Reverse time migration imaging of tunnels via the finite element method using an unstructured mesh*
Wang Jing 1,2,3, Liu Jiang-Ping ♦4, Cheng Fei 5, Yang Huai-Jie 6, Huang Yi-Fan 1,2,3
1. School of Civil Engineering, Shijiazhuang Tiedao University, Shijiazhuang 050043, China.
2. Key Laboratory of Roads and Railway Engineering Safety Control (Shijiazhuang Tiedao University), Ministry of Education, Shijiazhuang 050043, China.
3. Hebei Technology and innovation center on safe and effi cient mining of metal mines, Shijiazhuang Tiedao University,Shijiazhuang 050043, China.
4. Hubei Subsurface Multi-scale Imaging Key Laboratory, Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, China.
5. College of Marine Science and Technology, China University of Geosciences, Wuhan 430074, China.
6. Beijing Research Institute of Uranium Geology, Beijing 100029, China.
Abstract：
Wavefield extrapolation is critical in reverse time migration (RTM). The finite difference method is primarily used to achieve wavefield extrapolation in case of the RTM imaging of tunnels. However, complex tunnel models, including those for karsts and fault fracture zones, are constructed using regular grids with straight curves, which can cause numerical dispersion and reduce the imaging accuracy. In this study, wavefield extrapolation was conducted for tunnel RTM using the finite element method, wherein an unstructured mesh was considered to be the body-fitted partition in a complex model. Further, a Poynting vector calculation equation suitable for the unstructured mesh considered in the finite element method was established to suppress the interference owing to low-frequency noise. The tunnel space was considered during wavefield extrapolation to suppress the mirror artifacts based on the fl exibility of mesh generation. Finally, the influence of the survey layouts (one and two sidewalls) on the tunnel imaging results was investigated. The RTM results obtained for a simple tunnel model with an inclined interface demonstrate that the method based on unstructured meshes can effectively suppress the low-frequency noise and mirror artifacts, obtaining clear imaging results. Furthermore, the two-sidewall tunnel survey layout can be used to accurately obtain the real position of the inclined interface ahead of the tunnel face. The complex tunnel numerical modeling and actual data migration results denote the eff ectiveness of the finite element method in which an unstructured mesh is used.
2020, Vol. 17 
