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1

Elastic full waveform inversion based on equivalent staggered grid FDM and inter-parameter trade-off mitigation

Li Qingyang, Wu Guochen

Acta Geophysica

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2022

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Vol. 70, no 6

2555--2579

EN

Elastic full waveform inversion (EFWI) has increasingly been applied in seismic exploration as computer performance improves. EFWI significantly improves calculation efficiency, but requires very large computer storage space and suffers interparameter trade-off and local minima problems. Preconditioning the gradients based on elastic wave mode decomposition can effectively mitigate inter-parameter trade-offs, but the decomposition-based scheme may further increase the memory usage, which limits EFWI application. The equivalent staggered grid (ESG) scheme in acoustic medium requires less memory usage and generates results numerically equivalent to those using the standard staggered grid (SSG) scheme. In this paper, we extend the ESG scheme to second-order elastic wave equations in terms of velocity, producing results numerically equivalent to the SSG ones based on first-order velocity–stress wave equations while reducing memory usage by 45% compared with the SSG scheme. We then apply the ESG scheme to EFWI and derive the formula of the preconditioned gradient of the S-wave velocity. Finally, three numerical examples demonstrate that applying the ESG scheme to decomposition-based EFWI can significantly reduce computer memory usage and mitigate the trade-offs between the P- and S-wave velocities.

2

An optimal 125-point scheme for 3D frequency-domain scalar wave equation

Yang Lingyun, Wu Guochen, Wang Yunliang, Li Qingyang, Zhang Hao

Acta Geophysica

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2022

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Vol. 70, no 1

71--88

EN

To improve accuracy and efficiency of forward modeling in the frequency domain, a 125-point finite-difference scheme is proposed. At present, the optimized difference format based on the rotating coordinate system is widely used, but it only suitable for equally sampling interval, and the optimized difference format based on the average-derivative method can be applied to different spaced sampling while improving the sampling accuracy. In this paper, we firstly introduce a 125-point optimized scheme for the three dimensional scalar wave equation. Then, according to the optimized difference scheme, the 125-point optimized difference coefficient is calculated for different spatial sampling spacing ratios. Compared with the optimal 27-point scheme, grid points number reduces from 4 points to 2.5 per wavelength, higher efficiency and suitable for unequal directional sampling intervals. In addition, the higher accuracy of 125-point scheme means it requires more storage and computation cost. Numerical results show that the optimized 125-point difference format has higher accuracy than the classical 27-point difference format.

3

2D multi parameter waveform inversion of land refection seismic data obtained from the particle motion response from the vertical geophone

Li Qingyang, Wu Guochen

Acta Geophysica

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2020

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Vol. 68, no. 2

377--388

EN

Onshore seismic exploration analyzes seismic wave propagation in elastic media, which includes the conversion between P- and S-waves. The development of multi-wave and multi-component seismic exploration methods provides data that enable onshore elastic wave full-waveform inversion. However, most data sets of onshore exploration are single component obtained from the particle-motion response from the vertical geophone. When the aiming area has a low-velocity zone, the ray path of refected wave that propagates to the detector is nearly perpendicular to the ground surface, so that we call it P-wave data. In this paper, we focus on multi-parameter waveform inversion using P-wave refection seismic data. Although only P-wave data are received, it still contains the converted P-wave information, and the converted P-wave energy gradually increases as the ofset increases. As seismic acquisition technology, observation systems and science develop, the folds and acquisition ofset increase signifcantly, and the seismic data contain important converted P-wave information. In this paper, the frst-order elastic velocity–stress equation is decomposed to obtain the scalar-P-wave equation from which the S-wave velocity is included frstly. Then we present the theoretical framework for onshore multi-parameter full-waveform inversion using P-wave data. In order to explore the inversion potential of the P-wave data (extracting the S-wave velocity from the converted P-wave information) and accuracy and stability of the P- and S-wave velocity inverted by our method, we carry out numerical tests via diferent inversion strategies, by using the P-wave data regarded as containing converted P-wave information, and get successful results.