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<![CDATA[Detecting gas leakage using high-frequency signals generated by air-gun arrays]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/A7?rss=1
Recent field experiments have demonstrated that marine air-gun arrays create acoustic energy greater than 1 kHz. We have suggested to use the high-frequency signal as a source to look for gas leakage at, for instance, a producing hydrocarbon field, or a CO2 storage site in which the field is covered by permanent acoustic sensors at the seabed, often referred to as a permanent reservoir monitoring field. The only needed modification is that the temporal sampling interval for the receivers is decreased to 0.1 ms (in contrast to the normal sampling interval of 1 or 2 ms), to ensure that the system is capable of recording signals up to 5 kHz. We suggest using numerous fixed receivers at the seabed to detect a gas chimney by simple high-pass filtering and subsequent transmission type analysis of the recorded signals. We think this method might serve as an elegant, precise, and very cost-effective way to detect gas leakage into the water layer.
]]>2017-03-01T23:50:08-08:00info:doi/10.1190/geo2016-0483.1hwp:resource-id:gsgpy;82/2/A7Society of Exploration Geophysicists2017-03-01Geophysics Letters822A7A12<![CDATA[Contributors]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/Z7?rss=1
This article lists contributors to this issue and provides brief biographies of them.
]]>2017-03-06T05:47:35-08:00info:doi/10.1190/geo2017-0206-CONTRIB.1hwp:resource-id:gsgpy;82/2/Z7Society of Exploration Geophysicists2017-03-06Contributors822Z7Z12<![CDATA[Directional structure-tensor-based coherence to detect seismic faults and channels]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/A13?rss=1
Seismic coherence is widely used in seismic interpretation and reservoir characterization to highlight (with low values) faults and stratigraphic features from a seismic image. A coherence image can be computed from the eigenvalues of conventional structure tenors, which are outer products of gradients of a seismic image. I have developed a simple but effective method to improve such a coherence image by using directional structure tensors, which are different from the conventional structure tensors in only two aspects. First, instead of using image gradients with vertical and horizontal derivatives, I use directional derivatives, computed in directions perpendicular and parallel to seismic structures (reflectors), to construct directional structure tensors. With these directional derivatives, lateral seismic discontinuities, especially those subtle stratigraphic features aligned within dipping structures, can be better captured in the structure tensors. Second, instead of applying Gaussian smoothing to each element of the constructed structure tensors, I apply approximately fault- and stratigraphy-oriented smoothing to enhance the lateral discontinuities corresponding to faults and stratigraphic features in the structure tensors. Real 3D examples show that the new coherence images computed from such structure tensors display much cleaner and more continuous faults and stratigraphic features compared with those computed from conventional structure tensors and covariance matrices.
]]>2017-03-01T23:50:08-08:00info:doi/10.1190/geo2016-0473.1hwp:resource-id:gsgpy;82/2/A13Society of Exploration Geophysicists2017-03-01Geophysics Letters822A13A17<![CDATA[Oriented surface passive seismic location using local slopes]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/KS13?rss=1
A common acquisition scenario in microseismic monitoring is the deployment of large areal receiver arrays at or near the surface. This recording geometry has the advantage of providing coverage of the source’s focal hemisphere as well as characterization of the arrival time moveout curve; however, the accuracy of many location techniques applied to these data sets depends on the accuracy of the depth velocity model provided prior to location. We have developed a simple oriented time-domain location technique so that full knowledge of the velocity model is not required a priori. The applicability of the technique is limited to horizontally layered models and also to models with dipping interfaces of small angles; however, this restriction is acceptable in many unconventional reservoirs. Implementation of the technique includes three steps: (1) smoothing of the observed time arrivals by fitting a hyperbolic moveout curve with a broad set of constraints, (2) updating and restricting the constraints using a local-slopes-based location workflow, and (3) estimation of the focal coordinates of passive sources using the updated constraints for the final least-squares fitting of the moveout curves. We have tested the performance of the proposed technique on several 2D examples and a 3D field data set. The results from synthetic examples suggest that, despite the assumption of the method that the arrival moveout can be modeled using a constant effective velocity, a reliable event location is achieved for layered models without considerable lateral heterogeneities. Our tests on the field data set find that the focal point coincides with a previously derived estimate of the source location. To assess the uncertainty of the proposed technique, bootstrap statistics was also used and applied to the field data set.
