Guidelines for the good practice of surface wave analysis: a product of the InterPACIFIC project

Surface wave methods gained in the past decades a primary role in many seismic projects. Specifically, they are often used to retrieve a 1D shear wave velocity model or to estimate the VS,30 at a site. The complexity of the interpretation process and the variety of possible approaches to surface wave analysis make it very hard to set a fixed standard to assure quality and reliability of the results. The present guidelines provide practical information on the acquisition and analysis of surface wave data by giving some basic principles and specific suggestions related to the most common situations. They are primarily targeted to non-expert users approaching surface wave testing, but can be useful to specialists in the field as a general reference. The guidelines are based on the experience gained within the InterPACIFIC project and on the expertise of the participants in acquisition and analysis of surface wave data.Published2367-24205T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa

Caractérisation sismique de sites hétérogènes à partir de méthodes actives et passives : variations latérales et temporelles

Shear wave velocity (Vs) is an elastic parameter characterizing the stiffness of super- ficial formations. It is important both in applied seismology, for site effect or liquefaction estimation, and in civil engineering, for landslide investigation. This PhD work deals with the different seismic techniques available for deriving Vs values from ten to hundred meters depth. It aims at evaluating the capabilities of the different methods and defining a site characterization strategy adapted to their complexity. The first part of the work is devoted to the assessment of the surface wave inversion reliability for horizontally stratified medium. In collaboration with other European laboratories, active and passive seismic methods were applied at 20 European strong motion sites for which Vs profiles are known from borehole investigations. Comparison of results show that Vs profiles derived from surface (active and passive) seismics are similar for soft sites (Vs30 < 600 m/s) to those derived from borehole methods but present lower velocities for stiffer sites. Moreover, the information contained in dispersion curves appeared to be sufficient for EC8 site characterization. On the basis of the measured Vs profiles, a theoretical study on Rayleigh wave fundamental mode inversion was undertaken for sites with a strong contrast. A two step inversion procedure allowing bedrock depth estimation during the inversion process was defined on synthetic cases and successfully applied to horizontally layered real sites. More complex geometries are addressed in the second part. The inversion of dispersion curves obtained from active (synthetics and real data) and passive (synthetics) seismics over a dipping interface resulted in Vs profiles with a velocity gradient and misestimated bedrock depth and/or velocity. A procedure for detecting 2D/3D sites was defined from data of the 20 strong motion sites and of the Avignonet landlside (Isere, France). Based on active (seismogram, lateral spectrogram, dispersion curve comparison) and passive (shape of the H/V ratio, lateral variation of the H/V frequency peak) seismics, this procedure aims at adapting the choice of imaging technique during site characterization. In the third part, the ambient noise cross-correlation technique is applied to the Avignonet landslide. Thirteen seismological stations were settled on site during two weeks, and a tomographic inversion of arrival times measured with cross-correlation resulted in Rayleigh wave group velocity frequency maps. Their inversion allowed 3D Vs imaging of the landslide, down to 100 m depth. Finally, the evolution with time of ambient noise cross-correlations computed between two permanent stations on the landslide