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Records of incision history such as topographic data and landform dating can be gathered in inversion schemes to reconstruct base-level fall and uplift history. Here, we use a non-linear inversion scheme and the stream power incision model to study the landscape evolution of a mountainous area to quantify whether it responds to an uplift or a capture-induced local base-level fall. Our inversion is constrained by 10Be cosmogenic nuclide data. The probabilistic approach yields an ensemble of solutions made by a combination of model parameters. We apply our model to the Draa Canyon located in the Anti-Atlas of Morocco at the outlet of the Ouarzazate Basin which was internally drained during the Miocene. We show that incision rates in the Ouarzazate Basin and the southern margin of the High Atlas are compatible with a Pleistocene age for the opening of the Basin. The forcing to drainage integration may be due to capture by regressive erosion in the proto Draa river or tilting to the south of the High Atlas, Ouarzazate basin, and Anti-Atlas as a whole in response to mantle-related continental-scale uplift, or a combination of both. The southern border of the High Atlas in this region displays a transient landscape previously interpreted as evidence for recent shortening and rock uplift. Our results suggest that the rejuvenation of the southern Central High Atlas, in the northern margin of the Ouarzazate Basin, mainly occurred in response to the opening of the Ouarzazate Basin, with only several hundreds of meters of rock uplift localized along the South Atlas front during the late Cenozoic.

International audience; The critical zone is a region of the shallow subsurface that ranges from the top of the vegetation canopy to the base of superficial aquifers. It comprises rocks, soils, water, air, and living organisms; contains the vast majority of life-sustaining resources; and regulates the interaction between the atmosphere and aquifers (e.g., Binley et al., 2015; Parsekian et al., 2015). The combined use of geophysical multiscale probing and imaging techniques along with the integration of hydrological, hydrogeological, and geochemical data is widely practiced for the observation of the partially saturated region of the critical zone, that is, the vadose zone (e.g., Parsekian et al., 2015). This approach to geophysical subsurface characterization, referred to as hydrogeophysics (e.g., Hubbard & Linde, 2011; Rubin & Hubbard, 2006), is dominated by electrical and electromagnetic methods due to their strong sensitivity with regard to water content and salinity (e.g., Friedman, 2005). However, given that seismic waves are inherently sensitive to key hydraulic properties of porous media, such as, porosity, permeability

International audience; A new hydrogeophysical approach was developed to reveal the spatial and temporal dynamic flow pathways inside a river corridor as well as the pattern of transient surface water and the groundwater (SW-GW) exchanges. Obtaining such information from traditional hydrological data remains a challenge, both in terms of accuracy and validation. As there is no direct measurement of these fluxes, estimating dynamic pathways and SW-GW exchanges is frequently tackled with the help of numerical models. Yet, the definition of initial and boundary conditions is generally based on poorly constrained assumptions and restrained to the location of piezometers, due to the lack of knowledge of the water table (WT) geometry. We provide a methodology to build stronger constraints to the numerical simulation and the hydrodynamic parameters calibration, both in space and time, by using a multi-method approach. On the Orgeval Critical Zone observatory (France), we show how a thorough interpretation of high-resolution geophysical images, combined with geotechnical data, helps to resolve subsurface structural heterogeneity. This coupling provides a detailed distribution of hydrofacies, valuable prior information about the associated hydrodynamic properties distribution. The temporal dynamic of the WT table can be locally captured with high-resolution time-lapse seismic acquisitions. Time-lapse variations are interpreted as temporal changes in the saturated-unsaturated zone continuum. Each seismic snapshot is then thoroughly inverted to image spatial WT variations. This posterior geophysical information is then used to calibrate the hydrogeological modeling domain. The use of the WT improves the estimation of transient stream-aquifer exchanges, especially during the rain event.

International audience; – How geophysical data can inform the archaeological potential of urban land.– How geophysical prospecting strategies can be adapted to urban contexts.– How geophysical prospecting setups and results can be constrained with geotechnics and geomatics

As part of the understanding of the phenomena affecting tracks geometry on the Northern Europe high-speed line (LGV-Nord), a study was carried out on an area with occasional maintenance efforts. The goal is to identify the causes of structural disorders on railway embankment (RE). To better understand the origins of these issues, geotechnical characterization and bender elements (BE) tests were performed in the laboratory on soils collected on the field. Measurements of water content, density, and Atterberg limits cannot explain the anomalies observed along the tracks. However, when it comes to mechanical properties and more precisely to shear wave velocities (VS), we can clearly see strong contrasts between areas where the phenomena were observed and where it was not. Thus, VS obtained with BE tests seems to be a good indicator of the quality of soils constituting the RE. At the site scale, surface-wave methods were identified as a possible tool to estimate the mechanical properties of the RE. In this context, we dimensioned and designed specific acquisition setups. To estimate 1D profiles of shear wave velocities, we performed many tests to show the robustness of this method, when inversions are constrained by railway a priori. Those tests allowed us to determine the geometry of the platforms and to characterize the VS of the supporting soil. These results show a difference between the sections of RE with disorders and those without. The reliability of these results is also highlighted since they coincide with geotechnical data.

