In order to develop an appropriate model for long-term research efforts in seismic wave propagation through a complex near-surface, we built a geological model on an ultra-fine grid of a field area in Alaska. A rock physics and solid substitution workflow was used to calculate seismic rock properties in the ice-bonded sediments of the permafrost. The process of building the model has given us insight into the geological peculiarities of the permafrost layer, including how pore-filling ice impacts seismic rock properties and what impact solid-ice geomorphologies such as polygonal wedges might have on seismic scattering. We simulated synthetic elastic and acoustic seismic data to study wave propagation through the high-velocity near-surface, captured by vertical and horizontal component surface recordings at fine spacing. The model and synthetic datasets have had a significant influence on our understanding of seismic acquisition and imaging requirements in permafrost zones, which will impact how we design and process future datasets in the area.
Presentation Date: Thursday, September 28, 2017
Start Time: 9:45 AM
Presentation Type: ORAL
I propose a new 3D scalar TTI wave propagation method: TTI Fourier Finite Differences (FFD), a chain operator of FFTs and FDs. It can achieve pseudo-analytical accuracy with about 3 times the speed of conventional pseudo-spectral method for TTI media. It requires only 2 points per wavelength sampling in 3D space and is stable in complicated TTI models with similar conditions as the pseudo-analytical method. Furthermore, as a scalar propagator, FFD can naturally avoid the coupling of qP-waves and qSV-waves in TTI media.
Presentation Date: Monday, September 25, 2017
Start Time: 2:15 PM
Location: Exhibit Hall C, E-P Station 4
Presentation Type: EPOSTER
Accurate modeling of seismic wave propagation is a crucial component of many seismic processing algorithms. To this end, Etgen and Brandsberg-Dahl (2009) developed the pseudoanalytical method, a dispersion-free numerical algorithm for wave propagation. This method was presented as a modified pseudospectral method where the Fourier transform of the Laplacian is weighted to create a pseudo-Laplacian operator that compensates exactly for the error from explicit, second-order, finite-difference extrapolation in time. In this work we introduce a new method that reaps the computational benefits of short convolutional operators, while maintaining the dispersion-free solution of the pseudoanalytical method. This is accomplished by introducing a new weighting factor applied in the wavenumber domain that is based on the Fourier transform of a convolutional spatial Laplacian. We show that this new method is computationally more efficient than the pseudoanalytical method. Because this method demonstrates pseudoanalytical (PA) accuracy and implements convolutional derivatives in time (T) and space (X), we call our method the pseudoanalytical space-time (PA-TX) method. The PA-TX method is developed to solve the variable-velocity, variable-density, isotropic, acoustic wave equation. However, it may be extended to solve anisotropic problems as well.
Presentation Date: Wednesday, October 19, 2016
Start Time: 4:00:00 PM
Presentation Type: ORAL
We describe two different approaches to estimate angledomain illumination: model-based and data-driven. The estimated illumination fields are then applied to image angle gathers as weights to correct seismic images for poor and uneven illumination. By comparing the resulting images we conclude that the best illumination-compensated subsalt image produced in our tests is produced through a combination of the model-based and data-driven illuminations. The new technique can increase S/N and provide better image amplitude fidelity. Our study indicates that the conventional model-based approach may not always predict the illumination with sufficient accuracy due to the imperfect velocity model of the subsurface used for the prediction. A modified approach, such as the one proposed here, should thus be adopted to mitigate the model inaccuracies while correcting seismic images for the illumination effects.
Some degradation of the image in subsalt areas is always to be expected, even with the application of advanced modern imaging algorithms such as reverse-time migration, because, realistically, imaging cannot completely remove all the propagation effects due to a complex overburden. Illumination imprints in images, such as shadow zones, have always been a challenging problem especially in subsalt environments. Illumination analysis is thus often used as an aid in subsalt exploration to guide survey design and to potentially correct distorted images for the effects of uneven illumination. Various methods to correct for illumination-induced image distortions have been previously discussed (Schneider and Winbow, 1999; Muerdter and Ratcliff, 2001; Askim et al, 2010). Gherasim et al (2010) designed a one-way wave-equation based (Etgen, 2008) demigration-remigration workflow to generate illumination weights, which can be applied to migrated angle gathers to improve structural imaging in subsalt areas. Although one-way methods are efficient, they are dip-limited. Li et al (2012) proposed to use RTM 3D dip gathers of point diffractors for subsalt illumination analysis. Gherasim et al (2012) developed a two-way waveequation based “modeling-migration” workflow, during which they built a reflecting velocity model by combining the background velocity with reflectors following interpreted events across the field. The illumination weights generated from this workflow can be applied to angle-gathers to improve subsalt stacked images. Consequently, the resulting images can produce better well ties and provide a basis for more accurate amplitude analysis. Sufficient accuracy of the reflectivity model is, however, critical.
Zhang, Rui (Lawrence Berkeley National Laboratory) | Song, Xiaolei (BP) | Fomel, Sergey (The University of Texas at Austin) | Sen, Mrinal K. (The University of Texas at Austin) | Srinivasan, Sanjay (The University of Texas at Austin)
Time-lapse seismic surveys have been implemented for monitoring CO2 sequestration at Cranfield. Some of the observed misalignments between the time-lapse datasets could have resulted from seismic acquisition or processing errors. To address this, we applied a registration method to correct misalignment between time-lapse post-stack seismic datasets. Such misalignments also exist in time-lapse pre-stack datasets, leading to the application of the same registration method on the time-lapse pre-stack datasets. The application of registration on the pre-stack datasets demonstrates its effectiveness on successfully separating time shift from time-lapse pre-stack AVA datasets. The improved consistency in the data allows us to invert time-lapse pre-stack datasets to capture time-lapse elastic property changes, which can be reliably used for monitoring of CO2 plume.
We introduce a novel finite-difference approach for seismic wave extrapolation in time. Compared with those of conventional finite-difference (FD) methods, the coefficients of the new method are variables in space. We derive the finite-difference operator from a lowrank approximation of the space-wavenumber, wave-propagator matrix. We present a mathematical derivation of this method. Numerical examples confirm the validity of the proposed technique. The lowrank FD method can be applied to enhancing accuracy and stability in seismic imaging by reverse-time migration.