井间各向异性多波走时联合层析成像:以波场模拟拾取走时为例  

Multi-phase traveltime tomography in crosshole anisotropic media: taking the picked traveltime from wavefileld as an example

作　　者：何雷宇 白超英[1,2] HE Lei-yu;BAI Chao-ying(Department of Geophysics,School of Geology Engineering and Geomatics,Chang＇an University,Xi＇an 710054,China;Institute of Computational Geophysics,Chang＇an University,Xi＇an 710054,China)

机构地区：[1]长安大学地质工程与测绘学院地球物理系,西安710054 [2]长安大学计算地球物理研究所,西安710054

出　　处：《地球物理学进展》2018年第5期1796-1806,共11页Progress in Geophysics

摘　　要：为了解决复杂地学模型中正、反演问题(如起伏地表、不规则界面和不规则异常体),本文实现了三角网格剖分下各向异性介质模型中分区多步最短路径射线追踪算法,结合共轭梯度法求解带约束的阻尼最小二乘问题,结合直达波和反射波走时实现了二维各向异性中同时反演四个弹性参数的算法.鉴于各向异性介质中相、群速度关于各弹性参数(a11、a13、a33、a44)偏导数随相角不同而有所不同(每个敏感核函数随相角的变化而变化),而且偏导数的值有正值也有负值.据此,在qP波反演时我们对敏感核函数(Jacob偏导矩阵元素)进行了归一化处理.另外,为了解决qSV波反演时出现的多解性问题,我们提出了分步反演的策略.同时为了验证方法的有效性和正确性,文中采用从地震波场数值模拟中拾取的走时作为观测走时,数值模拟结果表明:波场模拟观测走时反演结果与射线法模拟观测走时反演结果类似,验证了反演过程中正、反演方法的正确性; qP波反演时对敏感核函数归一化处理后的反演结果要明显优于未做归一化的反演结果; qSV波分步反演的结果要优于四个弹性参数同时反演的结果.The cell in anisotropic seismic tomography have always been regular cell for a long time. Although the regular cell model are simple to understand and easy to realize by computer,it also has some bad aspects. Such as,it is hard to describe complex geological models,and the model cannot be divided into different parts by using regular cell. In order to overcome the difficulty in forward and inversion problem in complex geological model,including undulated topography and irregular subsurface interface,we in this paper realize a multistage triangular shortest-path method for multi-phase seismic ray tracing,and combine a nonlinear inversion solver（ constrained and damped least squares problem solved by a conjugated gradient method） to simultaneously invert four elastic parameters in anisotropic media by using combined direct and reflected arrival times. For trading off in different parameter updating,we normalize specific kind of sensitive function of qP-wave inversion,because the partial derivatives of phase velocity respective to different elastic parameters（ a11,a13,a33,a44） varying with different phase slowness angles, and the partial derivatives have positive and negative values. In addition,in order to solve the multi-solution problem of qSV-wave inversion,a strategy of stepwise inversion is proposed. For testing the efficiency and correctness of proposed inversion method, we use the picked traveltimes in synthetic seismograms as the observed traveltimes in the inversion. The results show that both inverted images by taking the picked traveltimes in synthetic seismograms as the observed data and by taking predicted traveltimes using ray tracing method as observed data have a similar imaging pictures,which verify the efficiency and correctness of the proposed inversion method. Furthermore, the inverted results by normalized kernel functions is better than that of not normalized kernel functions and the result of stepwise inversion is better than four elastic parameters simultaneous inversion in qSV-wave

关 键 词：三角网格模型 各向异性介质 弹性参数反演 井间层析成像 核函数归一化 最短路径 

分 类 号：P315[天文地球—地震学]