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73rd EAGE Conference and Exhibition incorporating SPE EUROPEC 2011
- Conference date: 23 May 2011 - 27 May 2011
- Location: Vienna, Austria
- ISBN: 978-90-73834-12-5
- Published: 23 May 2011
1 - 100 of 799 results
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Joint Inversion of Surface-wave Dispersion and P-wave Refraction Data for Laterally Varying Layered Models
Authors L. V. Socco, D. Boiero and C. CalzoniWe present here a joint inversion method to build P- and S-wave velocity models from surface-wave and P-wave refraction data, specifically designed to deal with laterally varying layered environments which can present strong velocity contrasts with depth. In this case, a smooth minimum-structure inversion produces smooth models even for geological models, which are overall layered. The proposed algorithm is also able to incorporate a-priori information available over the site and any physical law to link model parameters. This method presents advantages with respect to individual surface wave analysis and refraction tomography since it imposes internal consistency for all the model parameters, reducing the required a-priori assumptions and the ill-ness of the two methods. We describe the algorithm and we show its application to synthetic and field datasets.
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Shallow Seismic on Top of the European Roof – A Vibroseismic Experiment on Firn and Ice at Colle Gnifetti
Authors U. Polom, C. Hofstede, A. Diez and O. EisenIn the summer season 2010, a small shallow reflection seismic experiment was carried out on the firn and ice cover of the Colle Gnifetti, Monte Rosa group, Swiss/Italian Alps. At this site, the physical properties of ice are similar to polar regions, wherefore it is widely used for testing. The challenging experiment in 4500 m asl was designed to explore the scope of shallow vibroseis for seismic targets within and below the firn and ice mass. The small ElVIS vibrator system was used to generate shear waves and compression waves for SH-wave and P-wave receiver setups of two profiles. The resulting sections clearly show the boundary from ice to rock and deeper structures. The deepest features are estimated to 150 m for the SH-wave and 220 m for the P-waves. Reflections could be detected within the ice overburden, which are preliminarily interpreted as change of the crystal structure in the ice column. Furthermore, elastic parameters could be derived from seismic velocities, due to the clear basement reflections. The results of this unique experiment enable new insights in the internal structure of ice formations, and open a promising new investigation method for sub-ice structures and properties, such as basal sediments.
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Determination of Spatial Distribution of Topsoil Texture and Available Potassium Contents Using Gamma Ray Spectrometry
By A. M. FakeyeThere are often difficulties in adequately mapping spatial distribution of soil properties at small scale due to soil variations. Gamma-ray spectrometry is a new technique that can potentially address this while improving the mapping of soil texture and available potassium (avK) and minimising huge soil survey cost and maximizing economic returns. To achieve this, secondary spatial information is incorporated into the study. Using portable Exploranium GR 320 gamma ray spectrometer, concentrations of 40K, 238U and 232Th in sandstone washed materials were measured in three plots while representative soil samples were analysed. The result showed homogeneity in on-ground γ-rays concentrations derived from γK based on the geo-lithological nature of soils. The recorded γK values and total gamma ray intensities were related to the soil texture and avK properties. Strong linear relationships were identified between γK and avK (r2 = 0.78, P < 0.001); and soil texture (r2 = 0.48 to 0.67). The success of γ-ray spectrometry in predicting avK and soil texture classes relied on this strong relationship that transformed γK map into a 0.2m resolution soil property map by regression analysis, associated with 78% variance in avK and 51% clay.
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Algorithm for the Restoration of Clipped GPR Amplitudes
Authors A. Gulati and R. J. FergusonIt is common in Ground Penetrating Radar (GPR) imagery to have missing or corrupted traces. This can be either due to obstacles, noise, technical problems or economic considerations. Antenna-ground coupling is another reason for clipped amplitudes in GPR data. Most commercially available software use the famous "rubber band interpolation", which uses the spline polynomial to undo the clippings. This method is a simple polynomial based interpolation which performs declipping without considering any prior knowledge about the signal. In this paper, a modified Projection on convex set (POCS) method hybird with Non uniform fast Fourier transform (NFFT) is adopted for reconstruction of clipped amplitudes. Restoration of bandlimited GPR data which has undergone amplitude clipping is studied. This algorithm is tested on synthetic and then real clipped GPR data. To study the effectiveness of the technique, results obtained are compared with industry standard spline interpolation.
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Using GPR to Detect Bauxite Horizons in Laterite Deposits of Amazon Basin, Brazil
Authors M. A. Barsottelli-Botelho and J. G. LuizThe GPR was applied on areas with laterite deposits and bauxite occurrences to see its capability of discriminate the different weathered horizons and detect the presence of bauxite horizon and also measure its width. The GPR was tested on two bauxite occurrences in Pará State, Brazil: Paragominas Region and Trombetas River. The good quality of the radargrams was expected since the electric resistivity of the soil, evaluated by Vertical Electrical Sounding which reveals a resistivity varying from 450 to 3000 Ohm.m. The GPR data acquisition with constant-offset results in clear GPR sections where it was possible to separate four zones or horizons as well as its thicknesses. Each zone shows a different reflection pattern or radarfacies, which helps to differentiate them. The first horizon corresponds to the Belterra Clay, which is the most superficial layer, (AT); below, there is the laterite sequence composed by more three horizons, where the second one is a diffractions zone, which corresponds to a clay with concretionary laterites (ND); the third zone shows a homogeneous pattern and it corresponds to a massive ferruginous horizon (LT); an fourth zone shows discontinuous and small reflectors and/or chaotic pattern which corresponds to the main bauxite horizon (BX).
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Seismic Radar Using Time Reversal Mirrors
By S. M. HanafyA new application of Time Reversal Mirrors (TRM) is introduced in this work. It is similar to radar surveillance, detecting and tracking moving object(s) in a remote area using seismic waves; this includes the exact location of the moving object and its speed. Recordings of high signal-to-noise ratio reference Green's functions are required, where they are used as calibration records or moving sensors. The proposed method is tested with a field experiment.
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SEM Wave Propagation in Complex Media with Tetrahedral to Hexahedral Mesh
Authors M. Charara, A. Vershinin, D. Sabitov and G. Pekarated the ability to handle high-resolution simulations of seismic wave propagation in 3D complex domains. However, the exponential accuracy of the method and the reduction of the computational effort rely on the use of conformal hexahedral meshes. Generating an all-hexahedral mesh based upon the available meshing can be difficult. We propose to use a 3D unstructured tetrahedral mesh generator and then split the resulting mesh into hexahedral elements. This approach allows using the same modeling SEM program without any modifications, and it remains faster than dealing with tetrahedral spectral element due to the tensorisation property for spatial derivatives. With this approach, the SEM method doesn’t have restrictions to solving complex problems in sonic or seismic data with respect to methods using tetrahedral meshes. Two examples dealing, respectively, with sonic modeling and the other with surface seismic modeling illustrate the feasibility of the proposed method.
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Solving the 3D Acoustic Wave Equation with Higher-order Mass-lumped Tetrahedral Finite Elements
Authors E. Zhebel, S. Minisini, A. Kononov and W. A. MulderPresent-day computers allow for realistic 3D simulations of seismic wave propagation, as well as migration and inversion of seismic data with numerical solutions of the full wave equation. The finite-difference method is popular because of its simplicity but suffers from accuracy degradation for complex models with sharp interfaces between large impedance contrasts and for models with rough topography. A tetrahedral mesh offers more flexibility and maintains its accuracy if element boundaries are aligned with sharp interfaces. Higher-order finite elements with mass lumping provide a fully explicit time-stepping scheme. We have implemented elements of degree one, two, and three for the 3D acoustic wave equation. Numerical tests confirm the accuracy of the mass-lumped elements. There are two different third-degree elements that have almost the same accuracy, but one has a more favourable stability limit than the other. Convergence analysis shows that the higher the order of the element, the better the computational performance is. A low-storage implementation with OpenMP shows good scaling on 4-and 8-node platforms.
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3D Wave Propagation Modeling in Presence of Topography
Authors I. Tarrass, F. Aye, L. Giraud and P. ThoreWe studied and developed an efficient method to handle the wave simulation in presence of topography based on curvilinear finite difference. Foremost, we derived the modified equations on the continuous problem. We used afterwards optimized stencils and optimized selective filters adapted from aeroacoustics.
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Time-varying Boundary Conditions in Simulation of Seismic Wave Propagation
Authors R. P. Fletcher and J. O. A. RobertssonWe propose two new boundary conditions to regulate coherent reflections from the model boundaries in numerical solutions of wave equations. Both boundary conditions have the common feature that the boundary condition is varied with respect to time. The first boundary condition expands or contracts the computational model during a modeling simulation. The effect is to cause a Doppler shift in the reflected wavefield that can be used to shift energy outside a frequency band of interest. Additionally, when the computational domain is expanding, the range of possible incidence angles on the boundary is restricted. This can be used to increase the effectiveness of many existing absorbing boundary conditions that are more effective for incidence angles close to normal. The second boundary condition is an extension of random boundaries. By carefully changing the realization of a random boundary over time, a more diffusive wavefield can be simulated. We show results with 2D numerical simulations of the scalar wave equation for both these boundary conditions. While the first boundary condition has application to modeling, both of these boundary conditions have potential application within algorithms that rely upon modeling kernels, such as reverse-time migration and full-waveform inversion.
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Elastic Corrections to Acoustic Finite-difference Simulations – Plane-wave Analysis and Examples
Authors J. W. D. Hobro, C. H. Chapman and J. O. A. RobertssonAcoustic finite-difference modelling is playing an increasingly important role in seismic imaging (e.g. in reverse time migration) but the additional cost of elastic finite-difference modelling restricts its use in commercial imaging technology. The cost of full elastic finite-difference modelling can exceed the cost of acoustic modelling in the same velocity model by two orders of magnitude or more. A technique is discussed that corrects an acoustic finite-difference simulation for elastic effects. It is based on estimating the error incurred when using the acoustic wave equation as an approximate solution to the elastic wave equation. The errors are used to generate an effective source field for an additional acoustic simulation that calculates a correction to the wavefield produced in the original acoustic simulation. The cost of this approach is roughly twice that of an acoustic simulation and therefore much less than that of a full elastic simulation. Plane wave theory is used to examine the properties of the method and results from finite-difference simulations are presented to illustrate its accuracy.
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Wavefield Extrapolation in the Pseudo-depth Domain
Authors X. Ma and T. AlkhalifahWe introduce the pseudo-depth as an extension to vertical time representation of acoustic wavefield. By formulating the mapping relations between conventional depth and pseudo-depth, we derive an acoustic wave equation for isotropic media in the pseudo-depth domain. Because of a uniform vertical wavelength in the pseudo-depth domain, wavefield extrapolation requires less number of vertical samples to avoid aliasing. Reduction of samples implies a reduction of run time for finite-difference time extrapolation. To highlight its application potentials, we performed an exploding reflector modeling and zero-offset migration using pseudo-depth wavefield extrapolation. The results agree well with conventional vertical-depth modeling and migration. The pseudo-depth wavefield extrapolation also showed its potential in eliminating multiple artifacts resulting from using a discontinuous velocity model in migration.
