弹性波逆时偏移中的稳定激发振幅成像条件

本文针对弹性波逆时偏移,提出稳定的激发振幅成像条件.在震源波场的正向传播过程中,计算每个网格点的能量,并保存最大能量密度的时刻和相应的波场值;在检波器波场的逆时传播过程中,在每个网格点提取最大能量密度时刻的检波器波场值,并利用保存的最大能量震源波场做归一化,获得角度依赖的反射系数成像剖面.相比于归一化互相关成像条件,该成像条件在震源波场的正向传播过程中无需存储波场快照,节省大量磁盘空间和I/O吞吐任务,提高了计算效率;相比于弹性波的激发时间成像条件,该成像条件自动校正了水平分量在震源两侧的极性反转,在多炮叠加时避免振幅损失.数值试验表明,与归一化成像条件相比,稳定激发振幅成像条件具有更小的计算量,偏移剖面的低频假象更弱,水平分量的成像能力更优,具有更高的空间分辨率.

A stable excitation amplitude imaging condition for reverse time migration in elastic wave equation

A stable excitation amplitude (SEA) imaging condition for the elastic wave equation is proposed. In the propagation of the source wavefield extrapolation along a positive time axis, the energy density for total grid points are computed at each time-step, and the traveltime as well as wavefield values corresponding to the maximum energy density are saved as the excitation criterion. While in the propagation of the receiver wavefield extrapolation along a negative time axis, the excitation criterion is applied to obtain the imaging profiles at each grid point that satisfies the image time at each time step. Then the receiver wavefield is normalized by the source wavefield to form the angle-dependent reflection coefficient profiles. Compared to the normalized correlation (NC) imaging condition, the SEA imaging condition can eliminate the need of the hard disk, which saves a large volume of hard disk space (especially for three dimensions) and avoid a lot of I/O tasks. Consequently, the computational efficiency is enhanced significantly due to the application of SEA imaging condition. Compared to the excitation traveltime (ET) imaging condition in elastic media, the polarity reverse of horizontal component at opposite sides of the source will be corrected automatically to enhance the imaging when it involves in the stacking of many shot profiles, which is beneficial from the normalized amplitude (i.e. the angle-dependent reflection coefficients). Numerical tests validate the feasibility of the SEA imaging condition. When comparing the NC imaging condition with the SEA imaging condition in the numerical tests, the latter has a small computational amount than the former, and the latter can produce less low-frequency artifacts, and also has better imaging capability as well as higher spatial resolution than the former.