McMechan, George A.

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George McMechan is the Ida Green Professor of Geosciences and Director of the Center for Lithospheric Studies. He is considered an expert in seismology, geophysics, ground-penetrating radar, and wave fields. Learn more about Dr. McMechan from his profile and Research Explorer pages



Recent Submissions

Now showing 1 - 20 of 20
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    Reverse Time Migrations in Transversely Isotropic Media: A Comparison between Acoustic and Elastic Wave Equations with Two Wave Mode Separation Algorithms
    (Society of Exploration Geophysicists) Wang, W.; Hua, B.; McMechan, George A.; Williamson, P.; McMechan, George A.
    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 Method
    (Oxford University Press) Yang, Jidong; Zhu, Hejun; McMechan, George A.; Yue, Yubo; 0000-0002-7452-075X (Zhu, H); 103911551 (McMechan, GA); Yang, Jidong; Zhu, Hejun; McMechan, George A.
    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.
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    P- and S-Decomposition in Anisotropic Media with Localized Low-Rank Approximations
    (Society of Exploration Geophysicists, 2017-11-13) Wang, W.; Hua, B.; McMechan, George A.; Duquet, B.; 103911551 (McMechan, GA); McMechan, George A.
    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.
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    Combining Multidirectional Source Vector with Antitruncation-Artifact Fourier Transform to Calculate Angle Gathers from Reverse Time Migration in Two Steps
    (Society of Exploration Geophysicists, 2017-08-11) Tang, Chen; McMechan, George A.; 103911551 (McMechan, GA); Tang, Chen; McMechan, George A.
    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.
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    Characterization of a Coalesced, Collapsed Paleocave Reservoir Analog Using GPR and Well-Core Data
    (Society of Exploration Geophysicists, 2002-07) McMechan, George A.; Loucks, R. G.; Mescher, P.; Zeng, Xiaoxian; McMechan, George A.; Zeng, Xiaoxian
    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.
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    Estimation of Gas Hydrate and Free Gas Saturation, Concentration, and Distribution from Seismic Data
    (Society of Exploration Geophysicists, 2018-08-24) Lu, Shaoming; McMechan, George A.; Lu, Shaoming; McMechan, George A.
    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.
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    AVA Analysis and Interpretation of a Carbonate Reservoir: Northwest Java Basin, Indonesia
    (Society of Exploration Geophysicists) Adriansyah; McMechan, G. A.; Adriansyah; McMechan, G. A.
    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.
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    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
    (Society of Exploration Geophysicists, 2018-06-01) Wang, Wenlong; McMechan, George A.; Tang, Chen; Xie, F.; 0000-0002-0255-6147 (Wang, W); 103911551 (McMechan, GA); Wang, Wenlong; McMechan, George A.; Tang, Chen
    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.
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    Efficient Love Wave Modelling via Sobolev Gradient Steepest Descent
    (Oxford University Press, 2016-02-22) Browning, Matt; Ferguson, John; McMechan, George A.; 103911551 (McMechan, GA); McMechan, George A.
    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.
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    Removing Smearing-Effect Artifacts in Angle-Domain Common-Image Gathers from Reverse Time Migration
    (Soc Exploration Geophysicists, 2015-03-17) Jin, Hu; McMechan, George A.; 103911551 (McMechan, GA); Jin, Hu; McMechan, George A.
    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.
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    Vector-Based Elastic Reverse Time Migration
    (Society of Exploration Geophysicists, 2015-10-15) Wang, Wenlong; McMechan, George A.; 103911551 (McMechan, GA); Wang, Wenlong; McMechan, George A.
    Prestack elastic reverse time migration (RTM) of multicomponent seismic data requires separating PP and PS reflections before, or as part of, applying the image condition, and using image conditions that preserve the angle and amplitude information. Both of these requirements are best achieved when all operations are on vectors.We have created a new 2D migration context for isotropic, elastic RTM, which included decomposition of the elastic source and receiver wavefields into P- and S-wave vectors by decoupled elastodynamic extrapolation, which retained the same stress and particle velocity components as the input data. Then, the propagation directions of the incident and reflected P- and S-waves were calculated directly from the stress and particle velocity definitions of the P- and S-wave Poynting vectors. An excitation- amplitude image condition that scaled the receiver wavelet by the source vector magnitude produced angle-dependent images of PP and PS reflection coefficients with the correct polarities, polarization, and amplitudes. It thus simplified the process of obtaining PP and PS angle-domain common-image gathers (ADCIGs); it was less effort to generate ADCIGs from vector data than from scalar data. We found that the resulting prestack elastic images were nearly identical to the corresponding source-normalized crosscorrelation images and had improved resolution because the wavelet broadening that resulted from the crosscorrelation was not present. © 2015 Society of Exploration Geophysicists.
