Search

SUMMARYOver the past decade, ambient seismic noise has been used successfully to monitor various geological objects with high accuracy. Recently, it has been shown that surface seismic waves propagating within a sea dike body can be retrieved from the cross-correlation of ambient seismic noise generated by sea waves. We use sea wave impacts to monitor the response of a sea dike during a tidal cycle using empirical Green’s functions. These are obtained either by cross-correlation or deconvolution, from signals recorded by sensors installed linearly on the crest of a dike. Our analysis is based on delay and spectral amplitude measurements performed on reconstructed surface waves propagating along the array. We show that localized variations of velocity and attenuation are correlated with changes in water level as a probable consequence of water infiltration inside the structure. Sea dike monitoring is of critical importance for safety and economic reasons, as internal erosion is generally only detected at late stages by visual observations. The method proposed here may provide a solution for detecting structural weaknesses, monitoring progressive internal erosion and delineating areas of interest for further geotechnical studies, in view to understand the erosion mechanisms involved.

Identification of different wave types in a seismogram is an important step for the understanding of wave propagation phenomena. Because in most seismograms, different types of waves with different frequencies may appear simultaneously, separation of waves is more effectively achieved when a time–frequency analysis is performed. In this work, we propose a new time–frequency analysis procedure to identify and extract Rayleigh and Love waves from three‐component seismograms. Exploiting the advantage of the absolute phase preservation by the Stockwell transform, we construct time–frequency filters to extract waves based on the normalized inner product (NIP). Because the NIP is the time–frequency counterpart of the correlation, Rayleigh and Love waves can be identified depending on the NIP between the Stockwell transforms of the horizontal and vertical displacement components. The novelty and advantage of the proposed procedure is that it does not require specifying a priori the direction of propagation of the surface waves, but instead such direction is determined. Furthermore, it is shown that the NIP is a more stable parameter in the time–frequency domain when compared to the instantaneous reciprocal ellipticity, and thus it avoids smoothing (and with it, altering) the data. The procedure has been successfully tested with real signals, specifically to extract Rayleigh and Love waves from seismograms of one aftershock of the 1999 Chi‐Chi earthquake. With the proposed procedure, we found different directions of propagation for retrograde and prograde Rayleigh waves, which might suggest that they are generated by different mechanisms.

Site effects may be assessed using a standard soil classification parameter, VS30 (the harmonic average shear-wave velocity in the first 30 m); however, this index does not account for the complexity of the velocity profile, especially its variability at depth. In the present study, in addition to VS30, we propose consideration of the gradient of the VS profile from 0 to 30 m depth, denoted B30. A lower gradient value means low velocity increases with depth; a higher gradient indicates a rapid velocity increase with depth in the shallow layers. In addition, we consider the fundamental resonance frequency of the soil (f0), which has been shown to be a relevant parameter for site-effect assessment and which is obtained from the empirical site response. Using the Japanese KiK-net database, we analyze the variability of the VS profiles and the empirical borehole site responses of selected sites through the VS30, the velocity gradient B30, and f0. We select 289 sites for which the 1D linear numerical modeling is close to the empirical site response and a VS at the downhole station is greater than 1000 m=s. For a given VS30 class, B30, and f0 can be used to distinguish between two types of sites: deep sedimentary sites and sites with high velocity contrast at shallow depths. We find that, even if the gradient is calculated using shallow information, its use improves the site amplification characterization, compared to using only VS30, by reducing the intersite site-response variability. As expected, however, this improvement is limited for deep sedimentary sites. On the other hand, f0 is able to reduce the intersite response variability for deep sedimentary sites even though it is limited to specific VS30 classes. Thus, the combined use of VS30, B30, and f0 improves the assessment of linear site amplification.

In the construction of roads or railways, the capping layer is the last layer of the earthworks phase. This layer can be made of non bound aggregates or by a treatment of a soil with lime and/or hydraulic binder, such as cement or hydraulic road binders. In the latter case, the in situ testing of the capping layer performances should encompass treated soil modulus measurements, but sampling such materials is not often satisfactory, because of the risk of material degradation by the sampling itself. Consequently, a non destructive method, aiming at measuring the modulus of the treated materials, could be very useful. For this reason, we propose to study the feasibility of the seismic surface or guided waves dispersion analysis in order to recover the depth and the S wave velocity of the subgrade. Previous works provided results and analysis of the dispersion curves concerning the pavement auscultation (Ryden et al, 2004). However, in these cases, the subgrade was an underlying layer in the global zone of interest that includes the upper pavement layers where the measurement surface is the thin asphalt layer. In the present study, we focus on the subgrade layer in the case of under construction roads, before the shallower pavement layers are built because it should help to qualify the project acceptance concerning this earthworks phase. In this context, the issue deals with a two layers medium case where the investigated subgrade, whom the top is the measurement surface, lays above a low velocity zone, i.e. the natural soil. As described by Ryden et al. (2004), the resulting dispersion curves in the case of a high velocity upper layer should be typical of Lamb waves dispersion curves and could bring out higher modes that could be difficult to pick. In this case, they advocated the entire dispersion diagram inversion to avoid any subjective picking in the data (Ryden et al, 2006). However, the treated soil can contain heterogeneities unfavourable to the assumption of homogeneous layers presupposed to the dispersion diagram calculation. Thus a first feasibility stage, i.e. a field experimental data acquisition, was conducted to define the ability of seismic data to provide a coherent dispersion diagram in the spectral content required. The dispersion curve have been extracted and inverted with an iterative weighted least squares local minimization method (Hermann, 2002). In order to consider the possibility of inverting the entire dispersion diagram, a second feasibility stage consisted in analysing all the events that possibly occur in the dispersion diagram. For that, the measurement experience was reproduced at reduced scale in laboratory as a perfectly controlled experimental modelling approach. These data and more precisely the dispersion diagram is compared in one hand to the theoretical curves associated to the leakage attenuations and in an other hand to the theoretical dispersion diagram numerically calculated with an original method taking into account the source effects.

