Continental break-up history of a deep magma-poor margin based on seismic reflection data (northeastern Gulf of Aden margin, offshore Oman)

International audienceRifting between Arabia and Somalia started around 35 Ma followed by spreading at 17.6 Ma in the eastern part of the Gulf of Aden. The first-order segment between Alula-Fartak and Socotra-Hadbeen fracture zones is divided into three second-order segments with different structure and morphology. Seismic reflection data were collected during the Encens Cruise in 2006 on the northeastern margin. In this study, we present the results of Pre-Stack Depth Migration of the multichannel seismic data from the western segment, which allows us to propose a tectono-stratigraphic model of the evolution of this segment of the margin from rifting to the present day. The chronological interpretation of the sedimentary sequences is mapped out within relation to the onshore observations and existing dating. After a major development of syn-rift grabens and horsts, the deformation localized where the crust is the thinnest. This deformation occurred in the distal margin graben (DIM) at the northern boundary of the ocean-continent transition (OCT) represented by the OCT ridge. At the onset of the OCT formation differential uplift induced a submarine landslide on top of the deepest tilted block and the crustal deformation was restricted to the southern part of the DIM graben, where the continental break-up finally occurred. Initial seafloor spreading was followed by post-rift magmatic events (flows, sills and volcano-sedimentary wedge), whose timing is constrained by the analysis of the sedimentary cover of the OCT ridge, correlated with onshore stratigraphy. The OCT ridge may represent exhumed serpentinized mantle intruded by post-rift magmatic material, which modified the OCT after its emplacement

Modelling Seismic Wave Propagation for Geophysical Imaging

International audienceThe Earth is an heterogeneous complex media from the mineral composition scale (10−6m) to the global scale ( 106m). The reconstruction of its structure is a quite challenging problem because sampling methodologies are mainly indirect as potential methods (Günther et al., 2006; Rücker et al., 2006), diffusive methods (Cognon, 1971; Druskin & Knizhnerman, 1988; Goldman & Stover, 1983; Hohmann, 1988; Kuo & Cho, 1980; Oristaglio & Hohmann, 1984) or propagation methods (Alterman & Karal, 1968; Bolt & Smith, 1976; Dablain, 1986; Kelly et al., 1976; Levander, 1988; Marfurt, 1984; Virieux, 1986). Seismic waves belong to the last category. We shall concentrate in this chapter on the forward problem which will be at the heart of any inverse problem for imaging the Earth. The forward problem is dedicated to the estimation of seismic wavefields when one knows the medium properties while the inverse problem is devoted to the estimation of medium properties from recorded seismic wavefields

Inversion in viscoelastic media. Some effects of the memory kernel singularity on wave propagation and inversion in viscoelastic media, II: inversion

Geophysical Journal International, v. 158, n. 2, p. 426-442, 2005. http://dx.doi.org/10.1111/j.1365-246X.2004.02337.xInternational audienc

Seismic attenuation imaging with causality

Seismic data enable imaging of the Earth, not only of velocity and density but also of attenuation contrasts. Unfortunately, the Born approximation of the constant-density visco-acoustic wave equation, which can serve as a forward modelling operator related to seismic migration, exhibits an ambiguity when attenuation is included. Different scattering models involving velocity and attenuation perturbations may provide nearly identical data. This result was obtained earlier for scatterers that did not contain a correction term for causality. Such a term leads to dispersion when considering a range of frequencies. We demonstrate that with this term, linearized inversion or iterative migration will almost, but not fully, remove the ambiguity. We also investigate if attenuation imaging suffers from the same ambiguity when using non-linear or full waveform inversion. A numerical experiment shows that non-linear inversion with causality convergences to the true model, whereas without causality, a substantial difference with the true model remains even after a very large number of iterations. For both linearized and non-linear inversion, the initial update in a gradient-based optimization scheme that minimizes the difference between modelled and observed data is still affected by the ambiguity and does not provide a good result. This first update corresponds to a classic migration operation. In our numerical experiments, the reconstructed model started to approximate the true model only after a large number of iterations.GeotechnologyCivil Engineering and Geoscience

Inversion in viscoelastic media

13 pagesInternational audienc

Asymptotic waveform inversion for unbiased velocity and attenuation measurements: numerical tests and application for Vesuvius lava sample analysis

