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76th EAGE Conference and Exhibition 2014
- Conference date: June 16-19, 2014
- Location: Amsterdam, Netherlands
- Published: 16 June 2014
51 - 100 of 1028 results
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A Method for Unified Suppression of Surface-Related Multiple and Ghosts
More LessSummarySurface-related multiple (SRM) and ghosts are challenging tasks in marine data processing. The surface-related multiple elimination (SRME) method has been proven effective in many cases. But a deghosting process must be applied in advance. The absence of ghosts makes the multiple predicted by SRME less accurate and the adaptive subtraction struggles. We propose a method for unified suppression of SRM and ghosts (USMG). The ghosting operator is used as part of the predictor instead of discarding it. In this way, both the ghosts and SRM are predicted at the same time. Moreover, the predicted multiple is more accurate because of the ghost operator. Therefore, this results in better multiple elimination. Tests on synthetic and field datasets show the feasibility of this method.
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Multilevel Acceleration Strategy for the Robust Estimation of Primaries by Sparse Inversion
Authors T.T.Y. Lin and F.J. HerrmannSummaryWe propose a method to substantially reduce the computational costs of the Robust Estimation of Primaries by Sparse Inversion algorithm, based on a multilevel inversion strategy that shifts early iterations of the method to successively coarser spatial sampling grids. This method requires no change in the core implementation of the original algorithm, and additionally only relies on trace decimation, low-pass filtering, and rudimentary interpolation techniques. We furthermore demonstrate with a synthetic seismic line significant computational speedups using this approach.
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Multiple Attenuation Using MultiFocusing Technology
Authors M. Rauch-Davies, A. Berkovitch, K. Deev and E. LandaSummaryThe removal of multiples energy on seismic data has been a major issue on many datasets worldwide. The primary advantage of MultiFocusing (MF) is the enhancement of the signal-to-noise ratio of both stacked sections and prestack data through stacking a much larger number of traces than in conventional CMP processing. We present a modification of the MF-based approach when multiples are recognized directly in the MF attribute domain. First, they are predicted according to MF wavefront parameters and then they are subtracted using an adaptive least squares method. The key elements of the proposed procedure are the MF attributes. We identify and predict the multiples in the MF attribute domain through interpretation of the RMS velocity and emergence angle panels, which are determined from the pre-stack data during the MF multidimensional analysis. We compute a multiple model based on the partial coherent summation of the original data along the predicted traveltime surfaces. For the final stage, we adaptively subtract the predicted multiples from the original data using a least squares adaptive subtraction procedure similar to SRME-type multiple attenuation methodology. We presented a multiple attenuation methodology using MF applied on a real data example.
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Shallow Reverberation Prediction Methodology with SRME
Authors S.R. Barnes, R.F. Hegge, R. van Borselen and J. OwenSummaryIt is well known that surface-related multiple elimination (SRME) breaks down when applied to shallow water datasets. The prediction is distorted at the reconstruction stage by the NMO stretch of the seabed, progressing to the loss of seabed information beyond the critical distance. Furthermore, the adaptive subtraction (multiple elimination) struggles when several orders of the predicted short period reverberation are present, within a given design window for minimization, as the predicted amplitude (and phase) between multiple orders from a single convolution of the data with itself are incorrect.
This abstract describes a novel seabed modelled SRME approach with regards to predicting simultaneously and non-iteratively both the amplitude and phase of simple and pegleg source and receiver-side sea layer reverberation correctly with minimal distortion for moderately undulating shallow seabeds.
Using a shallow water dataset from the Central North Sea, it is demonstrated that the 3D approach can replace more limited 1D T-p shot-based deterministic multiple prediction techniques to form part of a multi-model multiple prediction strategy that includes iterative SRME where appropriate.
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Water-layer Demultiple Using Separated Wavefields From Variable-depth Streamer Data
More LessSummaryWe present an approach for water-layer multiple attenuation using up-going and down-going wavefields available from pre-migration deghosting of variable-depth streamer data. After extrapolation of the separated wavefields to the seabed, the down-going wavefield is used as a multiple model which is adaptively subtracted from the up-going wavefield. The water-layer related multiples removed by this procedure are forward propagated to the variable-depth streamer datum where they are finally subtracted from the deghosted data. We illustrate the concept using a synthetic example and shows results from a variable-depth streamer dataset acquired in the North Sea.
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Surface Multiple Attenuation Developments in the Clair Field
Authors A. Dawson, S. Wolfarth, C. Leone, M. Salgadoe and R. AlexandreSummaryMultiples are a key problem over the Clair area, mainly caused by the complexity in the overburden. The paper describes a much improved combination of demultiples techniques, which have been applied to the latest Clair South West HDOBC survey. The sequence consists a top-down approach removing successive multiples with PZ summation, Wavefield Extrapolation Multiples Modeling, GSMP and TauPQ deconvolution.
Water bottom WEMM plays a key role, effectively predicting water layer multiples in a 3D sense. GSMP complements the results of the other techniques against short period multiples, but more importantly it also predicts other long period surface multiples which are cutting through at reservoir level. The results are a much improved level of multiples suppression, although some surface multiples contamination is still present, and internal multiples have not been attenuated.
