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SinceMay 2018,Mayotte Island has been experiencing seismo-volcanic activities that could trigger submarine landslides and, in turn, tsunamis. To address these hazards, we use the HySEA numerical model to simulate granular flow dynamics and the Boussinesq FUNWAVE-TVD numerical model to simulate wave propagation and subsequent inundations. We investigate 8 landslide scenarios (volumes from 11.25£106 m3 to 800£106 m3). The scenario posing the greatest threat involves destabilization on the eastern side ofMayotte’s lagoon at a shallowdepth and can generate sea-surface deformations of up to 2 m.We show that the barrier reef surroundingMayotte plays a prominent role in controlling water-wave propagation and in protecting the island. The tsunami travel time to the coast is very short (a few minutes) and the tsunami is not necessarily preceded by a sea withdrawal. Our simulation results provide a key to establishing hazard maps and evacuation plans and improving early-warning systems, Depuis mai 2018, l’île deMayotte connaît une activité sismo-volcanique importante susceptible de déclencher des glissements de terrain sous-marins générant des tsunamis. Pour faire face à ces aléas, nous utilisons deux modèles numériques complémentaires : le modèle HySEA (simulant la dynamique des écoulements granulaires) et le modèle Boussinesq FUNWAVE-TVD (simulant la propagation des vagues et les inondations) pour étudier 8 scénarios de glissements sous-marins potentiels (volumes de 11,25£106 m3 à 800£106 m3). Les scénarios ayant le plus d’impact se situent à proximité de Petite Terre et à faible profondeur. Ils peuvent générer une élévation de la surface de la mer jusqu’à 2 m en zone habitée à Petite Terre. Nous montrons que la barrière de corail entourant Mayotte joue un rôle prépondérant dans le contrôle de la propagation des vagues et dans la protection de l’île. Le temps de trajet du tsunami jusqu’à la côte est très court (quelques minutes) et le tsunami n’est pas nécessairement précédé d’un retrait maritime. De telles observations sont essentielles pour construire des cartes d’aléas précises et des plans d’évacuation afin d’aider la population

Several studies have shown that global changes have important impacts in mountainous areas, since they affect natural hazards induced by hydrometeorological events such as landslides. The present study evaluates, through an innovative method, the influence of both vegetation cover and climate change on landslide hazards in a Pyrenean valley from the present to 2100. We first focused on assessing future land use and land cover changes through the construction of four prospective socioeconomic scenarios and their projection to 2040 and 2100. Secondly, climate change parameters were used to extract the water saturation of the uppermost layers, according to two greenhouse gas emission scenarios. The impacts of land cover and climate change based on these scenarios were then used to modulate the hydromechanical model to compute the factor of safety (FoS) and the hazard levels over the considered area. The results demonstrate the influence of land cover on slope stability through the presence and type of forest. The resulting changes are statistically significant but small and dependent on future land cover linked to the socioeconomic scenarios. In particular, a reduction in human activity results in an increase in slope stability; in contrast, an increase in anthropic activity leads to an opposite evolution in the region, with some reduction in slope stability. Climate change may also have a significant impact in some areas because of the increase in the soil water content; the results indicate a reduction in the FoS in a large part of the study area, depending on the landslide type considered. Therefore, even if future forest growth leads to slope stabilization, the evolution of the groundwater conditions will lead to destabilization. The increasing rate of areas prone to landslides is higher for the shallow landslide type than for the deep landslide type. Interestingly, the evolution of extreme events is related to the frequency of the highest water filling ratio. The results indicate that the occurrences of landslide hazards in the near future (2021–2050 period, scenario RCP8.5) and far future (2071–2100 period, scenario RCP8.5) are expected to increase by factors of 1.5 and 4, respectively.

Several studies have shown that global changes have important impacts in mountainous areas, since they affect natural hazards induced by hydro-meteorological events such as landslides. To estimate the capacity of mountainous valleys to cope with landslide hazard under global change (climate change as well as climate- and human-induced land use change), it is necessary to evaluate the evolution of the different components that define this type of hazard: topography, geology and geotechnics, hydrogeology and land cover. The present study evaluates, through an innovative methodology, the influence of both vegetation cover and climate change on landslide hazard in a Pyrenean valley from the present to 2100. Once the invariant features of the studied area, such as geology and topography, were set, we first focused on assessing future land use changes through the construction of four prospective socioeconomic scenarios and their projection to 2040 and 2100. These inputs were then used to spatially model land use and land cover (LUCC) information to produce multi-temporal LUCC maps. Then, climate change inputs were used to extract the water saturation of the uppermost layers, according to two greenhouse gas emissions scenarios. The impacts of land use and climate change based on these scenarios were then used to modulate the hydro-mechanical model to compute the factor of safety (FoS) and the hazard levels over the considered area. The results demonstrate the influence of land use on slope stability through the presence and type of forest. The resulting changes are significant despite being small and dependent on future land use linked to the socioeconomic scenarios. In particular, a reduction in human activity results in an increase in slope stability; in contrast, an increase in anthropic activity leads to an opposite evolution in the region, with some reduction in slope stability. Climate change may also have a significant impact in some areas because of the increase in the soil water content; the results indicate a reduction in the FoS in a large part of the study area, depending on the landslide typology considered. Therefore, even if future forest growth leads to slope stabilization, the evolution of the groundwater conditions will lead to destabilization. These changes are not uniform over the area and are particularly significant under the most extreme climate scenario, RCP 8.5. Compared to the current period, the size of the area that is prone to deep landslides is higher in the future than the area prone to small landslides (both rotational and translational). On the other hand, the increase rate of areas prone to landslides is higher for the small landslide typology than for the deep landslide typology. Interestingly, the evolution of extreme events is related to the frequency of the highest water filling ratio. The results indicate that the occurrences of landslide hazards in the near future (2021–2050 period, scenario RCP 8.5) and far future (2071–2100 period, scenario RCP 8.5) are expected to increase by factors of 1.5 and 4, respectively.

A key point of landslide hazard assessment is the estimation of their runout. Empirical relations linking angle of reach to volume can be used relatively easily, but they are generally associated with large uncertainties as they do not consider the topographic specificity of a given study site. On the contrary, numerical simulations provide more detailed results on the deposits morphology, but their rheological parameters can be difficult to constrain. Simulating all possible values can be time consuming and incompatible with operational requirements of rapid estimations. We propose and compare three operational methods to derive scaling power laws relating the landslide travel distance to the destabilized volume. The first one relies only on empirical relations, the second one on numerical simulations with back-analysis, and the third one combines both approaches. Their efficiency is tested on three case studies: the Samperre cliff collapses in Martinique, Lesser Antilles (0.5 to 4&times, 106 m3), the Frank Slide rock avalanche (36&times, 106 m3) and the Samperre cliff collapses in Martinique, Lesser Antilles (0.5 to 4&times, 106 m3) the Fei Tsui debris slide in Hong Kong (0.014&times, 106 m3). Purely numerical estimations yield the smallest uncertainty, but the uncertainty on rheological parameters is difficult to quantify. Combining numerical and empirical approaches allows to reduce the uncertainty of estimation by up to 50%, in comparison to purely empirical estimations. However, it may also induces a bias in the estimation, though observations always lie in the 95% prediction intervals. We also show that empirical estimations fail to model properly the dependence between volume and travel distance, particularly for small landslides (&lt, 20,000 &lt, 0.02&times, 106 m3).