In order to accurately study the properties of partially saturated unconsolidated media at the near surface scale, or be able to image deeper structures through them, accurate 2D and 3D wave propagation numerical modelling tools are required. The rheology/mechanical properties of such media are frequently extremely complex (nonlinear, anisotropic, ... ), even when considered at dry state and of homogeneous mixture. Experimental observations (both at the laboratory and field scales) show that the seismic wave-field in unconsolidated granular materials remains difficult to interpret within standard methodological frameworks. We present here a numerical study aiming at exploring possible alternative forward modelling approaches to better extract information from recorded signals. We first present a finite volume method (Asfour et al. 2021) in which exact Riemann solvers are introduced. Solutions are compared to high-order finite-differences (Seismic_CPML code) and spectral finite element (SPECFEM code) solutions. A first series of synthetic cases is shown to benchmark the code at the hundred meters scale with a 100-300Hz wavelet source content. Another synthetic and more reallistic case is then presented with a medium affected by a steep nonlinear velocity gradient in depth, typical of an unconsolidated granular medium (as previously considered at laboratory scale). For this model, a 1500Hz dominant frequency point source wavelet is considered and fluid saturation is also tested by applying a fluid-solid coupling. First arrival times and PSV-wave dispersion obtained from the different codes are compared. In a second step, and considering the real data recorded at the laboratory, we apply a more realistic source wavelet (obtained through signal spectrum ratio) and we perform parallelized high-order finite difference simulations (UNISOLVER code) to compare 2D and 3D elastic as well as poroelastic solutions on finely discretized meshes. Computed and observed data are compared. The poroelastic rheology provides better amplitudes in the seismograms and better exhibits some PSV modes in the phase velocity dispersion observations. Sensitivity kernels are also shown for the different rheologies. The different results obtained are now paving the way to seismic inversion at the near surface scale and to image shallow fluid/water saturated layers.

Estimation of water content in the shallow subsurface using seismic data is a complex task of increasing importance in the overall field of hydrogeophysics. In this context, the velocities of compressional (Vp) and shear (Vs) waves can be used to infer strong water content variations in unconsolidated soils, such as, the presence of the water table, by means of Vp/Vs ratio estimations. This approach, which is based on first-arrival time data, generally does not permit a proper quantification of the water content distribution in the partially saturated zone. Conversely, field and laboratory measurements indicate that surface-waves are indeed remarkably sensitive to both the water table depth and the saturation characteristics in the overlaying capillary fringe. This apparent difference in sensitivity between body and surface waves cannot be explained using conventional models. Observations and experiments show that the effective stress of unconsolidated porous media is not only affected by the overburden stress and pore pressures, as classic models assume, but also by capillary forces, which tend to stiffen the soil at relatively low saturations. In this work, we extend seminal rock physics models to include capillary suction effects in the effective stress of the soil. This approach provides effective elastic moduli and, thus, Vp and Vs, which are depth- and saturation-dependent. Then, we solve the quasi-static fluid flow equations in a porous medium and obtain saturation profiles for a given water table depth. This information, combined with the proposed rock physics model, permits to simulate simple seismic data sets, that is, body-wave first-arrival times and surface-wave phase velocities, for different water table depths and soil textures. Our results clearly show that capillary effects allow to explain the apparent difference in sensitivity between body- and surface-wave signatures in response to small water content variations in the partially saturated zone. Capillary effects are primarily relevant in porous media composed by relatively small characteristic grain sizes. We conclude that the proposed framework has the potential to fundamentally improve our characterization of near-surface environments using both active and passive seismic methods.

International audience; Estimating evolution of exchanges within the stream-aquifer interface is frequently tackled with the help of numerical models. Yet, the definition of boundary conditions is generally based on poorly constrained assumptions and restrained to the location of piezometers. We suggest here to stretch the modeling domain and build stronger constraints, both in space and time, by using a multi-method approach. On a hotspot of the Orgeval Critical Zone observatory (France), we show how a thorough interpretation of high-resolution geophysical images, combined with geotechnical data, helps describing the spatial heterogeneities of the shallow aquifer. It provides a detailed distribution of hydrofacies, valuable prior information about the associated hydrodynamic properties and makes it possible to expand the modeling window in space. We show how the local temporal dynamic of the water table can be captured with high resolution time-lapse seismic acquisitions. Time-lapse variations in seismic data are discriminated from noise or measurement errors to be interpreted, regarding hydrological observations, as temporal changes in the saturated-unsaturated zone continuum. Each seismic snapshot is then thoroughly inverted to actually image spatial water content variations and delineate the water table outside the limits defined by the piezometers. This posterior geophysical information is then suggested as initial and boundary conditions of the expanded hydrogeological modeling domain. We finally calibrate and provide plausible ranges of hydraulic parameters to reproduce the water table and improve the estimation of stream-aquifer exchanges.

