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80th EAGE Conference and Exhibition 2018
- Conference date: June 11-14, 2018
- Location: Copenhagen, Denmark
- Published: 11 June 2018
1 - 50 of 1073 results
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3D Seismic Forward Modeling from the Multi-Physical Reservoir Model at the Ketzin CO2 Storage Site
Authors Y. Zheng, B.U. Wiese, S. Lüth, R. Zeng and Y. WangSummaryAbout 67 kt of CO2 have been injected into the deep saline formation at the Ketzin pilot CO2 storage site from June 2008 to August 2013. During the injection, 3D seismic survey has been performed and repeated to monitor the migration of sequestered CO2. The results of seismic monitoring are limited by the acquisition and signal-to-noise ratio of the data. Reservoir simulation can provide information on the CO2 fluid behavior and the approximated model should be calibrated with the monitor results. In this work, the property models are delivered from the multi-physical model at the time of 3D repeated seismic survey. Seismic data modeled based on the models are compared with the real data and the results validate the effectiveness of the multi-physical inversion method. Time-lapse analysis shows the trend of the CO2 migration during and after the injection.
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Diffraction Imaging in 3D via Image Spectral Decomposition of Partial Images
Authors M. Protasov, K. Gadylshin, V. Tchevreda, A. Pravduhin and N. IsakovSummaryThe paper presents 3D diffraction imaging based on spectral decomposition of the different combination of selective or partial images. These images are got by the pre-stack asymmetric migration procedure which is weighted data summation. Spectral decomposition is done in Fourier domain with respect to spatial dip and azimuth angles. Numerical examples with application of different workflows for the synthetic and real data examples demonstrate detailed reliable reconstruction of the fractured zones and reliable reconstruction of fracture orientation on synthetic and real 3D data examples.
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Diffraction Modeling and Imaging of Sand Injectites
Authors M.A. Pelissier, D. Brethaut, R.L. Hartstra, M. Jaya and T.J. MoserSummaryIn this paper we use a conceptual model to investigate the diffraction response of sand injectites. Unlike conventional seismic attributes derived from a migrated image using a local averaging process, the diffraction image provides the full resolution of the wavefield. We model a dike representing a typical injectite wing. This is of particular interest due to the advantage in illumination provided by diffraction over reflection for the steep flanks of the dike. We show that the model produces three type of diffraction response. These are associated with the host rock reflector terminations, with discontinuities in reflectivity along the flanks of the dike due to layering of the host rock, and with the pinchout of the dike. In each case the diffraction response is the resultant of a pair of edge diffractors, and the interference of the imaged diffractors depends on the geometry of the injectite. These results illustrate the potential for diffraction imaging to provide additional resolution of injectite geometries.
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Application of Deep Learning Along Directional Image Gathers for High-Definition Classification of Subsurface Features
Authors Y. Serfaty, L. Itan, R. Levy and Z. KorenSummaryWe present a novel method for decomposing different geometrical characteristics of imaged seismic data. The automation of procedures for enhancing interpreted/classified image data is achieved by applying principle component analysis (PCA) to directional (dip/azimuth) gathers, followed by deep learning (convolutional neural network) classification. The subsurface geometrical objects to be classified are reflectors (continuous structural surfaces) and different types of diffractors (discontinuous objects such as small-scale fractures and faults). This approach shows great promise in identifying subsurface structural features with high accuracy, low cost (no processing preparation is needed) and simple yet scalable implementation. Our preliminary results show superiority over other methods involved in geometrical transformation (e.g., Radon) and specular/diffraction weighted stacks.
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An Outlook on Seismic Diffraction Imaging Using Pattern Recognition
Authors B. Lowney, I. Lokmer, C.J. Bean, G.S. O’Brien and M. IgoeSummaryA seismic image is formed by interactions of the seismic wavefield with geological interfaces, in the form of reflections, diffractions, and other coherent noise. While in conventional processing workflows reflections are favoured over diffractions, this is only beneficial in areas with uniform stratigraphy. Diffractions form as interactions of the wavefield with discontinuities and therefore can be used to image them. However, to image diffractions, they must first be separated from the seismic wavefield. Here we propose a pattern recognition approach for separation, employing image segmentation. We then compare this to two existing diffraction imaging methods, plane-wave destruction and f-k filtering.
Image segmentation can be used to divide the image into pixels which share certain criteria. Here, we have separated the image first by amplitude using a histogram-based segmentation method, followed by edge detection with a Sobel operator to locate the hyperbola.
The image segmentation method successfully locates diffraction hyperbola which can then be separated and migrated for diffraction imaging. When compared with plane-wave destruction and f-k filtering, the image segmentation method proves beneficial as it allows for identification of the hyperbolae without noise. However, the method can fail to identify hyperbolae in noisier environments and when hyperbolae overlap.
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Advanced PP and PS Model Building — An Offshore Mexico Example
Summary“Model building and depth imaging with PS converted-wave data is more complicated than with P-wave data alone. We must determine appropriate compressional and shear velocities such that PP and PS sections match, and accuracy of anisotropic parameters is crucial to minimize residual moveout on both data types. Fortunately, recent technological developments, notably joint PP-PS tomography, allow us to produce excellent PS depth-imaged results with reasonable turnaround time.
In this case study we describe the application of PP-PS model building and depth imaging to an OBC seismic survey. The area is characterized by complex structure combined with large velocity contrasts, both of which cause problems for PS model building. A state-of-the-art workflow was applied that included joint PP-PS tomography, maximum use of well data, and minimal interpretation. High-quality PP and PS results were produced in a reasonable timeframe.”
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Automated and Semi-Automated Depth Registration of PP and PS Images
Authors C. Bagaini, Y. Elmosharaf, A. Dhara, J. Mathewson and M. Acosta PerezSummaryThe registration of PP to PS images is an essential step of iterative model building of multicomponent data. Depending on the quality of the data and the accuracy of the initial model of the earth’s interior, this operation may or may not require identifying corresponding geological boundaries. We developed a method for PP to PS registration that operates directly on images defined in the depth domain. This method can use horizons (either manually or automatically interpreted) or perform this operation in a data driven (automated mode) without interpreter input using only PP and PS seismic images. The application to synthetic and real data demonstrates the value of this method.
