三维复杂黄土塬区变加密不等距网格初至走时计算与特征分析

Travel-time calculation and characteristic analysis for first arrival in 3D complex loess plateau area using variable density and unequal distance grid

三维复杂黄土塬区初至走时计算与特征分析,对于分析复杂黄土塬区地震波运动学特征、观测系统设计、初至拾取及层析反演等均具有重要意义.针对该问题:首先,采用三维分区局部变加密不等距网格剖分复杂黄土塬速度模型;然后,建立基于费马原理和迎风格式的双线性插值局部走时算法,采用修正后的群推进法作为波前扩展方式;最后,以理论模型考察了算法的精度与效率、分析了三维复杂黄土塬区地震初至波的走时特征.结果表明:变加密网格策略能在增加很少计算量的情况下,大幅降低因震源奇异性引起的计算误差问题,同时还能无条件稳定且灵活的适应三维复杂黄土塬区的复杂地震地质条件;和层状均匀模型相比,黄土塬区地震初至波的走时场和时距曲线较为复杂.

The travel-time calculation and characteristic analysis for first arrival in the 3D complex loess plateau area are of great significance to the analysis of the kinematic characteristics for seismic waves,the design of the seismic acquisition system,especially the first arrival picking and tomography inversion.To solve this problem:Firstly,the partition local variable density and unequal distance grid is adopted to divide the velocity model of 3D complex loess plateau;Then,a local travel-time algorithm of bilinear interpolation is established based on Fermat principle and upwind scheme and a modified group marching method is adopted as the scheme of wave-front expansion;Finally,the accuracy and efficiency of the algorithm and the characteristics of first arrival travel-time in the 3D complex loess plateau area are investigated by using the theoretical model.The results show that:We can greatly reduce the computational error caused by the singularity of the source and adapt to the complex seismic geological conditions of the 3D complex loess plateau unconditional stably and flexibly by using the scheme of variable density grid;Compared with the 3D layered uniform model,the travel-time field and time distance curve of seismic first arrival in 3D complex loess plateau area are more complex.