International audience; 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

International audience

International audience; Mapping subsurface clay minerals is an important issue because they have particular behaviors in terms of mechanics and hydrology that directly affects assets laid at the surface such as buildings, houses, etc. They have a direct impact in ground stability due to their swelling capacities, constraining infiltration processes during flooding, especially when moisture is important. So detecting and characterizing clay mineral in soils serve urban planning issues and improve the risk reduction by predicting impacts of subsidence on houses and infrastruc-tures. High-resolution clay maps are thus needed with accurate indications on mineral species and abundances. Clay minerals, known as phyllosilicates, are divided in three main species: smectite, illite, and kaolinite. The smectite group highly contributes to the swelling behavior of soils, and because geotechnical soil analyses are expensive and time-consuming, it is urgent to develop new approaches for mapping clays' spatial distribution by using new technologies, e.g., ground spectrometer or remote hyperspectral cameras [0.4-2.5 μm]. These technics constitute efficient alternatives to conventional methods. We present in this chapter some recent results we got for characterizing clay species and their abundances from spectrometry, used either from a ground spectrometer or from hyperspectral cameras.

International audience; The investigation and monitoring of shallow hazards due to the presence of underground cavities remain a challenge for geophysical approaches. Thus, seismic surface waves have been tested in several recent research projects in order to detect and localize voids as well as to determine their geometries. Among these works, numerous numerical studies have proved the feasibility of Rayleigh waves to detect cavities. However, most imagery processes adapted to R waves are faced with difficulties when applying them to real data. This limitation points to a major problem: the interactions between Rayleigh waves and a cavity are complex, particularly in the case of dispersing and attenuating surrounding media. Here, a combined approach based on numerical and experimental data obtained in a reduced-scale measurement bench is conducted to better understand the seismic wave propagation phenomena involved in the presence of a cavity and define robust observables that can be used in field measurements. The observables bearing the cavity signature are studied qualitatively and quantitatively on numerical and experimental recordings. The latter take into account all the propagation phenomena involved. The observations are carried out on the vertical and horizontal component of the Rayleigh wave displacement. The selected observables are studied depending on non-dimensional cavity's parameters versus the frequency, that is the wavelength-to-size ratio and the wavelength-to-depth ratio. The effects of the cavity's parameters on the observables show particularities as a function of these components, such as a higher rate of the amplitude on the horizontal component as well as a perturbation of the direct seismic surface wave amplitude above the cavity, also higher on the horizontal component. This latter feature is particularly visible on the variation of the elliptical particle motion recorded at the surface. It can be linked to the mode conversions that occur in the vicinity of the cavity and which predominate on the horizontal component when the signal is normalized.

World Landslide Forum (4º. 2017. Liubliana, Eslovenia), Exposure to hazards is expected to increase in Europe, due to rapid population growth in urban areas and the escalation of urbanization throughout many countries. In the framework of the European Geological Surveys (EGS), the Earth Observation and Geohazards Expert Group (EOEG) has carried out a survey based enquiry regarding the integration of geohazards (earthquakes, volcanoes, landslides, ground subsidence, floods and others) into urban and land-use planning. Responses from 19 European countries and 5 regions reveal heterogeneous policies across national borders. 17% of the countries have not yet implemented any legal measures to integrate geohazards into urban and land-use plans and half of the participating countries have no official methodological guides to construct geohazard maps. Additionally, there is a scarce knowledge about real social impacts of geohazards and resulting disasters in many of the countries, although they have a significant impact on their national economies. This overview stresses the need for a common legislative framework and homogenization of the national legislations as well as mutual guidelines which adopt the principles applicable to the management of geohazards and explain the process to be followed in the production of hazard documentation. This is especially relevant in case of landslide and subsidence hazards; although those are of great importance in Europe, there are no common guidelines and practices similar to Directive 2007/60/EC on the assessment and management of flood risk. Based on their expertise, EuroGeoSurveys (EGS) have the potential to coordinate this activity in European geohazard guidelines and to promote the interaction among stakeholders., Geohazards InSAR Laboratory and Modeling Group, Instituto Geológico y Minero de España, España, Bureau de Recherches Géologiques et Minières, Francia, Engineering Geology Department, Institute of Geology and Mineral Exploration, Grecia, GeoHazard team, Geological Institute of Romania, Rumanía, Geological Survey of Slovenia, Eslovenia, Croatian Geological Survey, Croacia, Geological Survey of Italy, Italia, Swiss Federal Office for the Environment, Suiza, Geological Survey of Austria, Austria, Polish Geological Institute, National Research Institute, Polonia, Geological Survey of Ireland, Irlanda, Czech Geological Survey, República Checa, Federal Institute for Geosciences and Natural Resources, Alemania, Geological Survey of Norway, Noruega, Cyprus Geological Survey, Chipre, Geological Survey of Sweden, Suecia, Institut Cartogràfic i Geològic de Catalunya, España, British Geological Survey, Reino Unido, Geological Survey of Slovakia, Eslovaquia, Geological Survey of Lithuania, Lituania, Federalni Zavod Za Geologiju, Nigeria, Geological Survey of Estonia, Estonia, Laboratorio Nacional de Energia e Minas, Portugal, Geological Survey of the Republic of Srpska, Bosnia y Herzegovina

International audience; The ANR-SAMCO project aims to develop a proactive resilience framework enhancing the overall resilience of societies on the impacts of mountain risks. The project aims to elaborate methodological tools to characterize and measure ecosystem and societal resilience from an operative perspective on three mountain representative case studies. To achieve this objective, the methodology is split in several points: (1) the definition of the potential impacts of global environmental changes (climate system, ecosystem e.g. land use, socioeconomic system) on landslide hazards, (2) the analysis of these consequences in terms of vulnerability (e.g. changes in the location and characteristics of the impacted areas and level of their perturbation) and (3) the implementation of a methodology for quantitatively investigating and mapping indicators of mountain slope vulnerability exposed to several hazard types, and the development of a GIS-based demonstration platform available on the web. The strength and originality of the SAMCO project lies in the combination of different techniques, methodologies and models (multi-hazard assessment, risk evolution in time, vulnerability functional analysis, and governance strategies) that are implemented in a user-oriented web-platform, currently in development. We present the first results of this development task, architecture and functions of the web-tools, the case studies database showing the multi-hazard maps and the stakes at risks. Risk assessment over several area of interest in Alpine or Pyrenean valleys are still in progress, but the first analyses are presented for current and future periods for which climate change and land-use (economical, geographical and social aspects) scenarios are taken into account. This tool, dedicated to stakeholders, should be finally used to evaluate resilience of mountainous regions since multiple scenarios can be tested and compared.

