Options
Wathelet, M.
Loading...
6 results
Now showing 1 - 6 of 6
- PublicationOpen AccessDerivation of Vs30 f rom dispersion curve: skipping the inversion step?(2010-09-06)
; ; ; ; ; ; ; ; ;Cornou, C.; LGIT, University J. Fourier, Grenoble, France ;Renalier, F.; LGIT, University J. Fourier, Grenoble, France ;Endrun, B.; University of Potsdam, Germany ;Di Giulio, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Ohrnberger, M.; University of Potsdam, Germany ;Savvaidis, A.; ITSAK, Thessaloniki, Greece ;Wathelet, M.; LGIT, University J. Fourier, Grenoble, France ;Bard, P.; LGIT, University J. Fourier, Grenoble, France; ; ; ; ; ; ; In the framework of the EU-NERIES project, 20 sites among all European strong motion sites in Italy, Greece, T urkey and France were selected to be representative of most common soil classes, and for which shear-wave velocities from borehole measurements (cross-hole and down-hole tests) are available. Passive (array noise) and active experiments have been carried out at these sites in order to evaluate the ability of surface waves technique to provide reliable estimates of shear-wave velocity profiles. In order to stay cheap and feasible, active seismic experiments involving 24 geophones and hammer source were carried out at all sites. Data were processed by using the MASW technique and Rayleigh and Love waves dispersion curves were retrieved from 5-10 Hz to 30-50 Hz. Passive array experiments were also performed by using 8 seismological stations linked with wireless connections and monitored with near real-time processing. Combining up to four different arrays with aperture ranging from 10 m and to 900 m, Rayleigh and Love waves dispersion curves were derived over a broad frequency range (from 0.5 Hz up to 45 Hz) by using the FK and MSPAC techniques. At about 75% sites, dispersion curves from ambient vibration and MASW are in good agreement over the overlapping frequency band. T he other 25% sites correspond to complex geometrical site structures. Whatever the site, passive experiments are shown to be very suitable to retrieve accurate estimates of phase velocities at high frequency (over 20-30 Hz). T his experiment also clearly outlined the limited penetration depth (comprised between 15 and 25 m) of the MASW technique. Inversion of dispersion curves to derive shear-wave profiles and EC8 site class (which is mainly based on Vs30) is a difficult and highly debated issue. Here we test an alternative to get average shear-wave profiles and especially Vs30 from the dispersion curves only. For these 20 sites, we show that site classes may be estimated directly from the dispersion curves. T heses results are confirmed by an extensive study involving about 800 velocity profiles from real sites.264 136 - PublicationRestrictedHigh-resolution shallow seismic tomography of a hydrothermal area: application to the Solfatara, Pozzuoli(2012)
; ; ; ; ; ; ; ; ;Letort, J.; ISTERRE, Institut des Sciences de la Terre, CNRS UMR 5275, Universit´e Grenoble 1, France. ;Roux, P.; ISTERRE, Institut des Sciences de la Terre, CNRS UMR 5275, Universit´e Grenoble 1, France. ;Vandemeulebrouck, J.; ISTERRE, Institut des Sciences de la Terre, CNRS UMR 5275, Université de Savoie, Chamb´ery, France ;Coutant, O.; ISTERRE, Institut des Sciences de la Terre, CNRS UMR 5275, Universit´e Grenoble 1, France. ;Cros, E.; ISTERRE, Institut des Sciences de la Terre, CNRS UMR 5275, Universit´e de Savoie, Chamb´ery, France ;Wathelet, M.; Dipartimento di Scienze della Terra, Universit`a di Perugia, Perugia, Italy ;Cardellini, C.; Dipartimento di Scienze della Terra, Universit`a di Perugia, Perugia, Italy ;Avino, R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia; ; ; ; ; ; ; The Solfatara is one of the major volcanoes of the Phlegrean Fields (Campi Flegrei) volcanic complex, and it is located in a densely populated area a few kilometres west of the city of Naples. It is an active resurgent caldera that has been characterized by a rich history of surface–ground deformation and soil diffuse degassing and fumarolic emissions, which are indications of the top of a hydrothermal plume. A seismic survey was completed in May 2009 for the characterization of the main subsurface features of the Solfatara. Using the complete data set, we have carried out surface wave inversion with high spatial resolution. A classical minimization of a least-squares objective function was first computed to retrieve the dispersion curves of the surface waves. Then, the fitting procedure between the data and a three-sedimentlayer forward model was carried out (to a depth of 7 m), using an improved version of the neighbourhood algorithm. The inversion results indicate a NE-SW fault, which is not visible at the surface. This was confirmed by a temperature survey conducted in 2010. A passive seismic experiment localized the ambient noise sources that correlate well with the areas of high CO2 flux and high soil temperatures. Finally, considering that the intrinsic attenuation is proportional to the frequency, a centroid analysis provides an overview of the attenuation of the seismic waves, which is closely linked to the petrophysical properties of the rock. These different approaches that merge complete active and passive seismic data with soil temperature and CO2 flux maps confirm the presence of the hydrothermal system plume. Some properties of the top of the plume are indicated and localized.326 29 - PublicationRestrictedDeriving Wavefield Characteristics and Shear-Velocity Profiles from Two-Dimensional Small-Aperture Arrays Analysis of Ambient Vibrations in a Small-Size Alluvial Basin, Colfiorito, Itaky(2006-10)