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New Model-Based PS Deghosting Boost Data Quality at Grane PRM
Authors S. Ostmo, H. Westerdahl, H. Mehdi Zadeh, M.S. Guttormsen, B. King and L.J. TveitoSummaryIn permanent reservoir monitoring (PRM) PS data, acquired with buried receivers, we have observed a number of notches in the spectra. These notches were not present in the spectra of receivers on the seafloor. To restore the PRM PS data quality, a model-based PS deghosting method for buried receivers is proposed and tested on real data, assuming that the variation in spectra is created by strong contrasts in the shear wave velocity in the very near surface. Only predominantly vertically travelling SS-reflections and transmissions are considered, including important contribution from internal multiples. A shallow layered model is inverted for using picked notches on receiver spectra. Inverse filters, one per receiver, is constructed from the model response and applied to receiver gathers. The method was tested within a complete processing sequence and applied to PRM data from the Grane field in the North Sea. Results indicated very low S-velocities close to the seabed, in the order of 20–40 m/s. The deghosting clearly improved the continuity and resolution in the final image.
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An Elastic Reverse Time Migration Method Based on the S-Wave Quasi-Tensor
More LessSummaryIn this manuscript, an alternative converted-wave image with elastic reverse-time migration is proposed. In the proposed method, PS image is generated by applying the imaging condition to the P- and S-wave stress wavefields. In decoupled wave equation, which is used to separate wavefields, we can obtain the P- and S-wave particle-velocity (or displacement) wavefields and P-wave stress, however we cannot obtain the pure S-wave stress. To fully excavate potential of using stress to produce converted-wave image, we construct a quasi S-wave stress from S-wave particle-velocity wavefield using the so called ‘acoustic shear wave equation’. We illustrate such S-wave stress can provide polarity-consistent converted-wave image theoretically and numerically.
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PZ Processing of a Signal-Apparition Triple-Source Field Test in the North Sea
SummaryWe present results from a triple-source signal apparition-style field test carried out over a PRM array in the North Sea during the summer of 2017. In addition to the triple-source line, two reference lines were acquired firing the central source only.
Assessing the quality of acoustic and elastic P/Z combination confirms high fidelity data quality throughout the bandwidth of the data. Comparing prestack migrated images of elastic P/Z combination against Amundsen demultiple images shows that multiples are a significant challenge in the area and that removing all surface related multiples as done in the Amundsen demultiple method is critical for unambiguous interpretation.
Amundsen demultiple requires the deconvolution of the downgoing wavefield from the upgoing wavefield. Thus, the process relies on excellent data fidelity in separated wavefield constituents. We find that comparisons of reference line on reference line display a similar level of residual as the reference line compared to the isolated source result corresponding to the same source location. The reference line compared to the decoded source result corresponding to the other offset source locations display a larger residual. Our results thus confirm that triple-source signal apparition-style acquisition enables highly productive acquisition of seismic data without compromising on data quality.
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Time Delays in Simultaneous-Source Acquisition and Its Impact on Low Frequencies
Authors K.E. Haavik, Å.S. Pedersen and M. ThompsonSummarySimultaneous-source acquisition enables more efficient and/or denser seismic data acquisition. While acquiring simultaneous source data is a relatively simple extension of conventional operations, separating the signal from the different sources is a more involved process. Modern simultaneous source acquisition is enabled by applying time delays to one or more of the sources in order for the separation process to work. Here we present a theoretical study on the effect of this time delay on the emitted signal from the simultaneous sources. Our findings suggests that small time delays are preferable over larger time delays when considering the amplitude and radiation pattern for low-frequencies (<15 Hz).
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A Seismic Apparition Experiment on Towed Streamer Seismic Data
More LessSummaryThis paper discusses a field test of seismic apparition, a simultaneous source technique for marine airgun sources that uses modulation to facilitate source separation. The first part of the paper focuses on the design of the modulation codes. The second part introduces a field test where a single line of 3D data was acquired using three two-string sources. This field test provides real data for testing current and future implementations of the seismic apparition method. In this paper we present initial separation results obtained after minimal pre-processing of the recorded data. Conclusions are drawn on the challenges and opportunities of towed-streamer seismic apparition for multi-source surveys.
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High-Multiplicity Simultaneous Sources and Marine Vibrator Acquisition Effects
Authors D. Halliday, R. Laws, A. Özbek and J. HopperstadSummaryHigh-fidelity marine seismic vibrators allow control of the phase of the emitted seismic wavefield. Phase control of the seismic source can allow new simultaneous-source encoding techniques. These techniques may allow highly efficient seismic acquisition without sacrificing data quality. One approach that uses phase control is phase sequencing, where the emitted source wavefield phase is changed in a non-random way from shot-to-shot, allowing manipulation of data in the frequency-wavenumber domain. Combined with wavefield reconstruction, phase sequencing allows separation of simultaneous sources with minimal residual crosstalk. An important consideration when evaluating such techniques is that marine seismic vibrator deployment and utilization will be different from an air-gun source. For example, a marine vibrator source array is likely to move while emitting energy, it may have different array elements distributed at different depths, it will emit swept rather than impulsive waveforms, and it may behave differently in the presence of a rough sea. We demonstrate the separation of high-multiplicity simultaneous sources using phase sequencing, before discussing marine-vibrator specific acquisition effects. The impact of these acquisition effects will be demonstrated in future publications.
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Compressed Sensing Based Land Simultaneous Acquisition Using Encoded Sweeps
Authors R. Kumar, S. Sharan, N. Moldoveanu and F. J. HerrmannSummarySimultaneous shooting methods using encoded sweeps can enhance the productivity of land acquisition in situations where deployment of many vibrators and larger receiver spread is not possible in the field due to obstructions or permit limitations. However, the existing framework requires shooting the full sequence of encoded sweeps on each shot point to reconstruct the complete frequency bandwidth Green’s function. Although this simultaneous shooting method reduces the sweeping time vs conventional sequential shooting, the gain in efficiency is limited. To further reduce the sweeping time, we propose to acquire randomly selected subsets of the encoded sweeps sequences followed by a rank-minimization based joint source separation and spectral interpolation framework to reconstruct the full bandwidth deblended Green’s function. We demonstrate the advantages of proposed sampling and reconstruction framework using a synthetic seismic line simulated using SEG-SEAM Phase II land velocity and density model.