International audience; Seismic data have been used to catalog the location and volume of most of the rockfalls that occurred at the Piton de la Fournaise volcano from May 2007, just after the major collapse of the Dolomieu summit crater floor, to May 2011. This catalog made it possible to compare the evolution of the number and volume of rockfalls at a high temporal resolution and to investigate their links with eruptions, seismicity, deformations and rainfalls affecting the Piton de Ia Fournaise volcano. Results show that the purge of unstable areas created by the Dolomieu crater floor collapse occurred in two phases: a first phase, lasting three months, during which the intense rockfall activity immediately following the collapse decreased abruptly and a second phase, lasting more than two years, during which the daily volume of the rockfalls slowly decreased before reaching a steady state. A detailed study of 4 time periods, including 3 eruptive cycles, indicates that strong seismicity can increase the number of rockfalls. Furthermore, when a dike reaches the surface at the summit of the central cone, giving birth to an eruption, the associated local forcing can in some cases increase the volume of rockfalls, possibly by creating or expanding weak zones. In most cases, this is not observed for dike intrusions that do not reach the surface. The newly created weak zones are most often far from the eruptive fissures of the ongoing eruption, but close to the location of the next eruption. This suggests that these distant zones are also weakened at depth, thus creating preferred paths for future dikes propagating toward the surface. The impact of rainfall on rockfall activity was also studied, given that La Reunion Island is subject to very intense rainfall events. Our results show that rainfall can in some cases trigger rockfalls rapidly, with response times of less than a day, but not systematically, and no triggering rainfall threshold was found.

Short-wave infrared (SWIR: 1100–2500 nm) reflectance spectra of soil samples, acquired under laboratory-controlled conditions, were used to estimate the swelling potentials of the soils and to use SWIR spectroscopy to improve a part of the swelling-risk map of France. A total of 332 samples were collected to the west of Orléans (France) in various geological formations and swelling-risk areas and along two (eastern and western) transects with different samples spacings. Comparisons between the swelling potentials of the soils and the swelling-risk areas of the map exhibit good correlation in the south of the sampling area; however, there are several inconsistencies in the north of the study area that highlight the necessity of locally redrawing the accepted swelling-risk map of France. The sampling interval (approximately 260 m) along the eastern transect was too sparse and does not appear to have effectively captured the smallest and/or isolated lithologies. The sampling interval along the western transect (approximately 50 m) revealed the presence of an unmapped swelling-potential zone, which was confirmed by several soil samples. The sample interval along the western transect appears to be more suitable for mapping the smallest lithologies. The presence of several samples in close proximity that exhibit the same swelling potential is a robust indication of the presence of a zone with constant swelling potential. A new swelling-risk map of the sampling area was produced based on the soil samples. The map produced by interpolation did not permit the representation of discrete lithological units, introduced spurious swelling-risk zones that however could be representative of tillage in agricultural zones. More samples are therefore needed to produce a reliable map on the scale of the sampling area. According to swelling potential uncertainty related to soil sampling and soil treatment, spectroscopy-based approach proposed here cannot be used to replace the existing swelling-risk map of France. This method permits however the rapid and low-cost estimation of the swell potentials of a large number of samples, which could be used at regional-scale to target areas where doubt remains or where infrastructure damages attributed to swelling behavior are known. At local scale, soil samples need to be properly and laboratory treated to accurately produce local revised and detailed swelling-risk maps.

International audience; When soil is affected by a heat source, some of its properties are modified, and in particular, the electrical resistivity due to changes in water content. As a result, these changes affect the thermal properties of soil, i.e., its thermal conductivity and diffusivity. We experimentally examine the changes in electrical resistivity and thermal conductivity for four soils with different grain size distributions and clay content over a wide range of temperatures, from 20 to 100 °C. This temperature range corresponds to the thermal conditions in the vicinity of a buried high voltage cable or a geothermal system. Experiments were conducted at the field scale, at a geothermal test facility, and in the laboratory using geophysical devices and probing systems. The results show that the electrical resistivity decreases and the thermal conductivity increases with temperature up to a critical temperature depending on soil types. At this critical temperature, the air volume in the pore space increases with temperature, and the resulting electrical resistivity also increases. For higher temperatures , the thermal conductivity increases sharply with temperature up to a second temperature limit. Beyond it, the thermal conductivity drops drastically. This limit corresponds to the temperature at which most of the water evaporates from the soil pore space. Once the evaporation is completed, the thermal conductivity stabilizes. To explain these experimental results, we modeled the electrical resistivity variations with temperature and water content in the temperature range 20-100°C, showing that two critical temperatures influence the main processes occurring during heating at temperatures below 100 °C.

International audience; Planning sustainable soil exploitation and land resource evaluation require up-to-date and accurate maps of soil properties. In that respect, geophysical techniques present particular interests given their non-invasiveness and their fast data acquisition capacity, which permit to characterize large areas with fine spatial and/or temporal resolutions. We investigated the relevancy of combining data from airborne hyperspectral (Hs), electromagnetic induction (EMI) and far-field ground-penetrating radar (GPR) for mapping soil properties, in particular soil clay content, at the field scale. Data from the three techniques were acquired at a test site in Mugello (Italy) characterized by relatively strong spatial variations of soil texture. Soil samples were collected for determining ground truth clay content. For the frequencies used in this study (200–650 MHz), the GPR surface reflection is mainly determined by soil dielectric permittivity, itself primarily influenced by soil moisture. In contrast, EMI is mostly sensitive to soil electrical conductivity, which integrates several soil properties including in particular soil moisture and clay content. Taking advantage of the complementary information provided by the two instruments, the GPR and EMI data were combined and correlated to local ground-truth clay content data to provide high-resolution clay content maps over the entire field area. Besides, a relationship was also observed between Hs data and clay content measurements, which permitted to produce a Hs-derived clay content map. EMI–GPR and Hs maps showed close spatial patterns and a relatively high correlation was observed between both clay content estimates, as well as between clay content estimates and ground-truth clay content measurements. Moreover, data fusion allowed constraining the EMI–GPR and Hs information and reduced the uncertainty of mapped clay content estimates. These results demonstrated great promise for integrated, digital soil mapping applications.

National audience; This work focuses on the use of a combined statistical-mechanical approach to predict changes in landslide displacement rates from observed changes in rainfall amounts. The forecasting tool associates a statistical impulse response (IR) model to simulate the changes in landslide rates by computing a transfer function between the input signal (e.g. rainfall) and the output signal (e.g. displacements) and a simple 1D mechanical (MA) model (e.g. visco-plastic rheology) to take into account changes in pore water pressures. The models have been applied to forecast the displacement rates at the Super-Sauze landslide (South East France), one of the most active and instrumented clayey landslide in the European Alps. Results indicate that the three models are able to reproduce the displacement pattern in the general kinematic regime with very good accuracy (succession of acceleration and deceleration phases); at the contrary, extreme kinematic regimes such as fluidization of part of the landslide mass are not being reproduced: this statement, quantitatively characterised by the Root Mean Square Error between the model and the observations, constitutes however a robust approach to predict changes in displacement rates from rainfall or groundwater time series, several days before it happens. The variability of the results, depending in particular on the fluidization events and on the location of displacement data is discussed.

International audience; This work aims at assessing the impact of local heterogeneities consideration in slope mechanical modelling. We choose to use different geophysical techniques to characterize these heterogeneities, and a fuzzy logic data fusion concept to create a geometrical model from these data. The first mechanical simulation, realized with basic elastic constitutive law, gives a surface displacement pattern in agree-ment with the one measured. This highlights the importance of the model's layer geometry on the results and also the suitability of geophysical imagery tools to constructs high resolution geometrical models.