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Seeing Coal Top Ahead of the Drill-bit Through Seismic-while-drilling – Results from Numerical Modeling
Authors B. Zhou, I. M. Mason and S. A. GreenhalghBlast damage to coal seams due to incorrect blast stand-off distances costs the equivalent of one unnecessary open cut mine for every 10 existing Australian mines. The current approach for mapping coal seam tops is through drilling and pierce-point logging. To provide appropriate depth control with ±0.2 m precision for blast hole drilling, a 50m x 50m grid is required in advance of overburden removal. Pierce-point mapping can be inaccurate and expensive as the seam is not flat. To date, there are no effective and economic techniques to map coal seam structures in the open cut environment where coal seams are shallow. In this paper, we propose two Seismic-While-Drilling (SWD)-based approaches to see the top of the coal seam ahead of the drill-bit: 1) reverse VSP recording, in which the geophones are planted on the surface near the drill rig; 2) in-seam seismic recording, in which the geophones are locked in the coal seam in a borehole. Numerical modeling demonstrates the feasibility of these two approaches for predicting the approach of coal seams during blasthole drilling. If these methods can be reliably realized in practice, significant coal resources can be saved and the mines can become more productive and profitable.
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Benchmark of Amplitude Preserved Imaging Algorithms Using Large Scale Synthetic Data
Authors H. Allouche, E. Crouzy, P. Thore, M. Brucher, G. Bailly and C. GodefroyIn the work we present, we analyze the amplitude preservation issue of imaging algorithms. We use for that purpose synthetic data that are generated by Full-wave seismic modeling. A particular emphasize is placed on the model building step which is very challenging because there are few robust tools that are adapted to build large and complex models. A 3D full elastic Finite Difference scheme is used to generate the seismic data. These generated data are primarily used to test the amplitude preservation of imaging algorithms and for other geophysical applications as well. The main results will be presented.
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Analysis of Patchy CO2 Saturation from Time-lapse Sonic Logs Using Rock Physics Modelling
Authors E. Caspari, T. M. Müller and B. GurevichWe compare time-lapse sonic and neutron porosity logs of the Nagaoka CO2 sequestration experiment against the uniform and patchy saturation models, which represent two end-members of the P-wave velocity and CO2 saturation relationship. Most of the data points fall between the two limits, suggesting that the relationship is somewhere between uniform and patchy saturation. The behaviour between these limits can be explained by the mechanism of wave-induced-fluid on mesoscopic fluid heterogeneities (porescale << mesoscale << P-wavelength), which causes wave attenuation and velocity dispersion. We model these fluid effects using the 1D and 3D continuous random media model (CRM). The log data approximately follow the predictions of the CRM models for fluid patch sizes of 1 to 5 mm. This heterogeneity scale is much larger than the porescale features of a reservoir thin section, indicating that the mechanism of wave-induced fluid flow on the mesoscale can occur in sonic log data and therefore controls the velocity-saturation relation.
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Borehole Transient Electromagnetics for Monitoring CO2 Sequestration in Saline Aquifers
Authors K. Spitzer, J. Börner, M. Afanasjew, R. U. Börner, O. Ernst and M. EiermannWe present a technique for monitoring CO2 migration in an underground gas storage by using a surface-to-borehole transient electromagnetic method (BTEM). As the targeted reservoir consists of a saline aquifer, the main idea is to detect the changes in the electric conductivity that go along with the resistive CO2 displacing the conductive pore fluid thus decreasing the rock conductivity significantly. There are several major problems to tackle: 1. the three-dimensional simulation of the transient EM fields, 2. the inversion of BTEM data in a time-lapse mode, 3. the description of the petrophysical processes associated with the CO2 migration, and 4. the experimental design to increase the sensitivity at the target depth as well as its practical implementation. Here, we mainly focus on the experimental setup employing BTEM and its interpretation technique using efficient vector finite element simulation procedures combined with Krylov subspace methods. This work is supplemented by extensive laboratory studies. Due to the wide range of practical and theoretical problems, our group is strongly interdisciplinary and consists of numerical mathematicians as well as laboratory, field, and theoretical geophysicists.
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Electromagnetic Characterization of CO2 Sequestration Sites – Feasibility Studies and First Field Results from Ketzin
Authors R. Streich, M. Becken and O. RitterThe electrical resistivity of sedimentary rocks depends strongly on pore fluid content and pore connectivity, and bulk resistivity increases where carbon dioxide displaces conductive saline pore fluid. Therefore, controlled-source electromagnetic (CSEM) methods are potentially a key geophysical technique for exploring and monitoring carbon sequestration sites. We investigate the feasibility of CSEM characterization of the CO2 sequestration pilot site at Ketzin, Germany. Simulation studies using newly developed, accurate 1D and 3D modelling tools indicate that surface-based CSEM surveys may well be able to image the background resistivity structure, but will probably not detect the small CO2 reservoir. Borehole-to-surface configurations exhibit much higher sensitivity to the reservoir and may thus be particularly well-suited for monitoring applications. A recently conducted large-scale CSEM field survey has demonstrated the feasibility of CSEM measurements at the site, and provided data that contain subsurface responses down to the target reservoir depth.
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Data-driven Hydraulic Barriers Delineation and Migration Pathway Detection within the CO2 Geological Storage of Sleipner
More LessThe deep saline aquifers constitute the strongest storage capacity for CO2 storage among the portfolio of possible geological opportunities. The upward and lateral migration of CO2 after being injected into such formations depends (i) on permeability and porosity of the storage formation, (ii) on the potential hydraulic barriers. At Sleipner, the CO2 is injected inside the Utsira formation, where hydraulic barriers correspond to thin intra-shale layers identified by logs without information on their lateral continuity. Furthermore, conventional interpretations of 3D seismic data cannot provide better constraints because the intra-shale layer thickness is smaller than seismic resolution. The proposed method consists in refining intra-shale layer delineation using 4D seismic data in the area invaded by CO2, but also in the Utsira formation part covered by time-lapse seismics. The shale bottom delineation is consistent with 3D-detailed geometry of the CO2 plume and with independent observations of shale at a well. To our knowledge such a detailed 3D description of the Utsira hydraulic barriers has not been published before. Hence, the intra-shale layer discontinuities, allowing the upward migration of CO2, can be identified and could be integrated, in further steps, in the geological static model, unavoidable stage for an efficient dynamic flow simulation.
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Traveltime and Attenuation Tomography of CO2 Plume at Sleipner
Authors G. Rossi, R. A. Chadwick and G. A. WilliamsWithin the European project CO2ReMoVe traveltime and attenuation tomography have been applied on one of the high resolution seismic line acquired in 2006 on the Sleipner CO2 geological storage site. The aim is to gain an accurate model in depth, of both velocities and attenuation, to constrain better the quantification studies. The results obtained from the tomographyc inversion of traveltimes and spectral-centroid frequency shift, respectively, confirm the cumulation of CO2 in the uppermost layer of the reservoir, whereas within the plume, lateral variations of both velocity and quality factor values are observed. The presence of fine shale lenses, and their impact on the CO2 distribution within the plume may be the reason for these lateral variations. Through the comparison of the velocity and Q tomographic values with the theoretical curves from petrophysics studies, it will be possible to interpretate them in terms of effective variations in CO2 saturation and heterogeneities within the plume.
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Wettability Determination of HVBB Coal – Water System with Injection of Model Flue Gas and CO2
Authors N. Shojaikaveh, E. S. J. Rudolph, J. Bruining and S. N. AshrafizadehGeological sequestration of carbon dioxide (CO2) in coal is one possible method to sequester CO2. CO2 or flue-gas injection into coal layers provides the dual benefits of enhancing coal-bed methane production (ECBM) and sequestering CO2. The success of this combined process depends strongly on the wetting behavior of the coal as a function of coal rank and roughness, system pressure and temperature, and also the composition of the gas. The wetting behavior can be evaluated from the contact angle of a gas bubble on the coal surface. In this study, we determine contact angles of a model flue gas (80/20 N2/ CO2 mixture) and pure CO2on a wet Warndt Luisenthal coal (WL), representing a high volatile bituminous coal (hvbB coal). Experiments were performed in the pressure range between 0 and 150 bar using a modified pendant drop cell at a constant temperature of 318 K. In general, contact angles of flue gas on WL coal are smaller than of CO2 on WL coal. The contact angle of flue gas against water on WL coal changes from water-wet to intermediate-wet with increasing pressure. With CO2 injection the wettability of WL coal becomes gas-wet for pressures above 90 bar.
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Data Reconstruction via Sparse Bi-focal Transformation
Authors H. Kutscha and D. J. VerschuurIn most reconstruction algorithms information about the subsurface can not be utilised, even if such is available. Focal transformation is a way to effectively incorporate prior knowledge of the subsurface in seismic data reconstruction. The basis functions of this transformation are the focal operators. They can be understood as one-way propagation operators from certain effective depth levels in a prior velocity model. A sparseness constraint is used to penalize aliasing noise. By using several depth levels simultaneously the data can be described with less parameters in the transform domain. This results in a better signal to noise separation and, therefore, improved reconstruction. In addition, we introduce a smart choice of focal operators, based on data-driven operator updating where we follow closely the reflection events in the data. This allows an even stronger compression of the data in the focal domain.
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Parametric Matching Pursuit Methods to Reconstruct Seismic Data Acquired with Multichannel Sampling
Authors A. Ozbek, M. Vassallo, K. Ozdemir, D. J. van Manen, K. Eggenberger and J. O. A. RobertssonIn marine seismic data acquisition, multichannel sampling can occur in various scenarios, including over/under streamers, multicomponent streamer, and multicomponent ocean-bottom node. In these various multichannel sampling scenarios, each channel outputs samples of a filtered version of a seismic wavefield. These filters are different for each channel; the data are often coarsely and irregularly sampled. The objective is to reconstruct the unknown wavefield from the multichannel samples. In this paper, we discuss how parametric matching pursuit methods can be extended to solve such reconstruction problems. We give two examples based on data that a multicomponent streamer could record and show the form the reconstruction problem takes when two- and three-component data are available.
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Matching Pursuit Methods Applied to Multicomponent Marine Seismic Acquisition – The Issue of Crossline Aliasing
Authors M. Vassallo, A. Ozbek, K. Eggenberger, K. Ozdemir, D. J. van Manen and J. O. A. RobertssonIn this work we analyze the theoretical aspects of spatial aliasing in the crossline direction in marine seismic acquisition. We also explain the benefits of the additional measurements acquired by a multicomponent towed streamer, able to measure the three components of the particle velocity vector in addition to the pressure wavefield. We propose matching pursuit based techniques to reconstruct a 3D full bandwidth seismic wavefield on a fine receiver grid. The techniques that we describe process multicomponent seismic data; they calculate the desired 3D wavefield with satisfactory quality despite the severe aliasing that affects each of the individual input measurements in the crossline direction.
