对隧道反射地震学的再认识

隧道反射地震法是利用放置在隧道边墙或者掌子面内的震源和接收器,来预测隧道前方的岩石特性。勘测人员应小心谨慎确保多分量的接收器与岩石牢固耦合,尽可能地避免记录到不需要的隧道波或空气波,同时,每一个震源点应尽可能产生几千赫兹的宽頻信号。这种方法有它的主要的约束条件。沿隧道轴线的空间分辨率受到信号频率和衰减影响的限制,而横向分辨率则主要受到小角度反射覆盖范围的制约。通过采用有限差分方法模拟弹性波的传播,计算人工合成的隧道地震反射记录。通过研究这些数据,得出如下一些处理隧道反射地震数据的规则:根据不同的质点运动轨迹和传播速度把压缩波和剪切波与隧道波、空气波和背景噪音分离开来。估计衰减模型、补偿振幅损失和反Q滤波都是必要的。通过振幅和保留相位的倾角滤波可以将反射的纵波和横波提取出来。空间可变的速度模型来源于对P、S波预先成像。建议用一个相容原理来导出速度模型,这就要求寻找一个合适的成像速度场,该速度场不受震源和接收器位置的约束而有相同的成像位置。空间叠前偏移是基于精确的通过不均匀速度模型的等时线和波的入射角。

Revisit Tunnel Reflection Seismology

Tunnel refection seismology employs sources and receivers that are mounted inside the tunnel wall or tunnel face and aims to predict rock properties ahead of the tunnel.Careful investigators make sure that multi-component receivers are stiffly coupled to the rock and avoid the reordering of undesired tunnel waves or air waves as much as possible and employ a source that provides a broadband signal of few kHz.The method has its principal limitations.Spatial resolution along the tunnel axis is limited by the signal frequency and attenuation and the lateral resolution mainly by the small angular illumination coverage.We perform finite-difference simulations of elastic wave propagation to compute a synthetic tunnel seismic reflection survey.We investigate this data and derive the following rules to process tunnel refection seismic data: Based on their different particle trajectories and propagation speeds and shear waves need to be separated from tunnel waves,air waves,and background noise.It is necessary to estimate attenuation models and to compensate amplitude loss as well as dispersion by an inverse Q-filtering.P and S-wave reflections can be extracted via amplitude and phase-preserving dip-filtering.Spatially variable velocity models need lo be derived for P-and S-waves prior imaging.We suggest a consistency principle to derive velocity models.It states that we should search for an imaging velocity field that yields the same image position independent of source or receiver position.The prestack migration into space should be based on accurate isochrones through the heterogeneous velocity model and the wave incidence angles.