International audienceRecovering physical properties such as attenuation and velocity of the Earth's heterogeneities is as important as recovering the shape and the surface reflectivity of the heterogeneities themselves. A reliable estimate of attenuation in the Earth is necessary to improve our knowledge of subsurface physical properties like the degree of fluid saturation. Moreover the attenuation may also be relevant to 'bright spot' studies in reservoir characterization. Within this context, this paper presents applications of asymptotic viscoacoustic waveform inversion to synthetic and ultrasonic laboratory data recorded to characterize the velocity and attenuation of a lava sample collected on the Vesuvius volcano. The waveform inversion method is based on a combination of ray theory and the Born approximation to linearize the relation between the scattered wavefield and the velocity and attenuation perturbation models. The iterative linearized inverse problem is solved using the classic least-squares criterion. Asymptotic local analysis of the Hessian operator published in a previous paper by Ribodetti et al. showed the theoretical uncoupling between the velocity and attenuation parameters. The method is first applied to realistic and calibrated synthetic data computed using the discrete boundary integral wavenumber method. The reliability of decoupling between the velocity and attenuation parameters during the inversion is confirmed through two case studies corresponding to a local velocity heterogeneity without any associated attenuation perturbation and a local attenuation heterogeneity without any associated velocity perturbation, respectively. Second, the method is applied to an ultrasonic laboratory data set that was recorded to determine the velocity and attenuation of a tephrite rock sample collected on the Vesuvius volcano. A velocity of 3200 m s −1 and an attenuation factor of 480 were found, which are consistent with the geological nature of the analysed sample. The numerical tests presented in this paper validate former theoretical development of asymptotic waveform inversion for characterization of viscoacoustic media. Application to ultrasonic data confirms that the proposed method can be used for reliable estimations of the velocity and attenuation properties of rock from laboratory experiments. Comparable methodology can be extended for use with data from multichannel seismic reflection surveys, providing an opportunity to compare results of laboratory and field experiments

Imagerie sismique de milieux anisoptropes par l'inversion des formes d'ondes

L utilisation du formalisme TG43 est courante dans la plupart des Systèmes de planification de Traitement (SPT) dédiés à la curiethérapie. Les grandeurs physiques issues de ce formalisme sont généralement obtenues à partir de simulation Monte Carlo et présentées sous forme d abaques servant de référence. L utilisation du code PENELOPE en curiethérapie est très récente ; Sa jeunesse a justifié autant notre choix que son absence de la littérature. Notre choix s est porté sur PENELOPE pour différentes raisons. La première est que sa physique est plus récente assurant une meilleure précision des résultats. La deuxième, quant à elle, s appuie sur l ouverture du code permettant la maîtrise de l ensemble des processus de la simulation. Ainsi, un des objectifs de nos travaux a consisté à montrer le potentiel que peut avoir ce code afin d enrichir des données caractérisant les sources de rayonnement de curiethérapie. Le code MCNPX a été utilisé en parallèle afin de valider les résultats des simulations avec PENELOPE. Les géométries de deux modèles de sources d IR 192 utilisés en curiethérapie, Microselectron HDR v2 et Flexisource, Haut Débit de Dose (HDD) ont été modélisées avec les deux codes de calcul. Pour les deux modèles de source, les résultats de nos simulations ont été comparés à ceux obtenus dans les travaux antérieurs. Une bonne concordance des résultats à proximité des sources jusqu à des distances inférieures à 4 cm est montrée. Les écarts entre les résultats observés au-delà de 4 cm résident dans les différences concernant les fonctions de dose radiale et d anisotropie.Exploring the solid Earth for hydrocarbons, as social needs, is one of the main tasks of seismic imaging. As a domain of the modern geophysics, the seismic imaging by full waveform inversion (FWI) aims to improve and refine imaging of shallow and deep structures. Theoretically, the FWI method takes into account all the data gathered from subsurface in order to extract information about the physical parameter of the Earth. The kernel of the FWI is the full wave equation, which is considered in the heart of forward modeling engine. The FWI problem is represented as a least-squares local optimisation problem that retrieved the quantitative values of subsurface physical parameters. The seismic images are affected by the manifested anisotropy in the seismic data as anomalies in travel time, amplitude and waveform. In order to circumvent mis-focusing and mis-positioning events in seismic imaging and to obtain accurate model parameters, as valuable lithology indicators, the anisotropy needs to integrated in propagation-inversion workflows. In this context, the aim of this work is to develop two dimensional FWI for vertically transverse isotropic media (VTI). The physical parameters describing the Earth are elastic moduli or wave speeds and Thomsen parameter(s). The forward modeling and the inversion are performed entirely in frequency domain. The frequency-domain anisotropic P-SV waves propagation modelling is discretized by the finite element discontinuous Galerkin method. The full waveform modelling (FWM) is performed for VTI and titled transverse isotropic (TTI) media by various synthetic examples. The gradient of the misfit function is computed by adjoint-state method. The linearized inverse problem is solved with the quasi-Newton l-BFGS algorithm, which is able to compute an estimated Hessian matrix from a preconditionner and few gradients of previous iterations. Three categories of parametrization type are proposed in order to parametrize the model space of the inverse problem. The sensitivity analysis on acoustic VTI FWI method is preformed by studying the partial derivative of pressure wave field and the grid analysis of least-squares misfit functional. The conclusions inferred from the sensitivity analysis of least-squares misfit functional. The conclusions inferred from the sensitivity analysis are verified by FWI experimental on a simple synthetic model. The anisotropic parameter classes that can be well retrieved by VTI FWI are recognized. Furthermore, the acoustic VTI FWI is applied on the realistic synthetic Valhall benchmark for a wide-aperture surface acquisition survey. The anisotropic acoustic and elastic FWI are performed on the three components of ocean bottom cable (OBC) data sets from Valhall oil/gas field that is located in North Sea.NICE-BU Sciences (060882101) / SudocSudocFranceF

