- Home
- Conferences
- Conference Proceedings
- Conferences
81st EAGE Conference and Exhibition 2019
- Conference date: June 3-6, 2019
- Location: London, UK
- Published: 03 June 2019
1 - 50 of 1010 results
-
-
Innovative Inversion Schemes for Model Building and Reflectivity Estimation: A Deep Water West African Case Study
Authors T. Martin, M. Barbaray, G. Venfield and V. ChavdaSummaryLate Cretaceous channel systems create structural uncertainty and impact amplitude fidelity of both Late and Early Cretaceous plays in deep water Côte d'Ivoire seismic data. We present a case study using a combination of Full Waveform Inversion (FWI) and image domain Least-squares migration (LSM) to resolve the impact of complex Late Cretaceous channel systems on deeper targets. A full wavefield FWI approach using a velocity kernel that eliminates the reflectivity imprint, created an accurate velocity model. This removed the structural uncertainty when used in the imaging step. A reflectivity estimate was then determined using LSM. The final dataset had improved amplitude fidelity by compensating for the loss of bandwidth and illumination associated with the Late Cretaceous channels.
-
-
-
A Deblending, Demultiple and High-Resolution Velocity Model Building Workflow Across the Senja Ridge
Authors S. Stokes, D. Manns, M. Romanenko, B. Kjølhamar, R. Myklebust and E. HendenSummaryTriple-source, continuous-recording acquisition is used to acquire a modern, high density 3D survey across the Senja Ridge and the surrounding salt provinces of the Tromsø and Sørvenstsnaget basins, in the Western margin of the Norwegian Barents Sea.
A robust imaging workflow is developed and presented, combining deblending technology with extensive demultiple methodology to precondition the data for velocity model building, utilising Full Waveform Inversion and high-resolution image guided tomography.
The final migrated image delivers a significant uplift in imaging across the area, with the advanced velocity model building techniques adding confidence to the positioning of events both within the Senja Ridge and the surrounding salt basins.
-
-
-
Application of Viscoelastic Pre-stack Time Migration in Thin Sand Reservoir Imaging
Authors H. H. Zeng, Q. Su, L. Lv, G. Yuan, H. Liu, H. J. Meng and B. SongSummaryWith the development of oil and gas fields in eastern China towards the stage of fine exploration, higher seismic resolution is required. How to improve the imaging accuracy of thin sand bodies and small faults is an important factor in identifying favorable reservoirs. Because of the narrow frequency band and small azimuth, conventional seismic exploration is difficult to meet the exploration needs of thin reservoirs and small faults of 3–5 meters. On the basis of broadband, wide-azimuth and high-density(BWH) data acquisition and processing, the viscoelastic pre-stack time migration can effectively compensate the attenuation of high frequency seismic wave absorption caused by viscous and thin layer scattering of the earth medium, restore the attenuated high frequency components, and further obtain high resolution migration imaging results. The application of actual data shows that the method can obtain higher resolution migration imaging results than conventional pre-stack time migration, and the result data effectively support the exploration deployment of horizontal wells for tight oil in this area.
-
-
-
Adaptive Traveltime Inversion: Mitigating Cycle-Skipping by Minimization of the Moment of the Matching Filter Distribution
Authors B. Sun and T. AlkhalifahSummaryWe present a method to obtain a misfit function for robust waveform inversion. This method, designated as adaptive traveltime inversion (ATI), computes a matching filter that matches the measured data to the predicted one. If the velocity model is accurate, the resulting matching filter reduces to an (approximate) Dirac delta function. Its traveltime shift, which characterizes the defocusing of the matching filter, is computed by minimization of the cross-correlation between a penalty function like $t$ and the matching filter. The ATI misfit function is constructed by the minimization of the least square error of the calculated traveltime shifts. Further analysis shows that the resulting traveltime shifts correspond to the first-order moment, the mean value, of the resulting matching filter distribution. We extend ATI to a more general misfit function formula by computing different order moment of the resulting matching filter distribution. Choosing the penalty function in adaptive waveform inversion (AWI) as $t$, the misfit function of AWI can be interpreted as the second-order moment. We use the Marmousi model to verify the effectiveness of the proposed method.
-
-
-
Elastic Full Waveform Inversion with Convolution-Based Crosstalk-Free Source-Encoding Algorithm
More LessSummarySimultaneous-source method can dramatically improve the efficiency of FWI but faces the problems of crosstalk noise and data mismatch. Besides, additional source wavelet estimation, which is critical for successful FWI, will counteract the efficiency of simultaneous-source algorithm. In this paper, different from conventional solutions, we propose the source-independent elastic FWI with the convolution-based crosstalk-free source-encoding algorithm. Time-domain blended simultaneous-source wavefield can be deblended and expressed as a few frequency-domain snapshots so that this algorithm can be naturally applied to unfixed-spread acquisition system, avoid the crosstalk noise and reduce the great memory requirement to a large degree. Synthetic examples verify the efficiency and accuracy of the propose algorithm with a strong robustness to different incorrect wavelets.
-
-
-
Fluid-Solid Coupled Elastic Full Waveform Inversion in the Curvilinear Coordinates for OBC Data
More LessSummaryThe marine seismic exploration with the ocean bottom cable (OBC) technology could record the compressional (P-) wave and shear (S-) wave information simultaneously. Elastic full waveform inversion (EFWI) uses both P- and S-waves of OBC data to invert multi-parameters with adequate amplitude information and complete illumination of the subsurface. We use different formats of wave equation in fluid and solid mediums, and the appropriate boundary condition applied enables the energy exchange on the interface, which is more stable and efficient than the traditional integrated simulation scheme. However, if the fluid-solid coupled medium has an extremely irregular seabed interface, the conventional rectangular-grid-based finite difference (RFD) method cannot obtain accurate source and receiver wavefields. We introduce the curvilinear coordinates to overcome the problem of the RFD method on handling the irregular fluid-solid interface. To reduce the crosstalk of inverted P- (Vp) and S-velocities (Vs), we derive the direction formulas of Vp and Vs based on P- and S-wave modes separation in the curvilinear coordinates, and finally propose a Curvilinear-grid-based Fluid-Solid Separated- wavefield EFWI (CFS-SEFWI) method. In the numerical examples, a modified Marmousi II model with irregular fluid-solid interfaces is used to test the proposed CFS-SEFWI method.
