) to measure the spatial variations of electrical potential. 23 V-FullWaver receivers with two 50 m dipoles have been deployed over an area of circa 2 km2; the current was injected between a fixed remote electrode and a mobile electrode grounded successively at 30 locations. The data uncertainty has been evaluated in relation to the accuracy of electrodes positioning. The software package BERT (Boundless Electrical Resistivity Tomography) is used to invert the apparent resistivity and model the complex data set providing the first 3-D resistivity model of the slope. Stability tests and synthetic tests are realized to assess the interpretability of the inverted models. The 3-D resistivity model is interpreted up to a depth of 500 m; it allows identifying resistive and conductive anomalies related to the main geological and hydrogeological structures shaping the slope. The high fracturation of the rock in the most active zone of the landslide appears as a resistive anomaly where the highest resistivity values are located close to the faults. A major drain formed by a fault in the unaltered micaschist is identified through the discharge of a perched aquifer along the conductive zone producing an important conductive anomaly contrasting with the unaltered micaschist.

Structure of the Séchilienne unstable slope from large-scale three-dimensional electrical tomography using a Resistivity Distributed Automated System (R-DAS)

T. Garin

Papers

Structure of the Séchilienne unstable slope from large-scale three-dimensional electrical tomography using a Resistivity Distributed Automated System (R-DAS)