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    Comparison of Two Algorithms for Isotropic Elastic P and S Vector Decomposition
    (Society of Exploration Geophysicists, 2015-06-10) Wang, Wenlong; McMechan, George A.; Zhang, Qunshan; Wang, Wenlong; McMechan, George A.
    P- and S-wavefield separation is necessary to extract PP and PS images from prestack elastic reverse time migrations. Unlike traditional separation methods that use curl and divergence operators, which do not preserve the wavefield vector component information, we did P and S vector decomposition, which preserves the same vector components that exist in the input elastic wavefield. The amplitude and phase information was automatically preserved, so no amplitude or phase corrections were required. We considered two methods to realize P and S vector decomposition: selective attenuation and decoupled propagation. Selective attenuation uses viscoelastic extrapolation, in which the Q-values are used as processing parameters, to remove either the P-waves or the S-waves. Decoupled propagation rewrites the stress and particle velocity formulation of the elastic equations into separate P-and S-wave components. In both methods, the decomposition is realized during the extrapolation of an elastic wavefield. These algorithms could also perform P and S decomposition in x-t gather data by extrapolating the data downward from the receivers, during which the decomposition is performed, and then back upward to record the decomposed P-and S-waves at the receivers. Comparisons of the two methods in terms of efficiency, accuracy, and memory showed that both could separate P-and S-waves in the vector domain. The decoupled propagation is preferable in terms of speed and memory cost, but was applicable only to elastic propagation.
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    Common-Reflection-Point Migration Velocity Analysis of 2D P-Wave Data From TTI Media
    (Soc Exploration Geophysicists, 2014-05) Oropeza, Ernesto V.; McMechan, George A.; McMechan, George A.
    We have developed a common-reflection-point (CRP)-based kinematic migration velocity analysis for 2D P-wave reflection data to estimate the four transversely isotropic (TI) parameters VPo, δ, and ε, and the tilt angle ϕ of the symmetry axis in a TI medium. In each iteration, the tomographic parameter was updated alternately with prestack anisotropic ray-based migration. Iterations initially used layer stripping to reduce the number of degrees of freedom; after convergence was reached, a couple of more iterations over all parameters and all CRPs ensured global interlayer coupling and parameter interaction. The TI symmetry axis orientation was constrained to be locally perpendicular to the reflectors. The VPo dominated the inversion, and so it was weighted less than δ and ε in the parameter updates. Estimates of δ and ε were influenced if the error in ϕ was >5⁰ estimates of VPo were also influenced if the error in ϕ was >10⁰. Examples included data for a simple model with a homogeneous TI layer whose dips allowed recovery of all anisotropy parameters from noise-free data, and a more realistic model (the BP tilted transversely isotropic (TTI) model) for which only VPo, delta, and ϕ were recoverable. The adequacy of the traveltimes predicted by the inverted anisotropic models was tested by comparing migrated images and common image gathers, with those produced using the known velocity models.
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    Comparison of Methods for Extracting ADCIGS from RTM
    (Soc Exploration Geophysicists, 2014-04-01) Jin, Hu; McMechan, George A.; Guan, Huimin; McMechan, George A.
    Methods for extracting angle-domain common-image gathers (ADCIGs) during 2D reverse-time migration fall into three main categories; direction-vector-based methods, local-plane-wave decomposition methods, and local-shift imaging condition methods. The direction-vector-based methods, which use either amplitude gradients or phase gradients, cannot handle overlapping events because of an assumption of one propagation direction per imaging point per imaging time; however, the ADCIGs from the direction-vector-based methods have the highest angle resolution. A new direction-vector-based method using instantaneous phase gradients in space and time gives the same propagation directions and ADCIGs as those obtained by the Poynting vector or polarization vector based methods, where amplitudes are large. Angles calculated by the phase gradients have larger uncertainties at smaller amplitudes, but they do not significantly degrade the ADCIGs because they contribute only small amplitudes. The local-plane-wave decomposition and local-shift imaging condition methods, implemented either by a Fourier transform or by a slant stack transform, can handle overlapping events, and produce very similar angle gathers. ADCIGs from both methods depend on the local window size in which the transforms are done. In small local windows, both methods produce ADCIGs with low noise, but also with low angle resolution; in large windows, they have high angle resolution, but contain smeared artifacts.