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Convergence Improvement and Noise Attenuation Considerations for POCS Reconstruction
Authors J. J. Gao, M. D. Sacchi and X. H. ChenIn recent years, a variety of algorithms for multidimensional seismic survey reconstruction have been proposed. Projection onto Convex Sets (POCS) seismic trace reconstruction appears to be an affective, uncomplicated and robust algorithm for the recovery of unrecorded seismic traces. Its slow convergence, however, could jeopardize its computational appeal. For this reason, we investigate the performance of POCS in terms of different threshold schedules. We show that a data driven threshold schedule leads to an efficient implementation of POCS where high-quality solutions can be obtained in a modest number of iterations (>30-40). In addition, we present a variant of POCS that permits concurrent reconstruction and denoising of seismic volumes. The latter is achieved by defining a weighted reinsertion trace strategy that alleviates the influence of the noisy traces in the final reconstruction of the seismic volume. We illustrate our findings with synthetic and real data examples.
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5D Seismic Volume Reconstruction Using HOSVD
Authors N. Kreimer and M. D. SacchiWe propose a tensor completion method to reconstruct sparse prestack data. An algorithm based on the High-Order SVD (HOSVD) is used to estimate a low rank decomposition of the seismic volume. The latter allows us to use rank-reduction of tensors to estimate missing traces and increase the SNR of the seismic volume. A real data set from the Western Canadian Sedimentary Basin is used to test the proposed algorithm.
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Minimizing Ellipsoidal Norm Seismic Data Interpolation with Radon Spectrum Constraints
Authors X. Wang, H. Z. Wang, D. H. Zhou and L. X. TianA data adaptive interpolation is designed under the Radon spectrum constraints for spatially aliased seismic data. The inverse process is formed through constructing an ellipsoidal norm, which can make full use of the information about the Fourier spectrum of the data. The ellipsoidal norm can enhance the relative distribution of the Fourier spectrum and is computed with the help of Radon spectrum. Radon spectrum can helpfully concentrate the energy distribution of the Fourier spectrum. Correspondingly, the aliasing components in the Fourier spectrum can be relatively attenuated. The solution of the interpolation inverse problem is sparser and the inverse process becomes more stable. The numerical examples show the effectiveness of the method. The method is applicable to 2D and 3D seismic survey data recorded sparsely in a horizontal plane. The most common pre-stack applications of the algorithm are common-mid-point and half-offset domain interpolation.
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Fast Generalized Fourier Interpolation of Nonstationary Seismic Records
Authors M. Naghizadeh and K. A. InnanenWe propose a fast and efficient method for the interpolation of nonstationary seismic data. The method uses the fast generalized Fourier transform FGFT to identify the space-wavenumber evolution of nonstationary spatial signals at each temporal frequency. The nonredundant nature of FGFT renders a big computational advantage to this interpolation method. A least-squares fitting scheme is used next to retrieve the optimal FGFT coefficients representative of the ideal interpolated data. For randomly sampled data on a regular grid, we seek a sparse representation of FGFT coefficients to retrieve the missing samples. A synthetic seismic data example was used to examine the performance of the method.
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How to Obtain a Seamless Dataset for a Pre-stack Multi-survey Merge Using 5D Data Reconstruction?
Authors J. L. Rivault and A. MotagallyPre-stack merging multi-survey datasets is often challenging in terms of processing effort as well as time and human resources. It is problematic not only due to issues arising from survey matching (due to differences in sources and receivers characteristics) but also from variations in sampling, which may relate to acquisition azimuth, bin size, shot/receiver line spacing. For such datasets, it is important to regularise the sampling of the data in order to harmonise fold and minimise migration artefacts. In this paper we demonstrate the use of 5D data reconstruction for the benefit of multi-survey merging by mapping all the different surveys to a single acquisition design. The result is a dataset with a single acquisition configuration which has a constant fold and a significantly reduced level of migration noise. It also allows the preservation of azimuthal information by migrating in the COV domain. Its shows that 5D data reconstruction fully preserves the original seismic aspect allowing to easily migrate pre-stack multi-survey vintages in a single pass, which reduces the work of the geophysicist.
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Determination of Shear-wave Velocity in Seabed from Dispersion Data of Love Waves
Authors H. Dong, G. Ke and K. DuffautEstimation of the shear (S-) wave velocity or shear strength in the seabed is of importance for geotechnical applications, characterization of subsurface, seismic exploration studies and geohazard investigation. S-wave velocities in the sediment can be estimated by invert the dispersion data of seismic interface waves, which will be considered in this study. Numerical study shows that Love wave sensitivity and inversion stability in linearized inversion is higher than that of the Rayleigh wave and the inversion of Love wave dispersion curve provides a more accurate results. However, in conventional underwater seismic experiments only Scholte wave can be generated. In a testing experiment S-wave sources were used and both Scholte wave and Love wave were generated and recorded. In this study the dispersion of Love waves are analyzed using three time-frequency analysis methods. The S-wave velocities are estimated by inverting the dispersion data. The inversion results show the maximum penetration depth of the estimates is around 25 m in the seafloor, where the estimated S-wave velocity is around 220m/s. This approach provides an alternate and supplementary means to estimate the seismic velocity structure that is valuable in both seafloor geotechnical engineering and the static correction in seismic exploration using reflection methods.
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PZ Summation of 3D WAZ OBS Receiver Gathers
Authors P. Hugonnet, J. L. Boelle, P. Herrmann, F. Prat and A. LaframPZ summation is a typical processing flow for 2-Component Ocean Bottom Seismic (OBS) data, which takes advantage of the simultaneous recording of the pressure and vertical velocity wavefields to separate the up-going and down-going wavefields and to attenuate the water bottom receiver-side peg-legs. However, practical implementations are often either monochannel/1D or designed for 2D receiver gathers, while modern acquisition geometries often lead to 3D, wide-azimuth (WAZ), densely sampled receiver gathers. Such 3D gathers can be processed in the tau-px-py domain with a monochannel algorithm. In this paper we extend the method to directly process 3D gathers, without the need for tau-px-py transforms. Some current implementations also attenuate the source-side multiples, but they rely on a first order approximation for the amplitudes and on the 1D geology (horizontally layered medium) assumption. By introducing a simultaneous 3D predictive deconvolution operator in the peg-leg attenuation formulation, we remove the first order approximation and we improve the attenuation in the case of deviations from the 1D assumption.
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Enhanced Wavefield Separation of OBS Data
Authors E. Zabihi Naeini, L. Baboulaz and S. GrionIn ocean-bottom seismic (OBS) data processing, wavefield separation results are sometimes affected by high levels of noise on the vertical component Z, while the pressure component P is in general of good quality. Nonetheless, Z is needed to achieve complete pre-stack wavefield separation and also to drive processes such as mirror imaging and up-down deconvolution. To address the problem of noise on Z affecting wavefield separation results, we propose a new method which first estimates a multiple model from the downgoing wavefield in a least-squares fashion. Next, this multiple model is used as a measure of seismic signal coherency to calculate an enhanced upgoing wavefield with minimal noise degradation. We show the benefits of this new method on a real data example from the Caspian Sea.
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Velocity-based Wavelet Corrections for Domain Transformation
By J. E. GaiserIt is well established that similar vertical wavelength ranges must be preserved in multicomponent data and that wavelengths of P- and S-waves must match in order to sample reflectivity in an equivalent manner. Conversion of a wavefield to another time or depth domain is described by transformation functions that depend on average VP/VS ratios and velocity. Although these functions align corresponding stratigraphic events of different wavefields, they distort the seismic wavelet because global average velocity properties are independent of local interval properties that define wavelength. In this study we develop a theory of velocity-based wavelet corrections for domain transformations, which are expressed as functions of interval and average VP/VS and velocity, to match wavelength of multicomponent wavefields. We examine the effects for both land and marine data examples and find that land surveys are affected more than marine, and may require spectral broadening of the wavelet. Data from the Marcellus shale in northeast Pennsylvania, USA, shows significant bandwidth improvements for C-waves when wavelet corrections based on velocity match their wavelengths with P-waves. Application of these wavelet corrections should benefit registration fidelity, joint AVO/A (offset and azimuth) inversions and attribute analyses.
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Polarity Reversal Correction for Multicomponent Joint Elastic Reverse Time Migration
Authors Q. Z. Du, Y. T. Zhu, G. Fang and X. F. GongMulticomponent joint elastic reverse time migration is one of important parts in multicomponent seismic data processing. We analyse the implementation of elastic reverse time migration and the polarity reversal in PS and SP images. We present a method using Poynting vector to correct the polarity reversal in elastic reverse time migration. The polarity reversal correction using Poynting vector can be done with the imaging conditions at every time step of the elastic RTM. And the whole correction procedure can be carried out in common shot domain rather than angle domain, which is low cost and convenient. Examples show this method can accurately locate the normal incidence point and correct the polarity reversal for both simple and complex models.
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Using 2D–3C Seismic Data to Discriminate the Sandstone Reservoir in Guang’an Gas Field
More LessThis paper describes the interpretation of 2D-3C seismic survey acquired in the Guang’an Gas Field of Sichuan Basin, which is located at the southwest part of China. Estimating reservoir parameters from geophysical data is vitally important in hydrocarbon exploration and production. We present a joint-inversion algorithm to estimate reservoir parameters directly, using both PP and PS seismic amplitude variation with offset (AVO) data. Reservoir parameters are linked to geophysical parameters through a rock-properties model. The joint inversion method for PP and PS seismic data solves approximated linear expressions of PP and PS reflection coefficients simultaneously using simulated annealing algorithm. By combining the two independent measurements (PP and PS seismic data) together, we stabilize the system of equations for PP and PS seismic data separately, leading to more robust parameter estimation.
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High-effort 3C 3D Seismic Survey in Northern Piceance Basin, Colorado, USA
Authors K. T. Lewallen, T. Johns and J. HeftiThe Upper Cretaceous Mesaverde Group in the Piceance Basin, northwest Colorado, USA, contains a large gas accumulation in a tight sand reservoir. Previous work using well log, borehole and surface seismic data demonstrated potential to utilize converted-wave images and attributes recorded on multicomponent data for fracture identification and lithology discrimination. A high-effort multicomponent 3D surface seismic survey was acquired that pushed the limits of available recording technologies. Preliminary processing results show PP images that overcome significant field noise issues and PS attributes consistent with previous assessments. The measured reservoir HTI attributes after layer stripping through the overburden indicate a principal axis direction of 130º and PS1 (fast) and PS2 (slow) traveltime delays between 10 and 40 ms consistent with pre-survey predictions. Processing and research continues to improve PP/PS images and attributes to support business decisions as well as to investigate the effects of fold, offset, azimuth and spatial sampling on data quality for future surveys in tight gas and other unconventional resource plays.