-
-
-
Robust FWI Updates in the Presence of Cycle Skipping
Authors J. Ramos-Martinez, A. Valenciano, X. Jiang and N. CheminguiSummaryFull Waveform Inversion (FWI) success depends on producing seamless short- and long-wavelength model updates while avoiding cycle skipping. In its traditional implementation, FWI risks converging to an inaccurate result if the data lacks sufficient low frequencies or the starting model is far from the true one. Additionally, the model updates may display a reflectivity imprint before the long-wavelength features are fully recovered. A solution to these fundamental challenges combines the quadratic form of the Wasserstein distance (W2-norm) for measuring the data misfit with a robust implementation of a velocity gradient. The W2-norm reduces the risk of cycle skipping whereas the velocity gradient effectively eliminates the reflectivity imprint and emphasizes the long-wavelength model updates. We illustrate the performance of the new solution on a field survey acquired offshore Brazil. There, we demonstrate how FWI successfully updates the earth model and resolves a high-velocity carbonate layer that was missing from the starting model.
-
-
-
Bayesian Uncertainty Estimation in the Presence of Tomographic Model Error
Authors A. Vlassopoulou, R. Felicio, C. Hagen, I. Jones, M. Ackers and S. SchjelderupSummaryWhether or not we build a parameter field model, or deliver a subsurface image, our industry has been sadly lacking in attempting to assign ‘error bars’ to any of the products created. Given that we can never obtain a “correct” model based on measured data, we need to assess how suitable the derived approximate model or resultant image, is. It transpires that this is an extremely difficult task to undertake in a quantitative manner. There are certain minimum acceptance criteria, which tell us that at least the derived model explains the observed data, namely, flat image gathers following migration with the obtained model, which also match all available well data (at least to within some specified acceptance threshold), but these criteria do not tell us how good the model or image is. Here we adopt a Bayesian analysis of tomographic model error so as to quantify image positioning uncertainty, but more specifically, in this work we consider the effect of the quality of the initial model on the final uncertainty estimation, demonstrating quantitatively how prior model uncertainty affects final posterior image positioning uncertainty estimates.
-
-
-
A Comparison Between Two Elitism-Based Global Optimization Methods for Estimating a Starting Velocity Model for FWI
Authors O.F. Mojica, P. Nogueira, R. Santana and D.E. ReveloSummaryThe performance of two global optimization methods (GOMs) with embedded elitism strategies for solving the 2D acoustic seismic inverse problem is compared here using synthetic acoustic data of the Marmousi model. A real-coded elitist genetic algorithm (GA) that had been used successfully in the past to this end is compared with a newly developed elitist-mutated particle swarm optimization (EMPSO) technique to estimate acoustic macro models of the P-wave velocity field (Vp). We find that EMPSO seems to have higher performance than GA with respect to the final attained value of the cost function when the completion of specified number of iterations is chosen as stopping criterion. The results further show that while both EMPSO and the GA obtain high-quality solutions, the computational effort required by PSO to arrive at such high-quality solutions is less than the effort required to arrive at the same high-quality solutions by the GA. Finally, multiple runs of descent-based full waveform inversion (FWI) started from either final GA or EMPSO models produce high-resolution final models.
-
-
-
Robust Wave Equation Migration Velocity Analysis Using a Normalized Differential Semblance Misfit Function
Authors A. Alali, B. Sun and T. AlkhalifahSummaryThe wave equation migration velocity analysis (WEMVA) techniques try to estimate an accurate subsurface velocity model for migration purposes by relying on the full-wave equation. A popular method in WEMVA is the differential semblance optimization (DSO). The objective function of DSO applies a penalty operator to extended subsurface images to minimize the energy residing in the non-physical extension. The penalty operator used in conventional DSO actually tries to reduce the energy instead of focussing it toward the subsurface zero offset. We introduce a normalization term to the DSO method in which we use a pseudo inverse Born operator to calculate the extended image and show that the new objective is more efficient and robust in focusing the energy and eliminating artifacts in the process of estimating the macro-velocity model.
-
-
-
Wavefront Tomography with Enforced Diffraction Focusing
Authors A. Bauer, B. Schwarz, L. Diekmann and D. GajewskiSummaryWavefront tomography is an efficient and stable tool for the generation of smooth velocity models based on first and second-order attributes, which describe slope and curvature of the measured wavefronts. While slopes are relatively stable to determine, curvatures can become unreliable in the case of strong lateral heterogeneity. Since wavefront tomography is mainly driven by the misfit of modeled and measured wavefront curvatures, its convergence may be compromised by curvatures of bad quality. A possible solution to overcome this problem are diffractions that have a unique property: all measurements belonging to the same diffraction are connected to the same subsurface region. In recent work, we introduced an event-tagging scheme that automatically assigns a unique tag to each diffraction in the data. We propose to use this information to constrain the inversion by enforcing all diffracted measurements with the same tag to focus in depth, thus overcoming the sole dependency of wavefront tomography on second-order attributes. Results for diffraction-only data with vertical and lateral heterogeneity confirm that it is possible to obtain depth velocity models for zero-offset data without using curvature information and that the suggested approach may help to increase the stability of wavefront tomography in complex settings.
-
-
-
First-Arrival Traveltime Tomography Based on the Adjoint State Method with Independence of Surface Normal Vectors
More LessSummaryThe first-arrival traveltime tomography based on adjoint state method has gained widespread attention in recent years. Compared with the traditional method, this method does not need to perform any ray tracing. Only the eikonal equation and the adjoint equation need to be solved in order to obtain the inversion gradient. However, the result of commonly-used adjoint state traveltime tomography is related to the boundary normal vector of the computational domain. In the cases of complex surface, the calculation of adjoint field via the boundary normal vector makes the traveltime residual distributed according to structure-irrelevant parameters. As a result, the tomography gradient deviates from the true gradient and consequently contributes to a untrue tomographic model. In need of solving this issue, this paper derives a new form of adjoint equation that is independent from the boundary normal vector. This tomography method adapts to any kind of observation system. In situations of tough topography, it produces better results compared to the commonly-used adjoint state tomography method. The gradient calculated by this method is similar to that of the traditional ray-based method, but it offers better continuity and smoothness with significantly improved inversion efficiency proved by the synthetic experiments.
