A nodal high-order discontinuous Galerkin method for elastic wave propagation in arbitrary heterogeneous media

International audienceWe present an extension of the nodal discontinuous Galerkin method for elastic wave propagation to high interpolation orders and arbitrary heterogeneous media. The high-order lagrangian interpolation is based on a set of nodes with excellent interpolation properties in the standard triangular element. In order to take into account highly variable geological media, another set of suitable quadrature points is used where the physical and mechanical properties of the medium are defined. We implement the methodology in a 2-D discontinuous Galerkin solver. First, a convergence study confirms the hp-convergence of the method in a smoothly varying elastic medium. Then, we show the advantages of the present methodology, compared to the classical one with constant properties within the elements, in terms of the complexity of the mesh generation process by analysing the seismic amplification of a soft layer over an elastic half-space. Finally, to verify the proposed methodology in a more complex and realistic configuration , we compare the simulation results with the ones obtained by the spectral element method for a sedimentary basin with a realistic gradient velocity profile. Satisfactory results are obtained even for the case where the computational mesh does not honour the strong impedance contrast between the basin bottom and the bedrock

PRENOLIN project. Results of the validation phase at sendai site

One of the objectives of the PRENOLIN project is the assessment of uncertainties associated with non-linear simulation of 1D site effects. An international benchmark is underway to test several numerical codes, including various non-linear soil constitutive models, to compute the non-linear seismic site response. The preliminary verification phase (i.e. comparison between numerical codes on simple, idealistic cases) is now followed by the validation phase, which compares predictions of such numerical estimations with actual strong motion data recorded from well-known sites. The benchmark presently involves 21 teams and 21 different non-linear computations. Extensive site characterization was performed at three sites of the Japanese KiK-net and PARI networks. This paper focuses on SENDAI site. The first results indicate that a careful analysis of the data for the lab measurement is required. The linear site response is overestimated while the non-linear effects are underestimated in the first iteration. According to these observations, a first set of recommendations for defining the non-linear soil parameters from lab measurements is proposed. PRENOLIN is part of two larger projects: SINAPS@, funded by the ANR (French National Research Agency) and SIGMA, funded by a consortium of nuclear operators (EDF, CEA, AREVA, ENL)

A look at the blind Kumamoto experiment: combining active and passive seismic observations to avoid Rayleigh-wave mode misidentification

International audienceAbstract We present our pathway through participation in the blind Kumamoto exercise, particularly the Step 1 of site characterization. The combination of passive and active seismic imaging techniques is used to image the velocity profile beneath the KUMA site. The estimation of the broadband Rayleigh wave dispersion curve is based on cross-correlations of ambient seismic noise and analysis of active seismic shots. We calculate correlations from the entire time series (only vertical components) of each seismic array after classical pre-processing of ambient noise data. Then, a passive seismic section is constructed using all available stations pairs and stacking the cross-correlation traces with similar interstation distances. The obtained passive seismic section is analyzed using a high-resolution Radon transform to obtain the dispersion image of Rayleigh waves traveling through the KUM-LL, KUM-M and KUM-SM arrays. Then, the information is merged and interpolated to obtain the final broadband dispersion curve. In addition, active source seismic data are used with the high-resolution Radon technique to constrain the model at shallow depths ( 1 km). After comparison of the submitted dispersion curve to the theoretical dispersion curve for the preferred model (Step 4 of the blind test), the authors note that there was a clear misinterpretation in the fundamental mode of their submitted results, especially at frequencies higher than 5 Hz. Using both fundamental (only visible in the passive data set) and first overtones of Rayleigh waves (only visible in the active seismic data set) a refined velocity model could have been inferred, but we decided to keep our first submitted result. This detailed interpretation should be further studied as dispersion images from forward and backward hammer shots are quite different, which may indicate strong variations in the geometry and/or shear-wave velocities of the first meters of the subsurface. Graphical Abstrac

Damage Evaluation Of RC Building With Soil-Structure Interaction By Seismic Interferometry: A Numerical Case Study

International audienc

Velocidades de ondas Scholte en sedimentos marinos: sensibilidad a las propiedades geoacústicas y petrofísicas

