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1

An efficient low-frequency artifact suppression method at excitation time for excitation amplitude imaging condition

Wang Rui, Wang Deli, Hu Bin, Zhang Weifeng, Zhou Jinju, Li Bowen

Acta Geophysica

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2023

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Vol. 71, no. 3

1225--1240

EN

The excitation amplitude imaging condition (EAIC) is a high-resolution, computationally efficient, and low-storage imaging condition in reverse time migration (RTM). However, when there are strong reflection interfaces in the velocity model, they will produce low-frequency artifacts, which seriously contaminate the RTM image. The artifacts can be removed by the wavefield decomposition algorithm, but this process always performed by analytic time wavefield extrapolation, which needs extra wavefield extrapolation. Furthermore, an extra source wavefield extrapolation is required to determine the excitation time before the migration. Thus, the additional wavefield extrapolations can seriously damage the computationally efficient advantage of the EAIC. By taking advantage of the directivity and low storage of excitation amplitude, we present a low-frequency artifact suppression method with no extra wavefield extrapolation. Poynting vector, reference traveltime and minimum amplitude threshold are combined to constraint the excitation amplitude updating process, and it makes the excitation amplitude more consistent with the definition of excitation criterion. We can directly obtain a noise-free excitation amplitude without the source wavefield decomposition. Instead of the analytic time wavefield extrapolation, the time-bin technique and the windowed Hilbert transform are combined to achieve the receiver wavefield decomposition only at the excitation time. The numerical results show that our method can effectively suppress the low-frequency artifacts in the image with no extra wavefield extrapolation.

2

A new acoustic traveltime approximation for attenuating transversely isotropic media

Xiao Han, Wang Deli

Acta Geophysica

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2021

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Vol. 69, no. 5

1611--1621

EN

Attenuation is one of the most important quantities in describing seismic wave propagation, which is also anisotropic because of the dispersion relationship between the seismic wave and the symmetry direction. Transverse isotropic media with titled symmetry-axis (TTI) is a widespread approximation of the Earth’s surface. For 2D TTI attenuating media, we frstly use the acoustic assumption to simplify the exact eikonal equation for the complex-valued quasi P-wave traveltime. Then we design a perturbation method to obtain the new approximation by solving the acoustic attenuating eikonal equation of TTI media and use Shanks transform to increase precision. Compared with former studies, the new approximation considers the symmetryaxis angles of the media as a factor, which will improve its robustness. The approximation is tested in several medium to demonstrate its efectiveness. The energy velocity which derived by the steepest-descent method is used to calculate the exact complex-valued traveltime. We test the accuracy of the approximations developed with and without Shanks transform in the following. Finally, we discussed the possibility to apply this approximation to the methods like fast marching methods.

3

Yang Fan, Wang Deli, Hu Bin, Zhu Hongyu, Sun Jing

Acta Geophysica

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2021

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Vol. 69, no. 5

1679--1696

EN

Considering the 3D propagation characteristics of seismic waves, theoretically, 3D surface-related multiples elimination (3D SRME) can suppress multiples with high accuracy. However, 3D SRME has strict requirements for acquisition geometry, which makes it difficult to be implemented in practice. In the process of 3D SRME, the multiple contribution gather (MCG) is a collection of wavefields with different propagation paths. The accuracy of the multiple propagation paths in the MCGs can be directly characterized by the inclination of the wavefields, which can achieve the weighted superposition of the wavefields. The direct summation of the sparse MCGs in the crossline direction produces serious spatial aliasing, which can easily cause the contamination of primaries. Based on the kinematic characteristics of multiple propagation, MCGs can be considered as a set of hyperbolas with temporal and spatial characteristics. Then, the direct summation of the sparse MCGs can be transformed into a process of superposition along the hyperbolic integration paths. However, as the stable phase points of the events, the apexes of the hyperbola have different spatial distributions in complex geological structures. Such hyperbolic stacking paths are difficult to be controlled by conventional Radon transform or constrained inversion. In this paper, we modify the apex-shifted hyperbolic Radon transform (ASHRT) to implement the summation of crossline MCGs with variable stable phase points along the hyperbolic integration paths. Improved ASHRT uses local similarity to locate the position of stable phase points, which can improve the stability of the algorithm and the efficiency of the computation. The proposed method is demonstrated on a 3D synthetic data set, as well as on a 3D marine data set, effectively avoiding the spatial aliasing caused by sparse crossline MCGs and improving the accuracy of multiple suppression.

4

Efcient mixed domain pure P wave equations for 2D and 3D tilted transversely isotropic media

Li Bowen, Wang Deli, Zhou Jinju, Zhang Qianxiang, Wang Rui

Acta Geophysica

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2020

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Vol. 68, no. 3

605--618

EN

In transversely isotropic (TI) media, accurate and efcient pure P-wave extrapolation is the basis of seismic imaging and inversion algorithms. For pure P-wave equations in TI media, combining the fnite-diference (FD) method with the pseudospectral (PS) method is an efective solution. However, if the mixed-domain pure P-wave equation contains multiple wavenumbers, using the hybrid approach will involve multiple Fourier transforms, which will result in high computational costs. Referring to the weak anisotropy approximation, we propose a new approximate P-wave phase velocity expression. We then use an optimization strategy to reduce the number of wavenumber terms in the corresponding dispersion relations and derive the mixed-domain pure P-wave equations in 2D and 3D tilted transversely isotropic (TTI) media. Through numerical experiments in 2D and 3D TTI models, we verify the feasibility and efciency of the proposed mixed-domain P-wave equations.

5

Multiples inversion imaging using a one way propagation operator

Li Qiang, Wang Deli

Acta Geophysica

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2019

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Vol. 67, no. 5

1341--1348

EN

The one-way propagation operator in the frequency-space domain has the advantages of fast calculation speed and good adaptability to medium with lateral velocity variation. The full wavefeld model constructed by the one-way propagation operator is iterative. As the number of iterations increases, the components of wavefeld are more and more abundant. In the full wavefeld model, the propagation and scattering processes are independent of each other. The former is determined by the propagation operator, while the latter is determined by the scattering operator. As each iteration increases, the wavefeld component will increase by one order. As an inverse migration operator, the full wavefeld model could feed back the imaging result to the data. By calculating the residual between the simulated data and the actual data, the refectivity is updated. This is an inversion process. In this process, multiples will be imaged. In this way, the subsurface information contained in multiples is utilized and the imaging quality is greatly improved. The L1-norm is used to constrain the imaging result, which further suppresses the artifacts and improves the imaging resolution. We have made some numerical examples in 2D case, explaining the principles and advantages of this methodology.