-
-
-
Inversion Velocity Analysis by One-Way Wave Equation
More LessSummaryMigration velocity analysis (MVA) is an image domain technique to determine large-scale structure of the subsurface velocity model. However, due to limited surface acquisition geometry and uneven illumination, migration smiles and spurious oscillations around the reflector positions may lead to local minimal in the inversion process. For more stable results, in this paper, we have developed new expressions based on true-amplitude, one-way wave-equation operators and used those expressions to direct velocity analysis which is called inversion velocity analysis (IVA). The differential semblance optimization (DSO) objective function is adopted to evaluate the inverted images quality. Compared to traditional MVA method, the modified inversion method attenuates migration smiles by compensating for geometric spreading and uneven illuminations. Also, the direct inversion has influence on the gradient, with slightly modified objective function, the spurious oscillations around reflectors in the gradient can be attenuated. Numerical examples illustrate the effectiveness of the IVA which offers new perspective to large-scale model construction.
-
-
-
A Robust Differential Semblance Optimization by Quantitative Migration
More LessSummaryWave equation migration velocity analysis is an image domain method which can be implemented to estimate the large-scale model. For classic migration and velocity analysis, the inversion results or stacked image sections often suffer from kinematic artefacts due to the limit of the acquisition geometry and uneven illumination or mathematically due to non-unitary of the wavefield extrapolation operator. In this paper, the modified differential semblance optimization (DSO) objective function with root mean squared image amplitudes, which can reduce the amplitudes sensitivity of DSO, is adopted to evaluate the inverted images quality. To obtain more stable results, we introduce a weight to the traditional migration and velocity inversion without ray tracing calculation. To some extent, the weighting function can automatically compensate for uneven illumination and remove migration artefacts. Thus the gradient of MVA has a smoother behavior and gives an optimal convergence. The results of numerical examples illustrate the robustness of the weighting function.
-
-
-
Optimal Surface and Subsurface Parameters for a Successful Migration Velocity Analysis
Authors T. ZHOU, H. Chauris and F. AudebertSummaryMigration velocity analysis is a method to estimate both reflectivity model and macromodel. We study here the sensitivity of the final result with respect to the selection of input surface offsets and subsurface parameters. We show that the velocity gradient used to update the macromodel highly depends on these parameters. We provide rules to ensure proper gradient by avoiding the dominance of the migration smiles. These rules are developed in the case of simple configurations. We apply them to the Marmousi model and show that final velocity model contains artefacts when the optimal parameters are not selected.
-
-
-
A High-Order Temporal and Implicit Spatial Staggered-Grid Finite-Difference Scheme for Modelling Scalar Wave Propagation
More LessSummaryFinite difference (FD) is a commonly used tool for modelling seismic wave propagation. Temporal high-order FD methods can enhance the accuracy and stability compared with the methods with second-order accuracy in time. The implicit calculation of spatial derivatives also contributes to the improvement in accuracy. We propose a new implicit staggered-grid FD (SFD) scheme based on a combined stencil, which is the combination of pyramid and cross stencils in 3D case. Our scheme calculates the temporal and spatial derivatives by using high-order temporal and implicit spatial FD operators, respectively. Based on the dispersion relations, we optimize the temporal and implicit spatial FD coefficients by using Taylor series expansion (TE) and least squares (LS). We formulate four implicit SFD operators: TE-TE, TE-LS, LS-TE and LS-LS operators. 2D and 3D modelling examples determine that our implicit scheme has greater accuracy than other schemes. Additionally, our implicit SFD scheme allows for a larger grid spacing, which can increase the computational efficiency.
-
-
-
3D Frequency-Domain Finite-Difference Acoustic Wave Modeling on Discontinuous Grids
More LessSummaryThe frequency-domain finite-difference (FDFD) method is an effective tool for implementing frequency-domain seismic modeling and full waveform inversion. However, the computational cost for the FDFD method dealing with 3D large models is prohibitive, limiting its application. As a common strategy to improve computational efficiency, a nonuniform grid is usually adopted in the time-domain finite-difference (TDFD) method instead of the FDFD method. We propose a strategy to implement 3D frequency-domain acoustic wave modeling on discontinuous grids efficiently. In the whole model area discreted by discontinuous grids, we apply the 3D average-derivative optimal scheme (3D-ADOS) to simulate wave propagation in each subregion and use the conventional second-order or rotated second-order finite-difference scheme to calculate the wavefield in the transition area. In this way, the accuracy of the wavefield in the global area remains the same as that obtained by the conventional 3D-ADOS while the computational cost is reduced significantly. The numerical example is shown to verify the feasibility and efficiency of our strategy.
-
-
-
Acoustic Wave Simulation by Lattice Boltzmann Method with D2Q5 and D2Q9 of Different Relaxation Times
More LessSummaryIn this abstract, the lattice Boltzmann method (LBM), a kind of mesoscopic method for modeling waves, is introduced to simulate acoustic wave propagation, which is usually simulated by finite difference method (FDM) in seismology. By choosing different discrete velocity models of D2Q5, D2Q9 and different relaxation times of LBM, with the help of the variable-controlling approach, various seismic wavefields can be obtained. Numerical experiments demonstrate that the wavefields of acoustic wave propagation simulated by LBM with D2Q5 of relaxation time as 0.51 can better coincide with those by acoustic FDM.
-
-
-
Seismic Traveltime Calculation Using Fast Marching Methods with a Rotated Staggered Grid Finite-Difference Scheme
More LessSummaryFast marching methods (FMM), with unconditonal stability and high efficiency, has become one of the most prevalent algorithms for traveltime computation. However, finite-difference (FD) schemes adopted by the existing FMMs define the velocity and traveltime on the same grid system, which doesn’t conform to the physical process of ray propagation.
In this article, by defining the traveltime and velocity on two sets of staggered grid system, we construct a new rotated staggered-grid FD schemes to approximate the eikonal equation. FMM with this staggered-grid FD schemes (RSFMM) can archive higher accuracy than classical FMM with 1st-order and 2nd-order FD schemes. Furthermore, RSFMM calculates the traveltime with the velocity at the centre of grid points involved in the FD operator, which perfectly conforms to the physical process of ray propagation. Therefore, RSFMM has better applicability in complex medium. Considering the Spherical wave characteristics in the vicinity of the source point, we apply a combination scheme of spherical approximation method (SAM) and RSFMM (SAM+RSFMM) to further improve the accuracy. Numerical experiments on homogeneous, layered and Marmousi model approve the high accuracy and applicability in complex media of RSFMM and SAM+RSFMM.