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Using RTM Angle Gathers for Velocity Model Building in a Structurally Complex Area - A Real Case Study
Authors O. Hermant, S. Hassan and J.P. MontelSummaryVelocity model building for pre-stack depth migration mainly relies on ray-based tomography methods. These methods usually produce good results when the moveout (RMO) information is reliable. From the imaging side, it is now established that Reverse Time Migration (RTM) is a higher fidelity imaging algorithm compared to Kirchhoff and Beam migrations and can provide the kinematic information needed for tomographic inversion. We show in this paper a real example of using RTM 3D angle gathers for ray-based tomography. The comparison between Kirchoff migrated and RTM migrated gathers shows the uplift of RTM on focusing and signal to noise ratio which in turn improves the RMO picking. We then show that nonlinear slope tomography can improve the velocity model and lead to better imaging provided the kinematic properties of the migration algorithm used are preserved during demigration of the RMO picks.
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Diffraction Imaging for Exploration of Seafloor Massive Sulfide Deposits - Case Study Solwara 1 Site
Authors K. Tertyshnikov, R. Pevzner, A. Bóna, F. Alonaizi and B. GurevichSummaryExtraction of mineral resources on land is becoming increasingly difficult. Recent discoveries of seafloor massive sulfides (SMS), which host significant amount of mineral commodities, appear as a new potential offshore mining sector. The marine 3D seismic exploration survey was carried out over a seafloor massive sulfide deposit at Solwara 1 site in the Bismark Sea, west of New Ireland, Papua New Guinea. Despite the fact that all prospective sulfide mineralization zones are concentrated close to the seafloor, knowledge of internal deep geological structures of seabed volcanic ridges and their genesis is important for understanding of the formation of mineral deposits. The steered migration with diffractions was applied to 3D seismic volume to emphasize deep geological structures and to enhance the signal to noise ratio of the seismic images. The post-stack steered migration utilizes coherency attributes obtained by a diffraction imaging algorithm in 3D to weight or steer the main Kirchhoff summation. The application of the steered migration to the investigation of the modern subduction zone at the Solwara 1 mine enhanced the signal to noise ratio of the final migrated images and helped to understand the formation mechanisms of seabed deposits in the region by exploring the deep structures.
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Subsalt Pore Pressure and Imaging Using Rock Physics Guided Velocity Modelling
Authors Y. Liu, D. Bhaskar and N.C. DuttaSummaryEarth models solely based on tomography may be non-unique, especially in presence of anisotropy or subsalt where incidence angles are small. The latter is a major problem for subsalt pore pressure prediction. In the method proposed here we constrain the subsalt tomography using geology in conjunction with thermal history modelling and rock physics principles. This is referred to as rock physics guided velocity modelling for migration and pore pressure prediction. A novel feature of this technology is to use predicted pore pressure as a guide to improve the quality of the Earth model. Thus, we produce a velocity model that not only flattens the CIP gathers, but also limits the velocity field to its physically and geologically plausible range without well control. This yields both a better image and pore pressure prediction below salt.
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The Seismic Response to Strong Vertical Velocity Change
By I.F. JonesSummaryConventional seismic data processing, whether it be pre-stack data conditioning or migration, is designed with the theory of P-wave reflected energy in-mind, for travel paths involving only a single reflection. Any energy propagating with other modes or travel paths will not be dealt with appropriately during conventional seismic data processing. It is primarily for this reason that we spend so much time preconditioning seismic data, so as to meet the assumptions of the subsequent migration. In this study, looking at shallow-water marine data from high velocity-contrast environments (such as found with basalt or carbonates), I assess the behaviour of some other classes seismic energy, when subjected to conventional processing, so as to better understand the anomalous events appearing in migrated CRP gathers and images, due to contamination of the data with remnant refraction and mode-converted energy.
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Compensating Attenuation Due to Gas Cloud Through QPSDM, a Case Study from Offshore Brunei
Authors K.H. Teng, J. Zhou, X. Wu, Y. Zhou, T. Brothers, S. Bergler and C. GibsonSummaryMarine seismic broadband solutions, as e.g. variable-depth streamer acquisitions, have shown great potential in providing high resolution seismic imaging and better low frequency penetration compared to conventional data ( Lin et al., 2011 ). The increased low frequency content as well as signal to noise ratio play an important role in velocity model building, especially in the presence of gas. It benefits the PreSDM with absorption compensation technology (QPSDM) as low frequency energy may penetrate better through gas clouds ( Xie et al., 2009 ). In this paper, we will show the methodology to compensate for Q absorption effects for a case study offshore Brunei. The step change in image quality shows that it is a viable solution to improve imaging through gas clouds by combining broadband acquisition and advanced depth imaging technology (Q tomography and QPSDM).
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Pre-salt Imaging Beneath Volcanic Intrusions in Brazils Santos Basin
Authors J. Langlois, Z. Yuan, H. Li, R. Kumar and B. BaiSummaryRecent offshore deep-water discoveries in the Santos Basin have drawn attention to pre-salt targets; however, seismic imaging in this area has proven to be challenging due to the complex geology and the depth of the pre-salt targets. Volcanic intrusions can also add intricacies to an already complicated geologic setting, creating very complex velocity profiles and limiting the effectiveness of typical tomographic solutions. For this survey in the Santos Basin, we found that even identifying the top of salt (TOS) can be challenging without first evaluating the velocity between the top of volcanic intrusion and the sediments below. Utilizing the improved bandwidth and imaging capabilities of variable-depth streamer acquisition, a new iterative tomographic approach has been developed to image beneath volcanic intrusions, providing a better understanding of the pre-salt targets.
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High Fidelity Imaging with Least Squares Migration
Authors B. Salomons, M. Kiehn, J. Sheiman, B. Strawn and F. Ten KroodeSummarySeismic acquisition is never perfect, and sparsity and irregularity issues lead to suboptimal images when using standard seismic imaging schemes. The remedy of Least-Squares Migration has been topic of research since the early eighties, but the take-up in the industry has been limited so far. This paper describes the experiences obtained for a number of real data cases, which show that the method does deliver and is worth the extra cost and effort.
