隧道反射波超前探测有限差分正演模拟与偏移处理

Detection simulation ahead of tunnel face and reverse-time migration with reflection wave method

针对地下工程领域隧道不良地质灾害超前探测问题,构建断层、倾斜软弱夹层等地质灾害体的隧道介质模型,研究隧道反射波超前探测的波场传播机理与偏移成像问题。利用一阶速度-应力弹性波方程和高阶交错网格差分计算方法,导出隧道工程反射波探测数值模拟的差分计算格式、数值稳定性条件以及边界条件,对上述模型进行正演模拟,得到相应模型不同时刻的波场快照与地震合成记录;通过对这些模拟结果进行分析,验证高阶交错网格差分法在隧道波场正演中的有效性。在导出弹性波方程逆时延拓交错网格差分计算格式基础上,通过求解程函数方程,获取激发时间成像条件,进而得到隧道全波场逆时偏移算法及偏移处理程序,并应用于隧道波场偏移成像中。研究结果表明:采用高阶交错网格有限差分逆时偏移算法能使反射波归位,绕射波收敛,从而大大提高了隧道工程反射波探测的分辨率,为隧道超前探测资料的解释提供理论依据。

The propagation and imaging problem of detection ahead of mining tunnel with the reflection wave method in underground engineering were discussed. Based on the fact that the detection and forecasting for badness geological structures ahead of the tunneling are difficult, two tunneling conceptual geological models with such geological hazards in tunnels as fault, tilted weak interlayer in front of tunnel face were constructed. After high-order finite-difference format of wave equation, the free boundary conditions and artificial boundary conditions were deduced based on one-order velocity-stress elastic wave equation, the detection simulation ahead of tunnel face for these two models was implemented. Then, the wave field snapshots and forward records were acquired and discussed, and the validity of high-order finite-difference forward simulation method in tunnel wave field was illuminated. Staggered-grid finite-difference scheme for inverse time calculation was derived from Eikonal equation, the seismic travel time at each underground point in these two models was computed by upwind-finite-difference format, which was the excitation-time imaging condition in reverse-time migration. Then the reverse-time migration algorithm of tunnel wave field and migration processing program were gotten, and synthetic data of those models were applied. The wave-field characters and migrated sections were analyzed with different models using reflection wave methods. The results show that this migration algorithm can make the reflection wave return to the original position and diffraction wave converge, which offers the logical proof for reflection wave method for detection ahead of tunnel.