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Survey Designing for Blended Acquisition with Irregularly Sub-Sampled Geometries
Authors S. Nakayama, G. Blacquière, T. Ishiyama and S. IshikawaSummaryWe introduce a workflow to derive survey parameters responsible for source blending as well as spatial sampling of detectors and sources. The proposed workflow iteratively performs the following three steps. The first step is application of blending and sub-sampling to an unblended and well-sampled data. We then apply a closed-loop deblending and data reconstruction enabling a robust estimate of a deblended and reconstructed data. The residue for a given design from this step is evaluated, and subsequently used by genetic algorithms (GAs) to simultaneously update the survey parameters related to both blending and spatial sampling. The updated parameters are fed into a next iteration till they satisfy given stopping criteria. We also propose repeated encoding sequence (RES) used to form a parameter sequence in GAs, making the proposed designing workflow computationally affordable. We demonstrate the results of the workflow using numerically simulated examples that represent blended dispersed source array data. Difference attributable only to a way to design parameters is easily recognizable. The optimized parameters yield clear improvement of deblending and data reconstruction quality and subsequently provide optimal acquisition scenarios. Additionally, comparison among different optimization schemes illustrates ability of GAs along with RES to efficiently find better solutions.
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Blended Noise Suppression Using a Hybrid Median Filter, Normal Moveout and Complex Curvelet Transform Approach
Authors L.Q. Dong, M.G. Zhang, C.H. Wang, Y.M. Zhang and Y. ZhuSummaryThe high density acquisition way can uplift the subsurface imaging accuracy, whereas the high cost limits the widely application in practice. Blending acquisition way has emerged as a promising way of significantly increasing the efficiency of seismic acquisition. However, there will exist a large challenge of severe interference noise and decrease S/N ratio. Therefore, with recent processing practices, the success of blending acquisition relies heavily on the effectiveness of de-blending to separate signals from simultaneous sources. In the paper, we proposed a blended noise suppression approach using a hybrid median filter, normal moveout (NMO) and complex curvelet transform (CCT) approach. Firstly, the large step median filter is applied to the initial data after NMO correction. Next, we continue to extract the residual energy to get the de-blended result by the CCT-based threshold method. Then, re-iterate the difference data by subtracting the original pseudo de-blended data and the pseudo de-blended data of the de-blended result from each iteration as the above processing flow. Finally, the final de-blended data is derived by adding the remained energy of each iteration until the S/N ratio satisfies the desired one. We demonstrate through a simulated field data the effectiveness of the approach.
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Triple Source Isolation: Results from a North Sea Field Test
Authors J.O.A. Robertsson, F. Andersson, D.J. van Manen, K. Eggenberger, R. Walker, C.R. Berg, J. Gateman and I. GimseSummarySeabed seismic data have proven critical for addressing long-standing challenges in seismic imaging both for exploration and reservoir development and management. Significant advances have been made to bring down the cost and increase efficiency for deploying and retrieving ocean bottom nodes, such that the efficiency on the source side increasingly becomes the bottleneck for efficient operations.
We present results from a recent field test in the North Sea where three source lines were acquired simultaneously from a single vessel without compromising inline sampling such that the fold is doubled compared to conventional flip-flop acquisition. The acquisition and processing technology is partially based on the science of signal apparition that allows for an optimally large part of the data corresponding to the individual sources to be exactly recovered while accurately reconstructing the full bandwidth beyond the region in frequency-wavenumber space of exact reconstruction. For the triple source field test presented here, the productivity increases 50% compared to conventional flip-flop acquisition or three-fold compared to conventional single-source full-fold acquisition suitable for reservoir development and management applications.
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Incorporating Stationary-Phase Implementation into QPSTM and Application in Daqing Oilfield
More LessSummaryWe have improved the so-called QPSTM by introducing a dip-angle domain stationary-phase implementation. QPSTM compensates absorption and dispersion via an actual wave propagation path using effective Q parameters. However, this strongly degrades the resolution gained by the compensation and requires more computational effort than conventional PSTM. Our stationary-phase implementation improves QPSTM through the determination of an optimal migration aperture based on an estimate of the Fresnel zone. This significantly attenuates the noises and reduces the computational cost of 3D QPSTM. We have estimated the 2D Fresnel zone in terms of two dip angles through building a pair of 1D migrated dip angle gathers using PSTM. Our stationary-phase QPSTM was implemented as a two-stage process. First, we used conventional PSTM to obtain the Fresnel zones. Then, we performed QPSTM with the Fresnel-zone-based optimized migration aperture. We apply the QPSTM to the actual 3D seismic data, obtain higher resolution imaging results, and verify the practicability of the method.
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Visco-Acoustic Reverse-Time Migration based on the Stereo-Modeling Operator
More LessSummaryViscous effects causes seismic energy attenuation and waveform distortion. Conventional acoustic migration cannot account for this effect, which may produce images with poor illumination, reduced resolution, and wrong placement of reflectors. Reverse time migration (RTM) based on the visco-acoustic wave equation is an effective way to image subsurface media. Solving the wave equation accurately and efficiently affects the RTM results. Stereo-modeling methods have great ability in suppressing numerical dispersion, thus can improve computational efficiency with coarse grids. Based on the stereo-modeling operator, we derive a nearly-analytic central difference (NACD) method (4th-order accuracy in both time and space) to solve the visco-acoustic wave equation. The efficiency test shows it can suppress the numerical dispersion effectively and is more efficient compared with Lax-Wendroff correction (LWC) method with the same accuracy. The acoustic and visco-acoustic RTM based on NACD is performed on the fault and Marmousi models. The results show that the application of NACD in RTM improves the accuracy and the resolution of images. Moreover, the images obtained by visco-acoustic RTM are clearer and have higher resolution compared with images obtained by acoustic RTM because the viscous effects are considered and compensated.