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FWI-driven High-fidelity Depth Imaging of a Large 3D Arctic Seismic Survey, West Greenland
Authors B. Du, S. Perrier, H. Roende and J. AndersenSummary3D pre-stack depth imaging was conducted for an 1800 km2 narrow-azimuth streamer survey in the Baffin Bay, West Greenland. The survey area is characterised by a hard, shallow and rugose sea bed as well as a high-velocity Quaternary layer immediately beneath. The seismic data are therefore plagued with serious multiple contamination and degraded signal-to-noise ratio. The velocity model building of the Quaternary layer with the conventional tomographic approach is challenging due to the lack of offset coverage and poor data quality on the whole. To tackle these challenges, the high-fidelity depth imaging presented here focuses on three key components: (1) enhanced multiple attenuation with the Shallow Water Demultiple (SWD) technique; (2) incorporating 3D Full Waveform Inversion (FWI) in the velocity model building flow to effectively resolve the details of Quaternary velocity variations - and (3) high-end imaging with the amplitude-preserving Controlled Beam Migration (CBM) to mitigate migration artefacts and further improve signal-noise ratio in the image domain.
We demonstrate the effectiveness of the methodology and show the step change in the resulting image quality, which contributed to robust and optimal structural interpretations throughout the survey area.
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Rock Mechanics template In a Shale Oil Play, Vaca Muerta Fm, Argentina and Their Impact in the Exploratory Strategy
More LessSummaryLocated in the Argentinean Neuquén Basin, the geological formation Vaca Muerta (Thitonian age), became in the last few years a unit that holds a huge potential as an unconventional resources play. Rock mechanics and reservoir characteristics in the Vaca Muerta Fm are extremely heterogeneous. This is due to the differences in depositional environment, petrophysical features and thickness of the unit along the whole basin. In the study area, the thickness of the Thitonian section consists of more than 350 m of marls with variable content of carbonate, quartz and clays. The TOC average in the upper section of the unit is 3.5% and in the lower section is among 6%. In order to understand the productivity behaviour within this formation, a shale rock mechanics template has been developed and implemented. This is done by estimating Lambda-rho/mu-rho for different scenarios, together with the estimation of pseudo brittleness and basic mineralogical characteristics of Vaca Muerta Fm. This work shows a successful integration between wells and seismic data for a shale oil play. The use of rock physics templeate, in an exploration phase, helps improving the understanding of the reservoir mechanical characteristics, optimizing the selection of well location and well completion.
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Improved Prediction of Source Rock Maturity in Liquid-rich Unconventional Plays with 3D Basin Modeling
Authors A. Hartwig, D.J. Jacobi, G.M. Walters and P.J. PerfettaSummaryThe recent unconventional exploration in North America has shown that the viable liquid-rich zone of most source rock plays is a narrow band. Accurate 3D basin modeling studies can be used to predict these in onshore basins with complex burial histories. This requires a rigorous quality checking of multiple thermal maturity indicators and the erosion estimates. The thermal maturity predictions and calibration to kerogen kinetics can be improved by integrating historic production data (API and GOR) and correcting erroneous thermal maturity measurements. This 3D basin modeling approach is shown on an example from the Permian Basin, USA.
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Accurate Velocity Estimation Through the Curvelet Domain
Authors R. Alai, G. Nyein, M.N.B.M. Isa, M.S.B. Sulaiman, B. Chowdhury and M. YusupSummaryWe present significant improvements in pre-migration seismic velocity estimation through the use of Curvelet transform in prestack noise attenuation. Seismic data is decomposed using the Curvelet transform, which has the capability of separating events having differing frequency, dipping angle and location. Curvelet transforms decompose data as a weighted sum of “Curvelets”, where each Curvelet is localised in both the f-k and t-x domains, and each weight consists of both amplitude and phase. The data are processed in the Curvelet domain by manipulation of these weights. Noise suppression via the Curvelet transform on prestack gathers has contributed to significantly improved pre-migration velocity estimation.
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Fluid Flow in Unconventional Reservoirs - Modelling and Implications
Authors V.M. Yarushina, Y.Y. Podladchikov, N.S.C. Simon and D. BercoviciSummaryA new fully coupled model of fluid flow through deformable viscoelastoplastic porous rock is developed. Constitutive evolution equations for porosity and densities of the solid matrix and the fluid are derived using effective media theory. Deformation and porosity-dependent permeability in the model lead to a strong pressure dependence of leakoff during hydraulic fracturing. Predicted rates of fluid loss are higher than those suggested by classical models. The model also allows investigation of the preferred upward flow pathways during fluid injection operations as observed in CO2 injection operations without invoking the hypothesis of a pre-existing fracture network. The model is applicable to soft and unconventional reservoirs whose mechanical behavior cannot be captured by simple elastic laws.
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Evaluation of Formation Permeability Using Minifrac Pre-closure Data - Case Study
Authors V.Y. Guk, M.N. Bychina and M.A. TuzovskiySummaryHydraulic Fracture Design relies strongly on formation permeability. For the fixed quantity of proppant, higher formation permeability will lead to higher optimal fracture width and lower optimal half-length. Low permeability and tight formations, however, in many cases would not flow without fracturing and thus cannot be tested for permeability using conventional techniques such as Pressure Transient Analysis. This is why a practical way to determine permeability for fracture design in such formations is Minifrac, or, Diagnostic Fracture Injection Test (DFIT).