International audience; Landslide analysis is increasingly based on the combination of multiple information sources at different spatial and temporal resolutions, spatial coverage, accuracy and acquired using various airborne and terrestrial platforms and geomorphological, geophysical, and geotechnical methods. Morphological mapping and morphostructure characterization are preliminary steps to analyze the observed and future landslide distributions. The objective of this work is to propose a morpho-structural model of an active landslide complex on the Normandy coast (north-west France) by combining different data sources. The methodology associates field investigation with interpretation of high-resolution topographic and geophysical images. The proposed morpho-structural model highlights the division of the landslide into several morphological features. This division may explain the spatial variability and temporal variability of the slope dynamics.

International audience; Since the collapse of the Dolomieu crater floor at Piton de la Fournaise Volcano (la Réunion) in 2007, hundreds of seismic signals generated by rockfalls have been recorded daily at the Observatoire Volcanologique du Piton de la Fournaise (OVPF). To study rockfall activity over a long period of time, automated methods are required to process the available continuous seismic records. We present a set of automated methods designed to identify, locate, and estimate the volume of rockfalls from their seismic signals. The method used to automatically discriminate seismic signals generated by rockfalls from other common events recorded at OVPF is based on fuzzy sets and has a success rate of 92%. A kurtosis-based automated picking method makes it possible to precisely pick the onset time and the final time of the rockfall-generated seismic signals. We present methods to determine rockfall locations based on these accurate pickings and a surface-wave propagation model computed for each station using a Fast Marching Method. These methods have successfully located directly observed rockfalls with an accuracy of about 100 m. They also make it possible to compute the seismic energy generated by rockfalls, which is then used to retrieve their volume. The methods developed were applied to a data set of 12,422 rockfalls that occurred over a period extending from the collapse of the Dolomieu crater floor in April 2007 to the end of the UnderVolc project in May 2011 to identify the most hazardous areas of the Piton de la Fournaise volcano summit.

International audience; The prediction of landslide movement acceleration is a complex problem, among others identified for deep-seated landslides, and represents a crucial step for risk assessment. Within the scope of this problem, the objective of this paper is to explore a modelling method that enables the study of landslide function and facilitates displacement predictions based on a limited data set. An inverse modelling approach is proposed for predicting the temporal evolution of landslide movement based on rainfall and displacement velocities. Initially, the hydrogeology of the studied landslides was conceptualised based on correlative analyses. Subsequently, we applied an inverse model with a Gaussian-exponential transfer function to reproduce the displacements. This method was tested on the Grand Ilet (GI) and Mare-à-Poule-d'Eau (HB) landslides on Reunion Island in the Indian Ocean. We show that the behaviour of landslides can be modelled by inverse models with a bimodal transfer function using a Gaussian-exponential impulse response. The cumulative displacements over 7 years of modelling (2 years of calibration period for GI, and 4 years for HB) were reproduced with an RMSE above 0.9. The characteristics of the bimodal transfer function are directly related to the hydrogeological functioning demonstrated by the correlative analyses: the rapid reaction of a landslide can be associated with the effect of a preferential flow path on groundwater level variations. Thus, this study shows that the inverse model using a Gaussian-exponential transfer function is a powerful tool for predicting deep-seated landslide movements and for studying how they function. Beyond modelling displacements, our approach effectively demonstrates its ability to contribute relevant data for conceptualising the sliding mechanisms and hydrogeology of landslides.

Swelling soils contain clay minerals that change volume with water content and cause extensive and expensive damage on infrastructures. Based on spatial distribution of infrastructure damages and existing geological maps, the Bureau de Recherches Géologiques et Minières (BRGM, the French Geological Survey) published in 2010 a 1:50 000 swelling hazard map of France, indexing the territory to low, moderate, or high swell susceptibility. At local scale, characterization of soil properties and identification of clay minerals using conventional soil analysis (DRX, chemical, and geotechnical analysis) are slow, expensive, and does not permit integrated measurements. VNIR (400-1100 nm) and SWIR (1100-2500 nm) spectral domains are characterized by significant spectral absorption bands that provide in France a largely unexploited tool for recognize swelling minerals. Hyperspectral laboratory (i.e. from samples) using an ASD Fieldspec Pro spectrometer provides thus a rapid and less expensive field surface sensing that permits to measure soil spectral properties. The BRGM aims to combine laboratory and remote (i.e. airborne measurement) sensing to apply and optimize hyperspectral reflectance imaging for mapping clay minerals and soil properties, so that local mapping of swelling clays susceptibility could be assessable from an economical point of view.

International audience; Swelling soils contain clay minerals that change volume with water content and cause extensive and expensive damages on infrastructures, including houses. This study aims to combine proximal (i.e. field measurement) and remote (i.e. airborne measurement) sensing to apply and optimize hyperspectral reflectance imaging for mapping clay minerals and soil properties, so that local mapping of swelling and shrinking clays susceptibility could be assessed from an economical point of view. Results and Discussion: Based on spatial distribution of declared damages and existing geological maps, the BRGM published in 2010 a 1:50 000 swelling hazard map of France. This map was drawn using locally incomplete or imprecise geological data that does not permit mapping of smallest and isolated clays lithologies, particularly in places where house building plans are foreseen over high susceptibility areas. At local scale, characterization of soil properties and identification of clay minerals using conventional soil analysis are slow, and expensive. VNIR and SWIR spectral domains are characterized by significant spectral absorption bands that provide a largely unexploited tool for identifying swelling clays. Hyperspectral sensing, used in a proximal way, using an ASD Fieldspec Pro spectrometer, provides a rapid and less expensive field surface sensing. The AGEOTHYP Research project financed by the BRGM, General Council of Loiret (45) and Géotec, which begin early 2013; aims to provide valuable information on the presence of clays over homogeneous and continuous scales. Airborne hyperspectral images will be acquired in the Loiret at the W of Orléans (France) in February 2013. The study area (ca. 280 km²) encompasses cultivated fields, urbanized areas, and forests. The three levels of clays hazard and many infrastructure damages recorded in the national database are also covered. Airborne hyperspectral imagery will contain 380 channels ranging from 400 to 2500 nm with a 1 m spatial resolution (VNIR) and 2 m spatial resolution (SWIR). Soil sampling will be used for calibration and validation. Soil samples will be taken on different land covers and on the three levels of expansive clays hazard. The reflectance measurements will be acquired on the field with the sun as the light source and in laboratory with a quartz halogen light source. The study area will be covered by standard laboratory analyses on soil samples in order to determine their mineralogy and swelling potential. Conclusion: All the geospatial data collected during the project will be integrated in a GIS project. Spatial data analysis will be carried out on the study area and the final high resolution map will be compared with the national swelling hazard map. Novelty Statement: This project is to use different types of data (geotechnical databases, hyperspectral field, and airborne measurements) that are integrated into an operational method to detect and quantify swelling potential of soils. Summary: The AGEOTHYP Research project plans to combine hyperspectral proximal and remote sensing with geotechnical engineering to identify and map swelling and shrinking clays on a study area covering an area of approximately 280 km² located to the west of Orléans (France). Hyperspectral images will cover cultivated areas, urbanized areas, forest, grassland, and Loire River. The analysis of the Hyperspectral images currently provides a largely unexploited tool for recognize swelling minerals. Therefore, we could expect various applications on other domains like agriculture, forestry, water...