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Angle Gathers from a Kirchhoff Pre-Stack Depth Migration using a Space-Lag Extended Imaging Condition
Authors G. O’Brien, S. Delaney and M. IgoeSummaryTraditional Kirchhoff pre-stack depth migration methods output surface offset common image gathers (CIG) but exploration workflows require subsurface reflection angle CIGs to extract accurate angle versus amplitude measurements. Providing any incremental uplift to the amplitude versus angle (AVA) attributes and the associated seismic inversions can significantly impact exploration and development success. Several high-end imaging techniques exist to extract subsurface angles directly, notably reverse time migration methods and common reflection angle migrations. Here, we explore the ability of the relatively cheap well-used Kirchhoff pre-stack depth migration to output subsurface reflection angle common image gathers via a local space-shift extended imaging condition. A layered synthetic model and a complex 2-D synthetic model are used to assess the space-shift image gathers output from such a migration scheme and to evaluate the seismic attributes relative to the traditional surface offset CIGs. The synthetic results show that the extended imaging condition clearly provides an uplift in the measured AVA over the surface offset migration. Finally, we show an example of a 3D space-lag gather from deep marine data and compare the resultant angle gathers with those generated from an offset migration and a time-shift imaging condition Kirchhoff migration.
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Image Domain Least-Squares Migration with Hessian Estimated by Non-Stationary Matching Filters
More LessSummaryThe key to image domain least-squares migration is the explicit calculation of the Hessian matrix. However, the full Hessian matrix is too big and expensive to compute and save. Guitton (2004) directly approximates the non-diagonal inverse of the Hessian with a bank of non-stationary matching filters, which can be seen as a low-rank approximation of the true inverse Hessian. The filters have the amplitude-balancing effect, but the ability to increase the resolution is missing. In this paper, to capture as much effect of least-squares migration as possible, we use non-stationary matching filters to approximate the non-diagonal Hessian first, and then solve a constrained optimization problem with the sparse and TV regularization for the result of the image domain least-squares migration. Numerical examples illustrate that the inverted images of the proposed method have both more balanced amplitudes and higher resolution than conventional migration images.
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Double Plane Wave Least Squares Reverse Time Migration with Sparse Frequency and Pane Wave Sampling
More LessSummaryLeast squares reverse time migration (LSRTM) is often formulated as an iterative updating process, where estimating the gradient of the misfit function is necessary. Traditional time domain shot profile LSRTM is computationally expensive because computing the gradient involves solving the two-way wave equation several times in every iteration. To reduce the computational effort of LSRTM, we propose to implement frequency domain LSRTM using double plane wave (DPW) data with sparse frequency sampling and sparse plane wave sampling. Theoretically, frequency sampling and plane wave sampling used in migration should obey the Nyqusit theorem. Otherwise, the well-known wrap-around artifacts and linear artifacts might contaminate images. In this research, we demonstrate that images with correct reflector amplitudes and reasonable resolution can be achieved even if frequency sampling and plane wave sampling are larger than that determined by the Nyquist theorem. The artifacts generated due to undersampling in frequency and plane wave can be effectively suppressed during iterations. Using sparse frequency and plane wave sampling for DPW LSRTM greatly reduces the number of wavefield computations. We show that the computational efficiency can be improved by an order of magnitude.
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Wavefield Transmission Imaging with Reverse Time Migration
Authors C. Willacy and M. KryvohuzSummarySteep salt boundaries can be poorly illuminated or completely absent in the migrated image. To provide a solution to this problem, we apply two reverse time migration imaging methods which use transmitted (refracted) wavefields. In the first technique, down-going waves, typically recorded in walkaway VSP surveys, are used to image the salt flank via the generation of aplanatic isochrones. It is demonstrated that this image can be generated in the absence of an explicit interpretation of the salt flank. In the second technique, we extend the basic theory to include imaging of up-going source wave fields which then refract at the base salt, as acquired by a surface acquisition geometry. This technique has similarities to the prism-imaging method, yet it uses transmitted instead of reflected waves at the salt boundary. We demonstrate that this is not only a viable technique for imaging of the salt boundary, but also to determine the correct salt velocity when used in conjunction with conventional reflection imaging. A combination of synthetic and field datasets is used to demonstrate the transmission imaging methodologies for imaging salt flanks with reverse time migration.
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A Separated Formulation of the Elastic Wave Equation in P- and S-potential Fields
More LessSummaryElastic wave imaging has been a significant challenge in the exploration industry due to the complexities in wave physics and in numerical implementation. In this paper, we derive the elastic wave equations without the assumptions of homogeneous Lamé parameters to capture the mode conversion between the P- and S-waves in an isotropic, constant-density medium. The resulting set of two coupled second-order equations for P- and S-potentials clearly demonstrates that mode conversion only occurs at the discontinuities of the shear modulus. Applying Born approximation to the new equations, we derive PP and PS imaging conditions as the first gradients of waveform matching objective functions. The resulting images are consistent with the physical perturbations of the elastic parameters, and hence are automatically free of the polarity reversal artifacts in the converted images. When implementing elastic reverse time migration (RTM), we show that scalar wave equations can be used to back propagate the recorded P-potential, as well as individual components in the vector field of the S-potential.
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A Wavefield-separation-based Elastic Least Squares Reverse Time Migration
More LessSummaryCompared with the elastic reverse time migration (ERTM), ELSRTM can produce images with higher spatial resolution, more balanced amplitudes and fewer artifacts. However, the crosstalk between compressional (P-) and shear (S-) waves can significantly degrade the imaging quality of ELSRTM. We developed an ELSRTM method to suppress the crosstalk artifacts. This method includes three crucial points. The first is that both the forward and backward wavefield are extrapolated based on the separated elastic velocity-stress equation of P- and S-waves. The second is that the separated vector P- and S-wave residuals are migrated to form reflectivity images of Lamé constants λ and μ independently. The third is that the reflectivity images of λ and μ are obtained by the vector P-wave wavefields achieved in the backward extrapolation of the separated vector P-wave residuals and the vector S-wave wavefields achieved in the backward extrapolation of the separated vector S-wave residuals respectively. Numerical tests with synthetic data demonstrate that our ELSRTM method can produce images free of the crosstalk artifacts.