Conventionally, Minifrac pre-closure data is used for determination of leak-off coefficient and fluid efficiency. After-closure data is in turn used for formation permeability evaluation. Low permeability formations, however, exhibit radial flow only after a considerable time after shut-in. This makes permeability evaluation using after-closure data not practically applicable in many cases.
In this study we propose a new approach to DFIT design and evaluation using pre-closure data for determination of formation permeability. The approach is based on the fact, that if water is used as an injection fluid, the dominant leak off mechanism will be reservoir fluid compressibility. This enables to explicitly relate permeability and leak-off coefficient. The leak-off coefficient can be determined using pre-closure data, which reduces required minifrac time considerably.
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Understanding Net Pay in Tight Gas Sands - A Case Study from the Lower Saxony Basin, NW-Germany
Authors B. Koehrer, K. Wimmers and J. StrobelSummaryWe present an integrated workflow to describe and characterise Upper Carboniferous tight gas sands by systematically integrating core-based macroscopic geologic elements (depositional rock types), microscopic observations (post-depositional diagenesis) and pore-scale properties like capillary pressure and nuclear magnetic resonance data.
Our workflow integrates multiple data sets and scales from a Wintershall-operated tight gas field in the Lower Saxony Basin of Northern Germany. The reservoir is of Westphalian C to Stephanian in age and consists of thick successions of fourth-order coarse- to fine-grained tight sandstone cycles separated by siltstones and in party by anthracite coal seams. The tight sandstones are intensely compacted and cemented with quartz, clay minerals and carbonate cements and generally characterized by low matrix porosities (< 10%) and very low permeabilities (<0.1mD).
A hydraulic rock-typing approach was selected to better define net pay of tight gas sands. In contrast to conventional petrophysical net pay criteria (porosity and water saturation log cut-offs), our approach makes use of the calculated pore throat radius (using Winland R35 equation) and relative permeability measurements (“mobile gas”). Our multi-scale rock typing study enables a better understanding of tight gas sand recovery factors and sweet-spot identification especially for new field development/appraisal projects.
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The Role of Geoscience and Engineering in Securing Social License to Operate in Europe
By S.W. GarrettSummaryExploration for Naturl Gas from Shale resources in Europe is at an early stage and it takes many years for these activities, if successful, to mature into pilot tests and full production.
As European Geoscientists and Engineers there is an emerging and increasing duty upon us to link our science and technology through communication to public dialogue and policy making. Companies, governments, trade associations, universities, national institutes, professional societies and scientific societies must to work together in partnership. Information must be disclosed transparently. For peer-review science to be trusted and impartial, it is critical to separate from industry funding and scientific governance. Findings must be communicated clearly to inform reporting to and dialogue with a wide range of press, public and policy-making stakeholders, many of whom may not have a technical background.
These will be increasingly important tasks as our profession helps in meeting the energy challenges of the 21st Century.
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Results of Deconvolution with Sparseness Constraints
Authors A. Guitton and J. ClaerboutSummaryWe tackle deconvolution within an inversion framework using the hyperbolic function for our data (i.e., reflectivity) misfit, thus enforcing sparse solutions.
This “sparse” deconvolution overcomes the minimum-phase assumption and converts the commonly seen three-lobed Ricker wavelet into an impulse. In addition, sparse deconvolution reveals the polarity of seismic events. Three datasets illustrate our findings.
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Seismic Reflectivity Inversion by Curvelet Deconvolution – A Comparative Study and Further Improvements
More LessSummaryCurvelet deconvolution refers to seismic deconvolution for reflectivity inversion based on curvelet transform. Curvelet transform is a multi-scale and multi-directional transform, and thus can provide a sparse representation for seismic reflectivity. When using it to model the reflectivity, the signal is represented effectively by large coefficients and random noise is represented by small ones. In this paper, we conduct a comparative study in the context of reflectivity inversion, to investigate the performance of curvelet deconvolution, the least-squares method and Lp-norm deconvolution. It is shown that by using curvelet deconvolution, the inverted reflectivity profiles have better noise suppression and higher resolution than those obtained by the least-squares method. On the other hand, its results excel those which are obtained by Lp-norm deconvolution in terms of the lateral continuity. Since curvelet deconvolution offers a good trade-off between the lateral continuity and the sparseness, the result obtained by this method can be used as the initial model to enhance the conventional Lp-norm deconvolution. Numerical results show that the lateral continuity of the inversed reflectivity profile has been further improved by the proposed method.
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Surface-consistent Residual Phase Corrections
By K. GarceranSummaryNear surface variations not only create time delays but can create a more complicated frequency dependent phase distortion. The estimation of surface-consistent residual statics can be generalized to the phase, with the same attempt to maximize the stack power. The method presented in this paper, based on cross-spectral analysis, allows performing surface consistent residual phase corrections. This approach has the advantages to avoid the phase unwrapping and to use the full phase spectrum. Used after a first pass of surface-consistent residual statics or not, the proposed algorithm shows very encouraging results.
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Robust Surface-consistent Phase Estimation
Authors P.W. Cary and N. NagarajappaSummaryWe have developed a robust method of estimating surface-consistent residual wavelet phase that is based on the simultaneous maximization of stack-power as a function of both statics and phase. Real data examples show that stack-power and image quality are improved in a robust fashion with the simultaneous estimation of statics and phase corrections. We typically apply the process after residual statics are applied, and we observe that the algorithm comes up with statics and phase corrections that are strongly anti-correlated. We explain the observed anti-correlation by the fact that previous residual statics steps in the processing flow were improperly trying to correct residual phase errors with statics corrections. Maps of phase errors often show good correlation with features of the surface topography. In addition, phase differences between different source types are reliably estimated with the new algorithm when compared with a standard method of phase estimation at overlapping CDP stack locations. These observations lead us to believe that the phase errors that are estimated with this method are real and are being robustly estimated.