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    2d Frequency-Domain Elastic Full-Waveform Inversion Using Time-Domain Modeling and a Multistep-Length Gradient Approach
    (Soc Exploration Geophysicists, 2014-02-17) Xu, Kun; McMechan, George A.; McMechan, George A.
    To decouple the parameters in elastic full-waveform inversion (FWI), we evaluated a new multistep-length gradient approach to assign individual weights separately for each parameter gradient and search for an optimal step length along the composite gradient direction. To perform wavefield extrapolations for the inversion, we used parallelized high-precision finite-element (FE) modeling in the time domain. The inversion was implemented in the frequency domain; the data were obtained at every subsurface grid point using the discrete Fourier transform at each time-domain extrapolation step. We also used frequency selection to reduce cycle skipping, time windowing to remove the artifacts associated with different source spatial patterns between the test and predicted data, and source wavelet estimation at the receivers over the full frequency spectrum by using a fast Fourier transform. In the inversion, the velocity and density re-constructions behaved differently; as a low-wavenumber tomography (for velocities) and as a high-wavenumber migration (for density). Because velocities and density were coupled to some extent, variations were usually underestimated (smoothed) for V_P and V_S and correspondingly overestimated (sharpened) for ρ. The impedances I_P and I_S from the products of the velocity and density results compensated for the under-or overestimations of their variations, so the recovered impedances were closer to the correct ones than V_P, V_S, and ρ were separately. Simultaneous reconstruction of V_P, V_S, and ρ was robust on the FE and finite-difference synthetic data (without surface waves) from the elastic Marmousi-2 model; satisfactory results are obtained for V_P, V_S, ρ, and the recovered I_P and I_S from their products. Convergence is fast, needing only a few tens of iterations, rather than a few hundreds of iterations that are typical in most other elastic FWI algorithms.
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    Tomography of Diffraction-Based Focusing Operators
    (2012-08-24) Santos, Luiz Alberto; Mansur, Webe Joao; McMechan, George A.; McMechan, George A.
    Diffractions carry the same kinematic information provided by common focus point operators (CFPOs). Thus CFPO and diffraction time trajectories may be used separately, or combined into a single unified tomography for velocity analysis. Velocity estimation by tomography of CFPOs reduces the depth-velocity ambiguity compared to two-way time tomography. CFPO estimation is complicated where there are event discontinuities and diffractions. This problem is overcome by using the kinematic information in diffractions in near-offset common-offset gathers. The procedure is illustrated using synthetic data, and a single-channel field seismic profile from the Blake Ridge (off the east coast of the United States). The results show the effectiveness of the proposed method for estimation of velocity from single channel seismic data, and for refinement of the velocity field from multichannel data. Both applications are cost-competitive.
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    Conductivity and Scattering Q in GPR Data: Example from the Ellenburger Dolomite, Central Texas
    (2012-06-27) Harbi, H.; McMechan, George A.; McMechan, George A.
    Total attenuation (Qt -1) in ground-penetrating radar (GPR) data is a composite of intrinsic and scattering attenuations (Qin -1 and Qsc -1). For nonmagnetic materials, Qin -1 is a combination of the effects of real conductivity and dielectric relaxation. The attenuation for real conductivity >1.0 mS/m in the GPR frequency band is a function of frequency while the dielectric relaxation is frequency-independent. These frequency behaviors allow separation of the attenuation types by attributing and fitting the Qt -1 decay shape with frequency to the conductivity, and by attributing the magnitude of Qt -1 to the sum of conductivity and dielectric relaxation attenuations at each frequency. Total attenuation is calculated from GPR data using spectral ratios, and Qin -1 is obtained by fitting a smooth lower bound to Qt -1; the difference between Qt -1 and Qin -1 estimates the scattering contribution Qsc -1. Scatterer size spectra are evaluated using KA=1 for 2D, and KA=1.5 for 3D, propagation (where K is wavenumber and A is the scatterer size). We illustrate with 2D synthetic data and three field 2D crosshole profiles from an outcrop of an Ellenburger collapsed paleocave environment in central Texas. Between the three pairs of holes, we estimate the breccia sizes from the scattering spectra Qsc -1. To image the anisotropic electrical conductivity distributions, we use simultaneous iterative reconstruction tomography. There is a correlation between the low wavenumber features of the results of the current conductivity tomography and those in previous velocity tomography, and with surface data results that are predicted and calculated from GPR data attributes. Low- and high-conductivity zones tend to follow either the GPR facies distributions, lithological boundaries, or the larger of the fractures. Correlations are not visible where the breccias are finer because these tend to be more randomly oriented, and/or below the resolution of the GPR data. © 2012 Society of Exploration Geophysicists.