]]>2017-01-04T02:55:45-08:00info:doi/10.1190/geo2016-0017.1hwp:resource-id:gsgpy;82/2/KS13Society of Exploration Geophysicists2017-01-04Passive Seismic Methods822KS13KS25<![CDATA[Efficient gravity inversion of basement relief using a versatile modeling algorithm]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/G23?rss=1
We have developed a novel approach to compute, in an efficient and versatile way, the gravity anomaly produced by an arbitrary, discrete 3D distribution of density contrast. The method allows adjustable precision and is particularly suited for the interpretation of sedimentary basins. Because the gravity field decays with the square of the distance, we use a discrete Green’s operator that may be made much smaller than the whole study area. For irregularly positioned observations, this discrete Green’s operator must be computed just at the first iteration, and because each of its horizontal layers presents a center of symmetry, only one-eighth of its total elements need to be calculated. Furthermore, for gridded data on a plane, this operator develops translation symmetry for the whole region of interest and has to be computed just once for a single arbitrary observation position. The gravity anomaly is obtained as the product of this small operator by any arbitrary density contrast distribution in a convolution-like operation. We use the proposed modeling to estimate the basement relief of a 200x100 km basin with density contrast varying along z only using a smaller Green’s operator at all iterations. The median of the absolute differences between relief estimates, using the small and a large operator (the latter covering the whole basin) has been approximately 9 m for a 3.6 km deep basin. We also successfully inverted the anomaly of a simulated basin with density contrast varying laterally and vertically, and a real anomaly produced by a steeply dipping basement. The proposed modeling is very fast. For 10,000 observations gridded on a plane, the inversion using the proposed approach for irregularly spaced data is two orders of magnitude faster than using an analytically derived fitting, and this ratio increases enormously with the number of observations.
]]>2017-01-04T02:55:45-08:00info:doi/10.1190/geo2015-0627.1hwp:resource-id:gsgpy;82/2/G23Society of Exploration Geophysicists2017-01-04Gravity Exploration Methods822G23G34<![CDATA[Low-frequency rotational isolator for airborne exploration]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/E27?rss=1
We have determined the performance of a passive rotational vibration isolator for a time-domain airborne electromagnetic (TDEM) receiver. The isolator uses neutrally buoyant flotation to provide very soft suspension and a very low resonant frequency of 0.065 Hz ± 0.005 Hz. One of the limitations of mapping deeper targets in areas of conductive overburden with TDEM systems is that low-frequency coil-vibration noise provides a lower bound to the transmitter base frequency (typically limited to 25 Hz). The purpose of this new isolator is to improve coil vibration related noise between 5 and 20 Hz to allow the transmitter base frequency to be reduced. A fixed-wing flight test determined that a receiver inside the new isolator had five times less rotational noise at 10 Hz than a current commercial system.
]]>2017-01-04T02:55:45-08:00info:doi/10.1190/geo2015-0521.1hwp:resource-id:gsgpy;82/2/E27Society of Exploration Geophysicists2017-01-04Electrical and Electromagnetic Methods822E27E30<![CDATA[Interrelation between Laplace constants and the gradient distortion effect in Laplace-domain waveform inversion]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/R31?rss=1
Laplace-domain waveform inversion (WI) is generally used to generate smooth initial velocity models for frequency- or time-domain full-waveform inversion. However, in the inversion results of Laplace-domain WI, anomalies such as salt domes are sometimes shifted. We evaluate the "gradient-distortion effect" that causes undesirable changes in parameter updates and found that this is caused by the relationship between the partial derivatives of Laplace wavefields with respect to two different parameters. By analyzing the gradient of the Laplace-domain misfit function, we found that the gradient distortion effect increases as the Laplace constants used in the Laplace-domain WI decrease. The velocity model inverted in the Laplace domain is generally blurred from shallower parameters to deeper parameters because the partial derivatives of the Laplace wavefields with respect to shallower parameters tend to be larger than those of deeper parameters. We found two solutions for suppressing the gradient distortion effect. The first one is the sequentially ordered Laplace constant approach with multiple Laplace constants. We discover that a dense, broad set of Laplace constants should be sequentially used in this approach. The second solution is the Gauss-Newton method, in which the Hessian matrix is considered. Numerical tests performed using a four-layer model and the BP benchmark model show the gradient distortion effect appeared in the inversion results and the effectiveness of the sequentially ordered Laplace constant approach. In addition, tests using an inverted BP benchmark model determine that the inversion results can be improved by applying a broad, dense set of Laplace constants to synthetic data. Finally, we verify the effectiveness of the Gauss-Newton method at suppressing the gradient distortion effect using the BP benchmark model.
