McMechan, George A.No Descriptionhttps://hdl.handle.net/10735.1/2490https://utd-ir.tdl.org/retrieve/6721b685-4aa2-4e55-b191-c5a262979d85/2024-06-25T23:01:03Z2024-06-25T23:01:03Z201Reverse Time Migrations in Transversely Isotropic Media: A Comparison between Acoustic and Elastic Wave Equations with Two Wave Mode Separation AlgorithmsWang, W.Hua, B.McMechan, George A.Williamson, P.https://hdl.handle.net/10735.1/71672020-01-18T09:01:45Zdc.title: Reverse Time Migrations in Transversely Isotropic Media: A Comparison between Acoustic and Elastic Wave Equations with Two Wave Mode Separation Algorithms
dc.contributor.author: Wang, W.; Hua, B.; McMechan, George A.; Williamson, P.
dc.description.abstract: 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 low-rank 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 S-waves in the former. Pseudoacoustic RTMs have diamond-shaped S-wave artifacts, which do not exist in elastic RTM images, provided that the P- and S-waves 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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Time-Domain Least-Squares Migration Using the Gaussian Beam Summation MethodYang, JidongZhu, HejunMcMechan, George A.Yue, Yubohttps://hdl.handle.net/10735.1/69692020-07-29T21:27:17Zdc.title: Time-Domain Least-Squares Migration Using the Gaussian Beam Summation Method
dc.contributor.author: Yang, Jidong; Zhu, Hejun; McMechan, George A.; Yue, Yubo
dc.description.abstract: 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 least-squares migration approach based on the time-domain 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 time-domain 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 L2-norm waveform misfit function. An L1-norm regularization is introduced to the inversion to enhance the robustness of least-squares migration, and an approximated diagonal Hessian is used as a pre-conditioner 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.
dc.description: Article is freely available on publisher's website. Use the Link to Article
P- and S-Decomposition in Anisotropic Media with Localized Low-Rank ApproximationsWang, W.Hua, B.McMechan, George A.Duquet, B.https://hdl.handle.net/10735.1/60802019-04-13T08:54:29Z2017-11-13T00:00:00Zdc.title: P- and S-Decomposition in Anisotropic Media with Localized Low-Rank Approximations
dc.contributor.author: Wang, W.; Hua, B.; McMechan, George A.; Duquet, B.
dc.description.abstract: We have developed a P- and S-wave decomposition algorithm based on windowed Fourier transforms and a localized low-rank 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; low-rank approximations and P- and S-decomposition are performed separately in each block. An overlap-add 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 S-decomposition possible in complicated 3D models. Tests with 2D and 3D synthetic data indicate good P- and S-decomposition results.
2017-11-13T00:00:00ZCombining Multidirectional Source Vector with Antitruncation-Artifact Fourier Transform to Calculate Angle Gathers from Reverse Time Migration in Two StepsTang, ChenMcMechan, George A.https://hdl.handle.net/10735.1/60792019-04-13T08:54:30Z2017-08-11T00:00:00Zdc.title: Combining Multidirectional Source Vector with Antitruncation-Artifact Fourier Transform to Calculate Angle Gathers from Reverse Time Migration in Two Steps
dc.contributor.author: Tang, Chen; McMechan, George A.
dc.description.abstract: Because receiver wavefields reconstructed from observed data are not as stable as synthetic source wavefields, the source-propagation vector and the reflector normal have often been used to calculate angle-domain common-image 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 Poynting-vector 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 source-angle-domain CIGs (SACIGs). After imaging, we use an antitruncation-artifact Fourier transform (ATFT) to convert SACIGs to ADCIGs in the k-domain. To achieve the new flow, another three innovative aspects are included. In the first step, we develop an angle-tapering scheme to remove the Fourier truncation artifacts during the wave decomposition (ofMPV) while preserving the amplitudes, and we use a wavefield decomposition plus angle-filter 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 plane-wave assumption. One uses an antileakage FT (ALFT) in local windows; the other uses an antitruncation-artifact 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 (source-side MPV plus ATFT) gives high-quality ADCIGs.
