地震各向异性——多组裂隙对横波偏振的影响

Frequency-dependent anisotropy: Effects of multiple fracture sets on shear-wave polarisations

通过对多分量地震资料的分析,我们发现随着频率的增加横波分裂时差减小.对于深部接收的VSP数据来说快横波的偏振方向保持不变,而对于浅层接收的VSP数据来说偏振方向却存在一个最大可以达到20°的旋转.尽管多尺度随机分布微裂隙岩石物理模型已经成功地模拟并解释了横波分裂时差随频率变化的现象,却不能解释与频率相关的横波分裂.据推测,如果微裂隙的排列方向和大裂隙的排列方向不同,利用低频信息获得的偏振方向将指示裂隙主方向,而利用高频信息获得的偏振方向则指示微裂隙方向.在背景多孔隙介质中存在多组裂隙的情况下,推导出垂直入射条件下横波偏振方向的解析式,给出了系统研究横波在介质中传播的方法.研究结果表明,横波偏振方向会随着频率的变化而变化,并且在入射方位、角度一定的条件下,是裂隙方位和密度的函数,这些认识可能有助于揭示观测到的、依赖频率变化的横波偏振现象.

From the analysis of a multi-component VSP for frequency-dependent anisotropy, we find that shear-wave anisotropy as inferred from measured time delays between splitting shear-waves decreases systematically as frequency increases. Whilst in general the polarisations of the fast split shear-waves remain constant for deep receivers, we notice systematic rotation （up to 20°） of polarisations with frequency for shallow receivers. The variation of time-delays with frequency has been successfully interpreted and modelled using a multi-scale rock physics model incorporating one aligned fracture set in a porous medium with randomly distributed micro-cracks. However this model fails to explain the frequency-dependent shear-wave polarisations. It is speculated that if aligned micro-cracks and aligned fractures are in different directions （i. e. the conjugate angle is not zero）, low frequency would indicate the direction of fractures and high frequency would give the orientation of microcracks. This speculation, however, has not been satisfactorily proven. In this study, we extend the previous model to allow multiple fracture sets inserted into background porous media. We then derive analytic expressions of polarisation of shear-waves at normal incidence, and present a systematic study of shear-wave propagation in media. Our study demonstrates that the shear-wave polarisations will vary with frequency and the variation depends on the azimuths and angle of incidence relative to the orientations and crack densities of individual sets, and thus may help to explain the observed dependence of shear-wave polarisation on frequency.