Summary An accurate initial model is crucial for full waveform inversion (FWI). In this paper, we propose a new way to build an initial model for conventional FWI by developing a new FWI algorithm based on a new time-shift nonlinear operator. We apply the new time-shift nonlinear operator to the waveform, define a new misfit function and derive the corresponding gradient operator. Numerical results using synthetic data from the Overthrust model demonstrate that compared with traditional FWI, the new algorithm is less sensitive to the traveltimes error. Using the inverted model of the new algorithm as the initial model, conventional FWI can obtain much better results.
The seismic modulation model analyzes a seismogram from the low and high-frequency information, which is different from the traditional convolution model. In the modulation model, a seismogram is regarded as a modulated signal and its envelope is the amplitude-modulation component containing the low-frequency information. On this foundation, the envelope of seismograms can be used to recover a very smooth background structure. However, amplitude demodulation methods can only obtain the absolute value of the envelope, which cannot reflect the polarity changes of the amplitude information in seismograms. To solve this problem, we consider the low-frequency modulation from both the amplitude and polarity points of view. We extract a modulator signal with smoothed apparent polarity, which contains the amplitude and polarity information in seismograms. The new approach can broaden the modulation model theory for seismic signals. Good results from examples for application to envelope inversion demonstrate the good performance and potential of the proposed method.
In this paper, we apply the one‐way dual‐domain wave propagators for the transversely isotropic media with a vertical symmetry axis (VTI) (Han and Wu, 2003a; Han and Wu, 2003b) to the imaging problem in such media. The one‐way propagator is used to perform both 2‐D poststack and prestack depth migration with a synthetic qP‐wave field in complex VTI media. The propagator is derived from an approximate dispersion relation of a scalar qP‐wave equation. It propagates in a homogeneous VTI background medium in the wavenumber domain and calculates the effect of velocity and anisotropy perturbations in the spatial‐domain. This research includes testing the ability of the propagator for imaging a qP‐wave field in media with strong anisotropy and complex structures, and the capability of the propagator for accurately propagating large angle waves. The effect of anisotropy on image quality is examined. The SEG/EAEG Salt model is modified from isotropic media to VTI media, while keeping its original velocity distribution and complex structures. Good quality images from both poststack imaging and prestack imaging support its validity and accuracy for imaging complex media with strong perturbations and strong anisotropy. When imaging the same anisotropic data, isotropic algorithms produce poor quality images with background artifacts, improperly focused energy and mispositioned reflectors, and may introduce false reflectors.
Strong-scattering inversion or the inverse problem for strong scattering has different physical-mathematical foundations from the weak-scattering case. In this paper I try to illustrate the connection between the Schrodinger inverse scattering (inverse problem for Schrodinger equation) by GLM (Gel'fand-Levitan-Marchenko) theory and the direct envelope inversion (DEI) using reflection data. The difference between wave equation and Schrodinger equation is that the latter has a potential independent of frequency while the former has a frequency-square dependency in the potential. I also point out that the traditional GLM equation for potential inversion can only recover the high-wavenumber components of impedance profile. I propose to use the Schrodinger impedance equation for direct impedance inversion and introduce a new parameter operator for the reconstruction of long-wavelength velocity structure which is closely related to the multi-scale decomposition and inversion in direct envelope inversion. Some examples of salt structure inversion are given to demonstrate the analysis. Presentation Date: Wednesday, October 17, 2018 Start Time: 8:30:00 AM Location: 207C (Anaheim Convention Center) Presentation Type: Oral
Abstract One-way approximations for regional phases slice the half-space crustal wave guide into a number of slabs perpendicular to the propagation direction; the Moho discontinuity can be easily treated as a perturbation from the crustal background. The advantage of one-way propagation methods is their great saving of computing time and memory compared to full-waveform numerical methods, especially for 3D simulations. In this article, we present general 2.5D SH formulas for the thin-slab method and a generalized screen propagator (GSP) for simulating SH wave propagation in the heterogeneous half-space crustal wave guide. Comparisons of numerical results with a finite-difference (FD) algorithm for a standard flat crustal model show excellent agreement, demonstrating the validity and accuracy of the 2.5DGSP for the half-space problem. We then compare the curves of energy attenuation calculated by the 2D and 2.5D formulas. The comparison shows that the energy attenuation of the 2.5D case is about 1.5–3 times greater than that of the 2D case when the trapped mode is formed. The SH screen propagator is also used to simulate two laterally unvarying (in the y direction) crustal models, showing the path effects on regional wave propagation.
Pre-stack reverse time migration (RTM) based on the two-way wave equation has been proved to be the most accurate seismic migration method theoretically.However, it requires reverse-order access to the wavefield calculated in forward time.In recursion computing, such out-of-order access requires that most of the recursion history should be stored on the hard disk.For massive amounts of seismic data, loading the saved wavefield data from the disk during imaging has been the bottleneck of RTM, restricting its wide application.To solve this problem, the wavefield in forward time must be reconstructed in reverse order.Although the random boundary can avoid the disk requirement by creating random velocity around the computational domain when propagate the source function.However, the random wavefield reflected from the boundary can generate unwanted artifacts in the final images.In this paper, we develop an attenuated and reversible random boundary condition which is implemented by mixing the reversible attenuation and random boundary conditions.Similar to the random boundary scheme, the proposed method just needs to save the last one or two wavefield snapshots into the memory in forward process.It then reconstructs the source wavefield in reverse order, while greatly reduces the disk input and output (I/O) requirements.Taking the attenuated property into consideration, the artificial events reflected from the boundary can be eliminated.Thus, our method can improve the imaging quality largely compared with the random boundary scheme.Numerical results demonstrate that the RTM images with our proposed attenuated and reversible random boundary condition can not only eliminate the unwanted artifacts, but also improve the computational efficiency greatly.
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