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Comparison of Two Semi-automatic Techniques for Seismic-to-well Tying
Authors R.H. Herrera, M. van der Baan and S. FomelSummaryTying well logs to seismic data is a highly subjective task that relies on the interpreter’s experience and the similarity metric used. Automated alternatives could help reduce this degree of subjectivity by making the tie reproducible. In this paper we compare two automated techniques: the dynamic time warping method and the local similarity attribute based on regularized shaping filters. These two methods produce superior tying in a guided stretching and squeezing framework. Results using a real well log example validate both approaches. Automated seismic-to-well tie algorithms can greatly aid in seismic interpretation. It is important to emphasize however that they are based on goodness-of-fit criteria and do not measure correctness of a fit. Best practices in well-tying have to be followed for their results to be meaningful.
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Improved Plane Wave Destruction for Iterative Local Slope Estimation
By K.J. HellmanSummaryThe estimation of local slopes has been an important topic for numerous seismic processing applications. Many local slope algorithms are based on the plane wave destruction approach, using the time derivative of the seismic trace along with a horizontal gradient trace to generate estimates of the dip. Differentiation increases high frequencies in the data, limiting the accuracy of these dip estimates. Alternatively, the plane wave destruction equation may be recast in terms of integration. In this case the original seismic trace is used along with the causal integration of the horizontal gradient trace. Since there is no differentiation to boost high frequency noise, and since numerical integration is itself a smoothing process, this approach leads to better-behaved dip estimates. In both cases, the finite difference approximation of the gradient leads to a biased estimate. However, as long as the local event slopes are not aliased, the dip estimates will be of the correct sign, and an iterative application of the algorithm will lead to accurate local slopes.
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Further Application of Frequency-diverse Filtering - A New 3D Deghosting Algorithm for Streamers
By Y.J. JiSummaryFrequency-diverse filtering is a method that combines array responses at several frequencies in order to overcome spatial aliasing. The method was recently introduced for simultaneous source separation. As a further application example, we present the formulation and results on synthetic data for 3D deghosting of marine streamer data.
We use a set of basis functions that include the ghost term and determine model space parameters (local frequency, slowness, time) using a one-norm (l1) optimization procedure. We illustrate the potential of the method with synthetic data from the SEM model, showing effective removal of receiver-side ghosts in a 3D sense for data acquired with coil acquisition.
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Premigration Ghost Wavefield Elimination on Different Cable Configurations - A Case Study in the Gulf of Mexico
More LessSummaryPremigration Ghost Wavefield Elimination (GWE), or premigration deghosting, has emerged as a key preprocessing step for broadening the bandwidth of marine streamer data. We apply a tau-p domain premigration bootstrap approach to eliminate the ghost wavefield on 2D data sets acquired using four different cable profiles in the Green Canyon planning area of the U.S. Gulf of Mexico. The effectiveness of the approach for all cable configurations is demonstrated, in particular in improved subsalt imaging and free-surface multiple attenuation.
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Full Waveform Inversion for Kinematic Equivalence
Authors J.K. Washbourne, K.P. Bube, K.T. Nihei, S.T. Kaplan, J.P. Stefani, N.K. Shah, G. Hampson, C. Manuel and D. BevcSummaryWe present an approach to 3D full waveform inversion that can handle noisy data deficient in refracted energy. We use time domain modeling and monochromatic inversion, and obtain excellent kinematic agreement of modeled data with observed data for a narrow azimuth towed streamer dataset from the northwest shelf of Australia. We describe how we apply regularization and preconditioners to obtain a robust solution with respect to an imperfect starting model and various sources of noise in the field data. Even though we employ very simple isotropic constant density visco-acoustic wave propagation, we obtain near kinematic equivalence of the data modeled using the inverted velocities with the field data. We discuss how this kinematic equivalence in the framework of robust norms and regularized nonlinear optimization may imply that non-uniqueness comes also from the choice of physics used for wave propagation.
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Full Waveform Inversion Using the Energy Objective Function
More LessSummaryWe suggest new objective functions for full waveform inversion in the time domain. These objective functions minimize the energy differences between observed and modeled data. We applied sequential time windows to the data and calculated the energy. A numerical example shows that the objective functions can be used to obtain a macrovelocity model. The recovered macro-velocity model can be used as an initial velocity for a subsequent inversion in the frequency domain to obtain high-resolution velocity information. Since we calculate the partial derivative of the energy objective function explicitly, heavy computational requirement is an important limitation of these objective functions.
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Two-dimensional Unwrapped Phase Inversion with Damping and a Gaussian Filter
Authors Y. Choi and T. AlkhalifahSummaryPhase wrapping is one of main causes of the local minima problem in waveform inversion. However, the unwrapping process for 2D phase maps that includes singular points (residues) is complicated and does not guarantee unique solutions. We employ an exponential damping to eliminate the residues in the 2D phase maps, which makes the 2D phase unwrapping process easy and produce a unique solution. A recursive inversion process using the damped unwrapped phase provides an opportunity to invert for smooth background updates first, and higher resolution updates later as we reduce the damping. We also apply a Gaussian filter to the gradient to mitigate the edge artifacts resulting from the narrow shape of the sensitivity kernels at high damping. Numerical examples demonstrate that our unwrapped phase inversion with damping and a Gaussian filter produces good convergent results even for a 3Hz single frequency of Marmousi dataset and with a starting model far from the true model.