2017-08-11T00:00:00ZCharacterization of a Coalesced, Collapsed Paleocave Reservoir Analog Using GPR and Well-Core DataMcMechan, George A.Loucks, R. G.Mescher, P.Zeng, Xiaoxianhttps://hdl.handle.net/10735.1/60352019-04-13T08:53:55Z2002-07-01T00:00:00Zdc.title: Characterization of a Coalesced, Collapsed Paleocave Reservoir Analog Using GPR and Well-Core Data
dc.contributor.author: McMechan, George A.; Loucks, R. G.; Mescher, P.; Zeng, Xiaoxian
dc.description.abstract: The three-dimensional architecture, spatial complexity, and pore-type distribution are mapped in a near-surface analog of a coalesced, collapsed paleocave system in the Lower Ordovician Ellenburger Group near the city of Marble Falls in central Texas. The surface area of the site has dimensions of about 350 × 1000 m. The data collected include about 12 km of 50-MHz ground-penetrating radar (GPR) data arranged in a grid of orthogonal lines, 29 cores of about 15-m length, and detailed facies maps of an adjacent quarry face. Electrical property measurements along with detailed core descriptions were the basis of integrated interpretation of the GPR data. Three main GPR facies are defined on the basis of degree of brecciation in the corresponding cores: undisturbed host rock, disturbed host rock, and paleocave breccia. This GPR facies division defined the major paleocave trends and the distribution of porosity types, which correlate with reservoir quality. Highly brecciated zones are separated by disturbed and undisturbed host rock. The breccia bodies that outline the trend of collapsed cave passages are up to 300 m wide: the intervening intact areas between breccias are up to 200 m wide. Understanding the breccia distribution in a reservoir analog will help in defining strategies for efficient development of coalesced, collapsed paleocave reservoirs.
2002-07-01T00:00:00ZEstimation of Gas Hydrate and Free Gas Saturation, Concentration, and Distribution from Seismic DataLu, ShaomingMcMechan, George A.https://hdl.handle.net/10735.1/60252019-04-13T08:53:56Z2018-08-24T00:00:00Zdc.title: Estimation of Gas Hydrate and Free Gas Saturation, Concentration, and Distribution from Seismic Data
dc.contributor.author: Lu, Shaoming; McMechan, George A.
dc.description.abstract: Gas hydrates contain a major untapped source of energy and are of potential economic importance. The theoretical models to estimate gas hydrate saturation from seismic data predict significantly different acoustic/ elastic properties of sediments containing gas hydrate; we do not know which to use. Thus, we develop a new approach based on empirical relations. The water-filled porosity is calibrated (using well-log data) to acoustic impedance twice: one calibration where gas hydrate is present and the other where free gas is present. The water-filled porosity is used in a combination of Archie equations (with corresponding parameters for either gas hydrate or free gas) to estimate gas hydrate or free gas saturations. The method is applied to single-channel seismic data and well logs from Ocean Drilling Program leg 164 from the Blake Ridge area off the east coast of North America. The gas hydrate above the bottom simulating reflector (BSR) is estimated to occupy Ο3-8% of the pore space (Ο2-6% by volume). Free gas is interpreted to be present in three main layers beneath the BSR, with average gas saturations of 11-14%, 7-11%, and 1-5% of the pore space (6-8%, 4-6%, and 1-3% by volume), respectively. The estimated saturations of gas hydrate are very similar to those estimated from vertical seismic profile data and generally agree with those from independent, indirect estimates obtained from resistivity and chloride measurements. The estimated free gas saturations agree with measurements from a pressure core sampler. These results suggest that locally derived empirical relations between porosity and acoustic impedance can provide cost-effective estimates of the saturations, concentration, and distribution of gas hydrate and free gas away from control wells.
2018-08-24T00:00:00ZAVA Analysis and Interpretation of a Carbonate Reservoir: Northwest Java Basin, IndonesiaAdriansyahMcMechan, G. A.https://hdl.handle.net/10735.1/58052019-04-13T08:53:17Zdc.title: AVA Analysis and Interpretation of a Carbonate Reservoir: Northwest Java Basin, Indonesia
dc.contributor.author: Adriansyah; McMechan, G. A.
dc.description.abstract: A detailed analysis is performed of amplitude variation with angle (AVA) observations in six common-midpoint gathers with reflection points that are over and beside carbonate reefs in the Parigi Formation in the northwest Java basin. Both empirical analysis and full-wavefield modeling of the AVA data suggest that the presence of gas affects AVA by reducing the bulk density of the reservoir, decreasing of the overburden V_{p} / V_{s} ratio and by local attenuation caused by gas sieving through the overlying sediments. The slopes of AVA curves for reflections from the top of the Parigi are negative for brine saturation and strongly positive for gas saturation.