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Channelized System Reservoir Characterization Using Diffraction Imaging - Practical Aspect of a Novel Technique
Authors J. Soldo, M. Koremblit, D. Lorenzo and M. SuarezSummaryThe seismic processing technique known as “Diffraction Imaging” has demonstrated to be an interesting method in order to map geological sub surface discontinuities, in special those high frequency features related to such as discontinuities. They have been study in the prestack time as well as in the prestack depth domain by several authors. Detection of geobodies (channel system) is important when developing continental type sandstone reservoirs. Understanding the location and orientation of such as features is important for optimal well placement and field delineation. Seismic diffraction can be used for imaging not only systems of natural fractures but also for mapping wedge limits caused by channels. Diffracted waves are created when an incident wavefield encounters small size objects or even discontinuities such as faults, sharp curvature geoforms or wedges. The diffractive portion of this wavefield can be extracted from its total in order to produce images that contain information regarding subsurface scattered energy due to discontinuities. This is the first time that this technique is applied in the company in order to characterise and improve the delineation of channel systems.
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A Holistic Approach to Model-Building in and around Injectites: A Case-Study Offshore Norway
Authors V. Valler, N. Payne, T. Hallett, M. Kobylarski, G. Venkatraman, J. Rappke and D. FaircloughSummaryThe presence of injectites is a common and often time consuming area to be addressed during velocity model building. In addition to their impact on deeper structures and prospects, re-worked injectites are increasingly being considered for hydrocarbon potential themselves. In order to handle the challenges above we should consider ways of producing an accurate velocity model of these structures within a framework that is efficient and commercially time-viable. Here we present a holistic approach and case study to model-building in and around injectites that utilizes robust broadband data pre-processing, a semi-automated identification and modeling of injectite bodies and subsequent high-resolution tomographic updating. Our results show that this method enables us to produce a highly accurate and detailed model of a complex injectite field and subsequent improvement on the deeper image within the timeframe of a conventional model building iteration.
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Separated Wavefield Imaging for Ocean Bottom Surveys - Feasibility Study of Receiver Decimation on a North Sea Dataset
Authors D. van der Burg, T. Martin, A. Asnaashari, S. Perrier, J. Fasterling and J. MusserSummaryWe present a method for imaging with separated wavefields from an ocean bottom cable acquisition to deliver images of the subsurface with enhanced illumination and angular diversity. Using this data, we investigate whether it is feasible to decimate the sources or receivers without degrading the structural image from separated wavefield imaging. We conclude by determining if this would be beneficial to reduce acquisition cost and to provide illumination below obstructions like a platform hole.
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Reservoir Imaging Using Fwi, Q-Tomography & Bayesian Classification - Black Sea Case Study from Acquisition to Drill Bit
Authors O. Costriiciuc, A. Fogg, R. Holden and J. MooreSummaryBlack Sea Oil and Gas’ concession XVa Midia is on the Romanian continental shelf of the Black Sea which the company operates on behalf of its partners Gas Plus International BV and Petro Ventures Resources SRL. This study demonstrates how a suite of seismic processing and QI (Quantitative Interpretation) technologies was optimised to fully resolve gas assets currently under development and highlight satellite exploration targets in Miocene to Pliocene age (late Pontian to Dacian) fluvio-deltaic sands and claystones. Near surface channel features and localised gas presence causes seismic imaging issues as amplitudes are attenuated and time structural push-downs are observed. A 3D survey was acquired during 2016 and processed together with three legacy 3D surveys. This data was used for exploration well planning for a drilling programme of two wells Q4 2017. During the seismic processing tests were run to investigate possible imaging improvements using Full Waveform Inversion (FWI) and Q-tomography. A subset of the 3D was reprocessed using these methods during Q4 2017. This presentation will demonstrate the imaging uplift achieved and the impact on the interpretation together with pre and post drill results guided by QI based on pre-stack seismic inversion and facies/fluid probability classification.
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Reviving a Mature Basin through High-End Imaging Technology
More LessSummaryThis paper highlights compelling imaging improvements achieved through modern high-end reprocessing in the Gippsland Basin. The area of study, largely represented by the Bass Canyon, has strong exploration potential as well as high risks. The major challenges in this region are related to geologic complexity and seismic imaging limitations, i.e.: a) extensive velocity anomalies leading to velocity uncertainties and false structural closures that increase drilling risk; b) strong noise interference and limited imaging clarity that affects AVO analysis. By integrating the modern techniques of 3D deghosting, full waveform inversion (FWI) and least-square Q pre-stack depth migration (LSQPSDM), the newly reprocessed data yields a substantial amount of added value over legacy datasets, resulting in an improved understanding of the subsurface geology and clearer prospect mapping. This reprocessing approach demonstrates, even in basins that are considered mature, that new ideas and technology can change long-held perceptions and rejuvenate exploration interest.
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Salt-Related Converted-Wave Attenuation – a Deep-Water Example
Authors M. Hegazy, A. Stewart, S. Hydal, K. Malave, O. Zdraveva and O. MataraciogluSummaryComplex shallow salt canopies, such as those found in offshore basins in the Gulf of Mexico, Brazil, and West Africa often suffer poor illumination, which results in poor subsalt imaging. The geometry of subsalt horizons truncating against the base of salt often determines if a trap exists and if it will be economically viable. It is, therefore, important to get an accurate image in these areas to reduce the seismic uncertainty. Unfortunately, the presence of converted mode energy can add spurious complexity to the subsalt image and degrade subsalt target interpretation. A novel C-wave attenuation workflow is introduced to address the C-wave energy. Synthetic and field data examples show a significant improvement in the image interpretability and demonstrates the importance of this application to earth model building and amplitude variation with offset AVO work
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Ultra-Large-Scale Wide-Azimuth Acquisition and Broadband Imaging in the Campeche Basin
Authors S.L. Yong, O. Zdraveva, E. Medina, S. Savoie and K. LyonsSummaryThe recent opening of Mexican acreage to foreign companies created a need for reliable regional interpretation over large prospective areas and lead to the wide-azimuth acquisition of new seismic data over ∼70,000 km2 in the Campeche basin. In this work, we discuss the overall acquisition and imaging strategy, allowing us to create high-quality, contiguous, broad-band images over large area of extreme geologic complexity in a short turnaround time. We describe all aspects of this strategy from the staged acquisition and fast-track products delivery, through broad-band processing and high-resolution anisotropic earth model building, to the use of complementary migration algorithms enabling supra-salt and subsalt target evaluation in this vast and prospective area located in the most southern part of the Gulf of Mexico.
