基于波场分离的弹性波逆时偏移

doi:10.6038/cjg20170726

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摘要尽管叠前逆时偏移成像精度高，但仅针对单一纵波的成像也可能形成地下介质成像盲区，由于基于弹性波方程的逆时偏移成像可形成多波模式的成像数据，因此弹性波逆时偏移成像可提供更为丰富的地下构造信息.本文依据各向同性介质的一阶速度-应力方程组构建震源和检波点矢量波场，再利用Helmholtz分解提取纯纵波和纯横波波场，使用震源归一化的互相关成像条件获得纯波成像，避免了直接使用坐标分量成像而引起的纵横波串扰问题.针对转换波成像的极性反转问题，文中提出一种共炮域极性校正方法.为有效节约存储成本，也提出一种适用于弹性波逆时偏移的震源波场逆时重建方法，在震源波场正传过程中，仅保存PML边界内若干层的速度分量波场，进而逆时重建出所有分量的震源波场.本文分别对地堑模型和Marmousi2模型进行了弹性波逆时偏移成像测试，结果表明：所提出的共炮域极性校正方法正确有效，基于波场分离的弹性波逆时偏移成像的纯波数据能够对复杂地下构造准确成像.

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	王维红
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关键词 ：成像, 弹性波逆时偏移, 矢量波场, 极性校正

Abstract：Compared with other imaging algorithms (e.g., ray-based and one-way wave equation), reverse time migration (RTM) based on two-way wave equation exhibits great superiority, especially in dealing with steeply dipping structures. However, imaging with conventional single-component seismic data may become imperfect in some complicated structures (e.g., gas clouds). The elastic reverse time migration based on the elastodynamic equation using multi-component seismic data can extract PP and PS reflectivity containing subsurface information, thus it can be more consistent with characteristics of elastic wave propagation in the real earth's medium, and resulting seismic images can more accurately characterize the subsurface. To begin with, we employ the first-order velocity-stress equations to implement extrapolation of elastic vector wavefields, and separate P-and S-wavefields by computing the divergence and curl operators of the extrapolated velocity vector wavefields. Then, imaging data with pure wave modes can be computed by applying the source normalized cross-correlation imaging condition, thus avoiding the crosstalk between the unseparated wave modes. To address the polarity reversal problem of converted images, we propose an alternative method in the shot domain. We also develop an efficient method that reconstructs source wavefields in the reverse time direction to save storage in CPU and avoid large input/output for elastic reverse time migration. During the forward modeling, the method only saves velocity vector wavefields of all time interval within an efficient absorbing boundary and total wavefields in the last time interval. When we extrapolate the receiver wavefields in reverse time direction, simultaneously, we reconstruct the total source wavefields by the saved wavefields. Numerical examples with a graben and Marmousi2 models show that the polarity reversal correction method works well and that elastic reverse time migration can generate accurate images for complicated structures.

Key words： Imaging Elastic reverse time migration Vector wavefield Polarity correction.

收稿日期: 2016-11-11

PACS:

P631

基金资助:国家自然科学基金项目（41574117，41474118）、黑龙江省杰出青年科学基金项目（JC2016006）、以及大连理工大学海岸和近海工程国家重点实验室开放基金（LP1509）联合资助.

通讯作者: 石颖,女,1976年生,教授,博士生导师,主要从事地震资料处理方面的研究.E-mail:shiyingdqpi@163.com E-mail: shiyingdqpi@163.com

作者简介: 王维红,男,1975年生,博士,教授,主要从事地震资料数字处理方面的研究.E-mail:wwhsy@sina.com

链接本文:

http://www.geophy.cn/CN/10.6038/cjg20170726 或 http://www.geophy.cn/CN/Y2017/V60/I7/2813

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