## Search

Now showing items 1-10 of 31

JavaScript is disabled for your browser. Some features of this site may not work without it.

Author

Alkhalifah, Tariq Ali (31)

Zhang, Zhendong (8)Wu, Zedong (6)Kazei, Vladimir (4)Choi, Yun Seok (3)View MoreDepartmentEarth Science and Engineering Program (31)Physical Sciences and Engineering (PSE) Division (31)Extreme Computing Research Center (1)King Abdullah University of Science and Technology (KAUST).. (1)Seismic Wave Analysis Group (1)JournalSEG Technical Program Expanded Abstracts 2018 (14)GEOPHYSICS (11)Geophysical Journal International (5)The Leading Edge (1)PublisherSociety of Exploration Geophysicists (26)Oxford University Press (OUP) (5)Subjectfull-waveform inversion (12)acoustic (4)inversion (4)anisotropy (3)elastic (3)View MoreTypeArticle (17)Conference Paper (14)Year (Issue Date)
2018 (31)

Item Availability
Open Access (31)

Now showing items 1-10 of 31

- List view
- Grid view
- Sort Options:
- Relevance
- Title Asc
- Title Desc
- Issue Date Asc
- Issue Date Desc
- Submit Date Asc
- Submit Date Desc
- Results Per Page:
- 5
- 10
- 20
- 40
- 60
- 80
- 100

A hybrid finite-difference/lowrank solution to anisotropy acoustic wave equations

Zhang, Zhendong; Alkhalifah, Tariq Ali; Wu, Zedong (GEOPHYSICS, Society of Exploration Geophysicists, 2018-12-05) [Article]

P-wave extrapolation in anisotropic media suffers from SVwave artifacts and computational dependency on the complexity of anisotropy. The anisotropic pseudodifferential wave equation cannot be solved using an efficient time-domain finite-difference (FD) scheme directly. The wavenumber domain allows us to handle pseudodifferential operators accurately; however, it requires either smoothly varying media or more computational resources. In the limit of elliptical anisotropy, the pseudodifferential operator reduces to a conventional operator. Therefore, we have developed a hybrid-domain solution that includes a spacedomain FD solver for the elliptical anisotropic part of the anisotropic operator and a wavenumber-domain low-rank scheme to solve the pseudodifferential part. Thus, we split the original pseudodifferential operator into a second-order differentiable background and a pseudodifferential correction term. The background equation is solved using the efficient FD scheme, and the correction term is approximated by the low-rank approximation. As a result, the correction wavefield is independent of the velocity model, and, thus, it has a reduced rank compared with the full operator. The total computation cost of our method includes the cost of solving a spatial FD time-step update plus several fast Fourier transforms related to the rank. The accuracy of our method is of the order of the FD scheme. Applications to a simple homogeneous tilted transverse isotropic (TTI) medium and modified BP TTI models demonstrate the effectiveness of the approach.

Frequency domain multi-parameter acoustic inversion for transversely isotropic media with a vertical axis of symmetry

Djebbi, Ramzi; Alkhalifah, Tariq Ali (GEOPHYSICS, Society of Exploration Geophysicists, 2018-12-04) [Article]

Multi-parameter full waveform inversion (FWI) for transversely isotropic (TI) media with a vertical axis of symmetry (VTI) suffers from the trade-off between the parameters. The trade-off results in the leakage of one parameter’s update into the other. It affects the accuracy and convergence of the inversion. The sensitivity analyses suggested a parameterization using the horizontal velocity vh, Thomsen’s parameter ϵ and the an-elliptic parameter η to reduce the trade-off for surface recorded seismic data. We aim to invert for this parameterization using the scattering integral (SI) method. The available Born sensitivity kernels, within this approach, can be used to calculate additional inversion information. We mainly compute the diagonal of the approximate Hessian, used as a conjugate-gradient preconditioner, and the gradients step lengths. We consider modeling in the frequency domain. The large computational cost of the scattering integral method can be avoided with direct Helmholtz equation solvers.We apply the proposed method to the VTI Marmousi II model for various inversion strategies. We show that we can invert the vh accurately. For the ϵ parameter, only the short wavelengths are well recovered. On the other hand, the η parameter impact is weak on the inversion results and can be fixed. However, a good background η, with accurate long wavelengths, is needed to correctly invert for vh.Furthermore, we invert a real data set acquired by CGG from offshore Australia. We invert simultaneously all three parameters using the proposed inversion approach. The velocity model is improved and additional layers are recovered. We confirm the accuracy of the results by comparing them with well-log information, as well as, looking at the data and angle gathers.