-
-
-
An Efficient Wave-Equation Based AVP Gather Simulation Using the Precondition Least Square Method
More LessSummaryA stable and fast numerical approach to synthesize the seismic prestack AVP gather based on the wave-equation in the frequency domain is presented in this abstract. As a powerful method to extract rock property information, wave-equation based seismic amplitude-variation-with-offset (WEB-AVO) inversion relies much on the corresponding forward modelling engine. To solve the state equations of Lippman-Schwinger type numerically, in the new method, we firstly convert the two integral formulas into a large linear system of equations, in which the total wavefields (pressure and particle velocity) are treated as the unknown. Then, the least-square method is used to solve the total wavefields. Further, introducing the pre-conditioner is essential, and the robustness of this scheme is great improved. For efficiency, parallel computing method is also used to solve the wavefield at different frequency points simultaneously. Two numerical examples demonstrate the effectiveness and superiority of the method.
-
-
-
Estimation of the Conversion Point Position in Elastic Orthorhombic Media
More LessSummaryIn order to estimate the conversion point in an elastic orthorhombic (ORT) medium, we define an explicit rational form approximation for the radial coordinate of the conversion point for the converted PS1, PS2 and S1S2 waves. In order to obtain the approximation coefficients, the Taylor series approximation in the corresponding vertical slowness for three pure wave modes is applied. The coefficients in our proposed approximation are computed within two vertical symmetry planes. The difference between the acquisition azimuth and the azimuth of the conversion point position is analyzed for different combinations of the wave modes. The accuracy in conversion point position estimation for three ORT models is illustrated in the numerical examples. One can see from the results that for converted PS1 and PS2 waves, the proposed approximation is very accurate in estimating the conversion point position in the tested ORT model. For a converted S1S2 wave, due to the existence of cusps, triplications and shear singularities, the error in conversion point estimation is relatively larger comparing with PS waves in the vicinity of the singularity point.
-
-
-
A Parallel Fast Sweeping Algorithm for Fast and Accurate Traveltime Computation in Anisotropic Media
By U. WaheedSummaryNumerical solution of the eikonal equation has been widely used to compute traveltimes in anisotropic media. Several techniques have been recently proposed to increase the accuracy of these solutions. These include factoring the unknown traveltime to tackle the source-singularity problem, using the Weighted Essentially Non-oscillatory (WENO) approximation of the traveltime derivatives, and the Discontinuous Galerkin method. Although these approaches yield highly accurate traveltimes but they also result in increased computational load. Therefore, I propose a parallel fast sweeping algorithm to compute fast and accurate solution of the anisotropic eikonal equation. High accuracy is achieved by using factorization followed by the WENO approximation of derivatives, whereas computational speed up is obtained by sweeping the computational domain in parallel. With a large number of CPUs, significant reduction in computational cost can be achieved for large 3D models.
-
-
-
A TTI Representation of a Heterogeneous Medium for Finite-Difference Seismic Wave Simulation
More LessSummarySince the computational time of FDM is proportional to the number of grid points, many higher-order and optimized schemes were developed to increase the spatial grid size while maintaining the dispersion error below predefined levels. However, mentioned techniques are applicable only to homogeneous media since large error due to the interfaces may arise when the large grid spacing is used. To solve this problem, various effective medium parameterization methods have been proposed. In our work, We develop a new TTI effective media parameterization method and analyze existing different effective media parameterization methods. Our tests show that the proposed TTI effective media parameterization method is better than other effective media parameterization methods and can be used with the higher-order scheme to utilize 3–4 PPW to produce satisfactory results.spacing is used.
We develop a new TTI effective media parameterization method for
-
-
-
Two-Phase Computational Model for Wave Propagation in Deforming Saturated Porous Medium
Authors G. Reshetova and E. RomenskiSummaryThe new two-phase model for a compressible fluid flow in deforming porous media is presented. The derivation of the model is based on the symmetric hyperbolic thermodynamically compatible systems theory, which is developed with the use of the first principles and fundamental laws of thermodynamics. The governing PDEs form the first order hyperbolic system and can be used for studying a wide variety of processes in saturated porous media, including small amplitude wave propagation. Our theory predicts the three types of waves: fast and slow pressure waves and a shear wave, as it is in Biot's model. The material constants of the model are fully determined by the properties of the solid and the fluid phases, and unlike Biot's model do not contain empirical parameters. The governing PDEs for small amplitude wave propagation in a saturated porous medium are presented, and the efficient numerical method has been developed for solving these PDEs, based on the finite difference staggered-grid scheme of fourth order spatial accuracy with modified coefficients to provide approximation in inhomogeneous media. An application of the developed computational framework to solving a series of test problems confirms the robustness and efficiency of the approach presented.
-
-
-
A Full Wavefield Approach to Survey Planning
Authors C. Lazizi, S. Jetschny, M. Pedersen and A. OrdoñezSummaryTowed streamer acquisition using typical 3D spreads can be problematic for shallow imaging due to strong acquisition footprints effects. These effects can be mitigated by imaging using separated wavefields whereby each receiver position acts as virtual source. We present a full-wavefield modeling and imaging workflow for de-risking of imaging application using separated wavefields in the survey planning phase of a marine seismic acquisition. Each of the applications share a modeling and migration step that considers only a few shots that can be duplicated to represent the migration response of a full 3D acquisition. The workflow allows us to estimate the depth range where separated wavefield imaging provides superior results over primary imaging in terms of sail line related footprint artefacts. We can further analyze the effect of cable separation on separated wavefield imaging and demonstrate how wavefield resampling mitigates these effects. Based on a horizontally layered model we can test and optimize survey geometries by analyzing migration footprint along plane reflectors and the resolution of fault features of various sizes in the image domain. Such analysis can be used to optimize sail line and cable separation to provide sufficient coverage to image the targets of interest as efficiently as possible.