International audience; Bedrock depth provides important information for many applications, such as groundwater modelling, the estimation of soil water availability and crop production potential. Direct estimates of bedrock depth from destructive soil observations are discontinuous and too expensive to be used in large areas. Geophysical methods are often cited as possible alternatives; however, their ability to provide reliable estimations of bedrock depth relies greatly on local site characteristics. Therefore, this study examines the ability of Electrical Resistivity Tomographie (ERT) in dry conditions, diachronic ERT and the Spectral Analysis of Surface Waves (SASW) method to determine bedrock depth (BD) in different geopedological contexts that are representatives of the Mediterranean landscape (Southern France). SASW was performed using the data that were measured in the field with classical seismic equipment (impulse source and geophones) along a transect in each study site. In the same place, transects of ERT (Wenner-Schlumberger array, 1-m electrode spaced) were measured under wet and dry conditions. To calibrate the geophysical measurements, 131 boreholes (from two to 5 m deep) were interpreted to determine the bedrock depth. Dry ERT, diachronic ERT and SASW were found to have highly variable performances for the estimation of bedrock depth in all geopedological contexts. SASW correctly estimated the bedrock depth (RMSE=0.3-0.7 m) in all situations except in the case of shallow soils. Conversely, dry ERT only estimated bedrock depth (RMSE=0.2 m) in the case of a high contrast of the resistivity between the soil and bedrock. By analysing the pattern of the water uptake by grapevine, diachronic ERT determined the bedrock depth in the case of very low contrasts in resistivity (RMSE=0.5). According to a prior pedological knowledge such soil maps, the combination with the use of a sensor based on different physical parameters may enhance the bedrock detection.

International audience; In this study, we demonstrate that cross-correlation gathers of multi-channel and multi-shot configurations provide accurate estimations of shear wave velocity (Vs) perturbations from Rayleigh wave data for the reconstruction of two-dimensional (2D), high-resolution velocity distributions without requiring the systematic calculation of surface wave dispersion, as in the spectral analysis of surface waves (SASW). Data acquisition for cross-correlation analysis is similar to that for a 2D seismic common midpoint reflection survey. The data processing involved is similar to the coda wave interferometry used for seismological data but differs in the sense that the cross-correlation of the original waveform is calculated for active source seismic data. Data processing in cross-correlation analysis consists of the following three steps: First, cross-correlations are calculated for every trace in each shot gather with the same offset trace as a reference shot to flatten the linearly sloping events of surface waves. A common receiver location stack section can then be obtained, which allows for the assessment of lateral variations in the elastic properties of the medium. Second, the maxima of the time-shifted cross-correlation gathers and the maxima of the shot gathers' envelope traces are picked for different frequencies, and trace values having the same receiver location are averaged to calculate Vs perturbations, as described in the theory of coda wave interferometry. Finally, a 2D Vs profile is reconstructed by applying the lateral Vs perturbation to a homogeneous Vs velocity profile obtained by inversion of the surface wave dispersion of the reference shot gather. Analyses of waveform data from numerical modelling and field observations indicate that this new method is valid and greatly improves the accuracy and resolution of near-surface imagery using surface waves and reconstructed subsurface velocity distributions compared with a conventional SASW or multi-channel application.

International audience; Knowledge about soil properties and their variation in space and time is a key challenge for understanding processes in the vadose zone. As we consider more complex models, applied to larger geographical regions, the demand for information on key soil parameters and thus input variables becomes harder to meet. In particular, classical soil maps and soil sampling has limitations in resolution, and information might be subjective. In the face of the new challenges in the context of climate and global change and the increasing need of multidisciplinarity, a more sophisticated use of soil information beyond the "classical" soil information is needed. Predictive soil mapping (Scull et al., 2003) or digital soil function mapping (Ugbemuna and Reuter, 2013) are promising ways to enhance the information content of soil maps. The application of pedometrical approaches, e. g., digital soil mapping (DSM) as one focus of pedometrics, provides the potential to overcome these issues. Today, DSM is well established in soil science, providing tools for improving, advancing, and increasing a more objective collection and spatial analysis of soil data. A broad variety of tools helping to improve the quality of diagnoses, to refine their resolution by using correlations between soil properties, sensor data, and environmental covariates is available and intensively discussed in the soil science community (Adamchuk and Viscarra Rossel, 2010; McBratney et al., 2003; Behrens and Scholten, 2006). However, Carre et al. (2007) asked the DSM community to be much more aware of end-user requirements and coined the term digital soil assessment for the quantitative modeling of soil attributes for assessing soil threats and soil functions. In addition, Grunwald (2009) identified a need for more sophisticated technologies to measure soil properties. Finke (2012) indicated the potential of integration of sensors in making maps and providing model input. Concerning soil moisture, Vereecken et al. (2008) described the need to develop strategies that combine hydrogeophysical measurement techniques with remote sensing methods and, moreover, the development of novel upscaling methods for predicting effective moisture fluxes and disaggregation schemes

International audience; Characterizing the internal structure of landslides is of first importance to assess the hazard. Many geophysical techniques have been used in the recent years to image these structures, and among them is seismic tomography. The objective of this work is to present a high resolution seismic inversion algorithm of first arrival times that minimizes the use of subjective regularization operators. A Quasi-Newton P-wave tomography inversion algorithm has been developed. It is based on a finite frequency assumption for highly heterogeneous media which considers an objective inversion regularization (based on the wave propagation principle) and uses the entire source frequency spectrum to improve the tomography resolution. The Fresnel wavepaths calculated for different source frequencies are used to retropropagate the traveltime residuals, assuming that in highly heterogeneous media, the first arrivals are only affected by velocity anomalies present in the first Fresnel zone. The performance of the algorithm is first evaluated on a synthetic dataset, and further applied on a real dataset acquired at the Super-Sauze landslide which is characterized by a complex bedrock geometry, a layering of different materials and important changes in soil porosity (e.g. surface fissures). The seismic P-wave velocity and the wave attenuation are calculated, and the two tomographies are compared to previous studies on the site.