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3D-3C Geomechanical Study of In-situ Bitumen Recovery, Alberta, Canada
By K. Wikel3D converted wave data have been used in the past as an indicator of fractures and differential stress, although the emphasis has historically been on fractures. Recently, industry has been analyzing stress directions and stress changes based on the direction of fast converted mode (PS1) and the time lag between the fast and slow converted modes (PS1/PS2) in the near surface. In the compliant near surface of NE Alberta the data provided from converted wave seismic data rotated into fast and slow shear directions allows us to evaluate cap rock integrity and areas where the cap rock may deviate from the regional faulting regime. This information is extremely useful in discerning the cap rocks ability to maintain integrity during shallow high pressure bitumen and heavy oil recovery. In addition to this, the changes in PS1 direction and PS1/PS2 time lag with time can assist us in monitoring reservoir changes and recovery of bitumen. This paper is an overview of a case study from Petrobanks pilot THAI facility near Conklin, Alberta, and the results of a converted wave cap rock integrity study.
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Preserved-traveltime Smoothing
Authors V. Vinje, A. Stovas and D. ReynaudMost ray-based migration- and tomography methods require some degree of smoothness in the depth velocity models. A drawback of the conventional smoothing is errors in the traveltimes at the discontinuities of the velocity models. These errors are offset-dependent and they cause errors in both depth and RMO. Here we propose a new method, Preserved-Traveltime Smoothing (PTS) that preserves traveltimes (and hence depths) at these discontinuities. This is done by smoothing of composite anisotropic velocity parameters using a specific convolution filter.
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High Resolution Velocity Model for Imaging Complex Structures
More LessWith the improvement of automatic CIG picking tool, the accuracy of tomography is mainly limited by the model representation (e.g. model grid size) and the regularization of the inversion solver. In practice, a dense model grid with the grid size less than 20 meters and a 100 meters’ smoothing are applied to achieve high resolution velocity updates in some well-conditioned areas (e.g. shallow sediment areas). The small grid size and small model smoothing may cause inversion artefacts in poorly conditioned areas (deep part) and increase the computational cost dramatically. In this abstract, we introduce a non uniform grid, which has a dense sampling in shallow and a sparse sampling in deep to reduce the computing cost. Also we propose a scheme for smoothing along geological structures to stabilize the inversion. We demonstrate with a production example in the Gulf of Mexico that our high resolution tomography is able to improve the subsurface images.
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RTM and Kirchhoff Angle Domain Common-image Gathers for Migration Velocity Analysis
Authors J. P. Montel and G. LambaréAngle domain common image gathers are recommended in Kirchhoff and reverse time migration for velocity model building in complex area. For these both approaches there is a general agreement that the tomographic ray pairs are fully defined by the reflection and azimuth angle information and the reflection dip and that if the velocity model is correctly updated down to a given horizon, it is not necessary to shoot the tomographic ray pairs upwards through this horizon. We show here through examples and a theoretical analysis that these both statements have to be mitigated when the common image gathers exhibit a significant residual move out on. We also show how to accurately compute the tomographic ray pairs allowing then for an accurate angle domain migration velocity analysis.
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Automatic Migration Velocity Analysis Using Reverse Time Migration
Authors W. W. Weibull and B. ArntsenThe objective of this paper is to describe an automatic velocity analysis method based on Reverse Time Migration and Differential Semblance Optimization. The velocity analysis is based on the solution of a nonlinear least squares problem aiming at the focusing of offset domain common image point gathers constructed by Reverse Time Migration. Because the method is based on the solution of the two-way wave equation, it can deal with strong and sharp velocity contrasts both in a stable and accurate manner. It is therefore expected that this method will help improve seismic imaging over complex geological settings. We illustrate the method with a simple synthetic 2D seismic example.
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Migration-velocity Analysis Using Image-space Generalized Wavefields
Authors C. Cardoso and B. BiondiIn areas of complex geology, migration-velocity analysis (MVA) should use methods that describe the complexity of wavefield propagation, such as focusing and defocusing, multipathing, and frequency-dependent velocity sensitivity. MVA by wavefield extrapolation has the ability to address these issues because it uses wavefields as carriers of information. However, its high cost and lack of flexibility with respect to model parametrization have prevented its routine industrial use. We overcome those limitations by using new wavefields: the image-space generalized wavefields. These wavefields are synthesized from a prestack image computed with wavefield-extrapolation methods, using the pre-stack exploding-reflector model. Cost of MVA by wavefield extrapolation is decreased because only a small number of image-space generalized wavefields are necessary to accurately describe the kinematics of velocity errors and because these wavefields can be easily used in a target-oriented way. Flexibility is naturally incorporated because modeling these wavefields have selected reflectors as the initial conditions, allowing the use of a horizon-based parametrization of the model space. In a 3D real data example, we show that using wavefields synthesized by the prestack exploding-reflector model greatly improves efficiency of migration-velocity analysis by wavefield-extrapolation, while yielding a final accurate migration-velocity model evidenced by good quality and reliable depth images.
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Well-based Interval Velocity Analysis Using Multi-parameter CIG
More LessThis study presents a unique approach of performing interval velocity analysis using multi-parameter CIGs for the 2D case. During the analysis, summation-free CIGs are generated and are analyzed as constant depth slices at every image location. When formation tops information is available, an efficient procedure for updating the velocity can be utilized. A specific pattern which can be used to identify correct and wrong velocity within each of the depth slices is used as the core of the analysis. The ability to analyze the structural parameters of each image point improves the accuracy of the velocity and at the same time provides valuable structural information at the analysis location. The procedure and the potential accuracy of proposed method are demonstrated for the 2D case.
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Anisotropic Tomography Using Rock Physics Contraints
Authors Y. Li, D. Nichols, K. Osypov and R. BachrachAnisotropic model building is a well-known challenging problem for its non-linear, ambiguous nature. To reduce the null-space and stabilize the inversion, we propose a new preconditioning scheme in the linearized tomography to include the rock physics prior information. The rock physics information in terms of covariance among P-wave vertical velocity (v0), epsilon and delta is generated by stochastic realizations of a compacting shale model. We design a VSP synthetic survey with the common industry geometry on two different examples to study the effect of applying prior rock physics prior information on proper and improper media, respectively. The results show that by utilizing the proper prior rock physics information, tomography can better resolve the anisotropy parameters, especially in the area where inversion is poorly constrained by the data. However, cautions should be taken when the lithology of the subsurface is unknown. Finally, a posterior uncertainty analysis is performed to evaluate the contribution of the rock physics prior information. The results show that the null-space is greatly reduced by introducing the prior information.
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Building Anisotropic Velocity Model Using Rock Physics and Local Tomography – A North Sea Case Study
Authors R. Bachrach and J. MathewsonAnisotropic velocity models are poorly constrained by surface seismic data alone. Rock physics anisotropic modelling of shale compaction and digenesis provides information which can be used to constrain anisotropic velocity model building. We demonstrate this new anisotropic model building methodology which incorporates shale compaction model, check shot data and tomography to build a 3D spatially varying anisotropic velocity model in the North Sea. The initial rock model is built by analysing well log data, temperature gradient and additional geologic information. Tomography with checkshots (local tomography) which takes into account both residual moveout of surface seismic data and vertical traveltime from checkshot data was used to calibrate rock model to predict a similar anisotropy profile. After calibration a 3D anisotropic model is built to produce spatially varying anisotropic parameters consistent with both rock model and checkshot data.
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Nonlinear, Full Waveform Modeling in Terms of Forward Multiple Scattering
Authors A. J. Berkhout and D. J. VerschuurThe WRW-model shows how to decompose seismic measurements in terms of responses of individual subsurface grid points that include multiple scattering and, optionally, wave conversion. These Grid Point Responses (GPRs) can be used in full waveform seismic modeling algorithms, allowing a better understanding of the complex (blended) codas of seismic reflections and, above all, utilizing these codas for imaging purposes.
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Seismic Diffraction Modeling with the Tip-wave Superposition Method
Authors A. M. Aizenberg, M. A. Ayzenberg and K. D. Klem-MusatovThe transmission-propagation operator theory provides mathematical tools for describing wave propagation in laterally inhomogeneous layered media. The theory is a hybrid of the integral representation (integral-equation) method and the spatial-frequency decomposition (slowness) method. The theory introduces a new wave-motion statement of the forward problem using two operators; the convolutional transmission operator and the feasible propagation operator. In this theory, the exact solution of the initial-boundary value problem is written as the sum of sequentially reflected and transmitted wave events. We further develop the tip-wave superposition method based on the approximations of the two operators in the seismic-frequency range. The feasible propagation operators inside the layers are approximated in the form of layer matrices. The transmission operators at the interfaces are approximated with effective reflection/transmission coefficients, which generalize plane-wave reflection/transmission coefficients to curved interfaces, non-planar wavefronts and finite frequencies. Each element of the layer matrix contains a feasible beam of the tip waves diverging from a small interface element with diffraction at concave parts of the layer boundary and reflecting/transmitting at the interface. We illustrate the potential of the tip-wave superposition method through diffraction modeling of the Green’s function in a layered medium.
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3D CRS-attributes Based Diffraction Imaging
Authors S. Dell and D. GajewskiImaging of diffractions is a challenge in seismic processing. Standard seismic processing is tuned to enhance reflections. Separation of diffracted from reflected events is frequently used to achieve an optimized image of diffractions. We present a method to separate diffracted events in the time domain based on the Common-Reflection-Surface approach. The Common-Reflection-Surface method provides a simulated zero offset section as well as kinematic wavefield attributes. Using synthetic data we show, how these attributes can be used to effectively separate reflected and diffracted energy in the 3D case. The objectives of this work are seismic event separation in 3D case and time migration velocity analysis for diffractions implemented in the poststack domain.
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Edge and Tip Diffraction Imaging in Three Dimensions
By T. J. MoserThe dimensionality of the diffracted wave is related to the dimensionality of the diffractor: a surface reflected ray is uniquely determined, edge diffracted rays strike out a one-parameter pencil, tip diffracted rays a two-parameter pencil of rays. These topological notions are important to analyse the effect of diffraction imaging by specularity suppression. In diffraction imaging reflectivity is fully suppressed, edge diffractivity partly suppressed, while tip diffractivity stands out unsuppressed. These observations have a significant impact on the structural interpretation of three-dimensional diffraction images. Edges constitute the skeleton of a structural model and tips indicate the joints of the skeleton.
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Diffraction Imaging Based on the Diffraction Operator
Authors J. J. S. de Figueiredo, F. Oliveira, E. Esmi, L. Freitas, A. Novais and J. SchleicherSeismic diffractions carry detailed structure information. Some of these structures are well-known as common hydrocarbon traps, such as faults, pinch-outs, unconformities and structures the size of which is in the order of the wavelength. Therefore, the development of computational resource capable of detecting diffractor points with a good resolution is desirable, but has been a challenge in the area of seismic processing. In this work, we present an approach to seismic diffraction imaging based on the diffraction operator, which can be used in both the time and depth domains, in accordance with the complexity of the area. This method that does not require any knowledge apart from the migration velocity field, i.e., rms velocities or interval velocities consists in application of pattern recognition to the amplitudes along the diffraction operator.
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Seismic Diffraction Separation in 2D and 3D Space
More LessSeismic diffractions contain valuable information of small objects underground. The main porosities in carbonate reservoirs with secondary storage space are dissolution caves and fractures, which generate diffraction waves. Therefore, it is valuable to research on diffraction separately. There exists a significant distinction between seismic diffractions and reflections in the post-migration dip-angle domain CRP. The difference response in dip-angle gather between the two types of waves is analyzed, and a flexible approach, which can separate the diffraction from refection efficiently, is described and illustrated in this abstract. The corresponding responses of diffraction and reflection in dip-angle gather for 2D and 3D cases are derived. This approach is proven to be effective and stable by applying it to two physical models data.