; ; ; ; ; ;Di Giulio, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Cornou, C.; Laboratoire de Geophysique Interne et Tectonophysique, Grenoble ;Ohrnberger, M.; Institut fur Geowissenschaften, Universitat Potsdam ;Wathelet, M.; Laboratoire de Geophysique Interne et Tectonophysique, Grenoble ;Rovelli, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; ; We analyze the dispersion characteristics of ambient noise vibrations. For this purpose, two-dimensional (2D) seismic array data were acquired in four different sites in the Colfiorito plain, an alluvial intramountain basin that exhibits strong site effects. Assuming seismic noise being mainly composed of surface waves, we derive one-dimensional (1D) shallow shear-velocity profiles through the inversion of dispersion curves measured by frequency–wavenumber (f-k) methods. The inverted shear-wave velocity profiles are consistent with a priori information for those sites that can be approximated by 1D simple models. In these cases, the use of passive records of seismic vibrations can be a valuable tool for determining the shallow velocity profile if a detailed depiction of velocity structure is not required. The theoretical dispersion curves for Rayleigh and Love waves were compared with the measured dispersion curves for vertical and horizontal components, respectively. This allows us to discuss qualitatively the composition of ambient vibrations (outlining a large proportion of Love waves in the noise wave field) and the effects of higher modes. We also use the single-station method for investigating the origin of the horizontal-to-vertical (H/V) peak in the plain of Colfiorito in terms of ellipticity of the fundamental Rayleigh mode.199 33 - PublicationOpen AccessInversion of surface wave dispersion at european strong motion sites using a multi-model parameterization and an information-theoretic approach(2010)
; ; ; ; ; ; ; ; ; ;Di Giulio, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Savvaidis, A.; ITSAK, Greece ;Theodoulidis, N; ITSAK, Greece ;Ohrnberger, M.; University of Potsdam, Germany ;Endrun, B.; University of Potsdam, Germany ;Wathelet, M.; LGIT, Grenoble, France ;Cornou, C.; LGIT, Grenoble, France ;Renalier, F.; LGIT, Grenoble, France ;Bard, P.; LGIT, Grenoble, France; ; ; ; ; ; ; ; Within the scope of the EC-projects NERIES and ITSAK-GR we have applied a procedure able to combine a multi-model space parameterization and an information theoretic approach in analysis of dispersion curve inversion. In detail we considered the dispersion curve assessed at 14 strong motion European sites. At each site we investigated the model space through four different parameterization groups within the wavelength range estimated by actual dispersion curves. In order to explore the influence of model space we increased progressively the number of layers for each parameterization. We therefore addressed the model evaluation among a set of competing models obtained by inversion following the corrected Akaike’s Information Criterion(AICc). By using such information-theoretic approach, we found an acceptable agreement between the inverted shear-velocity profiles of the best models and the available borehole results.177 527 - PublicationRestrictedGround structure imaging by inversions of Rayleigh wave ellipticity: sensitivity analysis and application to European strong-motion sites(2013-01)
; ; ; ; ; ; ; ; ; ; ; ; ;Hobiger, M.; Institut des Sciences de la Terre (ISTerre), IRD, IFSTTAR, CNRS, Université de Grenoble I, France ;Cornou, C.; Institut des Sciences de la Terre (ISTerre), IRD, IFSTTAR, CNRS, Université de Grenoble I, France ;Wathelet, M.; Institut des Sciences de la Terre (ISTerre), IRD, IFSTTAR, CNRS, Université de Grenoble I, France ;Di Giulio, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Knapmeyer-Endrun, B.; Institut für Geowissenschaften, Universität Potsdam, Germany ;Renalier, F.; Institut des Sciences de la Terre (ISTerre), IRD, IFSTTAR, CNRS, Université de Grenoble I, France ;Bard, P. Y.; Institut des Sciences de la Terre (ISTerre), IRD, IFSTTAR, CNRS, Université de Grenoble I, France ;Savvaidis, A.; Institute of Engineering Seismology and Earthquake Engineering (ITSAK - EPPO), Thessaloniki, Greece ;Hailemikael, S.; Dipartimento della Protezione Civile, Rome, Italy ;Bihan, N.; GIPSA-Lab, CNRS, Grenoble, France ;Ohrnberger, M.; Institut für Geowissenschaften, Universität Potsdam, Germany ;Theodoulidis, N.; Institute of Engineering Seismology and Earthquake Engineering (ITSAK - EPPO), Thessaloniki, Greece; ; ; ; ; ; ; ; ;; ; The knowledge of the local soil structure is important for the assessment of seismic hazards. A widespread, but time-consuming technique to retrieve the parameters of the local underground is the drilling of boreholes. Another way to obtain the shear wave velocity profile at a given location is the inversion of surface wave dispersion curves. To ensure a good resolution for both superficial and deeper layers, the used dispersion curves need to cover a wide frequency range. This wide frequency range can be obtained using several arrays of seismic sensors or a single array comprising a large number of sensors. Consequently, these measurements are time-consuming. A simpler alternative is provided by the use of the ellipticity of Rayleigh waves. The frequency dependence of the ellipticity is tightly linked to the shear wave velocity profile. Furthermore, it can be measured using a single seismic sensor. As soil structures obtained by scaling of a given model exhibit the same ellipticity curve, any inversion of the ellipticity curve alone will be ambiguous. Therefore, additional measurements which fix the absolute value of the shear wave velocity profile at some points have to be included in the inversion process. Small-scale spatial autocorrelation measurements or MASW measurements can provide the needed data. Using a theoretical soil structure, we show which parts of the ellipticity curve have to be included in the inversion process to get a reliable result and which parts can be omitted. Furthermore, the use of autocorrelation or high-frequency dispersion curves will be highlighted. The resulting guidelines for inversions including ellipticity data are then applied to real data measurements collected at 14 different sites during the European NERIES project. It is found that the results are in good agreement with dispersion curve measurements. Furthermore, the method can help in identifying the mode of Rayleigh waves in dispersion curve measurements.302 63 - PublicationRestrictedExploring the model space and ranking a best class of models in surface- wave dispersion inversion: Application at European strong-motion sites(2012-05-02)
; ; ; ; ; ; ; ; ; ;Di Giulio, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Savvaidis, A.; Institute of Engineering Seismology & Earthquake Engineering, EPPO-ITSAK, Thessaloniki, Greece ;Ohrnberger, M.; University of Potsdam, Institute of Earth and Environmental Science, Potsdam, Germany ;Wathelet, M.; Université Joseph Fourier, Institut des Sciences de la Terre, Grenoble, France ;Cornou, C.; Université Joseph Fourier, Institut des Sciences de la Terre, Grenoble, France ;Knapmeyer-Endrun, B.; University of Potsdam, Institute of Earth and Environmental Science, Potsdam, Germany ;Renalier, F.; Université Joseph Fourier, Institut des Sciences de la Terre, Grenoble, France ;Theodoulidis, N.; Institute of Engineering Seismology & Earthquake Engineering, EPPO-ITSAK, Thessaloniki, Greece ;Bard, P. Y.; Université Joseph Fourier, Institut des Sciences de la Terre, Grenoble, France; ; ; ; ; ; ; ; The inversion of surface-wave dispersion curve to derive shear-wave velocity profile is a very delicate process dealing with a non-unique problem, which is strongly dependent on the model space parameterization. When independent and reliable information are not available, the selection of most representative models within the ensemble produced by the inversion is often difficult. We present a strategy in the inversion of dispersion curves able to investigate the influence of the parameterization of the model space, and to select a ‘’best’’ class of models. We analyze surface-wave dispersion curves measured at 14 European strong-motion sites within the EC-project NERIES. We focus on the inversion task exploring the model space by means of four distinct parameterization classes composed of layers progressively added over a half-space. The classes differ in the definition of the shear-wave velocity profile; we consider models with uniform velocity as well as models with increasing velocity with depth. At each site and for each model parameterization, we perform an extensive surface-wave inversion (200100 models for 5 seeds) using the conditional neighbourhood algorithm. We address the model evaluation following the corrected Akaike’s Information Criterion (AICc) which combines the concept of misfit to the number of degrees of freedom (dof) of the system. The misfit is computed as least-squares estimation between theoretical and observed dispersion curve. The model complexity is accounted in a penalty term by AICc. By applying such inversion strategy on 14 strong-motion sites, we find that the best parameterization of the model space is mostly 3-4 layers over a half-space; where the shear-wave velocity of the uppermost layers can follow uniform or power-law dependence with depth. The shear-wave velocity profiles derived by inversion agree with shear-wave velocity profiles provided by borehole surveys at approximately 80% of the sites.175 38