International audience; The geometry of the bedrock, internal layers and shear surfaces/bands controls the deformation pattern and the mechanisms of landslides. A challenge to progress in the forecast of landslide acceleration in terms of early-warning is therefore to characterize the 3D geometry of the unstable mass at a high level of spatial resolution, both in the horizontal and vertical directions, by integrating information from different surveying techniques. For such characterization, seismic investigations are potentially of a great interest. In the case of complex structures, the measure and the processing of seismic data need to be performed in 3D. The objective of this work is to present the development of a 3D extension of a seismic refraction traveltime tomography technique based on a Simultaneous Iterative Reconstruction Technique (SIRT). First the processing algorithm is detailed and its performance is discussed, and second an application to the La Valette complex landslide is presented. Inversion of first-arrival traveltimes produces a 3D tomogram that underlines the presence of many areas characterized by low P-wave velocity of 500-1800 m.s−1. These low P-wave velocity structures result from the presence of reworked blocks, surficial cracks and in-depth fracture zones. These structures seem to extend to around 25 m in depth over a 80×130 m area. Based on borehole geotechnical data and previous geophysical investigations, an interface corresponding to an internal slip surface can be suspected near the isovalue of 1200 m.s−1 at a depth of −10 to −15 m. The stable substratum is characterized by higher values of P-wave velocity of 1800-3000 m.s−1. The features identified in the 3D tomogram allow to better (1) delineate the boundary between the landslide and the surrounding stable slopes, and (2) understand the morphological structures within the landslide at a hectometric scale. The integration of the 3D seismic tomography interpretation to previous geophysical acquisitions using a geostatistical approach allows to construct a 3D geometrical model of the middle and lower part of the La Valette landslide, and to estimate the volume of the unstable mass.

International audience; Many studies show that certain geophysical methods, such as seismic and electrical-resistivity imaging, appear to be well adapted for investigating the internal structures of landslides and understanding the related hydro-mechanical mechanisms. These are methods that allow the direct and non-intrusive measurement of acoustic (P) and shear (S) wave velocities and electrical resistivity (ρ), which are three physical parameters considered as essential for estimating the mechanical properties of moving reworked material. We applied these techniques to the La Valette landslide (Southern French Alps), a typical example of an intra-material landslide, carrying out measurements simultaneously along two profiles, 400 m and 300 m long and respectively perpendicular to and along the slide direction. We then used suitable inversion algorithms to estimate both the P- and S-wave velocity fields and the electrical resistivity field from the recorded data. The results, aided by field surface observations, show that a correlation exists between the state of the material and the seismic-velocity and/or electrical-resistivity data, thus confirming that the simultaneous use of the two methods provides complementary information on the geomechanical behavior of the landslide. More particularly, the seismic data provide information on fissure density variations and the presence of shear-bent material, whereas the electrical resistivity data provide information on the groundwater content. To enable a more integrated petrophysical interpretation, we applied a data-fusion strategy based on fuzzy subsets to the geophysical datasets. Through combining the tomograms we identified a surface layer of soft material along the two profiles; the bottom of this layer was also recognized in a borehole. From a methodological point of view, the results show the applicability of adopting geomechanical hypotheses as inputs of geophysical data fusion for identifying areas where sediment mobilization could occur.

International audience; This work presents an attempt to monitor water infiltration and subsurface flow within a clay-shale landslide using time-lapse electrical resistivity tomography (ERT). A rainfall experiment was carried out on a plot of 100 m2 at the Laval landslide in the Draix experimental catchments (ORE Draix, South French Alps) in order to characterize the spatial and temporal development of water circulation in the soil and to identify when steady-state flow conditions are reached. The experiment was conducted during 67 h with initial unsaturated conditions in the slope. The apparent electrical resistivity values were inverted with a time-lapse approach using several cross models. The results indicate a significant decrease in resistivity ( 18%) compared to the initial state in the rain plot. Downslope progression of negative resistivity anomalies is imaged suggesting that vertical and subsurface lateral flows have developed. About 21 h after the start of the rain experiment, a constant level of resistivity values is observed indicating that the hydrological system reached steady-state flow conditions. This observation is consistent with ground water level observations and chemical tracer analysis. Computed differences in time of steady-state conditions highlight possible preferential flows near the landslide toe. A hydrological concept of functioning of the slope is proposed, and apparent saturated hydraulic conductivity (Ks of 1Ð7 ð 10 4 m ð s 1) is computed from the steady-state times. This study demonstrates the potentiality of ERT monitoring to monitor water infiltration in clay-shale slopes and the high water transfer capacity of reworked clay-shale material.

International audience; This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: a b s t r a c t Bedrock depth provides important information for many environmental and agricultural applications, such as shallow groundwater monitoring, the determination of soil water availability, and the estimation of crop production potential. Direct estimates of bedrock depth from destructive soil observations are discon-tinuous and too expensive to be used in large areas. Geophysical methods are often cited as possible alternatives. However, their ability to provide reliable estimations of bedrock depth is known to depend greatly on local site characteristics. Therefore, combining geophysical methods based on different physical parameters may help to provide better predictions. This study examines the ability of the Spectral Analysis of Surface Waves (SASW) method combined with the classical high resolution Electrical Resistivity Tomography (ERT) method to predict soil depths in a 500 m ranged Mediterranean hillslope (Southern France) with increasing soil depths along the slope. SASW was performed using the data measured in the field with classical seismic equipment (impulse source and geophones distributed along a line). In the same place, eight transects of ERT (Wenner-Schlumberger array, 1 m electrode spaced) were measured under wet and dry conditions. To calibrate the geophysical measurements, 81 boreholes (from two to 5 m deep) were interpreted to determine the bedrock depth, which was defined as the occurrence in the depth of heterogeneous marine Miocene loose sandstone with centimetric laminations. ERT and SASW were found to have highly variable performances for predicting separately the bedrock depth along the hillslope. SASW correctly predicted the bedrock depth in the lower part of the hillslope, whereas the data from ERT were disrupted by shallow permanent groundwater. Conversely, ERT correctly predicted bedrock depth within the upper part of the hillslope, whereas a high variability of SASW data near the topsoil caused difficulties for bedrock depth prediction. From these results, it was possible to define an estimator of bedrock depth according to the presence of shallow groundwater, which varies along the slope, such that more importance is given to ERT estimates in the upper part of the hillslope and more importance is given to SASW in the lower part. This study shows the usefulness of such a sensor combination to estimate soil properties when the uncertainties of making predictions vary according to the geophysical methods.

4 pages; International audience; The aim of this work is to develop a methodology for integrating climate change scenarios into quantitative hazard assessment and especially their precipitation component. The effects of climate change will be different depending on both the location of the site and the type of landslide considered. Indeed, mass movements can be triggered by different factors. This paper describes a methodology to address this issue and shows an application on an alpine test site. The proposed approach is based on coupling a hydrological model (GARDENIA®) with a slope stability model (ALICE®), estimating safety factors spatially. From a DEM, land-cover map, geology, geotechnical data and so forth the program classifies hazard zones depending on geotechnics and different hydrological contexts varying in time. The methodology is applied to the Ubaye valley (France) using present and past climate conditions.

International audience; The Lesser Antilles have very high chemical weathering rates, with values that can reach 1290 t/km(2)/a. The tropical environment induces high precipitation rates, high temperature, dense vegetation, with sharp relief and thick soils. Because of volcanic activity, frequent pyroclastic flows produce very erodible and porous materials. In addition, agriculture induces important land use changes which replace existing native forest cover with banana and sugar cane plantations. Their surface can cover as much as 40% of the total area of a river basin. The aim of this study is to identify key parameters, either natural or anthropogenic, that control chemical weathering rates. Among the combined impact of all parameters (climate, runoff, slopes, vegetation etc.), basin age seems to be the control parameter: the younger the basin, the higher the weathering rate. A correlation between the chemical weathering rate and the basin age suggests that young volcanic rocks are more easily weathered than old ones: young fresh material is easily mobilized by erosion, while for older rocks with thick soil covers, chemical rates are much lower. A combined effect between the higher erodibility and a higher climate erosivity of the younger relief could be observed. Moreover, a correlation between banana plantations and the chemical weathering rates that can be explained by an increase of infiltration, due to stem flow processes is shown here. Banana plantations also have a correlation with the basin age, older basins being more favorable terrains for cultivation.

