McMechan, George A.
https://hdl.handle.net/10735.1/2490
20200929T07:25:41Z

Reverse Time Migrations in Transversely Isotropic Media: A Comparison between Acoustic and Elastic Wave Equations with Two Wave Mode Separation Algorithms
https://hdl.handle.net/10735.1/7167
Reverse Time Migrations in Transversely Isotropic Media: A Comparison between Acoustic and Elastic Wave Equations with Two Wave Mode Separation Algorithms
Wang, W.; Hua, B.; McMechan, George A.; Williamson, P.
Anisotropic reverse time migrations (RTMs) using pseudoacoustic or elastic wave equations are tested, and the migrated PP images, obtained using the same data set as input, are compared. In anisotropic elastic RTMs, both divergence operators, and localized lowrank approximations (LLA), are tested and compared for P/S separation. Tests with synthetic data indicate that elastic RTMs have better illumination apertures than pseudoacoustic RTMs in subsalt areas because of the involvement of converted Swaves in the former. Pseudoacoustic RTMs have diamondshaped Swave artifacts, which do not exist in elastic RTM images, provided that the P and Swaves are separated in the elastic wavefields. LLAs provide affordable, accurate P/S separations in anisotropic media, and the separation results are better than those obtained using divergence operators. Anisotropic elastic RTMs with LLA give the best quality images. ©2019 Society of Exploration Geophysicists.
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TimeDomain LeastSquares Migration Using the Gaussian Beam Summation Method
https://hdl.handle.net/10735.1/6969
TimeDomain LeastSquares Migration Using the Gaussian Beam Summation Method
Yang, Jidong; Zhu, Hejun; McMechan, George A.; Yue, Yubo
With a finite recording aperture, a limited source spectrum and unbalanced illumination, traditional imaging methods are insufficient to generate satisfactory depth profiles with high resolution and high amplitude fidelity. This is because traditional migration uses the adjoint operator of the forward modelling rather than the inverse operator.We propose a leastsquares migration approach based on the timedomain Gaussian beam summation, which helps to balance subsurface illumination and improve image resolution. Based on the Born approximation for the isotropic acoustic wave equation, we derive a linear timedomain Gaussian beam modelling operator, which significantly reduces computational costs in comparison with the spectral method. Then, we formulate the corresponding adjoint Gaussian beam migration, as the gradient of an L2norm waveform misfit function. An L1norm regularization is introduced to the inversion to enhance the robustness of leastsquares migration, and an approximated diagonal Hessian is used as a preconditioner to speed convergence. Synthetic and field data examples demonstrate that the proposed approach improves imaging resolution and amplitude fidelity in comparison with traditional Gaussian beam migration. © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society.
Article is freely available on publisher's website. Use the Link to Article

P and SDecomposition in Anisotropic Media with Localized LowRank Approximations
https://hdl.handle.net/10735.1/6080
P and SDecomposition in Anisotropic Media with Localized LowRank Approximations
Wang, W.; Hua, B.; McMechan, George A.; Duquet, B.
We have developed a P and Swave decomposition algorithm based on windowed Fourier transforms and a localized lowrank approximation with improved scalability and efficiency for anisotropic wavefields. The model and wavefield are divided into rectangular blocks that do not have to be geologically constrained; lowrank approximations and P and Sdecomposition are performed separately in each block. An overlapadd method reduces artifacts at block boundaries caused by Fourier transforms at wavefield truncations; limited communication is required between blocks. Localization allows a lower rank to be used than global lowrank approximations while maintaining the same quality of decomposition. The algorithm is scalable, making P and Sdecomposition possible in complicated 3D models. Tests with 2D and 3D synthetic data indicate good P and Sdecomposition results.
20171113T00:00:00Z

Combining Multidirectional Source Vector with AntitruncationArtifact Fourier Transform to Calculate Angle Gathers from Reverse Time Migration in Two Steps
https://hdl.handle.net/10735.1/6079
Combining Multidirectional Source Vector with AntitruncationArtifact Fourier Transform to Calculate Angle Gathers from Reverse Time Migration in Two Steps
Tang, Chen; McMechan, George A.
Because receiver wavefields reconstructed from observed data are not as stable as synthetic source wavefields, the sourcepropagation vector and the reflector normal have often been used to calculate angledomain commonimage gathers (ADCIGs) from reverse time migration. However, the existing data flows have three main limitations: (1) Calculating the propagation direction only at the wavefields with maximum amplitudes ignores multiarrivals; using the crosscorrelation imaging condition at each time step can include the multiarrivals but will result in backscattering artifacts. (2) Neither amplitude picking nor Poyntingvector calculations are accurate for overlapping wavefields. (3) Calculating the reflector normal in space is not accurate for a structurally complicated reflection image, and calculating it in the wavenumber (k) domain may give Fourier truncation artifacts. We address these three limitations in an improved data flow with two steps: During imaging, we use a multidirectional Poynting vector (MPV) to calculate the propagation vectors of the source wavefield at each time step and output intermediate sourceangledomain CIGs (SACIGs). After imaging, we use an antitruncationartifact Fourier transform (ATFT) to convert SACIGs to ADCIGs in the kdomain. To achieve the new flow, another three innovative aspects are included. In the first step, we develop an angletapering scheme to remove the Fourier truncation artifacts during the wave decomposition (ofMPV) while preserving the amplitudes, and we use a wavefield decomposition plus anglefilter imaging condition to remove the backscattering artifacts in the SACIGs. In the second step, we compare two algorithms to remove the Fourier truncation artifacts that are caused by the planewave assumption. One uses an antileakage FT (ALFT) in local windows; the other uses an antitruncationartifact FT, which relaxes the planewave assumption and thus can be done for the global space. The second algorithm is preferred. Numerical tests indicate that this new flow (sourceside MPV plus ATFT) gives highquality ADCIGs.
20170811T00:00:00Z