The simultaneous estimation of 2D pressure (P-) and S-wave velocities ([Formula: see text] and [Formula: see text], respectively) is a promising approach for imaging subsurface mechanical properties. It can be performed with a single acquisition setup by combining P-wave refraction and surface-wave (SW) analysis. Although SW methods are commonly applied for the 1D estimation of [Formula: see text], 2D profiling requires the implementation of specific processing and inversion tools not yet widely available in the community. We have developed an open-source MATLAB-based package that performs SW inversion and profiling (SWIP) so as to retrieve 1D to 2D variations of [Formula: see text] from any kind of linear active-source near-surface seismic data. Each step of the workflow involves up-to-date processing and inversion techniques and provides ready-to-use outputs with quality control tools. First, windowing and stacking techniques are implemented to enhance the signal-to-noise ratio and extract local dispersion images along the line. Then, dispersion curves are picked for each window with an uncertainty range in the phase velocity including higher uncertainties at low frequency. These curves are next inverted using a Monte Carlo approach with various parameterizations (e.g., user defined, refraction based). The best models are finally selected according to their fit to the data to build an average final model with a suggested investigation depth. As an example, we used SWIP to process data collected at a Yellowstone hydrothermal system. Our results show the benefits of estimating [Formula: see text] and [Formula: see text] from a single seismic setup to highlight subsurface gas pathways.

At the Grostenquin NATO airbase (France), a variety of geophysical techniques have been performed to address the relationship between DCP penetration resistance and several geophysical measurements at a few meters depth in order to characterize potential unimproved landing zones through continuous mapping, not only local measurements.

(IF 2.78 [2018]; Q1); International audience; In previous studies, the auxiliary differential equation (ADE) method has been applied to 2-D seismic-wave propagation modelling in viscoelastic media. This method is based on the separation of the wave propagation equations derived from the constitutive law defining the stress–strain relation. We make here a 3-D extension of a finite-difference (FD) scheme to solve a system of separated equations consisting in the stress–strain rheological relation, the strain–velocity and the velocity–stress equations. The current 3-D FD scheme consists in the discretization of the second order formulation of a non-linear viscoelastic wave equation with a time actualization of the velocity and displacement fields. Compared to the usual memory variable formalism, the ADE method allows flexible implementation of complex expressions of the desired rheological model such as attenuation/viscoelastic models or even non-linear behaviours, with physical parameters that can be provided from dispersion analysis. The method can also be associated with optimized perfectly matched layers-based boundary conditions that can be seen as additional attenuation (viscoelastic) terms. We present the results obtained for a non-linear viscoelastic model made of a Zener viscoelastic body associated with a non-linear quadratic strain term. Such non-linearity is relevant to define unconsolidated granular model behaviour. Thanks to a simple model, but without loss of generality, we demonstrate the accuracy of the proposed numerical approach.

National audience

International audience; The vadose zone is the main host of surface and subsurface water exchange and has important implications for ecosystems functioning, climate sciences, geotechnical engineering, and water availability issues. Geophysics provides a means for investigating the subsurface in a non-invasive way and at larger spatial scales than conventional hydrological sensors. Time-lapse hydrogeophysical applications are especially useful for monitoring flow and water content dynamics. Largely dominated by electrical and electromagnetic methods, such applications increasingly rely on seismic methods as a complementary approach to describe the structure and behavior of the vadose zone. To further explore the applicability of active seismics to retrieve quantitative information about dynamic processes in near-surface time-lapse settings, we designed a controlled water infiltration experiment at the Ploemeur Hydrological Observatory (France) during which successive periods of infiltration were followed by surface-based seismic and electrical resistivity acquisitions. Water content was monitored throughout the experiment by means of sensors at different depths to relate the derived seismic and electrical properties to water saturation changes. We observe comparable trends in the electrical and seismic responses during the experiment, highlighting the utility of the seismic method to monitor hydrological processes and unsaturated flow. Moreover, petrophysical relationships seem promising in providing quantitative results.

International audience; Time‐lapse applications of seismic methods have been recently suggested at the near‐surface scale to track hydrological properties variations due to climate, water level changes or permafrost thaw for instance. But when it comes to traveltime tomography or surface‐wave dispersion inversion, a careful estimation of the data variability associated to the picking process must be considered prior to any time‐lapse interpretation. In this study, we propose to estimate picking errors that are due to the inherent subjectivity of human operators using statistical analysis based on picking repeatability. Two seismic datasets were collected along the same profile under distinct hydrological conditions, across a granite‐micaschist contact at the Ploemeur hydrological observatory (France). Both datasets were recorded using identical equipment and acquisition parameters. A thorough statistical analysis is conducted to estimate picking uncertainties, at the 99 % confidence level, for both Pressure (P) wave first arrival time and surface‐wave phase velocity. With the suggested workflow, we are able to identify 33 % of the P‐wave traveltimes and 16 % of the surface‐wave dispersion data that can be considered significant enough for time‐lapse interpretations. In this selected portion of the data, point‐by‐point differences are highlighting important variations linked to different hydrogeological properties of the subsurface. These variations show strong contrasts with a non‐monotonous behaviour along the line, offering new insights to better constrain the dynamics of this hydrosystem.