suggests a slight decrease in velocity over the three years of recordings. These results enlighten the usefulness of cross-correlation techniques for 3D investigation of smooth heterogeneous sites and for landslide monitoring.La vitesse des ondes de cisaillement (Vs) est un parametre elastique permettant de caracteriser la compacite des formations superficielles. Il joue un role essentiel aussi bien en sismologie appliquee, dans l'estimation des effets de site ou du potentiel de liquefaction des sols, qu'en geotechnique pour la reconnaissance des glissements de terrain. Ce travail de these s'interesse aux differentes techniques sismiques permettant d'obtenir les valeurs de Vs a l'echelle de la dizaine a la centaine de metres de profondeur. Il vise a evaluer les capacites de ces methodes et a definir une strategie de reconnaissance des sites en fonction de leur complexite. La premiere partie du travail est consacree a l'evaluation de la fiabilite de l'inversion des ondes de surface pour des milieux stratifies horizontalement. En collaboration avec d'autres laboratoires europeens, les methodes de sismique active et passive ont ete appliquees sur 20 sites accelerometriques europeens pour lesquels des profils verticaux de Vs etaient disponibles a partir d'essais en forage. La comparaison des resultats montre que les profils de Vs obtenus par les techniques sismiques de surface sont comparables a ceux deduits des methodes en forage pour les sites mous (Vs30 < 600 m/s) mais fournissent des vitesses plus faibles pour les sites compacts. De plus, l'information fournie par les courbes de dispersion obtenues est apparue suffisante pour la caracterisation des sites selon la classification EC8. Sur la base des profils Vs mesures, une etude theorique sur l'inversion de la courbe de dispersion du mode fondamental de Rayleigh a ete realisee pour des sites presentant un contraste marque entre socle et sediments. Une strategie d'inversion en deux etapes, permettant de contraindre la profondeur du substratum durant l'inversion, a ete definie sur des cas synthetiques et a ete appliquee avec succes sur des cas reels stratifies horizontalement. Le cas de sites de geometrie plus complexe est aborde dans la deuxieme partie. L'inversion des courbes de dispersion obtenues par sismique active (donnees synthetiques et reelles) et passive (donnees synthetiques) au-dessus d'une interface en pente a fourni des profils de vitesse presentant un gradient de vitesse et une mauvaise estimation des vitesse et/ou profondeur du demi-espace. Une procedure a ete definie afin de detecter les sites 2D/3D, a partir des donnees des 20 sites accelerometriques europeens et des donnees du glissement de terrain d'Avignonet (Isere, France). Cette procedure, basee sur les observables de la sismique active (sismogrammes, spectrogramme spatial, comparaison de courbes de dispersion) et de la sismique passive (forme du rapport H/V, variations laterales de la frequence du pic H/V), vise a adapter le choix des techniques d'imagerie lors de la caracterisation de site. Dans la troisieme partie, la technique d'inter-correlation du bruit ambiant est appliquee au glissement de terrain d'Avignonet. Treize stations sismologiques ont ete deployees durant deux semaines sur le site et une inversion tomographique des temps d'arrivees mesures par inter-correlation a fourni des cartes frequentielles de vitesse de groupe des ondes de Rayleigh. L'inversion de ces donnees a permis d'obtenir une image 3D en Vs du glissement de terrain jusqu'a 100 m de profondeur. Enfin, l'evolution avec le temps des correlations calculees entre 2 stations permanentes sur le glissement suggere une legere diminution de la vitesse sismique au cours des trois ans d'enregistrement. Ces resultats montrent l'interet des techniques d'inter-correlation du bruit ambiant pour la reconnaissance tridimensionnelle de milieux heterogenes et le suivi temporel de l'activite des glissements de terrain