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A New Take on FWI - Wavefield Reconstruction Inversion
Authors T. van Leeuwen, F.J. Herrmann and B. PetersSummaryWe discuss a recently proposed novel method for waveform inversion: Wavefield Reconstruction Inversion (WRI). As opposed to conventional FWI – which attempts to minimize the error between observed and predicted data obtained by solving a wave equation – WRI reconstructs a wave-field from the data and extracts a model-update from this wavefield by minimizing the wave-equation residual. The method does not require explicit computation of an adjoint wavefield as all the necessary information is contained in the reconstructed wavefield. We show how the corresponding model updates can be interpreted physically analogously to the conventional imaging-condition-based approach.
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Wave-equation Based Inversion with the Penalty Method - Adjoint-state Versus Wavefield-reconstruction Inversion
Authors B. Peters, F.J. Herrmann and T. van LeeuwenSummaryIn this paper we make a comparison between wave-equation based inversions based on the adjoint-state and penalty methods. While the adjoint-state method involves the minimization of a data-misfit and exact solutions of the wave-equation for the current velocity model, the penalty-method aims to first find a wavefield that jointly fits the data and honours the physics, in a least-squares sense. Given this reconstructed wavefield, which is a proxy for the true wavefield in the true model, we calculate updates for the velocity model. Aside from being less nonlinear--the acoustic wave equation is linear in the wavefield and model parameters but not in both--the inversion is carried out over a solution space that includes both the model and the wavefield. This larger search space allows the algorithm to circumnavigate local minima, very much in the same way as recently proposed model extensions try to accomplish. We include examples for low frequencies, where we compare full-waveform inversion results for both methods, for good and bad starting models, and for high frequencies where we compare reverse-time migration with linearized imaging based on wavefield-reconstruction inversion. The examples confirm the expected benefits of the proposed method.
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Explicit High-contrast Surface Refinement Using Full Waveform Inversion
Authors J.W.D. Hobro and J.E. RickettSummaryFull waveform inversion (FWI) provides a powerful mechanism for resolving high-resolution structure within complex seismic velocity models. However, many models used in FWI contain complex bodies (e. g. salt intrusions) that generate a large, sharp contrast in seismic properties. Refining these with FWI can prove extremely difficult when they are represented implicitly within a conventional gridded velocity model. The constraints required to optimize convergence at large contrasts are different from those required elsewhere. It is difficult to allow for this when the surfaces have no explicit representation. This paper describes a new approach to refining complex 3-D surfaces directly using FWI. An explicit representation of the surface is updated during the inversion. The gradient required is obtained by projecting a conventional FWI gradient (for seismic parameters) to a position and shape parameter gradient for the 3-D surface. The method relies upon accurate gridded model generation in the vicinity of the surfaces and a linearization of this process to enable the gradient projection. We present 2-D and 3-D synthetic examples that resolve complex structure within a Gulf of Mexico top salt contrast. The examples demonstrate very rapid and stable convergence that resolves complex features beyond the resolution of the finite-difference grid.
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Full Waveform Inversion Based on the Optimized Gradient and Its Spectral Implementation
Authors Z. Wu and T. AlkhalifahSummaryFull waveform inversion (FWI) despite it’s potential suffers from the ability to converge to the desired solution due to the high nonlinearity of the objective function at conventional seismic frequencies. Even if frequencies necessary for the convergence are available, the high number of iterations required to approach a solution renders FWI as very expensive (especially in 3D). A spectral implementation in which the wavefields are extrapolated and gradients are calculated in the wavenumber domain allows for a cleaner more efficient implementation (no finite difference dispersion errors).
In addition, we use not only an up and down going wavefield decomposition of the gradient to access the smooth background update, but also a right and left propagation decomposition to allow us to do that for large dips. To insure that the extracted smooth component of the gradient has the right decent direction, we solve an optimization problem to search for the smoothest component that provides a negative (decent) gradient. Application to the Marmousi model shows that this approach works well with linear increasing initial velocity model and data with frequencies above 2Hz.
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Full Wave Inversion Using a Spectral-Element Discontinuous Galerkin Method
Authors J.R. Krebs, S.S. Collis, N.J. Downey, C.C. Ober, J.R. Overfelt, T.M. Smith, B.G. van Bloemen-Waanders and J.G. YoungSummaryWe have developed a flexible Discontinuous Galerkin (DG) toolkit for full-wave inversion (FWI) that operates on unstructured non-affine meshes using a variety of element types (quadrilateral, triangular, hexahedral). The code handles spatially-variable polynomial-order across the mesh, and includes two approaches: modal DG with exact adjoints and gradients, and spectral DG which, though computationally faster, has approximate adjoints and gradients. In this paper, we show that high-quality full wave inversion results are obtained using both the modal DG and the approximate spectral DG approaches. A 3D inversion of field data using spectral-element DG will be presented in the talk.
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Source Wavelet Estimation in Full Waveform Inversion
More LessSummaryIn full-waveform inversion (FWI), a small disturbance in the source wavelet may lead to large discrepancies in the inverted model, which becomes larger as the depth increases due to error accumulation. Hence, accurate source wavelet estimation becomes crucial in a successful inversion. On the other hand, an inaccurate model would jeopardize a wavelet estimation based on both simulated and observed data without proper constraints, which may in turn lead to wrong model updates and finally hazard the inversion.
To resolve this inversion dilemma, we propose a shallow-response based variable projection type of strategy to estimate the source wavelet alongside model parameters during FWI. Our approach embeds wavelet estimation naturally into FWI iteration as a standard variable reduction step, and restricts the computation to parts of the data that mainly consist of responses to shallow parts of a model. As the model becomes more and more accurate, one could always use more and more data, which might increase the robustness of wavelet estimation. To demonstrate the feasibility and robustness of our approach, we present inversion experiments with both synthetic and real data-sets, which suggest that shallow responses suffice to yield robust wavelet estimation that facilitates FWI.