International audience; VP/VS or Poisson’s ratio estimated from active seismic methods recently proved to be efficient in the imaging ofthe critical zone and associated hydrosystems. We suggest here a time-lapse application of this approach to provideboth spatial and temporal constraints on the hydrodynamic model of the Avenelles experimental basin (Seine etMarne, France). The preliminary studies of this hydrosystem relied on typical combined interpretation of sparsegeological and hydrological data. Geophysical surveys, performed throughout the watershed, helped delineatingthe different compartments and identifying their connectivity with the stream network. Once a basin-scale globalhydrogeological model established, hotspots were targeted with local high frequency monitoring stations to inves-tigate its stream-aquifer exchanges. At these stations, recorded data (bank piezometers, stream water temperatureand level, temperature profiles in the hyporheic zone) clearly showed contrasts in the dynamic of the hydrosystemalong the stream network. However, the nature of the compartments and their associated properties, observed at thebasin-scale, would not explain the data observed at the local scale. It highlighted the need for detailed descriptionof the hydrosystem, at the stream-aquifer interface. One specific hotspot was thus selected to perform soundingsand geophysical measurements of higher resolutions. Thanks to electrical resistivity tomography, P-wave refrac-tion and surface-wave seismic imaging, we provided a description of the local heterogeneities both in terms oflithology and water content. The seismic experiments were then repeated with a two-month time step. At eachtime step, pseudo-2D sections of Poisson’s ratio clearly showed strong spatial and temporal variations in satura-tion of the vadose zone. These results then helped providing updated constraints and boundary conditions to thehydrodynamic model

International audience; Le service d'observation H+ (hplus.ore.fr), créé en 2002, est un réseau de sites hydrogéologiques, pour la mesure et la modélisation du transfert et de la réactivité des eaux dans les aquifères hétérogènes. Sa mission première est de maintenir et de coordonner un réseau de sites expérimentaux capables de fournir des données pertinentes pour la compréhension du cycle de l'eau et des éléments transportés dans les aquifères à diverses échelles spatiales et temporelles.

International audience; Laboratory physical modelling and non-contacting ultrasonic techniques are frequently proposed to tackle theoretical and method- ological issues related to geophysical prospecting. We used an innovative experimental set-up to perform laser-Doppler acoustic probing of granular materials with varying water levels to target near-surface hydrogeological applications. The preliminary results presented here show a clear influence of the water level on both first arrival times and dispersion of guided waves, and significant differences in terms of amplitudes. They validate the use of such approach to benchmark recently developed methods for water saturation detection in hydrogeophysics.

International audience; Accurately quantifying surface-subsurface water and energy fluxes is challenging because a myriad of morphological and physical factors. The spatial and temporal variability of these factors and nested interfaces lead to patterns in residence time over multiple spatial scales. In these scope, we present here a new understanding of the stream-aquifer interface functioning. Our approach accounts for hydrology and thermal processes based on a combination of field methods and modelling tools. The concept of nested stream-aquifer interfaces is used to simulate the variation of the spatio-temporal surface-subsurface exchanges at the basin scale from LOcal MOnitoring Stations measurements of the stream-aquifer exchanges. This study was applied to the Orgeval basin over four years between 2012-2016. A fiber optic has been set up since 2016 along a 1 km river to improve our description of water and heat fluxes between the stream and the aquifers. This complementary monitoring allows to characterize Groundwater-Surface waters interactions at different spatial and temporal scales.

International audience; Laboratory physical modeling and laser-based experiments are frequently proposed to tackle theoretical and methodological issues related to seismic prospecting, e.g., when experimental validations of processing or inversion techniques are required. Lasers are mainly used to simulate typical field acquisition setups on homogeneous and consolidated materials assembled into laboratory-scale physical models (PMs) of various earth structures. We suggested the use of granular materials to study seismic-wave propagation in unconsolidated and porous media and target near-surface exploration and hydrogeologic applications. We designed and tested the reproducibility of an experimental procedure to build and probe PMs consisting of micrometric glass beads (GBs). A mechanical source and a laser-Doppler vibrometer were used to record small-scale seismic lines at the surface of three GBs models. When guided surface acoustic mode theory should prevail in such unconsolidated granular packed structure under gravity, we only considered elastic-wave propagation in stratified media to interpret recorded data. Thanks to basic seismic processing and inversion methods (first arrivals and dispersion analyses), we were able to correctly retrieve the gradients of pressure- and shear-wave velocities in our models. A 3D elastic finite difference simulation of the experiment offered, despite significant differences in terms of amplitudes, a supplementary validation of our approximation, as far as elastic properties of the medium were concerned.

