Influence of the Interface Fresnel zone on the reflected P-wave amplitude modelling

International audienceThe aim of the paper is to emphasize the importance of accounting for the Fresnel volume and for the Interface Fresnel zone (IFZ) for calculating the amplitude of the P wave emanating from a point source and recorded at a receiver after its specular reflection on a smooth homogeneous interface between elastic media. For this purpose, by considering the problem of interest as a problem of diffraction by the IFZ, that is, the physically relevant part of the interface which actually affects the reflected wavefield, we have developed a method which combines the Angular Spectrum Approach (ASA) with the IFZ concept to get the 3-D analytical solution. The variation in the reflected P-wave amplitude evaluated with the ASA, as a function of the incidence angle, is compared with the plane wave (PW) reflection coefficient and with the exact solution provided by the 3-D code OASES, for one solid/solid configuration and two dominant frequencies of the source. For subcritical incidence angles the geometrical spreading compensation is mostly quite sufficient to reduce the point-source amplitudes to the PW amplitudes. On the contrary, for specific regions of incidence angles for which the geometrical spreading compensation is not sufficient anymore, that is, near the critical region and in the post-critical domain, the ASA combined with the IFZ concept yields better results than the PW theory whatever the dominant frequency of the source, which suggests that the additional application of the IFZ concept is necessary to obtain the reflected P-wave amplitude. Nevertheless, as the ASA combined with the IFZ has been used only for evaluating the contribution of the reflected wavefield at the receiver, its predictions fail when the interference between the reflected wave and the head wave becomes predominant

The Interface Fresnel Zone revisited

We determine the part of reflectors which actually affects the reflected wavefield, which is of particular interest for the characterization of the interfaces from physical and seismic viewpoints, and for seismic resolution. We reformulate the concepts of Fresnel volumes (FV) and Interface Fresnel zones (IFZ), by accounting for all possible rays defining the isochrone for the source-receiver pair and the specular reflected wave. In the case of a plane homogeneous interface, the results obtained with our reformulation (in particular, the size of the IFZ) are identical to previous published works. Nevertheless, with the help of the lens formula of geometrical optics, we propose a correction to the classical expression for the depth penetration of the FV across the interface in the transmission medium, which can result in a depth penetration 50% greater than the classical one. Additionally, we determine a region above the interface in the incidence medium, which is also involved in the wave reflection. Finally, we propose a new definition for the minimal volume of integration and homogenization of properties above and beyond the interface, which is necessary to the evaluation of an effective reflectivity of interfaces with lateral change in physical and geometrical properties

Some reflections on reflectors and wave amplitudes

International audienceThe paper describes the refl ector from a seismic viewpoint, and investigates the imprint of such a description on the wave reflection process. More specifically, the spatial region in the vicinity of the interface which actually aff ects the refl ected wavefield is determined using the Fresnel volume and the Interface Fresnel zone (IFZ) concepts. This region is represented by a volume of integration of properties above and below the interface whose maximum lateral extent corresponds to the lateral extent of the IFZ, and whose maximum vertical extent corresponds to a thickness we evaluate accurately and which can be greater than the seismic wavelengths. Considering this description of a reflector, we then calculate the amplitude of the P-wave emanating from a point source and recorded at a receiver after its specular reflection on a smooth homogeneous interface between two elastic media. As the problem under consideration can be viewed as a problem of diff raction by the IFZ which is the physically relevant part of the interface which actually aff ects the refl ected wavefi eld in this simple case, we then apply the Angular Spectrum Approach (ASA) combined with the IFZ concept to get the 3D analytical solution. The variation in the refl ected P-wave amplitude evaluated with the ASA, as a function of the incidence angle, is fi nally compared with the plane-wave refl ection coeffi cient, and with the exact solution obtained with the 3D code OASES. Below but close to the critical angle, the prediction of our approximation better fi ts the exact solution than the plane-wave refl ection coefficient, which emphasizes the importance of accounting for the IFZ in amplitude calculations even for a very simple elastic model

Usefulness of the Interface Fresnel zone for simulating the seismic reflected amplitudes

The aim of the paper is to emphasize the importance of accounting for the Fresnel volume (FV) and for the Interface Fresnel zone (IFZ) for simulating the amplitudes of the spherical waves reflected from an interface between elastic media and recorded at the receiver. For this purpose, by considering the problem of interest as a problem of diffraction by the IFZ, we have developed a method wich combines the Angular Spectrum Approach (ASA) with the IFZ concept to get the 3D analytical solution. The comparison between the amplitude-versusangle curve predicted by our approximation with that predicted by the classical plane-wave theory, and also with the exact solution, clearly enlightens three points. First, for specific regions of incidence angles, for which the geometrical-spreading compensation is not sufficient anymore to reduce the point-source amplitudes to the plane-wave amplitudes, the additional application of the FV and of IFZ concept is necessary. Second, as our approximation is concerned only with the reflected wave, its predictions fit well the exact solution, provided there is no interference between the reflected wave and the head wave. Third, they exhibit oscillations in the postcritical region which result from the interference of the IFZ with the sharp edge of the reflection coefficient