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Unfolding Marine Multiazimuth Data from the Gulf of Mexico
Authors H. Roende, G. Hilburn and D. BateSummaryThe first evolution of WAZ seismic data acquired over 10 years ago provided a sizable uplift over previous NAZ data sets. The addition of orthogonal surveys, advances in data processing, and improved geological understanding have recently introduced another step change in the data quality. These improvements are now leading to a greater improvement in data analysis and interpretation than just structural imaging uplift. We can see 4 distinct behaviors in 6 azimuth M-WAZ marine data. When data is acquired over “simple” geology, we get common signal in all azimuths. We also notice that close to salt, there are major differences between the 6 azimuths over large depth ranges due to illumination. These two observations have been known and published for the last decade. Our findings show that there are untapped potential and knowledge in marine M-WAZ data if acquired and proccessed correctly. We believe that we can map azimuthal-amplitude variations, and therefore, map stress and fracture fields to facilitate the production of hydrocarbons. If the data is acquired with a significant time lapse between acquisition, there is also an opportunitiy to perform 4D studies.
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Seismic Re-processing and Q-FWI Model Building to Optimise AVO and Resolution in the Shallow Wisting Discovery
Authors G. Apeland, P. Smith, O. Lewis, S. Way, H. Veire, N. Stevens, L.M. Moskvil and J.R. GranliSummaryThe Wisting discovery in the south-west Barents Sea is a laboratory for geophysical studies due to its shallow reservoir and complex geological history. This discovery is constantly highlighting aspects of conventional seismic processing that require new approaches and technologies to allow AVO to be preserved and understood. Previous studies at the Wisting discovery have highlighted the importance of imaging in the depth domain with an accurate earth model for AVO preservation, including accounting for absorption within the imaging itself. In addition, AVO studies over the discovery have highlighted shortcomings with conventionally acquired and processed seismic datasets; The lack of near angles has made AVO studies over the discovery challenging and the AVO extracted from the seismic has not matched the modelled AVO at well locations. Following on from the recommendations of the previous AVO studies, OMV and partners initiated a reprocessing project that would focus on combining all previous experience with the Wisting seismic data. This paper describes the FWI model building and time reprocessing performed, and how we tailored the processing and QC to optimize the AVO response in the shallow section.
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Wave-Equation-Based AVO Inversion in the Presence of Coal Seams. A Synthetic Study from the Cooper Basin, Australia
Authors D. Gisolf, P. Haffinger, P. Doulgeris and S. SquireSummaryCoal seams, being very soft and light, cause strong internal multiple reflections and mode-conversions, as well as strong transmission effects. Conventional AVO inversion based on primary reflection coefficients only, will not be able to distiguish the primary from the multiple energy and consequently produce spurious results. Wave-Equation-Based AVO inversion, being based on the full elastic wave-equation, is able to account for all these effects and image the coal seams properly. Like all other AVO techniques, the WEB AVO is applied in the 1.5D data domain. In this paper we consider the logs from the well Moonanga-1 in the Cooper Basin in South Australia. This area is notorious for the presence of coal seams, hampering seismic interpretation. Full elastic synthetics are produced from the logs. The results from conventional Sparse-Spike-Inversion and from WEB inversion of the synthetic gather are compared, showing a clear improvement for the WEB inversion. This will lead to much better interpretability of the inverted data.
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Quantifying the Effect of Overburden Multiples on Miocene Reservoirs in the Levantine Basin
Authors P. Doulgeris, D. Anestoudis, P. Haffinger, A. Droujinina and D. GisolfSummaryThe Karish Main field was discovered in 2013 in the Levantive Basin, 75km offshore the northern coast of Israel. The Oligocene-Miocene submarine fan systems form the primary reservoirs of most commercial hydrocarbon discoveries to date in the region. At the end of the Miocene, the Mediterranean Sea became isolated from the Atlantic Ocean, which led to the deposition of up to 1,500 m of evaporites. The prospect was supported by seismic amplitudes and was proven by drilling well Karish-1. The aim of the current study is to model and understand the impact of the overburden evaporite on the seismic response of the reservoir. The wave-equation-based interbed multiples investigation technique deployed was able to identify the major multiple generators while quantifying the level of constructive interference expected on amplitudes at reservoir level from overburden multiples.
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AVAZ Quantitative Interpretation Based on Orthorhombic Medium-Part Two: AVAZ Inversion
More LessSummaryIn this paper, we mainly talk about orthotropic AVAZ inversion technique, it has proven to be important and effective for shale gas evaluation and development. We use pre-stack AVAZ inversion to predict elastic properties, it is carried on pre-stack azimuthal gathers with constrained wells. A slight modification was made on the basis of Bachrach’s (2004) equation in order to obtain elastic parameters such as P-wave impedance, S-wave impedance, combinations of anisotropic parameters and fracture orientation from inversion. This work could offer multiple elastic parameters that characterize the quality of shale reservoirs. This method is successfully applied in real data of Sichuan Basin, it shows that the results of inversion are effective and reasonable.
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Regional Porosity Estimation in Danish North Sea Chalk
Authors H. Wagner, H. Klemm and P. GeltingSummaryWhen performing seismic inversion over a large area we often face lateral changes in the seismic signature not related to the geology of the target. Therefore a methodology to equalize seismic data over large areas was developed. The methodology was used to prepare the data for absolute seismic acoustic inversion. The P-wave impedance result was subsequently transformed into porosity via a relationship from well data. The methodology builds on the ability to assess differences in data using an un-biased wavelet and well tie estimation method. The method is utilized to correct for data issues without removing geologic information. We did this by constructing a matching filter from a wavelet parametrization that varied with depth. The methodology was successfully applied for regional porosity estimation over an area of 6400 km2, or more than 40 million seismic traces, in the Danish North Sea. The regional nature of the results is beneficial for the large scale geological understanding of the area especially for exploration purposes. Furthermore, it provides insights into relations between porosity and geomorphology, and helps identifying new exploration opportunities.