-
-
-
Speeding up a Mass-Lumped Tetrahedral Finite-Element Method for Wave Propagation
Authors W. Mulder, S. Geevers and J. Van der VegtSummaryMass-lumped finite elements on tetrahedra offer more flexibility than their counterpart on hexahedra for the simulation of seismic wave propagation, but there is no general recipe for their construction, unlike as with hexahedra. Earlier, we found new elements up to degree 4 that have significantly less nodes than previously known elements by sharpening the accuracy criterion. A similar approach applied to numerical quadrature of the stiffness matrix provides a speed improvement in the acoustic case and an additional factor 1.5 in the isotropic elastic case. We present numerical results for a homogeneous and heterogeneous isotropic elastic test problem on a sequence of successively finer meshes and for elements of degrees 1 to 4. A comparison of their accuracy and computational efficiency shows that a scheme of degree 4 has the best performance when high accuracy is desired, but the one of degree 3 is more efficient at intermediate accuracy.
-
-
-
Spatially-Optimized Finite-Difference Schemes for Numerical Dispersion Suppression: an Implementation Using Symbolic Computation
By E. CauntSummaryIn this work, a 4th order DRP 2D elastic wave formulation with free surface boundary conditions is presented, extending methods of Tam and Webb (1993 ). Staggered first-derivative stencils are derived and applied to the P-SV formulation of Virieux (1986 ). Performance is compared to the Taylor-series-derived staggered scheme of equal extent, demonstrating the versatility and universal benefits of spatial optimization. Implementation of both FD schemes is carried out using Devito, a domain-specific Python module and compiler for FD applications. Devito allows for model specification with a handful of high-level symbolic Python objects to build an FD operator, used to generate highly optimized C++ code at runtime via a series of intermediate representations, allowing for complex multi-stage optimizations. The high-level, symbolic nature of Devito ensures concise, readable model code and expedites workflow compared to model building with low-level languages, enabling rapid prototyping in hours as opposed to weeks or months without sacrificing underlying code quality. This work showcases the potential of symbolic computation for implementing non-conventional FD stencils, and the straightforwardness of doing so with Devito.
-
-
-
Sparsity of Synthetic Wave Fields in Curvelet Space
Authors M. Henriques, G. Corso, J. Medeiros and L. LucenaSummaryThe Full Waveform Inversion (FWI) is based on the optimization of a physical model in order to fit the generated synthetic data with the empirical data collected by the receivers used in the field. At each iteration of the FWI computational procedure, the wave equation is solved and an update direction of the model is computed. This process requires a large amount of memory, a critical step in the performance of the method. The compression of the wave field data can effectively be performed with the curvelet transform, a modern multi-scale tool. Because of their dependence on orientation, curvelets are suitable to represent the anisotropy of wave patterns. Besides, it has been demonstrated that the solution operators of a wide range of wave equations are optimally sparse. In this work, we explore curvelets for data compression in the seismic FWI context. We compare the memory use of standard FWI processing with similar FWI processing using curvelets decomposition.
-
-
-
Time-Lapse Elastic Properties of Cracked Granite During Deformation Inferred from Laboratory Experiments Using 2D Waveform Modelling
Authors S. Lai, N. Fuji and I. KatayamaSummaryIn order to understand the physics of earthquake generation, we study the microscopic weakening process in a centimetre-scale laboratory experiment. A series of deformation experiment was performed to the granite sample from Aji region, Japan. During the experiments, active seismic wave propagation was measured. This study numerically models the observed waveform in order to infer to the time-lapse elastic structure changes. We first modify the finite difference code on collocated Cartesian introducing zig-zag free-surface discontinuities. We applied response function estimation to the synthetic waveform to consider all the effect (i.e. 3-D effect, instrumental response, attenuation). We use trial-and-error method to find the best-fit velocity models using full waveform synthetics with response. Then we use the same method to make a series of velocity models for the automated Monte Carlo scheme. Comparing automated velocity models using different methods, we find that L1 norm is the best fitting method. We further tested fault models that we find the trend of the changes of the velocity model after the creation and during the thickening of the fault. This method will detail a microscopic weakening process during the nucleation of earthquakes using full waveforms, which have an impact on fundamental understanding of earthquake physics.
-
-
-
Physical Modeling and Seismic Response Characteristics Analysis of Strike-Slip Faults
More LessSummaryIn the northwest exploration area of Sinopec, a series of strike-slip faults are formed due to the multi-stage structure and stress action. These faults are closely related to the formation of oil and gas reservoirs, so the accurate identification of these faults is of great significance to the discovery of the gas and oil reservoirs. Through the development of heterogeneous physical model materials, strike-slip fault physical models with different widths and different degrees of rupture are constructed; seismic data acquisition and response characteristics analysis are also carried out. The results show that the amplitude of the fault reflection is the comprehensive response of fault scale and internal filling characteristics. By summarizing the geological characteristics, a physical model of three typical strike-slip faults forms with positive, negative, and graben are constructed. The data analysis results show that it is difficult to accurately depict the fault by seismic response or single attribute for different strike-slip faults forms, but the low frequency data can obviously improve the ability of recognition.
-
-
-
Optimized and Cost-Efficient Visco-Acoustic Iterative Least-Squares Migration
Authors Ø. Korsmo, A. Osen, A. Pankov and O.A. SandstadSummaryIterative least-squares migration is an expensive process that requires several passes of migration and de-migration. In this paper we focus on how an optimized initial reflectivity model combined with a deconvolution imaging condition can ensure faster convergence. We also incorporate the visco-acoustic effects in the de-blurring process to improve image corrections across and below Q-anomalies. This workflow is demonstrated on a dual-azimuth North Sea dataset, where the aim is to improve the understanding of the Frosk and Bøyla fields, which are characterized by complex and steeply dipping sand systems and areas of weak reflectivity.
-
-
-
Reverse Time Migration with Random Space Shift
More LessSummaryThe classical imaging condition for reverse time migration (RTM) is to cross-correlate the forward propagated source-side wavefield with the backward propagated receiver-side wavefield at zero time- and spatial-lag, under the assumption that an accurate migration velocity model is available to correctly predict the kinematic information of the observed seismic data. When errors are present in the migration velocity model, the conventional RTM image may be defocused. Inspired by the work of the extended RTM with subsurface offset and considering the finite-frequency effects of wave propagation, we propose to introduce a random space shift to the source- and receiver-side wavefield at each time step before applying the cross-correlation imaging condition. The maximum random spatial shift is constrained by the theoretical lateral resolution limit of the half-wavelength after migration. The computational cost of our method is the same as the conventional RTM. Numerical examples on a 2D layered model demonstrate that our method could provide an image with better quality than the conventional RTM even the migration velocity model contains significant errors.