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Scattering Angle Base Filtering of the Inversion Gradients
More LessSummaryFull waveform inversion (FWI) requires a hierarchical approach based on the availability of low frequencies to maneuver the complex nonlinearity associated with the problem of velocity inversion. I develop a model gradient filter to help us access the parts of the gradient more suitable to combat this potential nonlinearity. The filter is based on representing the gradient in the time-lag normalized domain, in which low scattering angles of the gradient update are initially muted. The result are long-wavelength updates controlled by the ray component of the wavefield. In this case, even 10 Hz data can produce near zero wavelength updates suitable for a background correction of the model. Allowing smaller scattering angle to contribute provides higher resolution information to the model.
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Low Frequency De-Noising and De-Ghosting Marine Full Waveform Inversion
Authors P.P. Milcik, R.E. Plessix, K.H. Matson and V.L. GohSummaryIn this paper we shall present a number of processing steps that can be applied to improve the low-frequency signal-to-noise ratio such that the effects of the ghosts are taken into consideration in a consistent manner between the observed and modeled data. The discussion will include examples from wide azimuth marine streamer data (WAZ) and ocean bottom node (OBN) data sets and the impact these steps have on the FWI results.
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Full-waveform Inversion of Conventional Vibroseis Data Using Preconditioning Focused on Low Frequency Enhancement
Authors A. Adamczyk, M. Malinowski and A. GórszczykSummaryDespite the popularity that full-waveform inversion (FWI) has gained in recent years, its application to Vibroseis data is still challenging, due to the problematic low-frequency content. We present the results of acoustic frequency-domain FWI of Vibroseis data with sweep starting at 6 Hz, acquired with standard 10 Hz geophones in 2010 in south-east Poland. 4.5 Hz matching filter and curvelet denoising in the frequency domain are used to enhance the low-frequency content of the data. This, together with dense sampling of the frequencies in the first inversion group, resulted in a geologically plausible P-wave velocity model, which correctly reproduces the data, including the far-offset arrivals and wide-angle reflections.
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Effective Cycle Skipping Reduction through Adaptive Data Selection for Full Waveform Inversion
More LessSummaryFull waveform inversion (FWI) has proven that it has potential to provide high-resolution velocity parameters. However, for most cases, FWI suffers from an objective function with a local minimum instead of a global minimum due to cycle skipping between real data and predicted data. To avoid this issue, researchers have proposed an FWI work flow that uses offset stripping and inner mutes to limit the input for FWI to near offsets for the initial inversion iteration, and then gradually incorporates farther offsets in subsequent iterations as the velocity model accuracy improves with depth. However, this work flow is computationally expensive and cannot effectively avoid the cycle skipping issue. We propose a data selection algorithm that assures all input data for FWI is within a half-cycle difference compared with the predicted data. This data selection process is implemented in each iteration of the inversion to generate a velocity model with higher accuracy and fewer artefacts.
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A Probabilistic QC for Cycle-skipping in Full Waveform Inversion
Authors A.M.S. Martinez-Sansigre and A.R. RatcliffeSummaryFull waveform inversion (FWI) is now well established in the toolbox of velocity model building. That said, the cycle-skipping problem remains one of the primary practical limitations of FWI, especially in industry applications. A simple, effective and robust QC would be of benefit to determining the suitability of any given velocity model as a good starting point for FWI. Here we present a QC toward this goal based on a probabilistic interpretation of cycle-skipping. We discuss the theory and rationale behind our idea and demonstrate its effectiveness in a controlled experiment. We then show an application to a real data example from the Central North Sea, where it was able to distinguish between starting velocity models that were free from cycle-skipping and lead to an improved FWI update, and those that were cycle-skipped and gave an FWI update that degraded the migrated image.
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Statistical Sampling Enabled Full Waveform Inversion
Authors K. Jiao, A. Schiemenz and R. CoatesSummaryFull Waveform Inversion (FWI) has recently emerged as a promising method for refining seismic velocity models to achieve enhanced imaging. The algorithm involves iteratively updating the velocity model to improve the match between the recorded seismic data and the simulated waveforms. Each iteration typically requires multiple wavefield extrapolations. As a result the technique places significant computational burdens on even the largest computers when applied to large three-dimensional surface seismic datasets.
This paper discusses the application of two statistical sampling strategies to a time-domain FWI algorithm, with the aim of minimizing the computation costs while still ensuring that all the information in the data is utilized. Results are shown for a synthetic model and for a real data set acquired with a multi-vessel coil geometry, both of which show significant computational savings.
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Fast Uncertainty Quantification for 2D Full-waveform Inversion with Randomized Source Subsampling
Authors Z. Fang, F.J. Herrmann and C.D. SilvaSummaryUncertainties arise in every area of seismic exploration, especially in full-waveform inversion, which is highly non-linear. In the framework of Bayesian inference, uncertainties can be analyzed by sampling the posterior probability density distribution with a Markov chain Monte Carlo (McMC) method. We reduce the cost of computing the posterior distribution by working with randomized subsets of sources. These approximations, together with the Gaussian assumption and approximation of the Hessian, leads to a computational tractable uncertainty quantification. Application of this approach to a synthetic leads to standard deviations and confidence intervals that are qualitatively consistent with our expectations.