]]>2017-01-04T02:55:45-08:00info:doi/10.1190/geo2015-0670.1hwp:resource-id:gsgpy;82/2/R31Society of Exploration Geophysicists2017-01-04Seismic Inversion822R31R47<![CDATA[Efficient wave-mode separation in vertical transversely isotropic media]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/C35?rss=1
Wave-mode separation can be achieved by projecting elastic wavefields onto mutually orthogonal polarization directions. In isotropic media, because the P-wave’s polarization vectors are consistent with wave vectors, the isotropic separation operators are represented by divergence and curl operators, which are easy to realize. In anisotropic media, polarization vectors deviate from wave vectors based on local anisotropic strength and separation operators lose their simplicity. Conventionally, anisotropic wave-mode separation is implemented either by direct filtering in the wavenumber domain or nonstationary filtering in the space domain, which are computationally expensive. Moreover, in conventional anisotropic separation, correcting for amplitude and phase changes of waveforms by applying separation operators is also more difficult than in an isotropic case. We have developed new operators for efficient wave-mode separation in vertical transversely isotropic (VTI) media. Our separation operators are constructed by local rotation of wave vectors to directions where the quasi-P (qP) wave is polarized. The deviation angles between the wave vectors and the qP-wave’s polarization vectors are explicitly estimated using the Poynting vectors. Obtaining polarization directions by rotating wave vectors yields separation operators in VTI media with the same forms as divergence and curl operators, except that the spatial derivatives are now rotated to implement wavefield projections in accurate polarization directions. The main increase in computational cost relative to isotropic separation operators is the estimation of the Poynting vectors, which is relatively small within elastic-wave extrapolation. As a result, applying the proposed operators is efficient. In the meantime, the waveforms corrections for divergence and curl operators can be directly extended for our new operators due to the similarities between these operators. By numerical exercises, we have determined that wave modes can be well-separated with small numerical cost using the present separation operators. The conservation of energy in wave-mode separation by applying waveform corrections was also verified.
]]>2017-01-04T02:55:45-08:00info:doi/10.1190/geo2016-0191.1hwp:resource-id:gsgpy;82/2/C35Society of Exploration Geophysicists2017-01-04Anisotropy822C35C47<![CDATA[Locally solving fractional Laplacian viscoacoustic wave equation using Hermite distributed approximating functional method]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/T59?rss=1
Accurate seismic modeling in realistic media serves as the basis of seismic full-waveform inversion and imaging. Recently, viscoacoustic seismic modeling incorporating attenuation effects has been performed by solving a fractional Laplacian viscoacoustic wave equation. In this equation, attenuation, being spatially heterogeneous, is represented partially by the spatially varying power of the fractional Laplacian operator previously approximated by the global Fourier method. We have developed a local-spectral approach, based on the Hermite distributed approximating functional (HDAF) method, to implement the fractional Laplacian in the viscoacoustic wave equation. Our approach combines local methods’ simplicity and global methods’ accuracy. Several numerical examples are developed to evaluate the feasibility and accuracy of using the HDAF method for the frequency-independent Q fractional Laplacian wave equation.