Normalized nonzero-lag crosscorrelation elastic full-waveform inversion

Zhang, Zhendong; Alkhalifah, Tariq Ali; Wu, Zedong; Liu, Yike; He, Bin; Oh, Juwon (GEOPHYSICS, Society of Exploration Geophysicists, 2018-11-23) [Article]

Full-waveform inversion (FWI) is an attractive technique due to its ability to build high-resolution velocity models. Conventional amplitude-matching FWI approaches remain challenging because the simplified computational physics used does not fully represent all wave phenomena in the earth. Because the earth is attenuating, a sample-by-sample fitting of the amplitude may not be feasible in practice. We have developed a normalized nonzero-lag crosscorrelataion-based elastic FWI algorithm to maximize the similarity of the calculated and observed data. We use the first-order elastic-wave equation to simulate the propagation of seismic waves in the earth. Our proposed objective function emphasizes the matching of the phases of the events in the calculated and observed data, and thus, it is more immune to inaccuracies in the initial model and the difference between the true and modeled physics. The normalization term can compensate the energy loss in the far offsets because of geometric spreading and avoid a bias in estimation toward extreme values in the observed data. We develop a polynomial-type weighting function and evaluate an approach to determine the optimal time lag. We use a synthetic elastic Marmousi model and the BigSky field data set to verify the effectiveness of the proposed method. To suppress the short-wavelength artifacts in the estimated S-wave velocity and noise in the field data, we apply a Laplacian regularization and a total variation constraint on the synthetic and field data examples, respectively.

Full model wavenumber inversion: Identifying sources of information for the elusive middle model wavenumbers

Alkhalifah, Tariq Ali; Sun, Bing Bing; Wu, Zedong (GEOPHYSICS, Society of Exploration Geophysicists, 2018-10) [Article]

We recognized over the years that our conventional surface seismic recording can effectively identify two main features of the earth: its seismic propagation attributes and those attributes resulting in echoes or reflections. Thus, the resulting expression of the earth is dominated by the generally smooth (long-wavelength) features that control wave propagation, which we use to invert for the short-wavelength features causing reflections in a process we refer to as migration velocity analysis and imaging, respectively. The features of the earth that fall in between these two model scales - the middle wavenumbers - have been elusive, which is a dilemma for full-waveform inversion because we need to build the full velocity model (a broad band of model wavenumbers). We analyze the middle model wavenumber gap, but we focus more here on potential sources of information for such middle model wavenumbers. Such sources include regularization, objective function enhancements, and multiscattered energy. Regularization, provided by a total variation (TV) constraint admits middle and high model wavenumber components into the model to enforce the model's compliance with such a constraint. As opposed to minimizing the TV, such a constraint merely reduces the model space, and thus, these injected middle model wavenumbers are as good as the projected data information to the reduced model space. Such data information includes large offsets, but also multiscattered energy, in which the energy through wavepath and scattering updates can admit more of the elusive middle-wavenumber information that comes from the data. The combination of the right level of allowable model variations with the added data information from large offsets and multiscattering can help in filling the elusive middle model wavenumber gap and admit plausible models.