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Seismic Inversion for Statfjord Well Maturation
Authors J. Lippard and E.T. DelaneySummaryWe present a case study where a new seismic dataset’s improved data quality has enabled a geologically-constrained Bayesian pre-stack inversion algorithm to produce reliable and usable inversion results to uncover significant pockets of oil. The chosen inversion methodology was PCube+: A Bayesian algorithm that uses a local neighborhood approach to perform a one-step inversion to lithology-fluid classes. By using the resulting sand map together with production and well data, areas with remaining reserves can be identified for infill drilling. One such area that was identified led to the drilling of an oil producer that is one of the best performing wells on the Statfjord field in years.
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Amplitude Variation with Angle Inversion Based on Propagator Matrix Modelling
More LessSummaryMost existing amplitude variation with angle (AVA) inversions are based on the exact Zoeppritz equation and its linearized approximations. However, such modellings consider the primary (P-wave) waves only and ignore transmission loss, multiples and many other converted wave modes. It means that strict pre-processing is required to satisfy the requirement of the input data to inversions, which is difficult to accomplish. To overcome this problem, a new AVA inversion based on the propagator matrix method of Carcione (PMC) is proposed in this abstract. PMC is derived from the 1D wave equation and can accurately simulate full wave responses within horizontally multi-layered models. For computational accuracy, we use a Bayesian framework to define the augmented function and choose the L-BFGS optimization method to solve it. Through a model test and real data application, it is found that the proposed AVA inversion based on PMC can effectively eliminate transmission loss and internal multiples and helps to provide better S-wave velocity and density estimations.
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Anisotropic Inversion for VTI Media Based on a Modified Approximation of the PP-Wave Reflection Coefficient: Theory And
More LessSummaryAnisotropy has significant influence on the seismic amplitude variation with offset (AVO) response. Although analytical solutions and approximations of the PP-wave reflection coefficient for tranversely isotropic media have been explicitly studied, it is diffcult to apply these equations to anisotropic AVO inversion, because five parameters need to be estimated, and such an inverse problem is highly ill-posed. A modified approximation of the PP-wave reflection coefficient is proposed in this paper based on Rüger’s formula for VTI (tranverse isotropy with a vertical symmetry axis) media. The derived equation is formed using the attributes of acoustic impedance, anisotropic shear modulus, and an anisotropic P-wave velocity, which is proportional to the vertical P-wave velocity with the natural exponential function of Thomsens’s ε. Numerical tests show that the approximation has sufficient accuracy over a wide range of angles. The field-data application in a lower Silurian-age shale formation reveals that the inverted attributes are very useful for the characterization of a shale-gas reservoir. The Thomsen’s anisotropy parameter ε can be recovered subsequently. Inverted ε not only provides a reliable model for the anisotropic imaging and full waveform inversion, but also acts as an indicator of gas-saturated shale, which shows lower anisotropy than surrounding shales.
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Integrating Petrophysical Evaluations and Rock Physics
By I. EscobarSummaryWe have developed an optimization methodology to consistently integrate petrophysical evaluations and the construction of rock physics models using multiple wells. This approach allows exploring the different parameters/coefficients used both for the evaluation and rock physics model, while ensuring complete repeatability and traceability of results. It has been used to build a rock physics model for the Chalk member in the Danish North Sea area, allowing us to cover the transition from clean limestone in the upper-Cretaceous to the shale-rich lower-Cretaceous limestone. During this process, some of the initial petrophysical evaluations were also updated.
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Porosity and Pore-Type Estimation from Seismic Data Using the Xu-Payne Multi-Porosity Model for Carbonate Reservoir
Authors H.B. Li, J.J. Zhang, H.J. Pan and S.J. CaiSummaryCarbonate reservoir has complex pore structure. The popularXu-Payne multi-porosity model has been widely used to quantitatively characterize pore-type distribution from logging data and experimental data. But it remains a great challenge to perform the pore-type inversion from seismic data while using the Xu-Payne model because accurate porosity and saturation can not be pre-given like well observations. In this paper, we present a two-step method to estimate the porosity and saturation and quantitatively characterize pore-type distribution from seismic data for carbonate reservoirs. Firstly, the pore systems of carbonate reservoir are treated as single porosity system with an effective pore aspect ratio. The porosity, fluid saturation and effective pore aspect ratio are simultaneously inverted from the inverted elastic properties by integrating Gassmann equations and the differential effective medium analytical model. Secondly, the pore systems of carbonate reservoir are treated as triple-porosity system that the Xu-Payne multi-porosity model defines. The porosities of three pore types can be inverted using the inverted elastic and physical properties as input. The real application shows that the proposed rock physics modelling and inversion method can yield a good quantitative estimation of porosity and characterization of pore-type from well log data and seismic data.
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Digital Rock Physics Based Pore Type Effect Analysis on Acoustic Properties of Carbonate Rocks
More LessSummaryIn this paper, we first subsample numerous datasets from digital rock model, and then the subvolumes were used to simulate the elastic properties with FEM solver. We verified the FEM results through ultrasonic measurements, and presented the cross-plot of acoustic velocity versus porosity. Regardless of the pore shape, Vp-φ crosspiot showed chaotic and scatter. Based on 3D porous structure of each subvolume, we classified such data points into several groups by different pore type. We found that for a certain porosity, pore type greatly affects the acoustic velocities of carbonate rocks. For an instance, we can observe that the acoustic velocities of carbonate rocks with moldic pores are much higher than the ones of carbonate rocks with dissolved intercrystalline pores. Carbonate rocks with compliant pores, let’s say fracture or microcrack, exhibit the lowest acoustic velocity compared with the ones with stiffer pores like moldic pores. Through the classification approach in terms of pore type, one can obtain better correlations between acoustic velocity and porosity concerning with different pore-type carbonate. This probably enables us to gain profound insights into carbonate reservoir prediction based on reservoir inversion with digital rock physics knowledge.
