Summary Reverse-time migration (RTM) of seismic data at irregular surfaces is an effective way to avoid the impacts of the standard static correction errors on seismic migration. To simulate realistic seismic wave propagation under surface topography, one needs to consider the irregular free surface with specific boundary conditions. However, an absorbing boundary condition (ABC) should be applied in RTM to avoid the interferences caused by the free-surface related multiples. The ABC applicable to a horizontal surface cannot be used for irregular surfaces directly. We develop an efficient immersed absorbing boundary condition (IABC) for finite-difference (FD) modelling of seismic waves under topography. This ABC is based on the plane-wave transmitting boundary condition and it is modified to simulate wavefield absorption along the normal direction of the irregular surface. Unlike the curvilinear mesh method, IABC is implemented at the rectangular mesh directly. The only introduced computational cost by IABC is wavefield interpolation at a few fractional grid nodes, which makes it highly efficient. Based on IABC, we further develop a RTM method to migrate seismic data acquired at irregular surfaces. Numerical examples verify the effectiveness of IABC and IABC-based RTM method.
Fold-and-thrust belts are hot topics in the research of orogens. However, the single geological or geophysical discipline often provides multisolutions on the geometry of a fold-and-thrust belt. We introduce here a multidisciplinary method combining geological and geophysical methods, take the Hutubi River section as a case to study the geometry of the fold-and-thrust belt in the northern piedmont of Tianshan. Firstly, we have carefully taken geological survey and got structural data on the surface. Secondly, petroleum seismic profile was re-interpreted based on the surface data and drilled wells. However, the seismic profile is absent in the contact zone between the mountain and the Junggar basin. We therefore carried out gravity measurements and forward modeling along this profile with the densities of the basement and sediments. The result shows that the northern flank fault of Tianshan doesn't exist along the Hutubi River profile, and the basin sediments could be continually followed from the basin to the mountain interior. This indicates that the tectonic structures in the contact zones between the mountain and the basin are variable, as the western section in the Jingou River section shows that the Tianshan basement thrusts northward on the basin sediments. Based on the balance-section technology, the restored section shows 4. 8 km of shortening. Comparing with previous results, this indicates the heterogeneous deformation along the northern piedmont of Tianshan. It also implies that this multidisciplinary method could be widely used in the fold-and-thrust belt.
The modern Tian Shan (Central Asia) is considered as a direct consequence of the reactivation of a Paleozoic orogenic belt due to the India – Asia collision, during Cenozoic times. A detailed work has been investigated along the northern piedmont of Tian Shan, integrating the field work, structural analysis, seismic profile data, gravity anomaly measurements and drilled wells. Firstly, this study brings new and direct evidences, at different scales, for the existence of a major paleo-relief along the northern Tian Shan range during Mesozoic, and particularly during Jurassic times. Secondly, the calculated shortening amount in the northern piedmont of Tian Shan is rather small and the structural pattern of its front is heterogeneous along-strike. While, thrusting of the Paleozoic basement on the Mesozoic or Cenozoic sedimentary series of the basin is remarkably exposed along several river valleys, other sections display continuous Triassic to Jurassic sedimentary series from the basin to the range where they unconformably overlie on the Carboniferous basement. Four cross-sections are made by integrating multi-method data, showing that shortening amounts are less than 20% and could be even until to 6%. This suggests that the Tian Shan intracontinental range is rather “young” and still at a primary stage of its tectonic evolution. In other words, its front may be considered as a typical example of an immature range front.