Up/Down and P/S Decompositions of Elastic Wavefields Using Complex Seismic Traces with Applications to Calculating Poynting Vectors and Angle-Domain Common-Image Gathers from Reverse Time MigrationsWang, WenlongMcMechan, George A.Tang, ChenXie, F.https://hdl.handle.net/10735.1/58042019-04-13T08:53:18Z2018-06-01T00:00:00Zdc.title: Up/Down and P/S Decompositions of Elastic Wavefields Using Complex Seismic Traces with Applications to Calculating Poynting Vectors and Angle-Domain Common-Image Gathers from Reverse Time Migrations
dc.contributor.author: Wang, Wenlong; McMechan, George A.; Tang, Chen; Xie, F.
dc.description.abstract: Separations of up- and down-going as well as of P- and S-waves are often a part of processing of multicomponent recorded data and propagating wavefields. Most previous methods for separating up/down propagating wavefields are expensive because of the requirement to save time steps to perform Fourier transforms over time. An alternate approach for separation of up-and down-going waves, based on extrapolation of complex data traces is extended from acoustic to elastic, and combined with P- and S-wave decomposition by decoupled propagation. This preserves all the information in the original data and eliminates the need for a Fourier transform over time, thereby significantly reducing the storage cost and improving computational efficiency. Wavefield decomposition is applied to synthetic elastic VSP data and propagating wavefield snapshots. Poynting vectors obtained from the particle velocity and stress fields after P/S and up/down decompositions are much more accurate than those without because interference between the corresponding wavefronts is significantly reduced. Elastic reverse time migration with the P/S and up/down decompositions indicated significant improvement compared with those without decompositions, when applied to elastic data from a portion of the Marmousi2 model.
dc.description: "This paper is contribution no. 1283 from the Department of Geosciences at UT-Dallas."
2018-06-01T00:00:00ZEfficient Love Wave Modelling via Sobolev Gradient Steepest DescentBrowning, MattFerguson, JohnMcMechan, George A.https://hdl.handle.net/10735.1/55932019-04-13T08:52:23Z2016-02-22T00:00:00Zdc.title: Efficient Love Wave Modelling via Sobolev Gradient Steepest Descent
dc.contributor.author: Browning, Matt; Ferguson, John; McMechan, George A.
dc.description.abstract: A new method for finding solutions to ordinary differential equation boundary value problems is introduced, in which Sobolev gradient steepest descent is used to determine eigenfunctions and eigenvalues simultaneously in an iterative scheme. The technique is then applied to the 1-D Love wave problem. The algorithm has several advantages when computing dispersion curves. It avoids the problem of mode skipping, and can handle arbitrary Earth structure profiles in depth. For a given frequency range, computation times scale approximately as the square root of the number of frequencies, and the computation of dispersion curves can be implemented in a fully parallel manner over the modes involved. The steepest descent solutions are within a fraction of a per cent of the analytic solutions for the first 25 modes for a two-layer model. Since all corresponding eigenfunctions are computed along with the dispersion curves, the impact on group and phase velocity of the displacement behaviour with depth is thoroughly examined. The dispersion curves are used to compute synthetic Love wave seismograms that include many higher order modes. An example includes addition of attenuation to a model with a low-velocity zone, with values as low as Q = 20. Finally, a confirming comparison is made with a layer matrix method on the upper 700 km of a whole Earth model.
2016-02-22T00:00:00ZRemoving Smearing-Effect Artifacts in Angle-Domain Common-Image Gathers from Reverse Time MigrationJin, HuMcMechan, George A.https://hdl.handle.net/10735.1/51742019-04-13T08:51:14Z2015-03-17T00:00:00Zdc.title: Removing Smearing-Effect Artifacts in Angle-Domain Common-Image Gathers from Reverse Time Migration
dc.contributor.author: Jin, Hu; McMechan, George A.
dc.description.abstract: Local plane-wave decomposition (LPWD) and local shift imaging condition (LSIC) methods for extracting angle-domain common-image gathers (ADCIGs) from prestack reverse time migration are based on the local plane-wave assumption, and both suffer from a trade-off in choosing the local window size. Small windows produce clean ADCIGs, but with low angle resolution, whereas large windows produce noisy ADCIGs, which include smearing-effect artifacts, but with high angle resolution. The cause of the smearing-effect artifacts in LPWD is the crosscorrelation of plane waves obtained by decomposition of the source and receiver wavefronts, at points that do not lie on the source wavefront excitation time trajectory. The cause of the smearing-effect artifacts in LSIC is the decomposition of curved events of offset-domain common-image gathers (ODCIGs) at incorrect depth points at zero offset. These artifacts can occur even if the migration velocity model is correct. Two methods were proposed to remove the artifacts. In the LPWD method, the smearing-effect artifacts were removed by decomposing and crosscorrelating the resulting source and receiver plane waves only at image points and excitation (image) times. In the LSIC method, the artifacts were removed by decomposing curved events in ODCIGs into planar events only at zero-offset target image points. Numerical tests with synthetic data revealed the success of the proposed methods.
2015-03-17T00:00:00Z