Caractérisation sismique de sites hétérogènes à partir de méthodes actives et passives : variations latérales et temporelles

Shear wave velocity (Vs) is an elastic parameter characterizing the stiffness of super- ficial formations. It is important both in applied seismology, for site effect or liquefaction estimation, and in civil engineering, for landslide investigation. This PhD work deals with the different seismic techniques available for deriving Vs values from ten to hundred meters depth. It aims at evaluating the capabilities of the different methods and defining a site characterization strategy adapted to their complexity. The first part of the work is devoted to the assessment of the surface wave inversion reliability for horizontally stratified medium. In collaboration with other European laboratories, active and passive seismic methods were applied at 20 European strong motion sites for which Vs profiles are known from borehole investigations. Comparison of results show that Vs profiles derived from surface (active and passive) seismics are similar for soft sites (Vs30 < 600 m/s) to those derived from borehole methods but present lower velocities for stiffer sites. Moreover, the information contained in dispersion curves appeared to be sufficient for EC8 site characterization. On the basis of the measured Vs profiles, a theoretical study on Rayleigh wave fundamental mode inversion was undertaken for sites with a strong contrast. A two step inversion procedure allowing bedrock depth estimation during the inversion process was defined on synthetic cases and successfully applied to horizontally layered real sites. More complex geometries are addressed in the second part. The inversion of dispersion curves obtained from active (synthetics and real data) and passive (synthetics) seismics over a dipping interface resulted in Vs profiles with a velocity gradient and misestimated bedrock depth and/or velocity. A procedure for detecting 2D/3D sites was defined from data of the 20 strong motion sites and of the Avignonet landlside (Isere, France). Based on active (seismogram, lateral spectrogram, dispersion curve comparison) and passive (shape of the H/V ratio, lateral variation of the H/V frequency peak) seismics, this procedure aims at adapting the choice of imaging technique during site characterization. In the third part, the ambient noise cross-correlation technique is applied to the Avignonet landslide. Thirteen seismological stations were settled on site during two weeks, and a tomographic inversion of arrival times measured with cross-correlation resulted in Rayleigh wave group velocity frequency maps. Their inversion allowed 3D Vs imaging of the landslide, down to 100 m depth. Finally, the evolution with time of ambient noise cross-correlations computed between two permanent stations on the landslide suggests a slight decrease in velocity over the three years of recordings. These results enlighten the usefulness of cross-correlation techniques for 3D investigation of smooth heterogeneous sites and for landslide monitoring.La vitesse des ondes de cisaillement (Vs) est un parametre elastique permettant de caracteriser la compacite des formations superficielles. Il joue un role essentiel aussi bien en sismologie appliquee, dans l'estimation des effets de site ou du potentiel de liquefaction des sols, qu'en geotechnique pour la reconnaissance des glissements de terrain. Ce travail de these s'interesse aux differentes techniques sismiques permettant d'obtenir les valeurs de Vs a l'echelle de la dizaine a la centaine de metres de profondeur. Il vise a evaluer les capacites de ces methodes et a definir une strategie de reconnaissance des sites en fonction de leur complexite. La premiere partie du travail est consacree a l'evaluation de la fiabilite de l'inversion des ondes de surface pour des milieux stratifies horizontalement. En collaboration avec d'autres laboratoires europeens, les methodes de sismique active et passive ont ete appliquees sur 20 sites accelerometriques europeens pour lesquels des profils verticaux de Vs etaient disponibles a partir d'essais en forage. La comparaison des resultats montre que les profils de Vs obtenus par les techniques sismiques de surface sont comparables a ceux deduits des methodes en forage pour les sites mous (Vs30 < 600 m/s) mais fournissent des vitesses plus faibles pour les sites compacts. De plus, l'information fournie par les courbes de dispersion obtenues est apparue suffisante pour la caracterisation des sites selon la classification EC8. Sur la base des profils Vs mesures, une etude theorique sur l'inversion de la courbe de dispersion du mode fondamental de Rayleigh a ete realisee pour des sites presentant un contraste marque entre socle et sediments. Une strategie d'inversion en deux etapes, permettant de contraindre la profondeur du substratum durant l'inversion, a ete definie sur des cas synthetiques et a ete appliquee avec succes sur des cas reels stratifies horizontalement. Le cas de sites de geometrie plus complexe est aborde dans la deuxieme partie. L'inversion des courbes de dispersion obtenues par sismique active (donnees synthetiques et reelles) et passive (donnees synthetiques) au-dessus d'une interface en pente a fourni des profils de vitesse presentant un gradient de vitesse et une mauvaise estimation des vitesse et/ou profondeur du demi-espace. Une procedure a ete definie afin de detecter les sites 2D/3D, a partir des donnees des 20 sites accelerometriques europeens et des donnees du glissement de terrain d'Avignonet (Isere, France). Cette procedure, basee sur les observables de la sismique active (sismogrammes, spectrogramme spatial, comparaison de courbes de dispersion) et de la sismique passive (forme du rapport H/V, variations laterales de la frequence du pic H/V), vise a adapter le choix des techniques d'imagerie lors de la caracterisation de site. Dans la troisieme partie, la technique d'inter-correlation du bruit ambiant est appliquee au glissement de terrain d'Avignonet. Treize stations sismologiques ont ete deployees durant deux semaines sur le site et une inversion tomographique des temps d'arrivees mesures par inter-correlation a fourni des cartes frequentielles de vitesse de groupe des ondes de Rayleigh. L'inversion de ces donnees a permis d'obtenir une image 3D en Vs du glissement de terrain jusqu'a 100 m de profondeur. Enfin, l'evolution avec le temps des correlations calculees entre 2 stations permanentes sur le glissement suggere une legere diminution de la vitesse sismique au cours des trois ans d'enregistrement. Ces resultats montrent l'interet des techniques d'inter-correlation du bruit ambiant pour la reconnaissance tridimensionnelle de milieux heterogenes et le suivi temporel de l'activite des glissements de terrain

