水平层状VTI介质高效射线追踪计算方法  

High-efficient raytracing method in 1D horizontal layered VTI medium

作　　者：蒋星达 张伟 陈晓非 JIANG XingDa;ZHANG Wei;CHEN XiaoFei(Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou 511458,China;Shenzhen Key Laboratory of Deep Offshore Oil and Gas Exploration Technology,Southern University of Science and Technology,Shenzhen Guangdong 518055,China;Department of Earth and Space Sciences,Southern University of Science and Technology,Shenzhen Guangdong 518055,China)

机构地区：[1]南方海洋科学与工程广东省实验室(广州),广州511458 [2]南方科技大学深圳市深远海油气勘探技术重点实验室,广东深圳518055 [3]南方科技大学地球与空间科学系,广东深圳518055

出　　处：《地球物理学报》2022年第9期3540-3553,共14页Chinese Journal of Geophysics

基　　金：国家自然科学基金(U190602);深圳市深远海油气勘探技术重点实验室(ZDSYS20190902093007855);深圳市科技计划(KQTD20170810111725321)共同资助。

摘　　要：准确、快速计算水平层状VTI(vertical transverse isotropic)介质的走时和射线路径是实现VTI介质地震叠前时间偏移和微地震监测速度建模的基础.水平层状VTI介质射线追踪的常用算法包括打靶法和最短路径法.打靶法原理简单、实现容易,但在长偏移距时存在微小初射角度变化导致射线路径巨大扰动的问题;最短路径法原理直观,但需要沿射线群速度方向(群角)计算走时,计算特定群角对应的群速度值需要先搜索对应的相速度角度(相角),显著降低了计算效率.本文综合打靶法和最短路径法的优点,从震源同时出射一系列射线,以这些射线到达界面的交点构成稀疏的网格节点,自适应加密实现检波点周围网格节点间距高于精度要求,采用插值算法获得检波点的走时和动态网格方法获得不位于网格节点上的检波点的射线路径.本文方法严格计算已知相角对应的群角和群速度,未使用弱各向异性群速度近似公式,适用于任意强度各向异性VTI介质qP、qSV和qSH直达波和反射波射线追踪;以相角确定震源出射射线,不用遍历群角和群速度对应关系.同传统最短路径方法的数值实验对比表明本文方法具有高精度和高效性,非常适合于需要多次正演计算的地震叠前时间偏移和微地震实时监测问题.The calculation of traveltime and raypath in 1D layered VTI medium with enough accuracy and high efficiency is the foundation for seismic prestack time migration and microseismic velocity inversion.Two commonly used raytracing algorithms for 1D layered VTI medium are the shooting method and the shortest-path raytracing method.The shooting method is simple in principle and easy to implement,but a small change in the initial emergence angle can cause a large offset variation in the raypath at the longer offset.The principle of the shortest-path raytracing method is intuitive,but it requires calculating the traveltime along the ray group velocity direction(group angle)in anisotropic medium.It costs a lot of time to find the group velocity when the group angle is given.We combine the advantages of the above two methods and propose a high-efficient raytracing method for 1D layered VTI medium.A bunch of rays propagates from the source simultaneously and a dynamic grid model is set up based on the ray intersection with the velocity interfaces.The grid nodes around the receivers become denser adaptively to satisfy the traveltime accuracy requirement.The traveltime and raypath of the receivers are obtained by the cubic spline interpolation and the adaptive node method.Our proposed method calculates the group velocity and group angle by a given phase angle while no weak anisotropy assumption is employed.It can calculate the rays for the direct and reflected waves of qP,qSV and qSH in layered strong anisotropic VTI medium.Compared with the traditional shortest-path raytracing method,the numerical examples demonstrate that our method is more efficient and accurate,suitable for seismic prestack time migration and real-time microseismic monitoring.

关 键 词：强各向异性 层状VTI介质 射线追踪方法 最短路径方法 动态网格 

分 类 号：P315[天文地球—地震学]