4 p.; National audience; Cette étude s'inscrit dans le cadre du programme de Recherche et Développement initié par le Plan Cavité et a pour objectif le développement et l'amélioration de méthodes géophysiques non destructives pour la détection, la caractérisation et la surveillance des cavités souterraines et des désordres associés. La méthode géophysique concernée par cette étude est la tomographie sismique 3D en ondes de surface. Pour mieux comprendre les réponses sismiques provenant de cavités différentes, un code de modélisation élastique 3D en différences finies est utilisé. La méthode de différences finies a été choisie pour sa capacité de prendre en compte des modèles 3D hétérogènes pour pouvoir introduire des configurations géologiques complexes notamment la présence d'une cavité et simuler correctement la propagation des ondes de surface. Les sismogrammes synthétiques ainsi générées ont été utilisées pour tester l'efficacité du code d'inversion tomographique 3D des ondes de surface. Les résultats de l'inversion des temps d'arrivées montrent que les anomalies de perturbations de vitesse sont relativement cohérentes avec la profondeur et la forme réelles des cavités des modèles synthétiques. Si ces méthodologies fonctionnent dans un cadre théorique idéalisé, tel un milieu homogène en l'absence de bruit de mesure, l'ajout de bruit haute fréquence dans les données dégrade fortement les inversions, rendant leurs résultats incertains. Dans ce cas le couplage avec d'autres méthodes géophysiques, serait particulièrement intéressant.

International audience; Cette étude, associant l‘interprétation des données géologiques de surface (structures, lithostratigraphies) et de subsurface (sismiques et gravimétriques) de la région de Kalat Es Senan (Nord-Ouest de la Tunisie m‘ a permis d‘identifier et de connaître (i) la lithologie des formations sédimentaires entre l‘Aptien et l‘actuel, (ii) de les différencier de celles du salifère triasique et (iii) de préciser ainsi la structure des entités structurales de cette région en se réferent aux traces de la tectonique ayant affecté la région durant son histoire géologique. En plaçant la région d‘étude dans son cadre géographique qui est celui de la Tunisie, localisée sur la marge sud de la Téthys,il m‘a été possible d‘en conclure, à travers l‘analyse tectonique et structurale menée, qu‘au cours du Trias, le rifting a déclenché des évènements tectoniques distensifs au Crétacé et extensifs au Paléogène. Les corrélations lithostratigraphiques établies entre les affleurements et les données sismiques, calées sur celles des 6 puits pétroliers réalisées dans la région, nous ont permis de mieux comprendre la complexité structurale et le rôle des différents accidents tectoniques ayant contribué à la configuration actuelle, marquée par la présence de fossés d‘effondrement. En effet, trois directions de failles NW-SE, NNW-SSE à NS et NE-SW à EW ont eu un rôle majeur dans la genèse des plis et l‘ouverture des fossés d‘effondrement de direction NW-SE. Ces résultats ont été complétés par des données de sismique réflexion pour préciser la géométrie des zones méridionale et occidentale du fossé de Kalaa Khasba. Les lignes sismiques sélectionnées pour cette étude ont permis de caractériser les principales structures, avec les cartes en isochrone, isobathe et isovitesse de l‘horizon Serdj qui présente le principal réservoir de la région. La ligne sismique L2, calée par le puits 6, a permis de mettre en évidence la compression NW-SE qui s‘est traduite par des discordances persistantes largement identifiables sur sa colonne lithostratigraphique (fig. 1). L‘étude gravimétrique a confirmé l‘extension de la majeure partie des accidents profonds en subsurface dont l‘activité semble continuer jusqu‘à présent. La gravimétrie a également conforté l‘interprétation sismique en confirmant, au niveau du puits L2, que les deux flancs SW et NE du fossé sont cadrés par deux failles opposées qui tracent ainsi les limites du graben de direction NNW-SSE et dont la sédimentation est d‘âge Mio-pliocène et Quaternaire (fig. 2).