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Point and Edge Diffractions in Three Dimensions
More LessSmall geological objects – faults, fractures, karst, etc. – are presented by the diffracted component in the wavefield registered by seismic survey. Diffraction events analysis allows getting information about these objects. It this study we demonstrate how diffraction events behave in the migrated dip-angle domain. Using synthetic and real data we show that the event shape analysis makes it possible to detect a single diffraction point position, fractures direction and fault orientation.
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Fracture and Cavernous Reservoirs Prospecting by the CSP Prestack Migration Method
Authors A. N. Kremlev, G. N. Erokhin, L. E. Starikov and S. V. RodinResults of fracture and cavernous reservoirs forecasting using scattered waves extracted from multichannel seismic data are presented. The forecast is based on an original prestack migration method – the Common Scattering Point (CSP) method, which enables constructing two independent seismic cubes: a conventional cube with reflectors and a new cube with diffractors – the image of space distribution of acoustic impedance inhomogeneities. The last are joined with fracture or cavernous zones – strong sources of scattered waves. The method was tested on synthetic data and in areas with oil deposits in carbonate and shale rocks.
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Diffraction Imaging as an Interpretation Tool
Authors C. Tsingas, B. El Marhfoul and S. SattiThe main objective of imaging diffracted arrivals is to produce high resolution seismic sections in time or depth, which in turn will enhance the interpretation of fault edges, pinchouts, reef edges, fracture zones and other geological discontinuities. The accurate identification of these geological discontinuities plays an important role in the final interpretation of these features and their associated contribution in the formation of potential hydrocarbon traps. The seismic responses of these structural features can be observed in the diffracted part of the recorded wavefield. In the following we present, using real data examples the ability of the methodology to map fracture corridors and subtle geological discontinuities by focusing the associated scattering energy in the prestack domain. Finally, we conclude by highlighting the benefits of the application on land seismic data (i.e., sparse acquisition and low signal to noise ratio). The obtained diffraction image sections are interpreted in conjunction with the conventional reflection time migrated results and coherence type attributes.
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Diffraction Imaging Using 3D Azimuthally-anisotropic Velocity Continuation
Authors W. A. Burnett and S. FomelWe demonstrate 3D time-domain diffraction imaging using azimuthal velocity continuation on a field data set from the Piceance Basin, Northwest Colorado, USA. Azimuthal velocity continuation describes how a seismic image changes given a change in azimuthally anisotropic migration velocity. By using a spectral implementation of velocity continuation, we generate a suite of diffraction images corresponding to a range of azimuthally anisotropic velocity models. We use kurtosis to measure where diffractions become well focused among these images, and thus indicate the optimal anisotropic velocity model. Example timeslices from image and parameter volumes indicate subtle lateral heterogeneity in azimuthal anisotropy throughout the study area.
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Diffraction Imaging with Cross-well Seismic Data
Authors A. Nikitchenko, D. Kiyashchenko, B. Kashtan and V. TroyanScattering objects (diffractors), such as faults or salt inclusions, are of great interest for exploration. But the scattered wave amplitude may be so weak that it is difficult to see those objects in usual reflection images. The location of scattering objects requires special data pre-processing and/or imaging. We have developed the reflection and diffraction imaging methodology for cross-well data. It is based on the vector Kirchhoff migration with special weighting functions that allow us to discriminate reflecting interfaces and diffractors. We apply the noise imaging diagnostics to discriminate between signal and noise in the cross-well reflection and diffraction images. We verify the validity of reflectors and diffractors by building the Common Image Gathers (CIGs).
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Modeling and Observation of the 3D Diffraction Response of Faults and Fractures
Authors M. A. Pelissier, T. J. Moser, J. Pajchel and M. GrasmueckDiffractions not only mark the location of a local heterogeneity, their amplitude distributions carry important structural and stratigraphic information. We address the 3D diffraction response associated with faults and fractures and illustrate this by synthetic modeling, along with seismic and GPR data. In each of these cases, we observe similar phenomena. On vertical sections, edge diffractions are characterized by a phase polarity reversal across the reflection tangent point, and an amplitude asymmetry is often visible between the two legs of the diffraction hyperbola. On time slices, often diffractions appear in the form of concentric half-circles with different radii. The objective of our paper is to investigate these phenomena and provide a theoretical basis for them.
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Diffraction Multifocusing Stack
Authors A. Berkovitch, I. Belfer and N. ScharffDiffraction energy is an important part of the total seismic wavefield. It indicates the present of small scale geomogical objects such as faults, karsts, fractures in the subsurface. Multifocusing has a specific feature to adequately charachterize diffraction traveltime surfaces. This feature can be used to detect and to identify duffraction energy in the total wavefield. Diffraction Multifocusing Stack represents an imaging where diffraction events are enhances and specular reflections are attenuated. These type of images can be used for reliable detection and characterization of important subsurface elements.
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Fractured Basement Reservoir – Limitations of Seismic Evaluation
Authors S. Sengupta and N. C. NandaBasement fractures are cracks, fissures and joints present in the igneous and metamorphic rocks and are usually of sub-seismic scale to be imaged directly by seismic. However, indications of fractures, their potential distribution, density and orientations are possible from high-density-high-resolution (HDHR) seismic attribute studies of the basement reflection. But the extension of the fractures in depth, an important parameter for determining the reservoir thickness, is difficult to infer from seismic due to lack of reflections from below, and within the basement. With severe seismic limitations in delineation and characterization of basement fractures, large orders of uncertainties creep into the estimate of hydrocarbon reserves and the production profile. This makes the exploration for fracture basement reservoirs, as a primary target, a highly risky and expensive venture. However, in favorable geologic-specific situations, such reservoir may be explored as secondary/alternate targets.
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Deriving Microstructure and Fluid State within a Geothermal Reservoir, Reykjanes Peninsula (Iceland)
Authors M. Le Ravalec, M. Adelinet, C. Dorbath, J. Fortin and Y. GuéguenIn this presentation, we refer to a mechanical framework and develop an effective medium model for estimating velocities in porous media. It makes it possible to derive the distribution of crack density beneath the Reykjanes Peninsula from accurate tomography data. Outside the active hydrothermal areas, crack density is shown to decrease with depth. This can be explained mainly by two reasons: the closure of cracks because of the increasing overburden and the secondary filling of cracks because of hydrothermal flows. However, locally beneath the southwestern part of the Kleifarvatn lake, crack density increases with depth. This is consistent with the presence of a deep reservoir with supercritical fluids under pressure, which would trigger off hydrofracturing processes. This study stresses that capturing the link between seismic data and the physical properties of crust is very difficult. The combination of mechanical concepts and effective medium theory contributes to improve our understanding of the phenomena occurring within the Icelandic crust.
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Application of Spectral Decomposition and Ant Tracking to Fractured Carbonate Reservoirs
More LessThis paper uses the integrated application of ant tracking and spectral decomposition to detect minor faults and fractures based on the VSP-driven processing seismic data in fractured carbonate reservoirs. A new method is developed to improve imaging of faults and fractures using “discrete frequency ant tracking data”. Discrete frequency ant tracking data is computed using discrete frequency phase data obtained from spectral decomposition. Discrete frequency ant tracking data is more effective at detecting faults and fractures than full spectrum data. The different discrete frequency ant tracking attribute maps show that the high-frequency ant tracking data is more effective at detecting faults and fractures. When compared with the full spectrum data, the reconstructed ant tracking data produces more detailed images of faults and fractures. The results of our case study shows, this has the potential to be a highly effective and very valuable seismic method.
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3D Seismic Multi-attribute Analysis for Fault/Fracture Delineation and Porosity Prediction – A Kuwait Case Study
Authors T. Chen, O. Khaled, G. Al-Sahlan and T. Al-RashedAn existing exploration lead in the study area is considered to be high risk due to its lack of four-way closure, whereas nearby existing producing fields all have excellent four-way closures. However, stratigraphic traps could potentially exist in this area, which would make this lead attractive for exploration. We attempt to utilize seismic attributes, not only to delineate faults and fractures as the structural fabric, but also to delineate major stratigraphic features, including mapping of porosity distribution for the zone of interest. Post-stack inversion was conducted to get layer properties and to enhance the seismic resolution. The inverted acoustic impedance was combined with conventional migrated stack for neural network analysis to predict 3D porosity volume calibrated with well data. We observed that seismic attributes are useful for fault/fracture mapping in this area. Semblance, structure cube, and dip corrected structure cube are useful for regional fault mapping; whereas curvature, spectral decomposition, and wave form classification can potentially produce very detailed maps of faults and fractures. The porosity trend of the zone of interest appears to have a similar trend compared to the fault lineaments. This could imply that porosity generation and enhancement might be structurally related.
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Joint Interpretation Techniques with Different Azimuth Data to Depict Complex Fault Systems and Reservoirs
Authors B. Q. Zhang, Y. H. Li, T. H. Li, H. J. Zuo, Z. W. Wang and D. Y. LiThe Kongnan area is located in Huanghua depression of Bohai Bay basin in northeast China, where there are complex paleogene fault systems that dissect the area into many different types of reservoirs. It is very difficult to accurately identify the fault systems and to depict the boundaries and interior of the reservoirs with conventional seismic data and seismic interpretation techniques. In order to solve these problems high-density wide azimuth seismic data have been acquired in this area and through processing migration stack data and pre-stack migration gathers from different azimuths have been obtained. Based on these different azimuth seismic data complex fault systems have been accurately identified and reservoir boundaries have been accurately delineated by using a series of techniques including fault system optimal identification technique with different azimuth seismic data joint interpretation, reservoir characterization technique with different azimuth seismic data joint interpretation, special lithologic body recognition technique with different azimuth seismic data joint interpretation, azimuth AVO detection, and azimuth multi-attribute fracture detection. The success rate of drilling has been greatly improved by using these techniques, suggesting the feasibility and effectiveness of our methods.
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Fracture Prediction, Actual 2010 Drilling Results, and Full Wave Forward Modeling: An Integrated Technologies Approach
Authors B. H. Link, I. Y. Kromova and N. MarmalevskyiOver the last few years Lukoil has published the results of two exhaustive studies into the problem of fracture detection. Fracture prediction methods based on reflection amplitude, reflection curvature and its derivatives, coherency cube, spectral decomposition, ant-tracking technology, azimuthal anisotropy of P-wave velocity, and DWM amplitude cube analysis. We concluded that the DWM technique was the most reliable method for fracture permeability detection (Khromova et al., 2011). Lukoil planned a 2010 drilling program for horizontal wells based on DWM technology and this paper will report on the results of that 2010 drilling program. Two horizontal wells were drilled into a limestone interval and extensive well log measurements were taken. The well results illustrate that the location of the fracture systems as predicted by DWM were accurate to within 25 metres. The DWM amplitude maps and well log results will be shown to illustrate the success of this fracture prediction technique. Also, the ability to use these actual well results, along with full wave forward modelling, and DWM technology to push the boundaries of reservoir characterization prediction using an integrated seismic-based methodology is investigated.