Dépasser les conflits pour amener la coopération en CE2

This thesis presents the experimentation carried out in a CE2 class of a popular area, aiming the reflection on the concepts of tolerance and cooperation and their implementation. The children's literature album Sophie la vache musicienne (Geoffroy de Pennart) served as a cultural entry. Philosophical debates and discussions followed. At the same time, cooperative practices were initiated within the classroom, such as the use of I-messages or cooperation councils. The various sessions enabled the majority of students to understand situations of exclusion as well as the concepts of tolerance and cooperation. On the other hand, they have always had difficulty, and sometimes even failed to adopt the reflective posture necessary to have the listening and speaking skills essential for the development of collective thought within the class. We tried to make the link between our observations, the postures of students and teachers described in the literature, and the explicit teaching recommended by the instructors of the INSPE, to propose an adaptation of the upstream work of the teacher needed to guide students towards this attitude.Cet écrit scientifique réflexif présente l’expérimentation menée dans une classe de CE2 d’un quartier populaire visant la réflexion sur les notions de tolérance et de coopération et leur mise en pratique. L’album de littérature jeunesse Sophie la vache musicienne (Geoffroy de Pennart) a servi d’entrée culturelle. S’en sont suivis des débats et discussions à visée philosophique (DVP). En parallèle, des dispositifs coopératifs ont été initiés au sein de la classe, tels l’utilisation des messages clairs ou les conseils de coopération. Les différentes séances ont permis à la majorité des élèves de comprendre les situations d’exclusion ainsi que les notions de tolérance et de coopération. Ils ont par contre toujours eu du mal, voire n’ont parfois pas réussi à adopter la posture réflexive nécessaire pour avoir les qualités d’écoute et d’expression indispensables au développement d’une pensée collective au sein de la classe. Nous avons tenté de faire le lien entre nos observations, les postures d’élèves et d’enseignants décrits dans la littérature, et l’enseignement explicite recommandé par les formateurs de l’INSPE, pour proposer une adaptation du travail amont de l’enseignant nécessaire au guidage des élèves vers cette attitude

Geophysical investigation of a large landslide in glaciolacustrine clays in the Trièves area (French Alps)

International audienceSlope movements in clay deposits are spread all over the world and result from complex deformation processes, including internal strains in the landslide body and slipping along rupture surfaces. Such mass movements are likely to generate changes in the geophysical parameters characterizing the ground, which can be used to map the landslide body. In the last decade, geophysical techniques have been increasingly used for landslide investigation purposes. However, the success of any geophysical technique is overall controlled by the existence of a geophysical contrast differentiating the body to be mapped. For landslides affecting thick clay materials (from soft clay to shale or marl), electrical and seismic techniques have been mainly applied in the past. In this study, we attempt to physically characterize the deformation within a large slide (Avignonet) affecting laminated clays which were deposited in a glacially dammed lake during the Würm period. Clay deposits, which cover an area of 300 km2 south of Grenoble (French Alps) and have a maximum thickness of 200 m, overlay compact alluvial layers and marly limestone of Mesozoic age. Piezometric data at Avignonet show that the water table is very shallow, implying that the slide developed in saturated clay. Several seismic and electrical profiles were performed in order to tentatively correlate the variations of P-wave (Vp) velocity, S-wave velocity (Vs) and electrical resistivity with geotechnical data and morphological observations. In such saturated and fine material, it turned out that only the S-wave velocity exhibits significant variations with the displacement rates and the morphological features. Vs values at shallow depth were found to be inversely correlated with displacement rates measured by GPS, with a division by at least a factor of 2 between the zones unaffected and strongly deformed by the landslide. These results suggest that Vs mapping could provide valuable information on the deformation state of the clay material and that the evolution of Vs with time could be used as an indicator for characterizing the landslide activity in the subsurface, including the evolution into a flow

Characterization of site conditions (soil class, VS30, velocity profiles) for 33 stations from the French permanent accelerometric network (RAP) using surface-wave methods

International audienc