Variance-based model interpolation for improved full-waveform inversion in the presence of salt bodies

Ovcharenko, Oleg; Kazei, Vladimir; Peter, Daniel; Alkhalifah, Tariq Ali (GEOPHYSICS, Society of Exploration Geophysicists, 2018-09-07) [Article]

When present in the subsurface salt bodies impact the complexity of wave-equation-based seismic imaging techniques, such as least-squares reverse-time migration, and full-waveform inversion (FWI). Typically, the Born approximation used in every iteration of least-squares-based inversions is incapable of handling the sharp, high-contrast boundaries of salt bodies. We develop a variance-based method for reconstruction of velocity models to resolve the imaging and inversion issues caused by salt bodies. Our main idea lies in retrieving useful information from independent updates corresponding to FWI at different frequencies. After several FWI iterations we compare the model updates by considering the variance distribution between them to identify locations most prone to cycle skipping. We interpolate velocities from the surrounding environment into these high-variance areas. This approach allows the model to gradually improve from identifying easily resolvable areas and extrapolating the model updates from those to the areas that are difficult to resolve at early FWI iterations. In numerical tests, our method demonstrates the ability to obtain convergent FWI results at higher frequencies.

Subspace methods for time-lapse elastic full-waveform inversion

Zhang, Zhendong; Alkhalifah, Tariq Ali (SEG Technical Program Expanded Abstracts 2018, Society of Exploration Geophysicists, 2018-08-27) [Conference Paper]

The application of elastic full waveform inversion on time-lapse seismic data is arising from the boom in conventional full waveform inversion. In the past few years, many different inversion strategies are introduced for the time-lapse case, taking advantage of the power of FWI in capturing small changes. However, all methods tend to suffer from the imperfect repeatability of the data acquisition and the weakness in FWI in focussing on the affected areas (i.e. the reservoir). Thus, we modify the subspace method and apply it to time-lapse elastic full waveform inversion. A soft mask calculated using the gradients of the baseline and monitoring data, which acts as a pre-conditioner, is introduced to localize the update area to the affected regions. Specifically, we suppress the similarities of the two gradients and at the same time highlight their differences when calculating the soft mask. The calculated soft mask can reduce the dimensionality of the inverse problem with a delicately selected threshold, which provides a feasible way to calculate the reduced Hessian matrix. Besides, it is a data-driven approach free of human intervention or apriori knowledge. For comparison, we also use a hard mask surrounding the injection area. The numerical example shows that the proposed soft mask performs better than the hard mask.

Adaptive data-selection elastic full-waveform inversion

Zhang, Zhendong; Alkhalifah, Tariq Ali (SEG Technical Program Expanded Abstracts 2018, Society of Exploration Geophysicists, 2018-08-27) [Conference Paper]

The multiscale inversion strategy is widely used to mitigate the cycle-skipping problem in full waveform inversion. There are many different approaches to implement the multiscale inversion and the widely used low-to-high frequency continuation is not applicable when the observed data lack low frequencies. As an alternative to multiple frequencies, offsets continuation is also able to suppress the cycle-skipping problem when the initial model is not perfect. We improve the multiscale strategy of offset-selection by introducing a local similarity criterion. Thus, we formulate an adaptive data-driven selection process that is better than conventional offset continuation approaches. The global-crosscorrelation objective function used here aims to maximize the similarity of two data sets instead of subtracting one from another and it is more consistent with the selection strategy. Besides, the crosscorrelation-based objective function is more sensitive to the phase information of the data and thus is more applicable to field data. We use a modified elastic Marmousi example to verify the effectiveness of the proposed method.

A robust full-waveform inversion based on a shifted correlation of the envelope of wavefields

Wu, Zedong; Alonaizi, Faisal; Alkhalifah, Tariq Ali; Zhang, Zhendong; Almalki, Majed (SEG Technical Program Expanded Abstracts 2018, Society of Exploration Geophysicists, 2018-08-27) [Conference Paper]