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Seismic Azimuthal Anisotropy Analysis of Post-hydraulic Fracturing
Authors K. Zhang, Y. Guo and K. J. MarfurtThe Barnett Shale is one of the first and fully developed unconventional resource plays in North America. Since it has very low permeability, the Barnett Shale is now produced using multiple stages of hydraulic fracturing to provide pathways that allow hydrocarbons to reach the well bore through drilling horizontal wells perpendicular to the present-day azimuth of maximum horizontal stress. For geoscientists and engineers, the knowledge of induced fracture orientation and intensity is critical to evaluate the efficiency of the completion project and the possible need for restimulation. In this paper, we report the results of azimuthal anisotropy analysis of a shale-gas reservoir after hydraulic fracturing. In the study area, the organic-rich Barnett Shale has been extensively hydro-fractured by high pressure fluids to stimulate production prior to the acquisition of the 3D seismic survey. The objective of the seismic survey was to recognize the signature of gas- or water-charged fractured reservoir, and to identify any bypassed pay.
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Seismic Attribute Analysis of Fractures in Woodford Shale
Authors Y. Guo and K. J. MarfurtVolumetric seismic attributes allow interpreters to map the structural deformation and subtle stratigraphic details that are not readily observable on seismic data. In this study, we investigate fracture illumination in the Woodford Shale of the Arkoma Basin, U.S.A based on seismic attribute analysis. Coherence allows us to map relatively large offset discrete faults that appear to have a wrench component, while curvature allows us to map more subtle folds and flexures that are closely associated with fracture swarms. Low acoustic impedance anomalies have a strong correlation with structural lineaments given by most-negative curvature, strongly suggesting either natural fractures or diagenetic alteration. The proximity of microseismic events to ridge/dome features suggests that the formation of ridge/dome features generated paleo-zones of structural weakness.
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Gabor Filters for Segmentation of Salt Structures
Authors A. Solberg, A. Berthelot and L. GeliusThe suitability of Gabor filters for segmenting a salt structure present in a North Sea seismic section is investigated. A filter bank of 24 Gabor filters is designed and applied to the seismic data. Principal component analysis is applied to the resulting filter response images to extract a low number of relevant attributes. These attributes are used to train a statistical classifier to estimate the probability that each pixel is salt. The probability is then used as input to a segmentation algorithm that finds a continuous, smooth border between the salt and the surroundings. The resulting border corresponds well with a manually interpreted contour provided by an experienced interpreter.
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Application of Time-frequency Attributes Based on Generalized S-transform for Thin Bed Indication
Authors R. D. Han, Z. H. Wan, M. S. Chen and H. Y. ZhangDuring the last decade, spectral decomposition technique has proven to be an excellent tool to describe thin beds associated with channel sands, alluvial fans, and the like. However, the traditional spectral decomposition method based on short time Fourier transform, is difficult to acquire the accurate time-frequency spectrum for non-stationary seismic signals. In addition, it generates dozens of unwieldy iso-frequency cubes, not amenable for interpretation. We propose two new time-frequency attributes based on generalized S-transform and apply them to an oilfield of western China to characterize the Triassic reservoir distribution of fluvial deposits. Compared with the traditional short time Fourier transform, the generalized S transform results in more accurate time-frequency spectrum, and generate two attributes which can directly map reservoir thickness.
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Tackling the Stationary Zero-phase Assumption
Authors J. A. Edgar and J. I. SelvageThe assumption that the phase of seismic data is known, and is stationary with respect to time and space, is one of the few assumptions which underpin almost the entire spectrum of seismic interpretation methods. We demonstrate that the phase of 3D seismic data can be robustly estimated using an iterative kurtosis-based method. Our methodology is first applied to a noisy synthetic seismic volume and is shown to correct the non-stationary phase that is present. Results of the application of the method to a real seismic dataset indicate that non-stationary phase is present. We attempt to correct for this phase variation in 3D to leave only local apparent phase anomalies that may be attributable to geological variation. We believe that all seismic data should be subject to phase interrogation before applying techniques that require an assumption about phase to be made, and offer this method as a way to do so.
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Interpreter Guidance for Automated Seismic Image Segmentation
Authors A. Halpert, R. G. Clapp and B. L. BiondiWhile automatic segmentation of seismic images can dramatically speed up interpretation of salt bodies and alleviate a major model-building bottleneck, human expertise at this task is valuable and should be included. Here, we demonstrate a strategy to incorporate such expertise into the highly-efficient Pairwise Region Comparison (PRC) segmentation algorithm. By supplying a limited manual interpretation of the salt boundary, interpreters can correct local inaccuracies and guide the automatic result in both 2D and 3D. In the 3D case, accuracy of the segmentation improves even in areas far from the manual picks. Examples from a wide-azimuth Gulf of Mexico survey demonstrate the effectiveness of this procedure.
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Seismic Interpretation of Prograding Events in Tortonian Sands, Vienna Basin - Implications for Stratigraphic Modeling
Authors G. Gonzalez, M. Kornberger, T. Kuffner, E. Rieser and A. BallauriTortonian deltaic reservoir sands in the Vienna Basin contain substantial remaining HC potential. These reservoirs have been extensively drilled and produced, previous studies could not fully explain reservoir connectivity issues. The recognition of heterogeneities in reservoir modelling studies is a precondition to understanding fluid flow behaviour. This study presents the detailed mapping of prograding seismic elements using Relative Acoustic Impedance (RAI) and Spectral Decomposition (SD) attributes. These attributes facilitate the identification of geologic patterns, such as gently dipping clinoforms previously unrecognized in forced regressive stage reservoirs in this area. The morphological characteristics suggest a complex reservoir facies distribution. The patterns observed are linked to a fast and changing interplay between accommodation space and sediment supply, different feeders and/or source-shifting that are inferred to be acting over the deposition area. This intricate scenario makes ambiguous the definition of flow units based on a purely lateral well-to-well log correlation. Higher accuracy in interpretation of flow units is achieved using the aforementioned seismic attributes in addition to traditional stratigraphic modelling methods. The value added with the use of attributes certainly impacts the geological understanding during integrated multidisciplinary reservoir model workflows.
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Visualizing Turbidite Channels Using the View Locked Color Image Grand Tour in the Chicontepec Basin, Mexico
Authors B. C. Wallet and S. Chávez-PérezThe Chicontepec Formation is a deepwater reservoir of particular strategic importance to Mexico. Despite a large volume of proven reservoirs, development has been hampered by low permeability and uncertainty about controls upon production. In order to better understand the system and how geology may impact production, we have been doing detailed interpretation of a 3D seismic survey from the northern portion o f the Chicontepec Basin. Our analysis has revealed a complex system of channels and submarine fans that have proven difficult to interpret and map. In order to aid in the interpretation of channels, we applied the View Locked Color Image Grand Tour (VLCIGT) that we had previously developed. This technique is an interactive visualization technique used to combine a large number of attributes into a single, color image. In this abstract, we show the results of the application and demonstrate its utility in visualizing the complex channel structure that is frequently found in turbidite reservoirs.
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Stratal Domain Transformation - A Workflow
Authors G. A. D. Dorn and K. L. S. SherlockThis paper discusses the merits of 3D Stratal Domain Transformation that fully removes the 3D structural effects on a seismic volume and produces a Stratal Volume in which every horizontal slice represents a paleodepositional surface or genetic depositional unit. Up to now it has been generally accepted that any method to produce stratal slices would be difficult if not impossible to perform in 3D in areas of structural complexity. The workflow and transform described in this paper properly handles not only differential sedimentation, differential compaction, unconformities (including angular unconformities), salt bodies, canyons, carbonate platforms, but also 3D fault displacements. The resulting stratal volume can be used for interpreting all the depositional systems in the volume. The boundaries of the deposition can be captured 3 dimensionally using a Surface Wrapping method that allows multi-z mapping. The 3D meshes that represent these geobodies can then be inverse-transformed and merged with the structural interpretation. Not only does this method help unravel the stratigraphic and deformational history of any structure, it also provides a critical integrity check of the structural interpretation on which it is based.
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Investigation of Fluid Migration Pathways Using Volumetric Seismic Damage Zone Analysis
By G. PatonDetailed analysis of the geometry of damage zones and seismic properties in the immediate vicinity of faults is required to aid interpretation of fluid migration pathways. This level of analysis can be greatly facilitated by combining manually guided fault slicing with 3D seismic attribute driven Fault Damage Zone geobody delineation to generate fault based attributes, Allan diagrams and throw maps that accurately reflect the underlying data. A critical input to such a Fault Analysis workflow is the fault surfaces. To create simple models, fault surfaces are often created by linking small scale faults as a single fault surface leading to greatly simplified representations of the true fault geometries resulting in important fluid migration pathways being overlooked. A Fault Analysis workflow that avoids the issues of over simplification and allows more complete interrogation of the underlying data is presented by showing the results of applying the workflow to a highly faulted dataset from the Norwegian North Sea.
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Recovering Frequency-magnitude Statistics from Detection Limited Microseismic
More LessMicroseismic events are generally considered to follow a Gutenberg-Richter frequency-magnitude distribution. Previous interpretations have followed the practice in earthquake catalogue analysis of determining a Magnitude of Completeness, a magnitude above which all events appear to have been detected. In hydraulic fracture monitoring (HFM) we typically encounter single well observations; in this case, however, the Magnitude of Completeness approach can remove the majority of recorded events. We would like to provide a statistically robust estimation of the Gutenberg-Richter b-value from detection limited data, which includes as many microseismic events as possible in the calculation. Rather than defining a Magnitude of Completeness, the present method accounts for the detection limit in the calculation allowing detection-limited data to be included in the estimation. An analytical detection limit curve is introduced, based on simplifying assumptions; and it is shown that this detection limit estimation and appropriate statistical tests (the Kolmogorov-Smirnov test) can be coupled with the maximum likelihood estimate (known as the Aki estimate) of the Gutenberg-Richter b-value to provide a robust algorithm for determining a b-value across the maximum number of events.
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Linear Bi-wing Fracture Trends Do Not Indicate Induced Hydraulic Fractures – The Rock Mechanics of Source Mechanisms
Authors S. C. Williams-Stroud, W. B. Barker and K. L. Smith Inc.Microseismic activity was monitored during stimulation of a horizontal well in a tight gas shale using a wide-aperture array of geophones deployed on the surface above the well location. The lateral was drilled perpendicular to the presumed maximum horizontal stress direction, but long, linear, well-constrained microseismic event trends developed at an angle to the wellbore. Source mechanisms of the events show the failure planes of the events were parallel to the microseismicity trends. However, in-situ stress analysis from a crossed-dipole shear log acquired in the well showed the event trends are not parallel to the maximum horizontal stress direction. This result has important implications for stress interpreted from source mechanism analysis and for the impact of natural fractures on the stimulation treatment. Prior knowledge of existing fractures in the reservoir may be critically important for deciding the deviation of horizontal wellbores in order to optimize the stimulation treatment and placement of subsequent wells for field development.