Accuracy of qP Wave Modeling in Anisotropic Acoustic Media by a Finite-difference Frequency-domain Method

International audienceWe assess the kinematic and dynamic accuracies of a finite-difference frequency-domain method for qP wave modelling in transversally isotropic acoustic media with tilted symmetry axis. This method was developed as a tool for frequency-domain full-waveform inversion which requires accurate traveltime and amplitude modelling. The modelling method is based on the parsimonious mixed-grid method which requires 5 grid points per wavelength in homogeneous media to mitigate numerical dispersion. We compare seismograms computed with the acoustic frequency-domain method with that provided by the complete solution of the transversally isotropic elastic wave equation. As expected we observed strong traveltime and amplitude mismatches in the case of strongly anisotropic materials such as zinc crystals. For weak anisotropy, we obtain a reasonable agreement although slight delay of the acoustic wide-angle reflections was observed in the case of a two-layer medium. The footprint of these inaccuracies in full-waveform inversion will need to be assessed before considering application to real data

High-resolution seismic attenuation imaging from wide-aperture onshore data by visco-acoustic frequency-domain full-waveform inversion

International audienceHere we assess the potential of the visco-acoustic frequency domain full-waveform inversion (FWI) to reconstruct P-wave velocity (VP) and P-wave attenuation factor (Q) from surface onshore seismic data. First, we perform a sensitivity analysis of the FWI based upon a grid search analysis of the misfit function and several synthetic FWI examples using velocity and Q models of increasing complexity. Subsequently, we applied both the acoustic and visco-acoustic FWI to real surface wide-aperture onshore seismic data from the Polish Basin, where a strong attenuation of the seismic data is observed. The sensitivity analysis of the visco-acoustic FWI suggests that the FWI can jointly reconstruct the velocity and the attenuation factor if the signature of the attenuation is sufficiently strong in the data. A synthetic example corresponding to a homogeneous background model with an inclusion shows a reliable reconstruction of VP and Q in the inclusion, when Q is as small as 90 and 50 in the background model and in the inclusion, respectively. A first application of acoustic FWI to real data shows that a heuristic normalization of the data with offset allows us to compensate for the effect of the attenuation in the data and reconstruct a reliable velocity model. Alternatively, we show that visco-acoustic FWI allows us to reconstruct jointly both a reliable velocity model and a Q model from the true-amplitude data. We propose a pragmatical approach based upon seismic modelling and source wavelet estimation to infer the best starting homogeneous Q model for visco-acoustic FWI. We find the source wavelet estimation quite sensitive to the quality of the velocity and attenuation models used for the estimation. For example, source-to-source wavelets are significantly more consistent when computed in the final FWI model than in the initial one. A good kinematic and amplitude match between the early-arriving phases of the real and time-domain synthetic seismograms computed in the final FWI model provides an additional evidence of the reliability of the final FWI model. We find the recovered velocity and attenuation models consistent with the expected lithology and stratigraphy in the study area. We link high-attenuation zones with the increased clay content and the presence of the mineralized fluids