International audience; The Dead Sea (DS) coastal areas have been dramatically hit by sinkhole occurrences since around 1990. It has been shown that the sinkholes along both Israeli and Jordanian shorelines are linked to evaporate karst cavities that are formed by slow salt dissolution. Both the timing and location of sinkholes suggest that: 1) the salt weakens as the result of unsaturated water circulation, thus enhancing the karstification process; and 2) sinkholes appear to be related to the decompaction of the sediments above karstified zones. The location, depth, thickness and weakening of salt layers along the DS shorelines, as well as the thickness and mechanical properties of the upper sedimentary deposits, are thus considered as controlling factors of this on-going process. The knowledge of shear-wave velocities (Vs) should add valuable insights on mechanical properties of both the salt and its overburden. We have suggested Vs estimation using surface-wave prospecting methods, based on surface-wave dispersion measurements and inversion. Two approaches have been used. Along the Israeli shoreline, Vs mapping has been performed to discriminate weak and hard zones within salt layers, after calibration of inverted Vs near boreholes. It has been shown that there is a Vs increase in the DS direction. Initially examined weak zones, located near the salt edge, associated with karstified salt, are characterized by Vs values of 760–1,050 m/s, and extend 60–100 m from the salt edge in the DS direction. Hard salt zones with velocity Vs values greater than 1,500 m/s are located at distances of more than 100–220 m from the salt edge. Finally, transition zones (1,050 < Vs < 1,500 m/s) have a 40–160 m spread. On a Jordanian site, roll-along acquisition and dispersion stacking has been performed to achieve multi-modal dispersion measurements along linear profiles. Inverted pseudo-2-D Vs sections present low Vs anomalies in the vicinity of existing sinkholes and made it possible to detect decompacted sediments associated with potential sinkhole occurrences. Moreover, Vs profiles showed a high velocity unit at 40–50 m depth that can be interpreted as a salt layer.

International audience

International audience; Near-surface seismic methods are mainly used to determine the geometrical characteristics of hydrosystems (and to provide elements that are interesting for hydrogeologists such as separating aquifer layers, setting up systems boundaries, highlighting fractures etc.). Recent methodological advances suggest the high potential of seismic methods to investigate the mechanical properties of the Critical Zone (CZ), by exploiting the full wealth of seismic records. Indeed, the behavior of Shear (S) and Pressure (P) waves in the presence of water is partially decoupled, so that the ratio of their propagation velocities VP/VS is strongly linked to water saturation. We propose here a time-lapse application of this approach. Two seismic acquisitions were carried out under distinct hydrogeological conditions along the same line at the Ploemeur hydrogeological observatory (South Brittany, France). Vertical component seismic data were recorded to extract: (i) P-wave first arrival times and (ii) Rayleigh-wave phase velocities. The significant variations with time and space, of both datasets, indicate marked changes in mechanical properties of the CZ that have to be compared to soil moisture variations in the unsaturated zone and groundwater level variations.

International audience

The monitoring of materials elastic parameters during geological analogue experiments would highly improve the interpretation of involved processes. It would more particularly help the implementation of joint analytical or numerical analyses. We propose laser-Doppler vibrometry as a probing tool for the systematic characterisation of analogue models. We therefore develop an experimental set-up to record small-scale seismic lines at the surface of an unconsolidated granular medium in laboratory conditions. Pressure-wave first arrival times and surface-wave dispersion are then inverted for a one-dimensional Pressure- and Shear-wave velocity structure. Inferred profiles appear to match previously thoroughly estimated properties of the probed medium, thus validating our approach.

Surface-wave profiling techniques using active sources and linear arrays are often performed with short source receiver distances, compared to the involved wavelengths. Dispersion measurements however are usually performed by assuming body-wave amplitudes to be negligible and the recorded wave-field to be dominated by plane Rayleigh-waves. The estimated dispersion curves may then be corrupted by near-field effects. In this instance, both numerical and physical modeling has helped illustrate such effects,which are typically identified as a systematic underestimation of measured phase velocity at low frequencies. A normalized representation, based on theoretical phase velocities and spread length, has shown the apparent invariability of near-offset effects : the underestimation occurred as soon as the measured wavelength exceeded 50% of the spread length; homogeneous and normally-dispersive media provide the same limitation, regardless of the spread length value.

International audience; The characterisation and monitoring of aquifer systems mainly rely on piezometric and log data. Delineating spatial variations of lithology between piezometers is a delicate task, which inevitably generates errors possibly propagating into hydrogeological models. Seismic methods have been proposed to: (i) improve the low spatial resolution of borehole data, (ii) provide a characterisation of the subsurface geometry, and (iii) estimate the physical parameters of the medium influenced by the presence of water and the associated flow and transport processes. The joint study of pressure (P-) and shear (S-) wave seismic velocities (VP and VS, respectively), whose evolution is strongly decoupled in the presence of fluid, has been proposed through the estimation of the VP/VS ratio and Poisson's ratio. A specific methodology has been developed for the combined exploitation of P- and surface waves present on single seismic records. The use of this methodology in several geological and hydrogeological contexts allowed for estimating VP/VS ratio lateral and temporal variations in good agreement with a priori geological information and existing geophysical and piezometric data. Laser-based ultrasonic techniques were also proposed to put these processing techniques in practice on perfectly controlled physical models and study elastic wave propagation in partially saturated porous media.