-
-
-
Flank-Preservation De-Primary Reverse Time Migration, Part I: The Missing Flanks and a Modified Imaging Condition
Authors Y. Sun, T. Fei and R. Van BorselenSummaryDe-Primary Reverse Time Migration (dpRTM) is a recently proposed technology, which has the capability to remove incorrectly imaged structures caused by inaccuracies or complexities of the migration velocity model used in RTM. In spite of the success achieved by dpRTM, a bizarre effect is also observed, that is after dpRTM the flank structures in the image have been completely missing. We first point out the root cause leading to the missing flank structures in dpRTM, and next propose a Flank-Preservation De-Primary Reverse Time Migration (FPdpRTM) technology, which not only possesses the capability of dpRTM on removing false structures but also is capable of preserving flank structures in the final image. We use two synthetic examples to demonstrate the effectiveness of FPdpRTM using the true velocity models for migration.
-
-
-
Imaging Enhancement of Reverse Time Migration Using Dynamic Time Warping and Local Similarity
More LessSummaryConventional reverse time migration obtains the final image via simply stacking single-shot imaging result. Many factors such as the inaccurate migration velocity, the mad physics assumption and the imperfection sampling, cause the misalignment, amplitude difference and artifacts in different trace on the shot-index gathers. Mean stacking reduces the imaging quality. In the abstract, we propose a weighted stacking to improve the imaging quality. First, we apply the dynamic time warping method to correct the uneven shot gathers. The dynamic time warping is a nonlinear aligning method that effectively correct the residual misalignment on the shot gathers. Then, we use the local similarity to construct the weighted coefficient on the foundation of the flat shot gathers. The local similarity method can better estimate the correlation between different traces. Finally, we present results on synthetic and real data to illustrate our approach is valid.
-
-
-
A Stereotomography Velocity Inversion Method in Imaging Domain
More LessSummaryGenerally, the data space needed for velocity inversion of stereotomographic is based on the slant stacking of local events on common shot point gathers and common detection point gathers in the data domain. This is a stable but inefficient data space pickup method suitable for 2 dimensional seismic data. Using pre-stack gathers for slant stacking is effective for primary wave in interactive picking, but the diffracted wave in complex structures can not be distinguished. Therefore, in view of the problem of inaccurate data space picking and large workload, this paper chooses a method of obtaining accurate stereotomography data space in the imaging domain under the condition of incorrect velocity. Firstly, extracting the common imaging point gathers by DSR (double square root equation) prestack depth migration method, which includes angle information of underground ray in migration process. Then residual curvature and local half-offset ray parameters are obtained. Then travel time residuals and corresponding ray parameters at shot point and receiving point are obtained. Then, real stereo is calculated by specific correction formula. Model tests shows that stereotomographic velocity inversion can improve the accuracy and stability obviously. This method will have better application in the practice of stereotomography inversion in the future.
-
-
-
Visco-Acoustic Reverse Time Migration Using Lowrank Decomposition
More LessSummaryFor visco-acoustic reverse time migration, low-wavenumber noise removal, precision of extrapolation and stability of backward extrapolation are three important problems. In this paper, we propose an effective image condition for QRTM to suppress the low-wavenumber noise, which utilizes Hilbert transform to procure the upgoing and downgoing wavefield and sequentially uses normalized crosscorrelation to obtain the image value. And for the other two parts, we use lowrank decomposition and adaptive stabilization filter to guarantee the precision of extrapolation and the stability of backward extrapolation. Numerical experiments on a two-layered model and BP gas chimney model can demonstrate that our method can provide accurate and stable image results.
-
-
-
Absolute Acoustic Impedance Inversion Based on Reflectivity Estimated by Image Domain Least Squares Migration
More LessSummaryInstead of solving nonlinear FWI, the acoustic impedance (AI) model can be divided into two parts for linear inversion. One is the smooth background, and the other is the reflectivity.
Assuming the background AI model is obtained, true-amplitude and broadband reflectivity can be estimated by solving least-squares migration (LSM). We can calculate the corresponding broadband AI, and then the background AI is merged to get the absolute AI.
In this paper, we firstly solve the image domain LSM (IDLSM) for true-amplitude and broadband reflectivity inversion. Then the absolute AI model is inverted with the estimated reflectivity and the given the background AI. For IDLSM, we approximate the Hessian matrix with non-stationary matching filters. The image deblurring problem is solved with the sparse (L1 norm) and total variation (TV) regularization. With the estimated reflectivity and given background impedance, we solve a constrained optimization problem with combined first and second order TV regularizations for absolute AI model inversion. Numerical examples on Marmousi model illustrate that the IDLSM results solved by the proposed method have both more balanced amplitudes and higher resolution than conventional migration images. Absolute AI model can be inverted with the estimated reflectivity and given background impedance.
-
-
-
Travel Time Changes in the Groningen Gas Reservoir by Train Noise Interferometry of Borehole Data
Authors W. Zhou and H. PaulssenSummaryIn this study we show that time-lapse measurements of a reservoir can be made by noise interferometry of borehole data. We used borehole geophone array data of monitoring well SDM-1 in the Groningen gas field in the Netherlands. The ambient, anthropogenic noise allows accurate determination of the P and S velocity structure ( Zhou & Paulssen, 2017 ), but is not stable enough in time and space to measure temporal travel time variations. With deconvolution interferometry of high-frequency train signals, however, it is possible to detect small travel time decreases of ~0.05 ms (0.1%) over half a year associated with compaction of the reservoir. Moreover, we identified a strong travel time anomaly over a period of 1.5 months that is caused by drilling of a new borehole in the reservoir at ~5 km distance.