]]>2017-01-04T02:55:45-08:00info:doi/10.1190/geo2016-0269.1hwp:resource-id:gsgpy;82/2/T59Society of Exploration Geophysicists2017-01-04Seismic Modeling and Wave Propagation822T59T67<![CDATA[Signal extraction using randomized-order multichannel singular spectrum analysis]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/V69?rss=1
Multichannel singular spectrum analysis (MSSA) is an effective algorithm for random noise attenuation; however, it cannot be used to suppress coherent noise. This limitation results from the fact that the conventional MSSA method cannot distinguish between useful signals and coherent noise in the singular spectrum. We have developed a randomization operator to disperse the energy of the coherent noise in the time-space domain. Furthermore, we have developed a novel algorithm for the extraction of useful signals, i.e., for simultaneous random and coherent noise attenuation, by introducing a randomization operator into the conventional MSSA algorithm. In this method, which we call randomized-order MSSA, the traces along the trajectory of each signal component are randomly rearranged. Two ways to extract the trajectories of different signal components are investigated. The first is based on picking the extrema of the upper envelopes, a method that is also constrained by local and global gradients. The second is based on dip scanning in local processing windows, also known as the Radon method. The proposed algorithm can be applied in 2D and 3D data sets to extract different coherent signal components or to attenuate ground roll and multiples. Different synthetic and field data examples demonstrate the successful performance of the proposed method.
]]>2017-01-04T02:55:45-08:00info:doi/10.1190/geo2015-0708.1hwp:resource-id:gsgpy;82/2/V69Society of Exploration Geophysicists2017-01-04Signal Processing822V69V84<![CDATA[Stress-induced seismic azimuthal anisotropy, sand-shale content, and depth trends offshore North West Australia]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/C77?rss=1
Seismic azimuthal anisotropy is apparent when P-wave velocities vary with source-receiver azimuth and downward-propagating S-waves split into two quasi-S-waves, polarized in orthogonal directions. Not accounting for these effects can degrade seismic image quality and result in erroneous amplitude analysis and geologic interpretations. There are currently no physical models available to describe how azimuthal anisotropy induced by differential horizontal stress varies with sand-shale lithology and depth; we develop a model that does so, in unconsolidated sand-shale sequences offshore North West Australia. Our method naturally introduces two new concepts: "critical anisotropy" and "anisotropic depth limit." Critical anisotropy is the maximum amount of azimuthal anisotropy expected to be observed at the shallowest sediment burial depth, where the confining pressure and sediment compaction are minimal. The anisotropic depth limit is the maximum depth where the stress-induced azimuthal anisotropy is expected to be observable, where the increasing effects of confining pressure, compaction, and cementation make the sediments insensitive to differential horizontal stress. We test our model on borehole log data acquired in the Stybarrow Field, offshore North West Australia, where significant differential horizontal stress and azimuthal anisotropy are present. We determine our model parameters by performing regressions using dipole shear log velocities, gamma-ray shale volume logs, and depth trend data. We perform a blind test using the model parameters derived from one well to accurately predict the azimuthal anisotropy values at two other wells in an adjacent area. We use our anisotropy predictions to improve the well-tie match of the modeled angle-dependent reflectivity amplitudes to the 3D seismic amplitude variation with offset data observed at the well locations. Future applications of our method may allow the possibility to estimate the sand-shale content over a wide exploration area using anisotropic parameters derived from surface 3D seismic data.
]]>2017-03-06T05:47:35-08:00info:doi/10.1190/geo2015-0709.1hwp:resource-id:gsgpy;82/2/C77Society of Exploration Geophysicists2017-03-06Anisotropy822C77C90<![CDATA[A high-resolution weighted AB semblance for dealing with amplitude-variation-with-offset phenomenon]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/V85?rss=1
Velocity analysis is an essential step in seismic reflection data processing. The conventional and fastest method to estimate how velocity changes with increasing depth is to calculate semblance coefficients. Traditional semblance has two problems: low time and velocity resolution and an inability to handle amplitude variation-with-offset (AVO) phenomenon. Although a method known as the AB semblance can arrive at peak velocities in the areas with an AVO anomaly, it has a lower velocity resolution than conventional semblance. We have developed a weighted AB semblance method that can handle both problems simultaneously. We have developed two new weighting functions to weight the AB semblance to enhance the resolution of velocity spectra in the time and velocity directions. In this way, we increase the time and velocity resolution while eliminating the AVO problem. The first weighting function is defined based on the ratio between the first and the second singular values of the time window to improve the resolution of velocity spectra in velocity direction. The second weighting function is based on the position of the seismic wavelet in the time window, thus enhancing the resolution of velocity spectra in time direction. We use synthetic and field data examples to show the superior performance of our approach over the traditional one.
