【作者】 王妙月； 秦福浩； 郭亚曦；

【Author】 Wang Miaoyue Qin Fuhao Guo Yaxi (Institute of Geophysics,Chinese Academy of Sciences)

【机构】 中国科学院地球物理研究所；

【摘要】 复杂地质结构中地震波的传播变得非常复杂,常规的地震处理方法难以得到地下复杂结构的真实图象。因此,首先弄清复杂结构中地震波的传播特征就变得非常重要,这样才能由观测得到的地震波记录去很好地认识地下复杂结构的图象,同时也可以用来检验各种反演方法。复杂结构中地震波的传播没有解析解,当界面陡倾时,射线理论也失效,因此必须采用数值解,有限元方法则是有可能解决这一问题的一个工具。首先我们用16×22个节点的若干小模型检验了水平层状介质中有限元程序的可行性。导出了无界空间中无限长集中力线源源函数为钟型函数时弹性波的理论P波初动,使和相似情况下16×22个节点的有限元结果相比较表明,结果基本是可靠的。对各个小模型的有限元结果的分析表明,所使用的原始程序是可行的,于是进一步设计了几个柱体组合模型,模拟诸如盐丘、礁体等地质体的弹性波效应。对于每个模型,计算了各个节点的弹性波位移、速度和加速度。分析了若干水平剖面和垂直剖面的波的特征。在这些剖面上,分析了边界波的影响以及和内部柱体结构有关的反射波、透射波和角点衍射波,为如何认识地表和垂直地震剖面的实际资料解释提供了线索。为了由地表观测的二分量或三分量记录得到的地下构造的可靠位置,发展了P、S波同时偏移的方法,使用前面的两个模型的有限元资料和其它模型资料,检验了P、S波同时偏移的原理和设计程序的可靠性。讨论了弹性波速度的横向不均匀性对结果的影响和解决办法,表明在解决复杂结构中P、S波同时偏移比传统上使用的P波偏移更有发展前景。更多还原

【Abstract】 No analytic solution can be found for seismic wave propagation in media with complex stratified structures.When interfaces between layers are steep,the ray theorem even ceases to be effective.Then,numerical solutions are required and the finite element method is expected to be an effective approach in this case. To begin with,several small models(16×22 nodal points) are used to test the efficiency of the finite element program in horizontal layer media.Theoretical first motion of P wave is derived in time domain for a line force source with an infinite length and a bell-shaped time function.The theoretical distribution of the first motion then is compared with the result obtained by the finite element method under the similar conditions.A good agreement can be observed between them.We conclude that the finite element program used is effective in solving elastic wave propagation problem. More complicated models are then formed,each of which includes 51×51 nodal points and consists of several prisms.They are used to simulate such complex stratified structures as salt domes,reefs,etc..The computer program has been modified to calculate elastic wave response of these models.Elastic wave displacement,displacement velocity and displacement acceleration are calculated throughout the nodal points of each model.Wave features of some vertical and horizontal profiles are discussed,e.g.boundary effects,features of reflected wave,transmission wave and corner diffraction wave related to the prism configuration of the models,so that helpful clues were found for better understanding of the surface profiles and better interpretation of vertical seismic records. In order to determine the positions of structures beneath the surface through two or three component records on the surface,we developed a technique of simultaneous migration of P and S waves.Based on the results of the finite element method applied to the above mentioned models,we have examined the reliability of the principle and the feasiability of the computer program of this technique.Effect of lateral variations of velocity and the correction of this effect are also considered.It is indicated in this paper that the P and S wave simultaneous migration has brighter prospect than the P wave migration in solving problems related to media with complex stratified structures.更多还原