International audience; [1] The seismic signals of hundreds of rockfalls within Dolomieu crater, Piton de la Fournaise volcano, Réunion Island, have been analyzed to investigate a possible link between physical rockfall-generating processes and associated seismic signal features. Moreover, indirect observation of rockfalls via the seismic signals they generate can provide useful data for studying volcanoes and the temporal variations of their structure. An increase in the number of rockfall events and their volumes might be an indicator of structural weakness and deformation of the volcano associated with potential eruptive activity. The study focuses on a 10 month period following the 6 April 2007 crater floor collapse within Dolomieu crater, from May 2007 to February 2008. For granular flows a scaling law is revealed between seismic energy and signal duration. A semiempirical approach based on both analytical analysis and numerical simulation of these flows shows that a similar scaling law exists between the difference of potential energy computed for an event and its propagation times and also emphasizes the effect of local topography on this scaling law. Simulated and observed data were compared to evaluate the proportion of potential energy dissipated in the form of seismic waves and confirm a direct link between the seismic energy and potential energy of a given granular flow. The mean ratio of seismic to potential energy is of the order of 10 À4 , comparable to the range of values observed in previous studies. A simple method based on these ratios is proposed to estimate the volumes of rockfalls from their seismic signal. Observed seismic energy and the frequency of rockfalls decreased at the beginning of the studied period and reached a stable level in July, thus suggesting a postcollapse relaxation time of Dolomieu crater structure of about 2 months from seismic signal analysis, which is confirmed by deformation data. The total rockfall volume over the study period is estimated to be 1.85 Mm 3 .

International audience; From numerous studies, geophysical methods based on seismic surveying appear to be well-adapted to investigate the morpho-structure of landslides and to progress in understanding the related mechanisms. Indeed, these methods allow direct and non-intrusive measurements of acoustic (Vp) or shear (Vs) wave velocity, two important physical parameters for estimating mechanical properties of reworked moving materials. Different processing techniques and inversion strategies were applied on the La Valette and Super-Sauze mudslides (French South Alps) as well as on the Ballandaz landslide (Savoie, France) to retrieve these parameters. On each of these sites, measurements were recorded along 2D profiles of several hundred meters length, with sensor spacing from 2 to 5m. (of about few meters). A first approach, based on first breaks acoustic inversion for estimating Vp distribution on the Super-Sauze and La Valette sites was carried out; then, SASW (spectral analysis of surface waves) was performed to image Vs distribution on the same site. In order to produce a more geotechnical diagnosis of these sections, a fuzzy logic fusion was used to assimilate both of these parameters into a highest level of interpretation. This approach has (also) the advantage to take into account the resolution and accuracy of each individual method. Finally, a 2D elastic fullwaveform inversion test was carried out on a synthetic seismic data set computed from a conceptual Super-Sauze velocity model. This test highlights the difficulty to manage highly contrasted media in terms of velocity but also of topography. Nevertheless, the integration in the inversion process of the whole seismic signal produce a more coherent model in terms of Vp and Vs distribution compared to above-cited conventional techniques

The DIGISOIL project (FP7-ENV-2007-1 N°211523) is financed by the European Commission under the 7th Framework Programme for Research and Technological Development, Area “Environment”, Activity 6.3 “Environmental Technologies”. A comité de lecture, avec review; The multidisciplinary DIGISOIL consortium intends to integrate and improve in situ proximal measurement technologies for assessing soil properties and soil degradation indicators, moving from the sensing technologies themselves to their integration and application in (digital) soil mapping (DSM). The core objective of the project is to explore and exploit new capabilities of advanced geophysical technologies for answering this societal demand. To this aim, DIGISOIL addresses four issues covering technological, soil science, and economic aspects: (i) development and validation of hydrogeophysical technologies and integrated pedo-geophysical inversion techniques; (ii) the relation between geophysical parameters and soil properties; (iii) the integration of derived soil properties for mapping soil functions and soil threats; and (iv) the evaluation, standardisation, and industrialisation of the proposed methodologies, including technical and economic studies.

We study the stability of social and economic networks when players are farsighted. We first provide an algorithm that characterizes the unique pairwise and groupwise farsightedly stable set of networks under the componentwise egalitarian allocation rule. We then show that this set coincides with the unique groupwise myopically stable set of networks but not with the unique pairwise myopically stable set of networks. We conclude that, if groupwise deviations are allowed then whether players are farsighted or myopic does not matter, if players are farsighted then whether players are allowed to deviate in pairs only or in groups does not matter.

An alternative to seismic methods typically used in shallow subsurface prospecting (0-100m) consists in taking advantage of advances in quantitative imaging based on full waveform inversion, which is supposed to overcome their inherent limitations. In thi

International audience; We perform the full elastic waveform inversion in the frequency domain in a 2-D geometry. This method allows imaging of two physical seismic parameters, using vertical and horizontal field components. The forward problem is discretized using finite difference, allowing to simulate the full elastic wavefield propagation. Moreover, it is solved in the frequency domain, a fast approach for multisource and multireceiver acquisition. The non-linear inversion is based on a pre-conditioned gradient method, where Born and Rytov formulations are used to compute Fréchet derivatives. Parameter perturbations linearly depend on fields perturbations in the Born kernel, and on the generalized complex phases of fields in the Rytov kernel, giving different Fréchet derivatives. The gradient is pre-conditioned with the diagonal part of the inverse Hessian matrix, allowing to better estimate the stepping in the optimization direction. Non-linearity is taken into account by updating parameters at each iteration and proceeding from low to high frequencies. The latter allows as well to progressively introduce smaller wavelengths in parameter images. On a very simple synthetic example, we examine the way the inversion determines the Vp (P-wave velocity) and Vs (S-wave velocity) images. We highlight that, with a transmission acquisition, final parameter images weakly depend on the chosen formulation to compute Fréchet derivatives and on the inverted parameters choice. Of course, convergence strongly depends on the medium wavenumber illumination which is related somehow to the acquisition geometry. With a reflection acquisition, the Born formulation allows to better recover scatterers. Moreover, the medium anomalies are not well reconstructed when surface waves propagate in the medium. This may be due to the evanescent nature of surface waves. By selecting first body waves and then surface waves, we improve the convergence and properly reconstruct anomalies. This shows us that preparation of the seismic data before the inversion is as critical as the initial model selection.