International audience; The joint study of pressure (P-) and shear (S-) wave velocities (Vp and Vs ), as well as their ratio (Vp /Vs), has been used for many years at large scales but remains marginal in near-surface applications. For these applications, and are generally retrieved with seismic refraction tomography combining P and SH (shear-horizontal) waves, thus requiring two separate acquisitions. Surface-wave prospecting methods are proposed here as an alternative to SH-wave tomography in order to retrieve pseudo-2D Vs sections from typical P-wave shot gathers and assess the applicability of combined P-wave refraction tomography and surface-wave dispersion analysis to estimate Vp/Vs ratio. We carried out a simultaneous P- and surface-wave survey on a well-characterized granite-micaschists contact at Ploemeur hydrological observatory (France), supplemented with an SH-wave acquisition along the same line in order to compare Vs results obtained from SH-wave refraction tomography and surface-wave profiling. Travel-time tomography was performed with P- and SH- wave first arrivals observed along the line to retrieve Vtomo p and Vtomo s models. Windowing and stacking techniques were then used to extract evenly spaced dispersion data from P-wave shot gathers along the line. Successive 1D Monte Carlo inversions of these dispersion data were performed using fixed Vp values extracted from Vtomo p the model and no lateral constraints between two adjacent 1D inversions. The resulting 1D Vsw s models were then assembled to create a pseudo-2D Vsw s section, which appears to be correctly matching the general features observed on the section. If the pseudo-section is characterized by strong velocity incertainties in the deepest layers, it provides a more detailed description of the lateral variations in the shallow layers. Theoretical dispersion curves were also computed along the line with both and models. While the dispersion curves computed from models provide results consistent with the coherent maxima observed on dispersion images, dispersion curves computed from models are generally not fitting the observed propagation modes at low frequency. Surface-wave analysis could therefore improve models both in terms of reliability and ability to describe lateral variations. Finally, we were able to compute / sections from both and models. The two sections present similar features, but the section obtained from shows a higher lateral resolution and is consistent with the features observed on electrical resistivity tomography, thus validating our approach for retrieving Vp/Vs ratio from combined P-wave tomography and surface-wave profiling.

International audience; When applied to hydrogeology, seismic methods are generally confined to the characterisation of aquifers geometry. The joint study of pressure-(P) and shear-(S) wave velocities (V P and V S) can provide supplementary information and improve the understanding of aquifer systems. This approach is proposed here with the estimation of V P /V S ratios in a stratified aquifer system characterised by tabular layers, well-delineated thanks to electrical resistivity tomography, log and piezometer data. We carried out seismic surveys under two hydrological conditions (high and low flow regimes) to retrieve V S from both surface-wave dispersion inversion and SH-wave refraction interpretation, while V P were obtained from P-wave refraction interpretation. P-wave first arrivals provided 1D V P structures in very good agreement with the stratification and the water table level. Both V S models are similar and remain consistent with the stratification. The theoretical dispersion curves computed from both V S models present a good fit with the maxima of dispersion images, even in areas where dispersion curves could not be picked. Furthermore, V P /V S and Poisson's ratios computed with V S models obtained from both methods show a strong contrast for both flow regimes at depths consistent with the water table level, with distinct values corresponding to partially and fully saturated sediments.

International audience; A study of spatial heterogeneity of groundwater geochemistry is conducted within an abandoned underground quarry of chalk. An associated geophysical study (electromagnetic mapping at low induction number, and electrical resistivity tomography) is carried out on the surface. It allows to determine the structural geometry of superficial formations, in order to understand the link between the geochemical spatial variability of the lakes and the properties of the environmental medium.

A seismic survey involving two distinct acquisition setups, with vertical and horizontal component geophones, has been carried out along the same line on a site presenting a simple vertical structure (peat, quick clays and bedrock) and no strong lateral variations. SH-wave refraction tomography and Rayleigh-wave dispersion inversion provided the same shear-wave velocity gradient in the quick clays. SH-wave tomography failed to correctly depict the peat layer and to reach the bedrock. A collection of Rayleigh-wave dispersion images were extracted along the line using windowing and stacking techniques. A thorough analysis of these images made it possible to give a complete description of the site velocity structure, illustrating the complementarity of both methods.

International audience; Laboratory experiments using laser-based ultrasonic techniques can be used to simulate seismic surveys on highly controlled small-scale physical models of the subsurface. Most of the time, such models consist in assemblies of homogeneous and consolidated materials. To enable the physical modelling of unconsolidated, heterogeneous and porous media, the use of granular materials is suggested here. We describe a simple technique to build a two-layer physical model characterized by lateral variations, strong property contrasts and velocity gradients. We use this model to address the efficiency of an innovative surface-wave processing technique developed to retrieve 2-D structures from a limited number of receivers. A step by step inversion procedure of the extracted dispersion curves yields accurate results so that the 2-D structure of the physical model is satisfactorily reconstructed. The velocity gradients within each layer are accurately retrieved as well, confirming current theoretical and experimental studies regarding guided surface acoustic modes in unconsolidated granular media.