]]>2017-01-04T02:55:45-08:00info:doi/10.1190/geo2016-0047.1hwp:resource-id:gsgpy;82/2/V85Society of Exploration Geophysicists2017-01-04Signal Processing822V85V93<![CDATA[Improved eigenvalue-based coherence algorithm with dip scanning]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/V95?rss=1
Detection and identification of subsurface anomalous structures are key objectives in seismic exploration. The coherence technique has been successfully used to identify geologic abnormalities and discontinuities, such as faults and unconformities. Based on the classic third eigenvalue-based coherence (C3) algorithm, we make several improvements and develop a new method to construct covariance matrix using the original and Hilbert transformed seismic traces. This new covariance matrix more readily converges to the main effective signal energy on the largest eigenvalue by decreasing all other eigenvalues. Compared with the conventional coherence algorithms, our algorithm has higher resolution and better noise immunity ability. Next, we incorporate this new eigenvalue-based algorithm with time-lag dip scanning to relieve the dip effect and highlight the discontinuities. Application on 2D synthetic data demonstrates that our coherence algorithm favorably alleviates the low-valued artifacts caused by linear and curved dipping strata and clearly reveals the discontinuities. The coherence results of 3D real field data also commendably suppress noise, eliminate the influence of large dipping strata, and highlight small hidden faults. With the advantages of higher resolution and robustness to random noise, our strategy successfully achieves the goal of detecting the distribution of discontinuities.
]]>2017-01-04T02:55:45-08:00info:doi/10.1190/geo2016-0149.1hwp:resource-id:gsgpy;82/2/V95Society of Exploration Geophysicists2017-01-04Signal Processing822V95V103<![CDATA[Application of a complete workflow for 2D elastic full-waveform inversion to recorded shallow-seismic Rayleigh waves]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/2/R109?rss=1
The S-wave velocity of the shallow subsurface can be inferred from shallow-seismic Rayleigh waves. Traditionally, the dispersion curves of the Rayleigh waves are inverted to obtain the (local) S-wave velocity as a function of depth. Two-dimensional elastic full-waveform inversion (FWI) has the potential to also infer lateral variations. We have developed a novel workflow for the application of 2D elastic FWI to recorded surface waves. During the preprocessing, we apply a line-source simulation (spreading correction) and perform an a priori estimation of the attenuation of waves. The iterative multiscale 2D elastic FWI workflow consists of the preconditioning of the gradients in the vicinity of the sources and a source-wavelet correction. The misfit is defined by the least-squares norm of normalized wavefields. We apply our workflow to a field data set that has been acquired on a predominantly depth-dependent velocity structure, and we compare the reconstructed S-wave velocity model with the result obtained by a 1D inversion based on wavefield spectra (Fourier-Bessel expansion coefficients). The 2D S-wave velocity model obtained by FWI shows an overall depth dependency that agrees well with the 1D inversion result. Both models can explain the main characteristics of the recorded seismograms. The small lateral variations in S-wave velocity introduced by FWI additionally explain the lateral changes of the recorded Rayleigh waves. The comparison thus verifies the applicability of our 2D FWI workflow and confirms the potential of FWI to reconstruct shallow small-scale lateral changes of S-wave velocity.
]]>2017-03-06T05:47:35-08:00info:doi/10.1190/geo2016-0284.1hwp:resource-id:gsgpy;82/2/R109Society of Exploration Geophysicists2017-03-06Seismic Inversion822R109R117