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    Sensitivity of Estimated Elastic Moduli to Completeness of Wave Type, Measurement Type, and Illumination Apertures at a Receiver in Multicomponent VSP Data
    (2012-02-01) Rusmanugroho, Herurisa; McMechan, George A.; McMechan, George A.
    Inversion of phase slowness and polarization vectors measured from multicomponent vertical seismic profile data can yield estimates of all 21 density-normalized elastic moduli for anisotropic elastic media in the neighborhood of each 3C geophone. Synthetic test data are produced by direct evaluation of the Christoffel equation, and by finite-difference solution of the elastodynamic equations. Incompleteness of the data, with respect to illumination (polar and azimuth angle) apertures (qP and/or qS) wave types, wave-propagation directions, and the amount of data (e.g., with or without horizontal slowness components), produces solutions with variations in quality, as revealed by the distribution of model parameter correlations. In a good solution, with all parameters well constrained by the data, the correlation matrix is diagonally dominant. qP-only and qS-only solutions typically produce complementary distributions in their correlation matrices, as they are orthogonal in their sampling of the medium with respect to polarization. The elastic moduli become less independent as the data apertures decrease. If the other input data are relatively complete, the horizontal components of the slowness vector are not needed as the information they contain is redundant. The main consequence of omitting horizontal slowness components is slower convergence. When modest amounts of random noise are added to the slowness and polarization data, in otherwise adequately sampled apertures, the solution is still very close to the correct model, but with larger residual variance. © 2012 Society of Exploration Geophysicists.
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    3D, 9C Seismic Modeling and Inversion of Weyburn Field Data
    (2012-06-18) Rusmanugroho, H.; McMechan, George A.; McMechan, George A.
    Inversion of 3D, 9C wide azimuth vertical seismic profiling (VSP) data from the Weyburn Field for 21 independent elastic tensor elements was performed based on the Christoffel equation, using slowness and polarization vectors measured from field data. To check the ability of the resulting elastic tensor to account for the observed data, simulation of the 3C particle velocity seismograms was done using eighth-order, staggered-grid, finite-differencing with the elastic tensor as input. The inversion and forward modeling results were consistent with the anisotropic symmetry of the Weyburn Field being orthorhombic. It was dominated by a very strong, tranverse isotropy with a vertical symmetry axis, superimposed with minor near-vertical fractures with azimuth ∼55° from the inline direction. The predicted synthetic seismograms were very similar to the field VSP data. The examples defined and provided a validation of a complete workflow to recover an elastic tensor from 9C data. The number and values of the nonzero tensor elements identified the anisotropic symmetry present in the neighborhood of a 3C borehole geophone. Computation of parameter correlation matrices allowed evaluation of solution quality through relative parameter independence. © 2012 Society of Exploration Geophysicists.
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    Gas hydrate and free gas petroleum system in 3D seismic data, offshore Angola
    (Society of Exploration Geophysicists, 2012-09-26) Nyamapfumba, Martin; McMechan, George C.; McMechan, George A.
    Evidence of gas hydrate and free gas occurrences in a 3D seismic volume from the West-Central Coastal Province of the Congo Fan, offshore Angola, illustrates all the components of a complete petroleum system. Analysis and interpretation are based on the information in attributes calculated from three 3D time-migrated common-angle seismic volumes; the attributes include seismic amplitude, spectral components, dip magnitude, amplitude variation with angle, and instantaneous frequency. The source is organic-rich muds associated with late Cretaceous to early Tertiary channels, the migration paths are along growth faults, and the traps are partly defined by the gas hydrate stability zone (for the gas hydrate), partly by the bottom-simulating reflector (for the subhydrate free gas), and partly by faults (for both). The spatial distribution of free gas is further supported by the associated seismic bright spots, and also by the attenuation of high frequencies of P-waves that traverse the gas-saturated zone. Locally higher temperatures, associated with upward fluid circulation along fault zones, facilitate gas transmission through the gas hydrate and forms gas chimneys that extend to the sea floor.

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