Natural landscape analysis and numerical modelling point to a lack of physical data on relief dynamics. Experimental modelling is therefore an interesting approach for obtaining physical information on eroded systems with runoff transportation and topographic incision. The main technical challenge, in reproducing regional topography at the laboratory scale, is to obtain mm-scale incisions and a limitation of the smoothing action of diffusive transport processes. An experimental design using newly developed rain making apparatus and silica as a model material, satisfies the required conditions, and allows simulation of geomorphic instabilities. An example of “plateau instability” modelling is presented to illustrate the suitability of this experimental procedure.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]ARCEAU; International audience; A methodological framework is proposed for designing a multi-scale sampling system to quantify the stream-aquifer interactions in a multi-layer aquifer system. First, geophysical and drilling investigations are performed to assess the regional structure of the aquifer system and the local connectivity between streams and aquifer units. At the catchment scale, the investigations permit to define the composition of the upper aquifer unit. At the local scale, the connectivity status between streams and aquifer units is evaluated using various settings for electrical resistivity tomographies. These geophysical investigations are then used to select local monitoring stations (LMSs) along the stream network. Moreover, piezometric head maps representative of low and high flow regimes are interpolated using geostatistics, which provides distributions of both piezometric heads and standard deviations of the estimation error (STEE). The map of STEE is used to define the location of new piezometers. Altogether, the sampling system allows for monitoring water exchanges on a 40 km2 watershed along 6 km of the stream network, with a finer hydro-geophysical sampling at each LMS. Finally, temperature profiles in the HZ are interpreted with a coupled thermo-hydro finite element code at the upstream station of the domain. Multiple simulations indicate first proof of evidence for a gaining stream in the upstream part of the sampling domain.

International audience; Core Ideas - We tested the sensitivity of seismic measurements to water saturation variations. - We used ultrasonic techniques to reproduce a small-scale seismic acquisition setup. - Measurements were performed on glass bead layers with varying water levels. - Results of 3D numerical modelling were used to validate data at the dry state. - Data difference trends between the dry and wet models match with the water level.Physical properties of soils in the vadose zone, and especially their water content, are characterized by strong spatial and temporal variations mostly driven by weather and anthropogenic activities. To understand this variability and help water resource management, seismic methods have been recently suggested as a complement to electrical and electromagnetic techniques. The simultaneous in situ estimation of pressure (P) and shear (S) wave velocities (VP and VS, respectively) and their ratio (VP/VS) offers novel perspectives for the monitoring of space and time variations of vadose zone mechanical properties. However, the seismic response in partially saturated and unconsolidated soils remains complex and deserves to be studied both theoretically and experimentally. In this study, we tested the sensitivity of seismic measurements (i.e., P-wave travel times and surface-wave phase velocities) to water saturation variations using controlled physical models at the laboratory scale. Ultrasonic techniques were used to reproduce a small-scale seismic acquisition setup at the surface of glass bead layers with varying water levels. Travel times and phase velocity measurements obtained at the dry state were validated with both theoretical models and numerical simulations and serve as reference datasets. The increasing water level clearly affected the recorded wave field in both its phase and amplitude. In these cases, the collected data cannot yet be inverted in the absence of a comprehensive theoretical model for such partially saturated granular media. The differences in travel time and phase velocity observed between the dry and wet models show patterns that interestingly match the observed water level and depth of the capillary fringe, thus offering attractive perspectives for studying soil water content variations.

Laboratory small-scale physical models and non-contacting ultrasonic techniques are used to tackle theoretical or methodological issues of seismic wave propagation and seismic methods. Literature shows a wide range of experiments, both in terms of materials used for the production of the physical models but also regarding the models geometry and the recording techniques, which are chosen according to the issue that is being addressed. With a proper choice of granulometries and deposition processes, we managed here to create a two-layer granular physical model with a relatively complex geometry and characterized by 2D structures, property contrast and velocity gradients within layers. We performed several small scale seismic acquisitions using a mechanical source and a laser vibrometer. The acquired seismograms were interpreted by applying the surface-wave method and by extracting P-wave refraction data. We managed to correctly reconstruct the geometry of the model and estimated the parameters controlling the velocity gradients of P and S waves for both layers. The results we got are coherent with the different compaction degree we obtained for the two layers and with previous studies conducted over similar media.

In many seismic applications, a constant quality factor is used to describe the constitutive laws of viscoelastic materials, characterized by frequency-independent attenuation characteristics. In such cases, the frequency dependence of the medium’s properties is not taken into account. To overcome this drawback, we proposed an elegant finite difference time domain implementation, with an auxiliary differential equation technique to explicitly solve any stress-strain relation. This scheme is inherited from the formalism of electromagnetism and is based on the separation of the propagation equations from the constitutive law defined in the stress-strain equation. The conventional assumption of a constant quality factor assumption can then be easily avoided in the modeling of seismic-wave propagation in viscoelastic media. We developed such a method and simulated synthetic traces over a simple, 2D viscoelastic homogeneous medium using a Zener model. Wave propagation phase velocities were estimated by means of dispersion analysis and appeared to match theoretical values over a reasonable frequency range. We also measured the material’s attenuation behavior by studying the quality factor, thereby validating our approach.