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Elastic Full Waveform Inversion for Locations and Moment Tensors of Microseismic Events
Authors A. B. Droujinine, S. Oates and J. ItaAn Elastic Full Waveform Inversion method for estimating locations and moment tensors of microseismic events is presented and applied to a 4C multi-well data set of production induced microseismic activity from an existing field (Middle East). The algorithm minimizes differences between observed and synthetic elastic (P+S) vector waveforms in order to determine event coordinates and fault angles. It is shown to provide more accurate solutions when compared to conventional ray-based algorithms. The improved link to geomechanical modeling is seen as key to the future realization of reservoir management value from the microseismic technology.
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Localization of Seismic Events in 3D Media by Diffraction Stacking
Authors O. Zhebel, D. Gajewski and C. VanelleThe localization of seismic events is of great importance in exploration geophysics for monitoring of for instance hydraulic fracturing. It can be successfully implemented by diffraction stacking, where the source location is obtained from the maximum of the image function. Since the maximum of the image function is distinct, even in the presence of noise very weak events can be detected. Previous research showed that the method works reliably for homogeneous 2D media. In this paper we demonstrate the extension to 3D and present numerical examples in both homogeneous and heterogeneous media. Strongly heterogeneous media are intensely affected by triplications. We show that the localization of events in such media is nevertheless possible if the most energetic arrivals are taken into account. Moreover, by using geometrical spreading as weighting factors for the input data, separation of propagation and source effects is achieved. We have studied the source effects of radiation patterns. Finally, the method was tested on field data from Southern California. Both numerical and field data application confirm the potential of the method. Conventional source location methods by event picking locate the event with a maximum spatial deviation of 1 km when compared to the location method presented here.
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Automatic Microseism Location by Full-waveform Triangulation
More LessWe present a method for locating microseism hypocenters in both space and time, by correlating multiple reconstructed wavefields corresponding to different groups of receivers. The method is mathematically and conceptually similar to triangulation, but does not require arrival picking. The fundamental idea is that for correct velocity for the wavefield from all receivers arrives at the spatio-temporal location for the microseism simultaneously but on distinctly different paths. Thus, the wavefields for subsets of receivers do not coincide in space and time until the wavefields are at the hypocenter location. To capture the coincidence of the wavefields, we propose a new imaging condition for passive seismic datasets that is similar to the zero-lag cross correlation imaging condition but uses the cross-correlation of multiple receiver wavefields instead of the source and receiver wavefields. We demonstrate this idea on a synthetic microseismic dataset by reconstructing the wavefield using time-reversed propagation for a few groups of receivers that have distinctly different angular aperture and then applying the proposed imaging condition. This method is able to correctly image the microseisms in space and time, even though the signal-to-noise ratio is low.
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Building Effective Anisotropic Models from Microseismic Data Acquired in a Shale-gas Reservoir
Authors I. Das, P. Singh and V. GrechkaVelocity models for locating microseismic events recorded in the oil-industry applications are usually derived from sonic logs and perforation shots. Instead of fixing these models, as is conventionally done, we update them when locating the events. This added flexibility not only improves the accuracy of predicting traveltimes of the recorded P- and S-waves but also provides a convincing evidence for anisotropy of the examined gas-bearing shale formation. While we find that velocity heterogeneity does not need to be introduced to explain the data acquired at each stage of hydraulic fracturing, the obtained models suggest time-lapse changes in the anisotropy parameters characterizing the stimulated reservoir volume.
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Interaction of Neighboring Stages in the Set of Hydraulically Stimulated Horizontal Wells: Are the Stages too Close?
Authors A. M. Baig, T. I. Urbancic and A. GuestMicroseismic moment tensors are determined from a number of stages in a multiwell hydraulic fracutre treatment. These moment tensors are point measures of the strain in the reservoir and can be related to volumetric opening and closure of cracks. The data for these stages locate asymmetrically with respect to the treatment well in a region of the reservoir that was recently fractured. The strain from these data show volumetric increase and decrease indicating that this pre-existing fracutre network opened and closed as the treatment progressed. The dominance of closure for the later set of events implies that the stages were too proximal as previously stimulated regions were closed reducing the enhancement in permeability in these regions.
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Feasibility Study on Microseismic Recordings in Rotliegend Reservoirs
Authors M. Scholze and H. VosbergFew microseismic surveys were carried out at Lower Saxony’s Rotliegend reservoirs, but due to the depth of about 5000m of the Rotliegend reservoirs they showed limited success. Microseismic event magnitudes are estimated using a statistical approach and quality factor Q is estimated using different techniques from VSP data. Using spectral modeling of a synthetic signal with the previous estimated magnitude signal strengths at different depths are modeled. Furthermore noise levels are taken from VSP survey, the calculation of signal to noise ratio gives evidence to the feasibility of microseismic monitoring. The study shows that microseismic monitoring using conventional instrumentation is not possible at surface level.
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Design of Passive Seismic Monitoring for Underground Gas Storage in the Vienna Basin, Austria
Authors L. Eisner, C. Stotter, M. C. Mueller, P. M. Duncan and E. HerndlerWe present a case study for designing a passive seismic monitoring network in a large underground gas storage site. The study included measurements of background seismic noise at selected locations across the site and background seismic noise along a string of receivers cemented in a shallow monitoring borehole. We measured the noise levels over a period of three months and studied the background seismic noise reduction in the shallow monitoring borehole. Specifically this study reveals a significant decrease with depth of the temporal variations in background seismic noise levels at the shallow monitoring borehole relative to the surface. We compare location accuracy from the downhole and surface monitoring arrays, and combinations of these to determine the optimal monitoring array. We also evaluate the detection threshold for microseismic events in the proposed shallow borehole monitoring array based on measured noise levels and empirical observations of microseismic events in similar reservoirs.
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Microseismic Network Design - Estimating Event Detection
Authors D. G. Raymer and H. D. LeslieIn microseismic monitoring, a network of sensors is required to enable the seismic energy generated by the microseismic event to be measured and then used to obtain information about the event’s location and source characteristics. Modeling is required to assess how well a network will perform in detecting and measuring seismic energy from an event, and then how useful that information will be for obtaining an event’s location and source characteristics. A method for estimating the amplitude and frequency content of the signal from a modeled event at a receiver is described. This information, along with expected noise at the receiver, can be used to predict the detectability of an event.
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Utilizing Seismic Source Parameters for Characterizing Reservoir Deformation
Authors M. Prince, A. Baig, T. Urbancic and A. LaidlowThe microseismic events generated as a result of the steam treatment of a reservoir were monitored. The inflation of the reservoir was monitored by a pressure sensor in a neighbouring well and featured some rapid fluctuations as some of the microseismicity migrated out of zone representing a loss of steam chamber containment. The signature of this strain in and above the reservoir is encoded in the microseismic waveforms themselves. We show that using higher-order seismic source parameters that the deformation conditions indicated by the downhole pressure sensor is reflected in the cumulative strain, stress and the relative energy release of the events. This work suggests that deformation in the reservoir can be directly related to changes in the seismic waveforms.
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Geometric Control of Earthquake Magnitudes by Fluid Injections in Rocks
Authors S. A. Shapiro, O. S. Krüger, C. Dinske and C. LangenbruchPore fluids in rocks and pore-pressure relaxation can induce aftershocks of earthquakes. Sometimes rock stimulations by fluid injections into geothermal boreholes are able to trigger perceptible or even potentially damaging earthquakes. This seems to be not the case by hydraulic fracturing of hydrocarbon reservoirs. Reasons of such a difference and factors defining magnitudes of induced earthquakes (triggered tectonically as well as induced artificially) remain unclear. Here we show that one of the main factors limiting the probability to induce a large-magnitude event is the minimum principal axis of a fluid-stimulated rock volume. This geometrical scale controls the order of a largest possible magnitude. We analyze an impact of the geometry of a stimulated volume on the Gutenberg-Richter-type frequency-magnitude distribution of induced earthquakes. It seems that a rupture is only probable along a surface located mainly inside a stimulated rock volume. Therefore, monitoring of a spatial growth of seismicity in real time can help to constrain a risk of damaging induced earthquakes.
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Using Microseismicity to Image the Structure of the Basel Geothermal Reservoir
Authors A. Reshetnikov, J. Kummerow, S. A. Shapiro, H. Asanuma and M. HäringWe apply our microseismic imaging approach to waveforms recorded during and after the stimulation of the Geothermal Reservoir below Basel, Switzerland (2006). Firstly, the hypocenter of the microseismic events is precisely located. Secondly, these events are treated as a "pseudo-active" seismic sources and the reflections within the recorded wavefield are processed by using a directional migration algorithm in order to construct a high resolution image of the illuminated subsurface region. Since the number of the located events is much larger than the number of sensors we swap sources and receivers. Thus, one can consider the system as several "sources" and a cloud of spatially distributed "receivers". We obtain two images from two different downhole sensors. The first one is a large scale fault system in the vicinity of the injection borehole which is compared to the existing results. Second one is a high resolution image of the microseismic cloud interior. There are several reflectors which can be associated with the first and second order cracks revealed in the vicinity of the injection well open hole interval. The strongest reflectors in the image consistent with cracks orientation in the borehole.
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Mining Induced Seismicity - Monitoring of a Large Scale Salt Cavern Collapse
Authors E. Klein, I. Contrucci, N. T. Cao and P. BigarreTo improve our understanding in large scale ground failure phenomenon induced by old underground mining works, a field experiment was undertaken in collaboration with the SOLVAY mining company: a solution mine was instrumented in 2004 previously to its collapse which was triggered in February 2009, as part of the mining scheme. This solution mine is located in the Lorraine salt basin (France). To monitor the cavern collapse, a multi-parameter system featuring high resolution microseismic linked to ground surface leveling (tacheometer and GPS-RTK) was used. The data transmitted for on-line processing offered daily insight of the evolution of the underground cavity. The early signs of unstable evolution were detected during spring 2008: shifts in microseismic background regime end recurrent microseismic episodes were associated to a general upwards process of rock failure of the roof cavern, with no ground surface movement detected. The high microseismic regime of the cavern has convinced the operator to trigger the collapse by brine pumping. Three main microseismic regimes were then observed, each being well correlated with changes in both the surface subsidence rate and the brine level in the cavern.
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Locating High-productivity Areas of Tight Gas-sand Reservoirs Using LF Seismic Surveys
Authors B. Birkelo, A. Goertz, K. Cieslik and E. LaBarre Inc.A low-frequency (LF) seismic survey was acquired over the Jonah and Pinedale gas fields to verify whether changes in the LF passive wave field could discriminate the higher productivity areas. The 2D profiles of the survey crossed the field in places that show maximum contrast in well productivity. As part of the LF seismic survey processing, the anthropogenic noise generated by the operations and the infrastructure of the field was captured and analyzed. Results of this analysis show that the LF seismic attributes correlate well to reservoir properties like estimated ultimate recovery (EUR) and hydrocarbon pore thickness (HPT or SgPhiH) while the anthropogenic noise was not responsible for the measured LF seismic signal anomaly. This opens the door to the use of LF seismic surveys to extend existing tight gas-sand fields and to locating the high-productivity areas in tight gas-sand exploration plays.
