三角网格剖分下速度与反射界面的同时反演

Simultaneous inversion for velocity and reflector geometry with multi-phase traveltimes in 2D triangular cell model

为了解决复杂速度模型中的走时正、反演问题,例如:含不规则起伏地表、不规则地下波阻抗界面、以及不规则速度异常体的复杂地学模型,本文采用三角网格单元模型参数化下的分区多步改进型最短路径算法,实现了多震相地震射线的追踪计算,结合共轭梯度法求解带约束的阻尼最小二乘反演问题,实现了多震相走时联合同时反演成像的方法技术.当界面起伏较大时出现散射,从而造成散射点所在区域射线密度过密,导致该区域内速度和界面的过度更新.为了克服上述问题,我们在同时反演中引入了射线密度的概念,从而有效地解决了上述过度更新问题.数值模拟实验表明:采用三角网格单元进行模型参数化,可保证在复杂模型中的正演计算具有较高的计算精度;同时反演中可以准确地刻画不规则异常体和不规则反射界面.因此,本文提出的走时成像方法技术具有较广的实用价值.

To conduct forward and simultaneous inversion in a complex velocity model, including an irregular topography, or irregular reflector,or irregular velocity anomaly,we in this paper realize a multi-phase arrival tracking with a multistage modified shortest-path method under a triangular cell model,and combine an inversion algorithm,that is the constrained damped least squares problem solved by a conjugated gradient method,to simultaneously invert the velocity model and reflector geometry with multi-phase arrival time information. Meanwhile,we introduce a concept of an equal ray density to overcome the over-updated velocity region,where the ray density is high,due to the scatter phenomenon caused by undulated reflector. The numerical simulation results show that with the triangular cell model parameterization,it is possible to maintain a high computational accuracy for forward modeling in the complex velocity model,and capture the irregular velocity anomaly and reflector geometry in the simultaneous inversion. Therefore,the proposed simultaneous traveltime inversion has a wide application in the real problem.