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

Inversion in viscoelastic media

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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

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

Multiparameter Full-Waveform Inversion for Velocity and Attenuation – Refining the Imaging of a Sedimentary Basin

International audienceP276 Multiparameter Full-Waveform Inversion for Velocity and Attenuation – Refining the Imaging of a Sedimentary Basin M. Malinowski* (Institute of Geophysics PAS) A. Ribodetti (Geosciences Azur CNRS UMR) & S. Operto (Geosciences Azur CNRS UMR) SUMMARY This study deals with the extension of the frequency-domain full-waveform inversion/modelling (FWI) from the acoustic to the viscoacoustic case in application to the wide-aperture seismic data recorded in the Polish Basin. Attenuation was accounted for by introduction of complex velocities. Multiparameter inversion involving P-wave velocity and Q factor produces more focused and clearer images than obtained by the acoustic FWI however only the absorption ..

Estimating in Situ Rock Properties by Multiparameter Full Waveform Inversion. A Case Study

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Deep structure and seismic stratigraphy of the Egyptian Margin from Multichannel seismic data

International audienceRegional multichannel seismic reflection (MCS) profiles across the Egyptian continental slope, offshore the Nile delta, were recorded during the MEDISIS survey (conducted in 2002 on board the R/V Nadir). The results of this survey allow an interpretation of the overall structure and evolution of this passive continental margin. The MCS data were processed using an amplitude preserving pre-stack depth migration technique, which has the advantage of providing a quantitative, and geometrically correct, image of seismic horizons. Well-defined reflecting events allow the identification of three main seismic units. The upper unit (a 7 km thick) is interpreted as the post-rift sedimentary cover of the margin; it includes an undisturbed Middle Cretaceous to Upper Miocene sedimentary pile, covered by thick Messinian (latest Miocene) salt-rich layers and by Pliocene to Quaternary sediments, locally intensively deformed by gravity tectonics. The underlying intermediate acoustic unit (6 km thick on average) is interpreted as the Mesozoic syn-rift sedimentary cover of the margin; the end of the last rifting event is marked by a strong angular unconformity, tentatively of Aptian age. The lower unit may correspond to the thinned continental crust of Africa (12 km thick on average in the study area) and its pre-rift cover. Its base is identified by strong, discontinuous reflector packages about 23–25 km below sea floor, interpreted as indicative of the Moho

Integrated Prestack Depth Migration/Inversion and Simulated Annealing Optimization for Structural Model Building

International audienceWhether refining seismic images to evaluate opportunities in mature areas and exploit the maximum resource, or exploring in frontier areas, determining an accurate velocity model within the turnaround time constraints of reservoir management and exploration timeframes is critical. Speed, robustness, and accuracy are equally important. Seismic imaging has made great strides in recent years with the advent of so-called wave-equation migration imaging methods. Given the correct acoustic propagation velocity for seismic waves in the Earth's subsurface, these wave-equation methods yield the highest resolution and most accurate images of the earth. However, the process of determining the correct acoustic propagation velocity can be an elusive, time consuming, and costly procedure. We describe an approach both to shorten the process and to make the process less biased and more accurate. The process is shortened by automating the labor-intensive portion of the workflow, and made less biased and more robust and accurate by using much more data than is commonly used in manual picking approaches. ..

Ultrasonic seismic imaging of lava samples by viscoacoustic asymptotic waveform inversion: calibration and developments

International audienceThe object of this study is to estimate the viscoelastic parameters (compressionnal velocity and the intrinsic attenuation factor Q) of lava samples in acoustic tomography experiments by the use of an asymptotic viscoacoustic diffraction tomography method developped by Ribidetti et al 2000. 2.5 D common offset ultrasonic experiments are performed in a water tank laboratory

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