The standard full waveform inversion (FWI) attempts to minimize the difference between the observed and modeled data. When the initial velocity is kinematically accurate, FWI often converges to to the best velocity model, usually of a high-resolution nature. However, when the modeled data using an initial velocity is far from the observed data, conventional local gradient based methods converge to a solution near the initial velocity instead of the global minimum. This is also known as the cycle skipping problem, which results in a zero correlation when observed and modeled data are not correlated. To reduce the cycle-skipping problem, we can compare the envelope of the modeled and observed data instead of the original data. However, when the initial velocity is not good enough, the correlation of the envelope of the modeled and observed data do not contribute accurately to the gradient. To mitigate this issue, we suggest to maximize not only the zero-lag correlation of the envelope but also the non-zero-lag correlation of the envelope. A weighting function, which has its maximum value at zero lag and decays away from zero lag, is proposed to balance the role of the lags. The resulting objective function is less sensitive to the choice of the maximum lag allowed and has a wider radius of convergence compared to standard FWI and envelope inversions. The implementation has the same computational complexity as conventional FWI as the only difference in the calculation is related to the modified adjoint source. We implement this algorithm on the AMD GPU based on OPENCL and obtained about a 14 fold speed up compared to a CPU implementation based on OPENMP. At last, several numerical examples are shown to demonstrate the proper convergence of the proposed method. Application to the Marmousi model shows that this method converges starting with a linearly increasing velocity model, even with data free of frequencies below 3 Hz.

A partial-low-rank method for solving acoustic wave equation

Wu, Zedong; Alkhalifah, Tariq Ali; Zhang, Zhendong (SEG Technical Program Expanded Abstracts 2018, Society of Exploration Geophysicists, 2018-08-27) [Conference Paper]

Numerical solutions of the acoustic wave equation, especially in anisotropic media, is crucial to seismic modeling, imaging and inversion as it provides efficient, practical, and stable approximate representation of the medium. However, a clean implementation (free of shear wave artifacts and dispersion) of wave propagation, especially in anisotropic media, requires an integral operator, the direct evaluation of which is extremely expensive. Recently, the low-rank method was proposed to provide a good approximation to the integral operator utilizing Fourier transforms. Thus, we propose to split the integral operator into two terms. The first term provides a differential operator that approximates that can be approximated with a standard finite-difference method. We, then, apply the lowrank approximation on the residual term of the finite-difference operator. We implement the two terms in two complementing steps, in which the spectral step corrects for any errors admitted by the finite difference step. Even though we utilize finite-difference approximations, the resulting algorithm admits spectral accuracy. Also, through the finite difference step, the method can deal approximately with the free surface and absorbing boundary conditions in a straight forward manner. Numerical examples show that the method is of high accuracy and efficiency.

Image-guided wavefield tomography for VTI media

Li, Vladimir; Guitton, Antoine; Tsvankin, Ilya; Alkhalifah, Tariq Ali (SEG Technical Program Expanded Abstracts 2018, Society of Exploration Geophysicists, 2018-08-27) [Conference Paper]

Transversely isotropic (TI) models have become essential in generating accurate depth images from seismic data. Here, we develop image-domain tomography (IDT) for building acoustic VTI (TI with a vertical symmetry axis) models from P-wave reflection data. Based on a separable dispersion relation, the modeling operator extrapolates only P-wavefields without the shear-wave artifacts. The inversion algorithm includes least-squares reverse-time migration (LSRTM), which improves the quality of the extended images and accuracy of parameter estimation. Whereas the zero-dip NMO velocity (V) and anellipticity parameter η are updated by focusing energy in space-lag LSRTM gathers, the Thomsen parameter δ is constrained by image-guided interpolation between two or more boreholes. We also apply image-guided smoothing to the IDT gradients of V and η to steer the inversion towards geologically plausible models. To mitigate the trade-off between V and η, we adopt a multistage approach that gradually relaxes the constraints on the spatial η-variation. The robustness of the algorithm is demonstrated on the elastic VTI Marmousi-II model. We also present preliminary inversion results for a line from a 3D data set acquired in the Gulf of Mexico.

The export option will allow you to export the current search results of the entered query to a file. Different formats are available for download. To export the items, click on the button corresponding with the preferred download format.

By default, clicking on the export buttons will result in a download of the allowed maximum amount of items. For anonymous users the allowed maximum amount is 50 search results.

To select a subset of the search results, click "Selective Export" button and make a selection of the items you want to export. The amount of items that can be exported at once is similarly restricted as the full export.

After making a selection, click one of the export format buttons. The amount of items that will be exported is indicated in the bubble next to export format.