伴随状态法初至波走时层析

初至波走时层析成像方法通常被用来反演近地表速度结构。传统的射线层析成像方法计算效率低，且在复杂模型计算中存在不稳定性问题。为了快速、稳定地进行初至波走时层析，本文基于程函方程的有限差分形式，利用快速扫描算法实现初至波走时的快速计算。在此基础上，采用伴随状态法计算目标函数的梯度，进而实现伴随状态法初至波走时层析。将该方法与传统射线层析成像方法应用于理论模型实验和实际资料的处理，结果表明基于程函方程的伴随状态法初至波走时层析可以取得与传统射线层析近似的反演结果，但计算效率得到大幅提升。

First arrival traveltime tomography based on the adjoint state method

Traveltime tomography always uses first arrival traveltime to invert near surface structures of the earth. Conventional ray-based tomography method is limited by low efficiency and poor stability in complex structures. In this paper, we use the fast sweeping method (FSM) to get the first arrival traveltime stably and efficiently based on the finite differential operator of the Eikonal equation. Then, the adjoint state method is applied to calculate the gradient of the misfit function. The combination makes the traveltime tomography more efficient. Both this method and the conventional ray-based tomography are applied to theoretical models and field data. Inverted results indicate that the adjoint state method can retrieve velocity structure with similar accuracy as conventional ray-based tomography, while the cost of computation is much less.