-
-
-
A Novel Array Acquisition and Processing Methodology for Microseismic Monitoring
Authors B. Witten, A. Booterbaugh and R. SegstroSummarySurface microseismic monitoring has converged on a few acquisition designs, notably the “star” and “patch” arrays, to detect and locate low signal-to-noise ratio (SNR) microseismic events. The principal ideas behind both these designs are the same: to be capable of attenuating surface noise and harnessing the power of stacking a large number of sensors to increase the SNR. We present an alternative acquisition design, consisting of hexagonal subarrays, and noise attenuation methodology that is easily scalable, adaptable to local site conditions, and cost effective. We propose this array design as it is easy to deploy in any environment since it does not require lines and each subarray has a very small footprint. In addition, we show that the hexagonal array is capable of removing coherent noise even with a limited number number of sensors through a semblance-weighted stack which attenuates surface wave noise while preserving the signal energy that arrives vertically. We demonstrate this capability through a pilot field test that co-located hexagonal arrays within a patch acquisition.
-
-
-
Selecting a Seismically Safe Formations for Salt Water Disposal in Delaware Basin
Authors Z. Jechumtalova, L. Eisner and D. AnikievSummaryThe recent increased level of the observed seismicity in Delaware Basin, USA, raises an urgent question of how we can safely dispose polluted water (salt water disposal - SWD). There are many active injection wells within this basin and water is injected into many separated formations. We applied a waveform similarity technique to data from regional stations to detect weaker events than those reported by USGS during years 2013-2017. We were able to detect 20 times more events than USGS. We have performed spatio-temporal correlation between this extended catalogue and SWD injected volumes for various injection formations and distances of events from injectors. Four formations targeted for SWD correlated with seismicity, injection rates into two of these formations correlates with the seismicity with less than one month delay between the injection and seismicity and two of these formations show significant delay of seismicity behind the injection (4 and 10 months) which may indicate indirect connection between SWD and seismicity.
-
-
-
Automatic Arrival Time Uncertainty Assessment for Downhole Microseismic Monitoring Data
Authors I. Abakumov and S.A. ShapiroSummaryArrival time uncertainties intrinsically define the accuracy of all arrival time-based measurements, e.g. the precision of wave arrivals in microseismic registration. They are used to weigh the data in inversion algorithms and to define the resolution of reconstructed velocity models. Although a wide range of methods for arrival time uncertainty estimation has been proposed in the literature, the physically most prominent ones are based on the probabilistic formulation. We review two probabilistic approaches for assessment of the lower boundary of picking error which are well-known in the radio signal processing - the Cramer-Rao Bound and the Ziv-Zakai Bound. These classic bounds require explicit knowledge of the spectral variance of the signal that is often hard to determine in microseismic experiments. Hence, we present reformulations of the bounds suitable for the case of downhole microseismic monitoring. The resulting easy-to-use analytical expressions require the signal and noise parameters that can be directly obtained from microseismic data and, hence, can be efficiently applied to the data in a semiautomatic fashion. We further demonstrate that the proposed analytical expressions provide realistic values of the arrival time uncertainties, and that the obtained uncertainties can be used to improve further processing steps, e.g., microseismic source localization.
-
-
-
Microseismic Event Location Using Time Reversed Imaging Based on Decoupled Wavefields in VTI Media
More LessSummaryLocating microseismic events in a reservoir monitoring system is of great significance due to its capacity of delineating the induced fractures. Time reversed imaging (TRI) method, which refocuses the back-propagated seismic energy to its real origin, has been demonstrated as a reliable location technique especially in noisy data processing. However, the time reversed images are often contaminated with strong imaging artifacts due to the cross talk of coupled wave modes coinciding in time and space especially in anisotropic media, leading to unreliable location estimations. To minimize the interference of strong imaging artifacts, we present a TRI technique based on decoupled wavefields. Spatial filters constructed based on Christoffel equation are applied to separate qP and SV wave modes. The auto and cross-correlations of decoupled qP and SV potentials are then applied as the image conditions to form the final location images. The synthetic example has shown that the spatial filters can fully separate the wave modes in 2D VTI medium compared to Helmholtz decomposition, leading to an improved location images with a much sharper focus.
-
-
-
A Comparative Study of Ray-Based and Waveform Local Earthquake Tomography: an Application in Albania Thrust Belts
Authors A. Soni and R. PlessixSummaryIn fold and thrust belt settings, local earthquake events often occur and can be recorded when we lay down a patch of seismometers. We can then invert those seismic events to retrieve velocity information as it is done in global seismology. A passive data has been recorded over an exploration block in Albania where we can detect local earthquake events. After identifying them, we have applied a ray-based traveltime inversion and an acoustic waveform inversion. Because of the large velocity variations and the illumination issues, ray-based traveltime inversion seems to give bias results. The waveform inversion appears more robust and geological possible, as expected because some of the scattering effects that may happen within the Fresnel zone are accounted for. This comparative study suggests than waveform inversion applied on passive data can help in such a complex geological setting.
-
-
-
A Field Data Application of Regularized Elastic Passive Equivalent Source Inversion With Full Waveform Inversion
Authors H. Wang, Q. Guo, T. Alkhalifah and Z. WuSummaryUsing full waveform inversion (FWI) to locate passive events allows for an automatic process. Passive seismic data are often acquired on solid surfaces including the bottom of the sea, in which multi-component measurement under the elastic assumption is important. We develop a regularized elastic FWI of passive seismic events to invert for the source image, source function and the velocity model, simultaneously, without any a prior information about the source. We reformulate the elastic problem by representing the source images by $P$-wave and $S$-wave perturbation coefficients. The unknown source ignition time is mitigated by convolving reference traces with the observed and modeled data. A total variation regularization is applied to improve the robustness of the velocity inversion considering the limited sources and illumination angles of microseismic experiments. We also applied a focusing function to the source to overcome the possible limited aperture coverage of the acquisition, especially in well recording. The adjoint-state method is used to derive the gradient for the source image, source function and velocity. The resulting inversion framework is capable of handling limited aperture data and limited sources. Application to synthetic and real data with limited recording aperture along a well demonstrates the effectiveness of the approach.
