基于优化交错网格有限差分法的VSP逆时偏移

doi:10.11720/wtyht.2020.1426

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Abstract

Compared with conventional surface seismic data,VSP seismic data have many advantages,such as abundant wavefield information,high resolution and signal-to-noise ratio information.Reverse time migration (RTM) method based on two-way wave equation is considered to be the most accurate imaging method for seismic data at present.The combination of the VSP data and RTM method is helpful to describing the structures beside wells and identifying the complex geological structures accurately.Based on the two-dimensional (2D) variable density acoustic wave equation,the authors studied the high-precision RTM method of VSP data using the optimal staggered-grid finite-difference method.For different aspects of this VSP RTM method,different measures were adopted.First,the authors used the optimal staggered-grid finite-difference method to realize high-precision wavefield extrapolation.Second,the authors used the PML absorbing boundary condition to suppress boundary reflections caused by the limited computing space of model.Third,the authors used the effective boundary storage strategy to reduce the storage requirements of source wavefields.Fourth,the authors used the normalized cross-correlation imaging condition of sources to handle RTM imaging of VSP data.Finally,the high-order Laplacian filtering method was used to suppress the low-frequency noises of RTM imaging results.The different model test results show that the VSP RTM method proposed in this paper can achieve high-precision RTM imaging for VSP data.Compared with the conventional RTM method of surface seismic data,the RTM method of VSP data can more accurately identify the underground complex geological structures, such as the high-steep structures and the structures with sharp velocity changes,which verifies the effectiveness of the proposed method.

Key words： VSP optimal staggered-grid finite-difference reverse time migration storage strategy noise suppression

Received: 13 December 2019 Published: 29 December 2020

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	Wei LIU
	Yan-Chun WANG
	Chen-Chen BI
	Zhong-Bo XU

Cite this article:

Wei LIU,Yan-Chun WANG,Chen-Chen BI, et al. Reverse time migration of VSP data based on the optimal staggered-grid finite-difference method[J]. Geophysical and Geochemical Exploration, 2020, 44(6): 1368-1380.

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https://www.wutanyuhuatan.com/EN/10.11720/wtyht.2020.1426 OR https://www.wutanyuhuatan.com/EN/Y2020/V44/I6/1368

Variations of dispersion error with wavenumber and different operator lengths by using different staggered-grid finite-difference methods
a—conventional staggered-grid finite-difference;b—optimal staggered-grid finite-difference

Variations of dispersion error with different propagation angles by using different staggered-grid finite-difference methods (M=5)
a—conventional staggered-grid finite-difference;b—optimal staggered-grid finite-difference

Snapshots by using different staggered-grid finite-difference methods and different operator lengths
a—conventional staggered-grid finite-difference for M=5;b—optimal staggered-grid finite-difference for M=5;c—conventional staggered-grid finite-difference for M=10;d—optimal staggered-grid finite-difference for M=10;from left to right time at 1s and 2.5s

Curves of stability factor of different staggered-grid finite-difference methods

Simple sketch of PML absorbing boundary condition

Simple sketch of effective boundary storage strategy

A multilayer model

Snapshots at different time for the multilayer model
a—forward wavefields;b—reconstructed wavefields;c—difference between the forward and reconstructed wavefields;from left to right time at 0.4,0.8,1.2 s

2D SEG/EAGE salt model

RTM results of the 57^th VSP gather for the 2D SEG/EAGE salt model
a—cross correlation imaging condition;b—normalized cross correlation imaging condition of sources

RTM results of conventional surface seismic data (left) and VSP data (right) for the 2D SEG/EAGE salt model
a—surface data before low frequency noise suppression;b—VSP data before low frequency noise suppression;c—surface data after low frequency noise suppression;d—VSP data after low frequency noise suppression

Amplitude spectrum and phase spectrum of VSP RTM results for the 2D SEG/EAGE salt model
a—amplitude spectrum before noise suppression;b—amplitude spectrum after noise suppression;c—phase spectrum before noise suppression;d—phase spectrum after noise suppression

Marmousi model

RTM results of the 59^th VSP gather for the Marmousi model
a—cross correlation imaging condition;b—normalized cross correlation imaging condition of sources

RTM results of conventional surface seismic data and VSP data for the Marmousi model
a—surface data before low frequency noise suppression;b—VSP data before low frequency noise suppression;c—surface data after low frequency noise suppression;d—VSP data after low frequency noise suppression

The smoothed Marmousi model and its corresponding RTM results
a—velocity;b—density;c—RTM results of conventional surface seismic data;d—RTM results of VSP data

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