Time domain elastic least-squares reverse time migration (LSRTM) is formulated as a linearized elastic full waveform inversion (FWI) problem. The elastic Born approximation and elastic reverse time migration (RTM) operators are derived from the time-domain continuous adjoint-state method. Our approach defines P- and S-wave impedance perturbations as elastic images. Our algorithm is obtained using continuous functional analysis where the problem is discretized at the final stage (optimize-before-discretize approach). The discretized numerical versions of the elastic Born operator and elastic RTM operator pass the dot product test. The conjugate gradient least squares (CGLS) method is used to solve the least-squares optimization problem. The diagonal of the pseudo-Hessian is used for preconditioning the elastic LSRTM. Elastic LSRTM provides higher resolution images with fewer artifacts and a superior balance of amplitudes when compared to elastic RTM. Moreover, elastic LSRTM can remove cross-talk between P-and S-wave impedance perturbations. Presentation Date: Monday, September 25, 2017 Start Time: 2:40 PM Location: 361A Presentation Type: ORAL
We propose a time-domain matrix-free elastic Gauss-Newton fullwaveform inversion (FWI) algorithm. At the core of our elastic Gauss- Newton FWI is an iterative elastic least-squares reverse time migration (LSRTM) problem. The proposed algorithm consists of two nested iteration loops: the outer Gauss-Newton nonlinear iterations and the inner conjugate gradient least-squares (CGLS) iterations. The Gauss- Newton search direction in each outer FWI iteration is computed using the CGLS method. This step is equivalent to applying elastic LSRTM on data residuals. We use the proposed algorithm to simultaneously invert for P- and S-wave velocities. The algorithm retrieves moderately improved models than the nonlinear conjugate gradient (NLCG) method. We observe that the elastic Gauss-Newton FWI converges faster than the elastic NLCG FWI. Presentation Date: Wednesday, October 17, 2018 Start Time: 1:50:00 PM Location: 207C (Anaheim Convention Center) Presentation Type: Oral
Baoshan is a world-class skarn Cu-polymetallic deposit located at the junction of the Nanling and Qin-Hang metallogenic belts in China. While there has been extensive research on the mineralogy and geochemistry of skarn deposits, studies on the fluid characteristics and evolutionary history from the early to late skarn stages in such deposits are still limited. In this study, we analyzed garnet and pyroxene from the early skarn stage and scheelite from the late skarn stage of the Baoshan deposit. We distinguished two generations of garnet (Grt1 and Grt2), one generation of pyroxene, and three generations of scheelite (Sch I, Sch II, and Sch III) on the basis of mineral assemblages and microscopic characteristics. Grt1 appears coarse-grained, and Grt2 cuts through Grt1 as veinlets. In Grt1, the andradite end-member increases from the core to the rim, while the grossular portion decreases (Ad35–36Gr59–61Sp3–4 to Ad59–61Gr36–37Sp2–3), and in Grt2, the andradite end-member significantly increases (Ad41–73Gr25–55Sp2–3). Grt1 and Grt2 have similar trace element compositions, with enrichment in Zr and depletion in Nb and Hf, depletion in LREE, enrichment in HREE, and weak negative Eu anomalies. Pyroxene coexists with Grt1 and is similarly cut by Grt2, with its composition mainly being diopside (Di82–99Hd0.6–15Jo0–3.2). Sch I and Sch II appear as anhedral to subhedral grains, while Sch III is predominantly found in veinlets. In Sch I and Sch II, most REEs enter the scheelite lattice via the Na-REE coupled substitution mechanism, with a smaller portion substituting Ca vacancies. In Sch III, the substitution mechanism involving Ca site vacancies may dominate. During the early skarn stage, the oxygen fugacity of the fluid gradually decreased from Grt1 and pyroxene to Grt2. In the late skarn stage, fluid oxygen fugacity remains stable from Sch I and Sch II to Sch III in shallow parts but significantly decreases in deeper parts. The garnet and pyroxene from the Baoshan deposit align with typical skarn Cu deposit compositions, while scheelite in the late skarn stage shows Sch I, Sch II, and shallow Sch III as skarn-type and deep Sch III as vein-type scheelite. Early skarn stage fluids were weakly acidic. Sch I, Sch II, and Sch III originated from fluids related to the Baoshan granite porphyry, with Sch III also showing evidence of water–rock interaction. This study reconstructed the fluid evolution history from the early to late skarn stages at the Baoshan deposit, providing insights into the ore-forming processes of other skarn deposits.