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Seismic Interferometry with Ambient Noise from Mizil Area, Romania
Authors D. Draganov and I. PaneaThe Green’s function between two receivers could be retrieved using seismic interferometry (SI) with ambient noise. The retrieval of surface waves has proved easy and robust. The retrieval of reflections has been more difficult and successful results have been demonstrated only lately. It has been shown that to retrieve reflections, the surface waves should be suppressed or eliminated during recording. Even though reflections were retrieved, their frequency content was lower then the frequency content of active data from the same sites. We applied SI to six hours of ambient noise recorded in the Mizil area, Romania, aiming at obtaining subsurface reflectivity without active sources. The site was chosen as relatively little anthropogenic surface-waves sources were expected. To further suppress the undesired retrieval of surface waves, the recorded noise was divided into parts dominated by surface waves and parts without dominant surface waves. We applied SI reflection-retrieval procedure to the parts, in total four hours, without dominant surface waves, which resulted in retrieval of nearly horizontal arrivals. We compared these arrivals with active data from the same area and confirmed the retrieved events to be reflections, including reflections from two geological markers, with frequency content comparable with the active-data one.
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Characteristics of Seismic Noise and the Average Velocity Structure Beneath the Annerveen Gas-field, The Netherlands
Authors W. Kimman, X. Campman and J. TrampertTo characterize the subsurface using noise cross-correlations on a scale relevant to hydrocarbon exploration, we designed and installed a temporary array above the Annerveen gas field in the Netherlands. We study the noise recordings in the period range 3 – 20 s by beamforming and by studying the characteristics of Green’s functions obtained by ambient noise seismic interferometry. Dispersion curves extracted from the beams and from the recovered Green’s functions are complementary: the beamforming results are used for the low frequency (0.05-0.2 Hz) content, and the Green's function method for higher frequencies (> 0.15 Hz). From the average phase velocity curves a 1-D shear wave speed model is constructed. The model clearly requires transverse isotropy to explain the dispersion profiles on both the vertical and transverse components and provides evidence that part of the noise consists of the first surface wave overtone.
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SVD and Seismic Interferometry for Attenuation of Ice-sheet Flexural Waves
Authors C. Bellezza, F. Poletto and G. Del MolinoIn recent years seismic interferometry was extensively investigated and studied from theoretical point of view. Many applications using interferometry by crosscorrelation and by crossconvolution have been proposed to redatum the seismic wavefields, to extract new Green’s functions, and to estimate and subtract noise wavefields. In this work we study an interferometry application with noisy data recorded on ice plates and affected by strong flexural noise. We perform the dispersion analysis and use a singular value decomposition (SVD) to extract the flexural noise from the eigenimages of the wavefields recorded in an Alaska on-ice seismic-reflection experiment. Similar filtering methods are used for attenuation of the ground roll. The SVD noise-estimation method gives improved results with respect to the original data, which are then combined using seismic interferometry to obtain redatumed sea-bottom signals in the shallow water layer below ice. The SVD results allow us to better identify the seismic reflections in the interferometry shots without need of surgical bandpass filters and to use improved signals for subsequent signal stacking and imaging.
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On the Relation between Seismic Interferometry and the Simultaneous-source Method
By K. WapenaarIn seismic interferometry the response of a virtual source is created from responses of sequential transient or simultaneous noise sources. In the simultaneous-source method (also known as blended acquisition), overlapping responses of sources with small time delays are recorded. Clearly seismic interferometry and the simultaneous-source method are related. We make this relation explicit by deriving deblending as a form of seismic interferometry by MDD. The discussed representation for the simultaneous-source method has two interesting limiting cases. When each source group consists of a single source, then we obtain the original expressions for interferometry with sequential transient sources. On the other hand, when there is only one source group containing all sources and when the source wavelets are replaced by mutually uncorrelated noise signals, then we obtain the expressions for interferometry with simultaneous noise sources.
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Accurate and Efficient Pure Acoustic Wave Propagation in VTI Media
Authors B. K. Macy, C. Chu and P. D. AnnoWe substitute isotropic and elliptically anisotropic dispersion relations into the P-SV dispersion relation, under the pseudo-acoustic approximation, to derive pure acoustic VTI wave equations. These pure acoustic VTI wave equations can be conveniently solved using the pseudospectral method and produce highly accurate results, completely free from shear wave artifacts. Straightforward generalization of this methodology gives pure acoustic TTI wave equations.
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An Equivalent Medium for Fractured Carbonate Reservoirs
Authors Q. Z. Du, X. M. Wang, Y. T. Zhu, Q. Zhang and H. Z. YanApproximately 50% of the world’s known hydrocarbon reserves are contained in carbonate rocks; therefore, wave studies on fractured carbonate reservoirs are very important for application in exploration activities. In this paper, we present a fractured porous equivalent medium to describe fractured carbonate reservoirs in oil seismic exploration frequency bands (<100Hz). Fractures are described in terms of fracture-induced anisotropy, and they are embedded in a porous background. Then we use Biot’s equations of poroelasticity and anisotropic Gassmann equations to derive the effective constants of the equivalent medium. It is shown that the fractures and pores strongly influence wave propagation, induce anisotropy and cause poroelastic behaviour in the wavefields. We observe that the presence of fractures gives rise to changes in phase velocity and attenuation.
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Estimation of Thomsen Anisotropy Parameters Using the P-wave Velocities on a Cylindrical Shale Sample
Authors D. Nadri, J. Sarout, A. Bóna and D. DewhurstIn this paper we present a new approach to the estimation of Thomsen anisotropy parameters from laboratory data on cylindrical rock samples. Using tomography-style transducers array, ultrasonic P-wave ray velocities are measured on a transversely isotropic shale sample. This approach is applied to core samples cut along and normal to the bedding plane. Synthetic and actual laboratory data from an anisotropic shale specimen are used as examples. The fast simulated re-annealing method is used to search for the anisotropy parameters.
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Influence of Source Frequency on Shear Wave Splitting - An Experimental Approach
Authors J. J. S. de Figueiredo, N. Dyaur, O. Omoboya, R. Wiley, A. William, J. Schleicher and R. R. StewartElastic anisotropy due to aligned cracks has been the subject of many seismic physical modeling experiments. Different experimental approaches related to sizes, shapes and density of cracks has been taken into account in earlier investigations. In this paper we present a physical study of shear-wave splitting in anisotropy induced aligned cracked media. In this experiment, rubber discs were used as inclusions in a solid epoxy resin matrix. Pulse transmission measurements were carried out on a reference model (without inclusions) and two other models with different aperture of cracks and dissimilar crack densities. The seismic records were measured using three different S-wave source transducers with dominant frequency of 0.1 MHz (low frequency), 0.5 MHz (intermediate frequency) and 1 MHz (high frequency). Crack apertures to seismic wavelength ratio were varied from 1.3 to 13.3 in one model and 2.3 to 23.5 in the second cracked model. Our results show that effects associated with acoustic scattering, attenuation and velocity dispersion interfere directly in shear wave splitting, which in turn is a function of crack aperture and source frequency.
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Joint Laterally Constrained Inversion of CSEM and MT Data
By F. MiorelliOne-dimensional modeling of CSEM and MT data represents the initial step in an increasingly complex interpretation workflow, where lower dimensional methods are used as assessment tools to better constrain the full 3D inversion schemes. The low complexity of 1D modeling enables the geophysicist to obtain results and model scenarios in a very short time. However, the inability to account for lateral variations limits its applicability. To overcome this limitation, while preserving the advantage of low computational complexity, a pseudo-2D technique using 1D forward modeling and lateral constraints is presented. This approach is shown to model 2D structures with a good degree of approximation, constituting a tool for quick imaging. The inversion procedure exploits spatial correlation to resolve model parameters, combining multiple measurements in a single cost function. The joint laterally constrained inversion algorithm was successfully applied on CSEM and MT datasets, both in marine and land environments, showing good consistency with 3D inversion results.
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Enhancing the Resolution of mCSEM Using a Hybrid-based Inversion Workflow
Authors A. Zerilli, M. P. Buonora and T. LabruzzoWe present a fast “hybrid”-based inversion workflow for interpreting mCSEM data, which reconstructs the resistivities and shapes of regions of interest using an image-based inversion approach followed by a “structure”-based inversion algorithm where the model is parameterized using both open and closed “structures”. These a priori information may come from independent measurements (e.g., seismic) or from the results of the image-based inversion. We show that the “hybrid”-based inversion is capable of quantitatively reconstruct the resistivities and shapes of the regions of interest such as prospective reservoirs to obtain models that are consistent with both the mCSEM and the seismic data and that considerable improvement in resolution is achieved compared to the current state-of-the-art. We illustrate the advantages and drawbacks of the “hybrid” inversion using both synthetic and real data. The real data were acquired in the Santos Basin offshore Brazil as part of a co-operation project between Petrobras and Schlumberger to evaluate the technique’s strengths and limitations when applied to the Brazilian deep offshore and to bring new insights toward the development of novel and cost effective solutions and establishment of new interpretation technology.
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Gauss-Newton-based 3D Joint Inversion of Marine CSEM and MT Data
By Y. SasakiAs marine controlled-source electromagnetic (CSEM) methods have been increasingly used for offshore hydrocarbon exploration in complex geologic settings, there is growing interest in developing and applying 3D inversion methods to detect and delineate hydrocarbon reservoirs. The previous work on 3D inversion of marine CSEM and magnetotelluric (MT) data uses some type of conjugate gradient methods, avoiding forming the full Jacobean matrix. This paper presents and tests a Gauss-Newton-based 3D inversion algorithm for marine CSEM and MT data that is implemented on a PC. The main advantage of this approach is that the inverse solution is stable compared to the conjugate gradient-based approaches when near-surface resistivity variations occur and practically is not dependent on the starting model. A synthetic data example shows the effectiveness of the algorithm and the merit of jointly inverting CSEM and MT data.
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Large-scale 3D Inversion of EarthScope MT Data
Authors M. S. Zhdanov, R. B. Smith, A. V. Gribenko, M. Cuma, A. M. Green and G. A. WilsonWe have developed efficient workflows for large-scale 3D MT inversion which address two common problems encountered in 3D MT inversion: computational time and memory requirements. To minimize computational time, our modelling is based on a parallel implementation of the integral equation method. To minimize memory requirements, we have implemented a receiver footprint which dramatically reduces the memory needed for storing Fréchet derivatives for large 3D models. We have applied our 3D MT inversion methodology to EarthScope data acquired over the western United States. In this paper, we present the first 3D earth models of the upper mantle beneath Yellowstone National Park as independently revealed by both 3D MT inversion and 3D seismic tomography. These earth models show a highly conductive region associated with the plume of hot material rising from the mantle towards the Yellowstone volcano. The plume is identified as a west-dipping conductive structure in the 3D conductivity model of geometry similar to the low velocity structure in the 3D P-wave velocity model.
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