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Temporal Variation in Subsurface Stress Estimated from Seismic Scattering
Authors K. Okamoto, H. Mikada, T. Goto and J. TakekawaSummaryWe focus on seismically scattered waves that bring the information of the earth crust where the seismic waves travel through to estimate stress field in the subsurface. Since seismic scattering is strongly related to the crustal inhomogeneities such as faults, cracks, etc., which are also created by the stress in the crust, we could be able to estimate stress field in the deep subsurface using the seismic scattering.
We employ coda-Q (Qc) as a parameter to detect the variation of stress field. Qc is a parameter reflecting the inhomogeneities.
Here we hypothesize that Qc could be used to estimate regional-scale stress accumulation in the crust without local and shallow disturbances since Qc is estimated from the scattered seismic waves travelling over a wide range of the crust. We first obtain a relationship between Qc and the stress change using numerical simulations. We calculate the static stress changes associated with the Iwate-Miyagi Nairiku earthquake in 2008 (Mw 6.9) in the subsurface using earthquake dislocation model and the surface deformation for comparison. As the result, it is found that the stress change inferred from Qc shows the similarity of stress change in the deep subsurface calculated by the earthquake dislocation model.
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Passive Seismic Monitoring at the Ketzin CCS Site - Magnitude Estimation
Authors B.F. Paap and T.P.H. SteeghsSummaryIn order to allow quantification of the strength of local micro-seismic events recorded at the CCS pilot site in Ketzin in terms of local magnitude, earthquake data recorded by standardized seismometers were used. Earthquakes were selected that occurred in Poland and Czech Republic and that were detected both on the Ketzin array and on nearby situated regional seismometers operated by BGR. Analysis of the waveforms of both types of data suggests that they could be combined to broaden the useful spectral bandwidth for earthquake waveform analysis. By identifying and isolating the overlapping frequency spectrum of both data types (1–3 Hz), linear relations were obtained that were used to define local magnitude for a given peak amplitude value. Extrapolation of these relations to smaller local magnitudes can serve as an estimate for local magnitudes of local high frequency micro-seismic events that are detected at the Ketzin site.
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Observation and Signatures of Injection-induced Repeating Earthquake Sequences
Authors J. Kummerow, C. Dinske, H. Asanuma and M. HäringSummaryWe detect and analyze repeating earthquake sequences in the Basel 1 microseismic data set. Using a combination of waveform similarity and estimates of source radius we identify 144 repeating earthquake sequences with a total of 422 events. The specific spatio-temporal behaviour of the repeating events indicates clearly that they are more sensitive to pore pressure changes than the total induced seismicity, which is at larger distances (more than about 250–300m from the injection source in the Basel reservoir) not only affected by pore pressure variations, but presumably also by stress transfer from the larger (Mw>2.0) events. The results suggest that repeating earthquakes may contribute to identifying and differentiating the dominant triggering effects of recorded microseismicity.
Furthermore, this study shows that a significant percentage of the fractures stimulated in the Basel reservoir ruptured repeatedly: More than 15% of all located events are repeaters. We anticipate that the occurrence of repeating events is a common feature in injection induced microseismic data sets, and that it is useful to incorporate repeating events in future statistical analyses of microseismic data, which so far generally assume that each potential source in the stimulated reservoir can only fail one-time.
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Rupture Propagation Imaging at Microseismic Scale at the Basel EGS Project
Authors J. Folesky, J. Kummerow, S.A. Shapiro, M. Häring and H. AsanumaSummaryWe further develop the Back Projection technique for tracing the rupture propagation of microseismic events. We lay out the basic idea of Back Projection imaging and show the results for three synthetic datasets obtained using finite-difference modeling. The synthetic rupture models in use where generated according to microseismic events that occurred at the Basel EGS. They help us to understand the influences of the station geometry and the station weighting process which need to be applied in Back Projection imaging. The focus of the work lies in the analysis of real events and the extraction of their respective properties. We show the corresponding results for the four largest real events of local magnitudes M=3.1–3.4 from the Basel EGS site and discuss the validity and interpretation of the outcome. We find that the obtained rupture dimensions are consistent with the independent magnitude derived estimates. The rupture directions which are obtained fit reasonably well to the shape of the microseismic cloud and to one of the respective fault planes obtained from source mechanism analysis. This supports the validity of our approach. In addition we are hereby capable of solving the fault plane ambiguity.
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Seismic Efficiency and Overshoot of Fractures Associated with Stimulation in Heavy Oil Reservoirs
Authors L.N. Meighan, T.I. Urbancic and A.M. BaigSummaryTo better understand the different fracturing process of two passive seismic datasets collected over a 30 month period for two heavy oil reservoirs, utilizing steam injection and water flooding, we investigate radiated energy and seismic efficiency. We calculate the Savage-Wood Efficiency (ŋsw) and overshoot of microseismic events recorded in two reservoirs with different stimulations and find under similar geological conditions and time period, the seismic efficiency, for steam injection compared to water flooding are unique across the datasets implying that temperature and fluid pressure has a different impact on the fracturing dynamics. Reservoir A (stream injection), has 4069 events (Mw = −2.2 top −0.2) with low efficiency (ŋsw=0.01 to 0.48) and considered in overshoot (ε=0.1 to 0.5). Reservoir B (water flooding), has, 1763 events, but higher magnitude range (Mw = −1.4 to 1.8); with a combination of low efficiency (overshoot ε=0.002 to 0.49) and high efficiency (undershoot ε=−415 to 0.49) events. Overshoot occurs when the dynamic strength of the fault is relatively higher than the final stress acting on the fault and less energy is radiated; where undershoot occurs when the dynamic strength is relatively lower than the final stress on the fault and in a state of dynamic weakening.
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