Geophysical methods such as seismic surveying or electrical resistivity imaging appear to be well adapted to investigate landslide structure and understand related mechanisms. They allow direct and non-intrusive measurements of acoustic (P), shear (S) waves velocity and electrical resistivity, three physical parameters considered as essential to define the properties of reworked moving materials. Both methods were applied at the “Super-Sauze” site, in the French South Alps, where a typical example of an intra-material mudslide can be observed. Measurements were taken simultaneously along a profile of 325 m in length, perpendicularly to 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 velocities as well as resistivity tomographies are presented and discussed in term of reliability. Preliminary interpreted results show a correlation between the seismic velocities and electrical resistivity data, confirming that the simultaneous use of both methods gives complementary information on the geomechanical behaviour of the landslide. The seismic data provide information on the variations of fissure density and on the presence of deformed material whereas the electrical resistivity data provide information on the variations of water content within the mudslide. In order to go deeper into the interpretation of the geomechanical behaviour of the mudslide from geophysical data, a data fusion strategy based on fuzzy subsets theory is developed. The computed fuzzy cross-sections show the possibility of geomechanical hypotheses to be realized in specific areas of the tomographic cross-sections highlighting the places where plastic or solid-body deformations could occur. This information is consistent with the geotechnical data and the borehole inclinometer measurements available for the mudslide.

Space-borne remote sensing imaging allows precise coverage of vast regions with basic data for geological exploration while significantly reducing exploration costs. These data provide valuable information for geological mapping and mineral exploration through highlighting geological structures such as lineaments, faults, and lithological contacts. The use of radar constitutes a very useful complement to optical remote sensing data, especially as regards to the surface but also the subsurface detection of structures. Specifically in arid desert areas, radar waves penetrate the dry surface sand sheet and highlight concealed geological structures or fossil river systems. We present a synthesis of two experiments designed to compare the contribution of radar and optical images to geological exploration using the Akjoujt region in Mauritania and the Bir Safsaf region in Egypt as study sites. Ground-Penetrating Radar (GPR) is used to evaluate volume scattering coming from subsurface dielectric heterogeneities in cases of radar penetration of the dry sand. This evaluation is based on correlating the reflectivity of the GPR section with backscattering coefficients extracted from radar images. Radar clearly demonstrates superior capability for precise detection of surface structures associated with dikes, lithological variation, or paleo-hydrogeological features. The effectiveness of radar for mapping in arid environments lies in the fact that geological features can be distinguished on the basis of their morphological expression and surface roughness. We also confirm that low frequency radar (L-band) can retrieve information about subsurface structures down to a few meters’ depth, where optical images cannot.

A new stable and accurate, finite-difference 2D elastic seismic wave propagation modeling algorithm is developed, with a Perfectly Matched Layer (PML) absorbing boundary condition to avoid reflections from the edges of the numerical model. This is used to study the interaction of seismic surface waves with near-surface heterogeneities. The effects of different empty cavity shapes and depths and altered zones is evaluated from the direct and diffracted seismograms and corresponding Rayleigh wave dispersion images. Differential seismograms calculated for models both with and without a cavity show a complex, non-symmetrical, diffraction pattern and their dispersion images show well-localized scattering from heterogeneities. Missing coherent energy appears in specific frequency bands, related to the cavity depth, shape and degree of modification of the surrounding medium. The cavity more strongly affects seismograms when it is shallower. A [Formula: see text] deep rectangular cavity generates more severe perturbations than a circular section, due to non-symmetrical back-scattered patterns recognized in dispersion images. Furthermore, low velocity zones around and above the cavity, due to soil alteration, trap waves and lead to increased ground roll attenuation and strong footprints in dispersion images that can possibly mask the cavity signature. If the altered zone extends to the surface as a cone, trapping phenomena completely dominate seismograms. The detection of altered zones is a very useful indication in natural hazard assessment, making it possible to distinguish the signature of a ‘safe’ cavity from a potentially dangerous one.

We study the capabilities of low frequency radar systems to sound the subsurface in arid countries. This approach is based on the coupling between two complementary radar techniques: the airborne/spaceborne Synthetic Aperture Radar (SAR) used in L-band (1.2 GHz) for imaging large scale subsurface structures, and the Ground-Penetrating Radar (GPR) used at 900 MHz for sounding soils from the surface down to several meters. Recent results obtained on the well-known site in southern Egypt (Bir Safsaf) are recalled and constrain a numerical model for quantitatively interpreting the SAR/GPR data. This model is based on electric field extrapolation in the frequency domain, taking into account the aperture and the incidence angle of the antenna for both the SAR and GPR systems. Simulated signals from given realistic geological cross sections can then be analysed in order to understand the contribution of volume backscattering on both of radar systems. The model is tested on the Bir Safsaf site, and gives a good description of the dielectric behaviour, since observed and simulated backscattered signals are quite comparable. Discussion on the domain of validity for the model is presented as well as the perspectives that it could offer in the future missions of quantitative mapping of the subsurface in arid deserts.

We present the capabilities of low-frequency radar systems to sound the subsurface for a site located in south-central Egypt, the Bir Safsaf region. This site was already intensively studied since the SIR-A and SIR-B orbital radars revealed buried paleodrainage channels. Our approach is based on the coupling between two complementary radar techniques: the orbital synthetic aperture radar (SAR) in C and L bands (5.3 and 1.25 GHz) for imaging large-scale subsurface structures, and the ground-penetrating radar (GPR) at 500 and 900 MHz for sounding the soil at a local scale. We show that the total backscattered power computed from L-band SAR and 900-MHz GPR profiles can be correlated, and we combined both data to derive the geological structure of the subsurface. GPR data provide information on the geometry of the buried scatterers and layers, while the analysis of polarimetric SAR data provides information on the distribution of rocks in the sedimentary layers and at the interface between these layers. The analysis of 500-MHz GPR data revealed some deeper structures that should be detected by lower frequency SARs, such as a P-band system.

International audience; A seismic refraction experiment has been carried out off the coast of northern Brittany (NW France) to investigate the deep structure of the North Armorican block. The seismic source was an array of airguns and the shots were recorded simultaneously both by ocean bottom seismometers and land stations. We present 2D velocity models computed from two crossing seismic refraction profiles (labelled P1 and P2 and each about 135 km long) oriented NE-SW and NNW-SSE, i.e. parallel and perpendicular, respectively, to the regional basement structures. Wide-angle reflection profiles were also performed to extend the 2D structures to a 3D crustal interpretation. Observed and calculated P-wave traveltimes were matched by forward modelling prior to their inversion in order to produce two crustal velocity models. The model which best fit the data consists of: (1) high basement velocities at the top of the crust, except in an area with shallow seismic velocity which can be as low as 3.0 km/s at the surface and possibly related to the existence of shallow Paleozoic sedimentary basins; (2) the mean crustal velocity ranges from 5.5 to 6.8 km/s at a depth of 15 km depth; (3) lower crustal velocities range from 7.0 to 7.5 km/s at a depth of 35-38 km and then from 7.5 to 8.1 km/s at the crust-mantle boundary located between 35 and 38 km depth. Comparisons with the crustal section imaged by the coincident seismic vertical reflection profile SWAT 10 show a good correlation between velocity gradients and lower crustal reflectivity. When compared to surrounding basement areas, the North Armorican Cadomian crust displays a slightly thicker crust and also a thicker reflective lower crust which can be related to the relative preservation of this Upper Proterozoic block from later Paleozoic and Mesozoic tectonic episodes. The crustal structure of the North Armorican block thus appears to be an intercalated feature between the surrounding Paleozoic areas and older shield or platform domains.