-
-
-
Cross-Correlation Migration of Microseismic Source Location with Multiplication Imaging Condition
More LessSummaryLocating microseismic sources are critical in hydraulic fracturing monitoring for unconventional oil/gas exploration. Waveform-based methods can reliably and automatically image microseismic source locations, but they usually need to scan the source excitation time. The cross-correlation migration (CCM) avoids excitation time scanning and reduces the total scanning dimensions from 4D (3D in space and 1D in time) to 3D. The conventional CCM sums all the contributions obtained from the virtual trace gathers, which leads to low resolution source location image. In this study, we propose to use the multiplication imaging condition (MIC) to replace the summation imaging condition which is used in CCM, and name it as CCM-MIC. Instead of summing all contributions obtained from the virtual trace gathers, the CCM-MIC multiply them to form the final image. By using CCM-MIC, it is more meaningful to select less receivers with good azimuthal coverage, rather than to use a large amount of indiscriminate receivers, which is required by CCM. Our approach is also beneficial for crustal scales seismology applications
-
-
-
Joint Using PP-Wave and PS-Wave Elastic Impedance to Construct Reservoir Sensitive Elastic Parameters
More LessSummaryDue to the different seismic amplitude information carried by PP-wave and PS-wave, the sensitive of PP-wave and PS-wave to reservoir are different. The joint using of PP-wave and PS-wave information can reduce the multi-solution of reservoir prediction. The construction of conventional elastic parameters is mainly based on PP-wave seismic information. This study investigates elastic parameter construction by using PP-wave and PS-wave elastic impedance jointly. Based on the previous research on PS-wave elastic impedance, a new form of PS-wave elastic impedance equation which can keep consistency with the PP-wave elastic impedance equation is derived. According to the construction formula of conventional elastic parameters, we use PP-wave and new PS-wave elastic impedance instead of P-wave and S-wave impedance to construct a new kind of elastic parameter with the concept of angle elastic parameter. Compared with the conventional elastic parameters, the angle elastic parameters have incidence angle and PS-wave information and can be used as sensitive indicators of reservoir. Both the model and real data show that the angle elastic parameters have high sensitivity to the beneficial reservoir.
-
-
-
The Analysis of the Influence of Vs/Vp on the Elastic Impedance Inversion
More LessSummaryElastic impedance (EI) inversion as a popular pre-stack seismic inversion method has been studied in the past years. However, the robustness of estimating elastic parameters of P-wave velocity, S-wave velocity, and density in conventional EI inversion is still controversial, especially the accuracy of the S-wave velocity. In this paper, we show that the obvious inversion error of the Vs is associated with the accuracy of the ratio of VS to VP (VS/VP) when the three parameters are inverted. The deviation of the VS/VP in the linearized Zoeppritz equation provided by Aki and Richards results in the mismatch between the forward and backward modeling. A new kind of inversion formula containing VS/VP as the inversion parameter is proposed, by which VS can be estimated in terms of the new inverted parameter. Due to the nonlinear relationship between the new inverted term and the VS, the Newton downhill method is used to solve the nonlinear problem. The synthetic data inversion examples demonstrate the performance of the new formula.
-
-
-
Azimuthal Amplitude Difference AVAZ Inversion for Orthotropic Parameters
More LessSummaryAnisotropic azimuthal amplitude variation with offset (AVO) inversion works as an important tool to estimate elastic parameters and anisotropic parameters of underground medium from pre-stack and azimuthal seismic data in the geosciences. Shale formation with parallel vertical or near-vertical fractures is often described with orthogonal anisotropic medium. However, the problems, which more model parameters, higher condition number of forward solver and relatively stronger dependence on the initial model for traditional orthotropic azimuthal AVO inversion, render the poor stability of the estimated parameters. In this paper, we carry on the study on the approaches with wide azimuthal pre-stack seismic data to establish the azimuthal amplitude difference AVO forward solver of orthotropic media, and implement the azimuthal amplitude difference AVO inversion for the orthotropic parameters under Bayesian framework with smooth background constraint to reduce the condition number of forward solver and improve the robustness and stability of inversion. Besides, Cauchy and Gaussian probability distributions are respectively utilized as priori information of the model parameters and the likelihood function for enhancing the inversion precision. Finally, field data examples are finally used to verify the feasibility and stability of the proposed approach in the estimation of the anisotropic parameters in orthotropic medium.
-
-
-
DE-MCMC Based Stochastic Seismic Inversion Incorporating Gaussian Mixture Model
More LessSummaryStochastic seismic inversion works as an important technology to estimate elastic parameters of subsurface media to guide the lithology prediction and fluid discrimination. In this study, an improved stochastic simulation is proposed to invert seismic impedance and lithofacies simultaneously. The Gaussian mixture priori probability density (PDF) is initially utilized to describe the distribution of model parameters influenced by subsurface lithofacies. Furthermore, a novel expression of multi-dimensional posteriori PDF conditioned with time and frequency joint-domain seismic data is derived. And, the differential evolution Markov Chain Monte Carlo (DE-MCMC) algorithm is utilized to implement the optimizations of multi-dimensional posterior PDF in our approach, which runs multiple Markov chains in parallel and estimates the multiple solutions of model parameters with the theory of population evolutionary. The lithofacies is clearly discriminated according to the weights of different Gaussian components in the posterior PDF and the model parameters are sampled from the selected Gaussian components to realize the simultaneous prediction of these two parameters. Finally, the feasibility and robustness of DE-MCMC model are illustrated by several synthetic examples and field datasets. The estimated P-wave impedance and lithofacies classification results of maximum conditional probability density (Cpd) coincide with the well logging curves and interpreted lithofacies.
-
-
-
One Novel Elastic Impedance Inversion for Brittleness Sensitive Parameter in Sichuan Basin
More LessSummaryAt present, accurate prediction of high brittleness regions is an important issue in the development of shale gas field. Elastic impedance inversion works as an effective technology to estimate elastic parameters of subsurface media to guide the identification of reservoir characteristics. In this study, an improved elastic impedance inversion is proposed to establish a workflow for brittleness prediction on shale gas field. Based on previous research results, an expanded elastic impedance equation is derived from reflection approximation and established relationship between brittleness sensitive parameter Eρ with elastic impedance. After that we advance a workflow for target parameter extraction based on Bayesian theory. Then the novel elastic impedance equation is accepted the accuracy test and verified the reliability. Finally, the workflow is utilized to implement the applicability on shale gas field data of Sichuan Basin, which runs well even on the influence of the SNR=3. and the Eρ section also shows that it is coinciding with the well logging curves in the destination layer.
-