煤层厚度变化时地震槽波理论频散曲线计算方法及频散特征分析

Calculation method for theoretical dispersion curves of seismic channel waves considering variation of coal-seam thickness and analysis of dispersion characteristics

槽波地震勘探利用槽波的频散特性反演煤层的结构特征,故理论频散曲线的计算是一个重要方面.使用水平层状模型假设下的面波频散曲线计算方法能够计算煤层厚度恒定模型地震槽波频散曲线;但当煤层厚度变化时该方法不再适用.基于前人水平层状均匀介质模型的面波理论频散曲线计算方法,对于含煤三层模型,本文发展了煤层厚度变化情况下的地震槽波理论频散曲线计算方法,并使用该方法计算分析了不同厚度函数模型的频散曲线形态特征.研究表明:与稳定厚度煤层相比,煤层厚度变化使得地震槽波群速度成为与频率及传播射线在水平面投影路径相关的二元函数;射线路径上煤层厚度的变化使得频散曲线在群速度方向上压缩,群速度变化范围变小,且使处于最小值位置的埃里相群速度增大;而煤层厚度的线性变化模型频散曲线只与射线首、尾处的煤层厚度有关,与煤层厚度恒定模型相比,曲线形态不发生改变;煤层厚度呈非线性变化时,频散曲线形态上可能发生改变.

In channel-wave seismic exploration,dispersion features are used to invert structures of coal seams.So calculation of theoretical dispersion curves is an important subject.We can calculate dispersion curves when the coal-thickness keeps unchanged by utilizing the calculation method for surface wave dispersion of a horizontal stratified model.But this method is inapplicable when the coal-seam thickness changes.For a coal-bearing three-layer model,this work proposes a calculation method for theoretical dispersion curves when the coal-seam thickness fluctuates,which is based on the calculation method for dispersion curves of horizontal stratified models.Wealso analyze the characteristics of dispersion curves for different models of varied coal-seam thickness.Our research shows that unlike unchangeable coal-thickness model,group velocity of a changeable coal-thickness model becomes a function related to frequency and projective ray paths in horizontal plane.The change of coal-seam thickness along the ray route leads to squeezing effect of dispersion curves in velocity′s direction,narrowing the range of group velocity and increasing the minimal group velocity at airy phase.The dispersion curve of the linearly changing coal-seam thickness model relates only to the thickness at the start and end of ray paths,while the curve shape does not change compared with the unchangeable coal-seam thickness model.The dispersion curve shape of a nonlinearly varied coal-seam thickness model may case change in the shape of dispersion curves with respect to the model with constant coal-seam thickness.