THIS COMMUNICATION IS CANCELLED (PAPER IS AVAILABLE). Laboratory benchmarks vs. Synthetic modeling of seismic wave propagation in complex environments (BENCHIE Project)

International audienceAccurate simulations of seismic wave propagation in complex geological structures with great and rapid variations of topography are of primary interest for environmental & industrial applications. Unfortunately, difficulties arise for such complex environments, due essentially to the existence of shadow zones, head waves, diffractions & edge effects. Usually, methods and codes are tested against " validated " ones, but one might wonder which method/code ultimately approaches the " real " solution. An original approach for seismics is to compare synthetic seismic data to controlled laboratory data for a well-described configuration, in order to analyze the respective limitations of each method/code. This is one of the objectives of the BENCHIE project. In this presentation we will present some preliminary results provided by both laboratory experiments conducted in a tank and numerical simulations of wave propagation. The laboratory data have been obtained by zero-offset acquisitions at different ultrasonic frequencies on the Marseille model which is made up of anticlines, fault and truncated pyramid. The numerical results have been obtained by two methods: the Spectral-Element Method and the Tip-Wave Superposition Method

Wavefield extraction using multi-channel chirplet decomposition

International audienceIn acoustical and seismic fields, wavefield extraction has alwaysbeen a crucial issue to solve inverse problem. Depending on the experimentalconfiguration, conventional methods of wavefield decomposition might nolonger likely to hold. In this paper, an original approach is proposed based ona multichannel decomposition of the signal into a weighted sum of elementaryfunctions known as chirplets. Each chirplet is described by physical parametersand the collection of chirplets makes up a large adaptable dictionary,so that a chirplet corresponds unambiguously to one wave componen

Compaction of a bed of fragmentable particles and associated acoustic emission

La fabrication des combustibles nucléaires actuels met en oeuvre un procédé de métallurgie des poudres qui comporte trois grandes étapes : la préparation des poudres, leur mise en forme et le frittage des compacts. C’est aussi le procédé de référence pour la fabrication des combustibles contenant des actinides mineurs à haute activité et à vie longue. Toutefois, compte tenu de la radiotoxicité de ces derniers, ils ne pourront être fabriqués qu’en cellule blindée. Il convient donc de simplifier au maximum le procédé de fabrication en limitant la dissémination et la rétention de matière et en recherchant des moyens de contrôle du procédé, simples à mettre en œuvre et robustes dans un milieu hostile. Une des voies envisagées pour la fabrication est l’utilisation de particules calibrées en taille, en forme et en tenue mécanique, à la place des poudres microniques actuellement utilisées pour la fabrication des combustibles à base d’oxydes d’uranium et de plutonium. La mise en œuvre de ces particules d’une taille de quelques centaines de micromètres doit concourir à limiter la dissimination/rétention de la matière et faciliter le remplissage des moules de presse. Toutefois, l’évolution sous contrainte de ce type de milieu granulaire en fonction des caractéristiques des particules reste peu étudiée et mal connue. Par ailleurs, la caractérisation de cette évolution fait généralement appel à des techniques expérimentales qui ne pourront pas être développées en cellule blindée. Ainsi, l’émission acoustique, qui est un des moyens utilisés pour suivre la compaction de poudres pharmaceutiques, est une technique simple à nucléariser. L’objectif est alors de suivre par émission acoustique, la compaction des particules céramiques, en l’occurrence d’UO2 pour notre étude. Nous présentons l’évolution du milieu granulaire lors de sa compaction en caractérisant l’évolution de la porosité par analyse d’images et porosimétrie mercure. Nous enregistrons les paramètres d’émission acoustique lors de la compaction des particules d’UO2

The EU Center of Excellence for Exascale in Solid Earth (ChEESE): Implementation, results, and roadmap for the second phase

publishedVersio

Wave diffraction by a periodic array of in-plane cracks at the interface between dissimilar elastic media: a multi-scale problem in geophysics

International audienc

What is a seismic reflector like?

International audienceThe spatial region in the vicinity of an interface which actually affects the interface response and, hence, the reflected wave field is of particular interest for the characterization of reflectors from a seismic viewpoint. This region is represented by a volume of integration of the medium properties above and below the interface whose maximum lateral extent corresponds to the lateral extent of the Interface Fresnel zone, and whose maximum vertical extent is equal to a thickness we evaluate approximately for subcritical incidence angles for a plane interface, and also for curved interfaces of anticline and syncline type. The maximum vertical extent may be larger than the seismic wavelengths for subcritical incidence angles close to the critical angle and for a strong impedance contrast at the interface. Although the part of the reflector volume laying below the interface and aff ecting the traveltime measurements is actually smaller than described by previous studies, the whole part of the reflector volume which affects the amplitude of the reflected wavefi eld is larger than described by previous estimates which considered only the spatial region below the interface. For a syncline (respectively, an anticline) it is larger (respectively, smaller) than described for a plane interface. In addition to providing more physical insights into the wave reflection process, this study may have signifi cant implications on seismic interpretation using AVA methodologies