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

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

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

Consistent seismic event location and subsurface parameters inversion through slope tomography : a variable-projection approach

We revisit the hypocentre-velocity problem, which is of interest in different fields as for example microseismics and seismology. We develop a formulation based on kinematic migration of two picked kinematic attributes in the 2-D case, the traveltime and the slope (horizontal component of the slowness vector), from which we are able to retrieve the location and subsequently the origin time correction and the subsurface parameters mainly velocity. We show how, through a variable projection, the optimization problem boils down to a physically consistent and parsimonious form where the location estimation is projected into the subsurface parameter problem. We present in this study a proof of concept validated by a toy test in two dimensions and a synthetic case study on the Marmousi model. The method presented in this study is extendible to three dimensions by incorporating the crossline slope or the backazimuth as a supplementary attribute

Revisiting the hypocenter-velocity problem through a slope tomography inspiration

International audienceWe revisit the hypocenter-velocity problem which is of interest in different fields as for example microseismics and seismology. We show how, through a formulation based on kinematic migration focusing using two picked kinematic attributes in the two-dimensions case, the travel time and the slope (horizontal component of the slowness vector), we are able to invert jointly for the location, the origin time correction and the subsurface parameters mainly velocity. We present in this study a proof of concept validated by a toy test in two-dimensions. The method presented in this study is extendable to three-dimensions by incorporating the back-azimuth as a supplementary attribute

From slope tomography to FWI: Is the conventional workflow viable in complex settings?

International audienceUltra-long offset seabed acquisitions implemented with sparse array of ocean bottom nodes (OBN) are emerging as the goto strategy for velocity model building. These stationaryrecording geometries provide the flexibility to record a plethora of wave arrivals and in particular diving waves that undershoot the deepest targeted structures. These seabottom acquisitions are also amenable to the recording of frequencies as low as 1.5Hz. Full-Waveform Inversion (FWI) can be fed with this wide variety of low-frequency wave types to build broadband velocity models. Ultra-long offset surveys provide also a suitable framework to revive well proven tomography methods such as first-arrival slope+traveltime tomography to build kinematically-accurate initial velocity model for FWI. Here, we revisit the challenging 2004 BP salt benchmark by assessing a workflow combining first-arrival slope tomography and FWI against an ultra-long OBN survey involving a maximum offset of 97 km. Starting from a crude starting velocity gradient model, we manage to capture accurately the geometry of the salt and the subsalt structure down to 7km depth using 1.5-Hz starting frequency. Below 7km, the deficit of wide-angle coverage near the ends of the targeted structure combined with the inaccuracy of the tomography model in these areas prevents accurate reconstruction of subsalt structures by FWI when frequencies smaller than 1.5Hz are not used. The remedies to reconstruct subsalt structures in area suffering from deficit of wide-angle illumination should come from sophisticated sparsifying regularization in FWI and/or by the combination of first-arrival and short-spread + post-critical reflection slope tomography. The velocity model built by firstarrival slope tomography should be also beneficial for any variant of FWI involving more convex distances than the leastsquares norm of the differences

Finite-difference frequency-domain modeling of viscoacoustic wave propagation in 2D tilted transversely isotropic (TTI) media

A 2D finite-difference, frequency-domain method was developed for modeling viscoacoustic seismic waves in transversely isotropic media with a tilted symmetry axis. The medium is parameterized by the P-wave velocity on the symmetry axis, the density, the attenuation factor, Thomsen's anisotropic parameters delta and epsilon, and the tilt angle. The finite-difference discretization relies on a parsimonious mixed-grid approach that designs accurate yet spatially compact stencils. The system of linear equations resulting from discretizing the time-harmonic wave equation is solved with a parallel direct solver that computes monochromatic wavefields efficiently for many sources. Dispersion analysis shows that four grid points per P-wavelength provide sufficiently accurate solutions in homogeneous media. The absorbing boundary conditions are perfectly matched layers (PMLs). The kinematic and dynamic accuracy of the method was assessed with several synthetic examples which illustrate the propagation of S-waves excited at the source or at seismic discontinuities when epsilon delta. These instabilities are consistent with previous theoretical analyses of PMLs in anisotropic media; they are removed if the grid interval is matched to the P-wavelength that leads to dispersive S-waves. Comparisons between seismograms computed with the frequency-domain acoustic TTI method and a finite-difference, time-domain method for the vertical transversely isotropic elastic equation show good agreement for weak to moderate anisotropy. This suggests the method can be used as a forward problem for viscoacoustic anisotropic full-waveform inversion

Finite-difference frequency-domain modeling of viscoacoustic wave propagation in 2D tilted transversely isotropic (TTI) media

International audienceA 2D finite-difference, frequency-domain method was developed for modeling viscoacoustic seismic waves in transversely isotropic media with a tilted symmetry axis. The medium is parameterized by the P-wave velocity on the symmetry axis, the density, the attenuation factor, Thomsen's anisotropic parameters delta and epsilon, and the tilt angle. The finite-difference discretization relies on a parsimonious mixed-grid approach that designs accurate yet spatially compact stencils. The system of linear equations resulting from discretizing the time-harmonic wave equation is solved with a parallel direct solver that computes monochromatic wavefields efficiently for many sources. Dispersion analysis shows that four grid points per P-wavelength provide sufficiently accurate solutions in homogeneous media. The absorbing boundary conditions are perfectly matched layers (PMLs). The kinematic and dynamic accuracy of the method wasassessed with several synthetic examples which illustrate the propagation of S-waves excited at the source or at seismic discontinuities when epsilo

Matrix-free anisotropic slope tomography: Theory and application

International audienceSlope tomography uses traveltimes, source, and receiver slopes of locally coherent events to build subsurface velocity models. Locally coherent events by opposition to continuous reflections are suitable for semiautomatic and dense picking, which is conducive to better resolved tomographic models. These models can be further used as background/initial models for depth migration or full-waveform inversion. Slope tomography conventionally relies on ray tracing for traveltimes and slopes computation, where rays are traced from scatterers in depth to sources and receivers. The inverse problem relies on the explicit building of the sensitivity matrix to update the velocity model by local optimization. Alternatively, slope tomography can be implemented with eikonal solvers, which compute efficiently finely sampled traveltime maps from the sources and receivers, whereas slopes are estimated by finite differences of the traveltime maps. Moreover, a matrix-free inverse problem can be implemented with the adjoint-state method for the estimation of the data-misfit gradient. This new formulation of slope tomography is extended to tilted transverse isotropic (TTI) acoustic media, in which the model space is parameterized by four anisotropic parameters (e.g., vertical wavespeed, Thomson’s parameter δ, ϵ, and tilt angle) and the coordinates of the scatterers. A toy synthetic example allows for a first assessment of the crosstalk between anisotropic parameters and scatterer coordinates. A more realistic synthetic example indicates the feasibility of the joint update of the vertical wavespeed and ϵ. The slope tomography is finally applied to real broadband towed-streamer data to build the vertical velocity and the scatterers, while anisotropic parameters ϵ and δ are used as background parameters. The velocity model quality is assessed through common-image gathers computed by TTI Kirchhoff prestack-depth migration