En este trabajo se propone la implementación de un flujo de trabajo basado en modelos de física de rocas para la estimación de las propiedades elásticas de las rocas sedimentarias poco consolidadas de la corteza oceánica, el ajuste de velocidades sónicas compresionales, la predicción de las velocidades de corte y el cálculo y análisis de las curvas de dispersión de velocidades de ondas Scholte Vsh asociadas (para el modo fundamental y el primer modo superior). Para ilustrar el procedimiento se utilizaron datos de dominio público correspondientes a dos pozos del programa IODP en el Golfo de Bengala (océano Índico). Para la calibración del modelo se utilizan las mediciones de velocidad sónica compresional, densidad, porosidad, fracciones mineralógicas y espesores de la secuencia sedimentaria, además de las propiedades físicas del basamento basáltico, del agua de mar y su profundidad. A partir del modelo calibrado realizamos diversos análisis de sensibilidad de las velocidades de ondas Scholte y sus curvas de dispersión, en un rango de frecuencias que abarca las ondas sismológicas, el ruido sísmico y la sísmica de exploración, variando las propiedades geoacústicas y petrofísicas de las diferentes capas. La sensibilidad de Vsh ante cambios en la velocidad de onda S, como se espera, es mucho más marcada en comparación a la de onda P, teniendo correlación directa con ambas para el rango de frecuencias analizado. Con respecto a la porosidad, observamos alta sensibilidad de Vsh para ambos modos, exhibiendo correlación inversa entre ambas magnitudes. Del presente análisis se establece la factibilidad de invertir a partir de velocidades de ondas Scholte no sólo velocidades de corte y espesores (aplicaciones ya conocidas), sino también información petrofísica de interés para la caracterización del fondo marino.Facultad de Ciencias Astronómicas y Geofísica

Mettre en évidence l'impact de l'histoire de la construction d'un bâtiment du patrimoine culturel grâce à un modèle aux éléments finis calibré à partir de mesures vibratoires via une une optimisation par essaims particulaires

International audienceNumerical models play a primary role in Cultural Heritage preservation. Nevertheless, the design of a realistic model remains challenging due not only to the complex behavior of masonry but also to the asynchronous building phases, the damage induced by natural and anthropic aggression, and the associated repairs. This paper discusses the impact of the information provided by an in-depth analysis of the construction history on the updating process of a Finite Element building model. The case study is the church of Sant'Agata del Mugello (Italy); for this building, a previous historical-archaeological study identified and recorded the asynchronous construction phases, the repair techniques, and the damage induced by three historical earthquakes (1542, 1611, and 1919)—moreover, a dense ambient vibration survey allowed to identify the modal parameters. The information from previous works is summarized in five Finite Element models with increasing complexity. A vibration-based model updating methodology based on a Particle Swarm Algorithm is developed. This work shows that the best minimization of the difference between the numerical and experimental modal parameters is obtained with the numerical model considering the identified construction techniques, repair phases, and connection relations between the bell tower and the nave

Postseismic Survey of a Historic Masonry Tower and Monitoring of Its Dynamic Behavior in the Aftermath of Le Teil Earthquake (Ardèche, France)

International audienceOn 11 November 2019, an Mw 4.9 earthquake struck the middle Rhône valley (South-East France) producing moderate to severe damage in the town of Le Teil and its surroundings. This unexpected event stressed the vulnerability of the French cultural built heritage to a moderate seismic hazard. Commonly applied to modern civil engineering structures, passive seismic methods are still lacking on historic constructions to understand properly the different factors driving their dynamic behavior. In this article, the results of a two-month seismic monitoring survey carried out shortly after the Le Teil mainshock in a historic masonry tower are presented and discussed. Located only 5 km south of the epicenter, the Gate Tower of Viviers (eleventh century) was instrumented with four highly sensitive seismic nodes. Ambient vibrations, as well as aftershocks and quarry blasts from the nearby Le Teil quarry, were recorded and used in the analysis. Through vibration-based analysis, the article addresses three relevant aspects of the dynamic response of ancient masonry structures. We discuss first the differences in the building’s response induced by the three reported types of vibrations, focusing on the particular signal characteristics of shallow aftershocks and quarry blasts. Then, we apply the Random Decrement Technique (RDT) to track the dynamic behavior variations over two months and to discuss the role of the environmental conditions in the slight fluctuations of the structural modal parameters (natural frequencies, damping coefficients) of unreinforced masonry structures. We also show evidence of the nonlinear elastic behavior under both weak seismic and atmospheric loadings. The correlation between the presence of heterogeneities in the construction materials and the nonlinear threshold supports the relevance of such types of monitoring surveys as a valuable tool for future modeling works and conservation efforts

Imaging an Underwater Basin and its Resonance Modes using Optical Fiber Distributed Acoustic Sensing

International audienceDistributed acoustic sensing is an ideal tool for ambient noise tomography owing to the dense spatial measurements and the ability to continuously record in harsh environments, such as underwater. We demonstrate the ability to image a complex underwater basin using ambient noise recorded on a fiber deployed offshore Greece. A two-dimensional shear-wave velocity model was derived by analyzing Scholte-wave dispersion. In addition, extremely detailed frequency-dependent resonance and wave propagation characteristics were revealed by computing power spectral densities (PSD) and auto-correlations (AC), respectively. These observations provide crucial information on lateral and vertical wave propagation, and were used to further constrain the velocity model. The analysis reveals significant lateral variations across the short 2.5 km long fiber segment, including basin edge effects and scattered waves. Waveform simulations further support the obtained model. Our results demonstrate the advantages of incorporating PSD and AC observations into ambient noise-based imaging