This paper analyzes the performance of several seismic data denoising algorithms, measured in the extent of noise suppression and the level of unintended signal distortion. It shows that the error in the filtered signal consists of residual noise, and signal distortion due to model-order mismatch and subspace corruption by noise. The contributing effects of these components to the overall denoising performance depend on algorithm parameters, input signal versus noise ratio (SNR), as well as the general signal and noise structure. The analysis focuses on singular spectrum analysis (SSA) and its robust version, and f-x deconvolution. However extension to other denoising algorithms is possible. While the main goal is to improve understanding of denoising performance and behavior, the analysis also hopes to provide a basis for adaptive adjustment of denoising parameters, paving the way for automated processing of large-scale field data where signal and noise structure and SNR level can vary significantly from window to window, or cube to cube. Presentation Date: Wednesday, October 19, 2016 Start Time: 8:50:00 AM Location: 148 Presentation Type: ORAL
Time-domain elastic least-squares reverse time migration (LSRTM) can provide higher spatial resolution images with fewer artefacts and a superior balance of amplitudes than elastic reverse time migration (RTM). More important, it can mitigate the crosstalk between P- and S-wave images. In previously proposed elastic LSRTM algorithms, density is either assumed to be constant or known. In other words, the density perturbation is not part of the least-squares inversion formulation. Neglecting density in elastic LSRTM may lead to crosstalk artefacts in the P- and S-wave images. In this paper, we propose a time-domain three-parameter elastic LSRTM algorithm to simultaneously invert for density, P- and S-wave velocity perturbation images. We derive the elastic Born approximation and elastic RTM operators using the continuous adjoint-state method. We carefully discretize the two operators to assure that they pass the dot-product test. This allows us to use the conjugate gradient least-squares method to solve the least-squares migration problem. We evaluate the proposed algorithm on two synthetic examples. We show that our proposed three-parameter elastic LSRTM can suppress the multiparameter crosstalk among density, P- and S-wave velocity perturbation images. Moreover, including density image in the elastic LSRTM inversion can improve the convergence of the least-squares inversion.
Internal solitary waves exist widely in the oceans, and their generations, propagation evolutions, and dissipations have profound effects on the ocean environment, topography, and marine structures. Typically, two basic theoretical models are now being developed to govern the evolutions of internal solitary waves at the interface of two immiscible inviscid fluids. One is a unidirectional wave propagation model described by the KdV (Korteweg-de Vries) equation, and the other is a bidirectional wave propagation model depicted by the Miyata-Choi-Camassa (MCC) equation. Neither of them, however, can effectively characterize the course of the evolution of large-amplitude internal solitary wave. In this paper, a modified unidirectional internal solitary wave model is established by adjusting the coefficients of the original unidirectional model. The adjusted coefficients are determined through asymptotic analysis by matching with the MCC model. The efficacy of the modified coefficients is investigated by comparing the modified model with the original model. The experiments on the generation of internal solitary waves with varying amplitudes are carried out by comparing the internal solitary wave solution of the modified equation. It is shown that the modified model is suitable for describing the waveform of internal solitary waves with small, medium, and large amplitudes within the limiting amplitude of the MCC model. By quantitatively analyzing the agreement of the effective wavelength, wave speed, and waveform of steady-state internal solitary waves between the unidirectional model and the MCC model, the applicability of the modified model in characterization of the properties of steady-state internal solitary waves is further investigated. In addition, the stability of unidirectional theoretical model is analyzed for simulating the propagation of large-amplitude internal solitary wave under flat bottom condition. It is found that the unidirectional model is suitable for initiating its own internal solitary solution provided that the numerical scheme is stable. It is shown that the modified unidirectional model can be used to characterize large-amplitude internal solitary waves, and is also expected to be applied to the study of marine structure hydrodynamics.
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