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Quantification of Pore Type System in Carbonate Rocks for Rock Physics Modelling
Authors J. Sharifi, M. Mirzakhanian, M.R. Saberi, M. Moradi and M. SharifiSummarySeismic responses of carbonate reservoirs are not well understood due to complex nature of pore system in these rocks. Therefore, it has been a challenge for researchers to develop an accurate rock physics model for carbonate rocks. In this study, a method was proposed for quantification of pore system in carbonate rocks for rock physics modeling purposes. Accordingly, Wyllie time-average equation (1956) and the methods introduced by Schlumberger (1974) and Lucia and Conti (1987) were used to obtain pore type in a given reservoir. With the aim of proposing a method for determining pore system, parameters of the rock physics model proposed by Xu and Payne (2009) were evaluated. Next, the results were verified based on core studies and thin section and SEM analyses, indicating a good agreement between estimated pore type from well log data and laboratory analysis results. The proposed methodology can help develop rock physics models for estimating continuous pore type logs in professional software packages.
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Predicting Seal Brittleness of Conventional Hydrocarbon Reservoirs Using LMR - a Case Study from the Norwegian Barents S
By N.H. MondolSummaryAmong other causes (e.g. pressure release and gas expulsion, tilting and spillage from pre-uplift hydrocarbon accumulation, cooling of source rock etc.) failure of seals linked to the uplift have been proposed the main reason for the lack of success in finding commercial petroleum accumulation in the Norwegian Barents Sea. This study predicts seal quality of Upper Jurassic Fuglen Formation from well log data of three exploration wells Skrugard (7220/8-1), Drivis (7220/7-3S) and Havis (7220/7-1) in the tectonically complexed, uplifted Norwegian Barents Sea using the LMR (LamdaRho-MuRho) rock physics templet. A simple rock physics model which allowed to compute theoretical values of dynamic elastic parameters for common constituents of shales and sandstones are utalized to constructe the LMR templet. The template shows the variation in LMR for a combination of mineralogical mixtures versus porosity. Results show that overall the Fuglen Formation in all three wells is a good seal though the caprocks/top seals have significant variations of brittleness values where a small upper sandy unit of Havis well shows high brittleness compared to the shale dominated sections. Brittleness indices estimates based on elastic parameters are easy to use but require calibration of lab observation.
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A New Rock Brittleness Index Based On Energy Dissipation in the Process of Uniaxial or Triaxial Compression Test
More LessSummaryThe previous rock brittleness evaluation methods based on stress- strain curve generally simplify the complex curve into several lines, and establish the evlauation equation by the characteristics of simplified curve. These methods have such problems as the loss of effective informaion, the difficulty of curve simplification and the multi solution of the calculation results. In order to solve these problems, a new brittleness index based on the conservation of energy transformation of the pre-peak and post-peak stages in compression test is presented in this paper, which has a clear physical implication. The new method does not simplify the stress- strain curve, uses the full information of curve, and then increases the accuracy of the calculation results. In addition, this paper further analyzes the relationship between the new brittleness index and seismic elastic parameters and establishes their correlation expression by using the experimental data, which is of great significance to the practical application of the new index in the oilfield.
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Effects of Cementation and Compaction on Elastic Wave Velocities of Sandstone
More LessSummaryCompaction and cementation are important properties that influence the elastic wave velocities of sandstone. In this work, we propose a theoretical model to quantify their effects. By relating the compaction rate to the grain coordination number and the cementation rate to the cement layer radius respectively, their effects on the elastic wave velocities can be studied using the Contact Cement Theory (CCT). To validate the proposed model, we measure the P- and S- wave velocities on the synthetic sandstone samples through the ultrasonic pulse transmission method. The theoretical predictions by the proposed model using the sample parameters are then compared to the experimental data, which shows good agreement between them. Furthermore, it also shows that without considering the compaction effects, the velocities can be obviously underestimated.
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Evolution in Physical Properties of Quartz-bearing Rocks with Evolving Degree of Microfissuration
Authors L. Pimienta and M. ViolaySummaryRocks are complex media that may contain very different geometries of pores. The two extreme families are the spherical pores, making up most of the total porosity, and the thin elongated micro cracks, controlling most of the rock elastic response. While the two families may exist in crustal rocks, they were found to be present in most sandstones. Using a technique to induce a set amount of micro cracks (thermal cracking) in a target rock, the purpose of this work is to investigate the relative effects of the porosity populations on the elastic and transport properties. We will show that, while elastic properties are similarly affected for all studied rocks, a strong effect of the rock initial porosity is observed on the resulting transport property. Moreover, consistently with existing theories, transport properties are additionally affected by the opening of micro cracks, which depends on the temperature of treatment.
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Sparse Frequencies Data Inversion: an Application to a Near Surface Experiment
Authors T. Alkhalifah, B. Sun, Y. Choi, F. Alonaizi and M. AlMalkiSummary“With an objective to invert for the subsurface velocity in the near surface rather than developing an image, we substitute the commonly used broadband acquisition scenario with a novel narrow band acquisition at coarse shot locations. We conduct the acquisition of narrow band seismic data, with an effect of 3 simultaneous sources vibrating at different bands (14–15 Hz, 24–25 Hz, and 49–50 Hz) of the frequency spectrum. The separation of the shot gathers corresponding to the simultaneous sources becomes natural as the shots fall in different bands of the frequency spectrum. The narrow band acquisition allows us to inject more energy of these frequencies using the same conventional vibrator sweep time (6 seconds). We mute regions of low signal-to-noise ratio, and then insert the data into a frequency domain waveform inversion algorithm. The inverted model down to 250 meters depth showed structure corresponding to a low velocity zone at around 80 meter depth. For comparison a conventional full sweep acquisition (30–170 Hz) at a dense shot spacing we recorded. We migrated this conventional dataset using the inverted model. The agreement between the inverted model and the image, extracted from the two independent datasets, supports the accuracy of the inverted model.”
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