New robust observables on Rayleigh waves affected by an underground cavity: from numerical to experimental modelling

International audienceThe 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

Small scale modeling, a tool to assess subsurface imaging methods - application to seismic full waveform inversion

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Assessment of physical properties of a sea dike using multichannel analysis of surface waves and 3D forward modeling

International audienceSeismic surface waves analysis methods have been widely developed and tested in the context of subsurface characterization and have demonstrated their effectiveness for sounding and monitoring purposes. Given their efficiency, surface waves methods have been used in a variety of contexts, including civil engineering applications. However, at this particular scale, many structures exhibit 3D geometries which drastically limit the efficiency of these methods since they are mostly developed under the assumption of a semi-infinite 1D layered medium without topography. Taking advantages of wave propagation modeling algorithm development and high-performance computing center accessibility, it is now possible to consider the use of a 3D elastic forward modeling algorithm for the inversion of surface wave dispersion. We use a parallelized 3D elastic modeling code based on the spectral element method which allows to obtain accurate synthetic data with very low numerical dispersion and a reasonable numerical cost. In this study, we choose a sea dike as a case example. We first show that their longitudinal geometry and structure may have a significant effect on dispersion diagrams of Rayleigh waves. Then, we investigate the sensitivity of the dispersion diagrams to small velocity and layer thickness perturbations, and show the limitations of the standard 1D surface wave methods approach. Finally, we demonstrate in this context the benefits of using both a 3D forward modeling engine and the whole dispersion diagram, instead of the dispersion curves only

Ground penetrating radar in the medieval oyster shell middens of Saint-Michel-en-l'Herm (Vendée, France)

International audienceThis study presents the integrated results of GPR (Ground Penetrating Radar) and sedimentological analysis, performed on the giant shell middens of the medieval site of Saint-Michel-en-l'Herm (Vendée, France). Avoiding the use of destructive methods like large-scale digging, the purpose of this study was to determine the internal structure of the northern midden and the links with its substrate, and to obtain insight on the manner of its construction. Four GPR profiles were obtained along the main axis of the midden, on the shell mound, and at its foot, in an area leveled by the former extraction of shells. Sedimentological analysis is also presented from study of the extant parts of the midden and with hand auger cores taken along one of the GPR profiles. The integrated results show that each sedimentological facies is related to a specific radar facies, allowing the mapping of lateral and vertical facies variations along the GPR sections. Within these deposits, the major tilted reflectors correspond to thin soils and trampled surfaces. Their geometry and the stacking pattern highlight multiphase deposition triggered by successive inputs of large volumes of shells (in the range of 100 m3), from west to east. Below the exploited parts of the midden, the data are interpreted to show the presence of undisturbed shells, although, an ancient dug area was also identified by GPR. Finally, sedimentological and GPR data both show evidence of a midden established on the west bank of a channel, progressively infilling the channel eastward. These results illustrate the relevance of GPR in answering major questions common to all large shell middens. © 2018 Elsevier Lt

Numerical Modeling of Surface Waves Over Shallow Cavities

International audienceA 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 welllocalized 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 2-m deep rectangular cavity generates more severe perturbations than a circular section, due to non-symmetrical backscattered 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

Towards multidiffusive ultrasonic propagation for non destructive evaluation of concrete; theoretical overview

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3-D crustal VS model of western France and the surrounding regions using Monte-Carlo inversion of seismic noise cross-correlation dispersion diagrams

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