The surface-wave profiling method has been carried out for the estimation of subsurface shear-wave velocities at the Ghor Al-Haditha sinkholes area, in Jordan. Roll along acquisition and dispersion stacking were used to achieve multi-modal dispersion measurements along two profiles. The estimated dispersion showed strong lateral variations, apparently correlated with sinkholes. Preliminary blind inversion results confirmed these correlations: low shear-wave velocity anomalies were noticed in the vicinity of existing sinkholes and in the currently most active collapsing zone. Such anomalies could be explained by a systematic decompaction of shallow sediment layers, associated to the sinkhole development process.

International audience; In sedimentary basins, the emplacement of magmatic stills tends to occur within rock of low mechanical competence and permeability, such as shale. This often contains pore fluidds at abnormalyy high pressure. We first theoretically show that, in anisotropic media, the higher the pore fluid pressure, the deeper the sill empalcement. Then we introduce a new technique for analogue modelling of such intrusive bodies under conditions of fluid overpressure (greater than hydrostatic), which corroborate the teoretical analysis. As an analogue of brittle sediment, we use a diatomite powder. this material is a dry, fine-grained, frictional, cohesive and permeable material. As an analogue for magma, we take a silicone putty (RTV silicone), which is at first Newtonian, but then solidifies at room temperature. We use compressed air as a pore fluid. Under these experimental conditions, we investigate the iontrusion of magma into the host powder under various fluid overpressures. In homogeneous diatomite powder, having uniform cohesion, intrusive bodies are segmented dykes. These become feeders to sills, if the fluid overpressure exceeds the weight of overburden. Where teh sedimentary column has a horizontal discontinuity in strengh, the transition from dyke to sill occurs at a smaller overpressure (hydrostatic < ?ex < 1). As a possible illustration of these results, we consider sills within source rocks of the Neuquén Basin and of the Parana Basin, both in South America, where overpressure may have resulted from maturation of organic materiel.

A 2nd order accurate time, 2nd or 4th order accurate space, 2D P-SV staggered-grid finite-difference scheme is proposed to simulate seismic-wave propagation in unbounded viscoelastic media, for both seismology and engineering applications. The anelastici

Multichannel analysis of surface waves (MASW) can be implemented along linear sections to obtain profiles of subsurface layers shear waves velocities. However, one often overlooks the assumption of horizontally layered medium inherent to the inverse problem required during dispersion curves interpretation. Unfortunately, dipping layers or pronounced interfaces topography can invalidate this assumption. That is why we attempted to evaluate the reliability and the relevance of shear waves velocity profiling using Rayleigh waves in such situations. As a first approach to this problem we used two dimensional finite elements models from which synthetic seismograms were computed. We used a typical in situ investigations source-receivers configuration and compared computed dispersion curves to theoretical ones. Then a parametric study on the first interface slope was performed. Thereby we estimated its influence on Rayleigh waves phase velocity determination.

Surface-wave dispersion inversion is growing in popularity for geotechnical applications, due to its noninvasive character, relative straightforward field procedures and interpretation, especially when the subsurface structure is locally assumed to be one-dimensional (1D). Here, laser-Doppler physical modeling of surface-wave propagation is used to address issues of surface-wave depth penetration, the presence of dipping layers, and the associated limitations and systematic errors propagated in conventional 1D surface-wave inversion. Flat-layered models show that, with an active source and linear spread, the maximum resolvable wavelength of the Rayleigh-wave fundamental mode is on the order of 40% of the spread length. Linearised inversions confirm the rule of thumb that the depth penetration is 20–25% of the spread length, and that correct a priori layer interface depths from refraction analysis allow more accurate results. However, even under optimal conditions, failing to account for a dominant higher mode at low frequency when a stiff shallow layer is present, causes an overestimate of deeper layer shear-wave velocity. Moreover, a layer dip of only a few degrees can significantly bias the surface-wave inversion. If the incorrect a priori information from a single-shot refraction analysis is incorporated in the inverse problem, estimated interface depth depends on the shot position and deeper layer shear-wave velocity is underestimated. Even if correct a priori constraints are used, an underestimate of half-space shear-wave velocity of up to 25% remains.

While surface-wave prospecting methods are classically applied for the one-dimensional (1D) estimation of shear (S-) wave velocities (Vs), two-dimensional (2D) profiling still requires implementing specific processing and inversion tools that are not yet widely available in the community. We present here a free and open-source tool performing surface-wave inversion and profiling (SWIP) in order to retrieve 2D lateral variations of Vs from typical seismic shot records. Windowing and stacking techniques are implemented to compute dispersion images with smooth lateral variations and enhanced signal-to-noise ratio. Dispersion curves are extracted for each window with an error in phase velocity taking into account the higher uncertainties at low frequency. These curves are then inverted for each window position using a Monte Carlo approach and a refraction tomography-based parameterization. Models matching the observed data within the error bars are selected to build a misfit-weighted final model and estimate the investigation depth. Finally, 1D models obtained for each window position are merged into a 2D Vs section.