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 flexibility 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 effectiveness of the finite element method in which an unstructured mesh is used.

波场外推法对于逆时偏移（RTM）至关重要。对于隧道的RTM成像，有限差分法主要用于实现波场外推。但是，复杂的隧道模型（包括岩溶和断层断裂带的隧道模型）是使用具有直线曲线的规则网格构造的，这会导致数值离散并降低成像精度。在这项研究中，使用有限元方法对隧道RTM进行了波场外推，其中在复杂模型中非结构化网格被认为是人体拟合的分区。此外，建立了适用于有限元法中考虑的非结构化网格的Poynting矢量计算方程，以抑制由于低频噪声引起的干扰。在波场外推过程中考虑了隧道空间，以基于网格生成的灵活性来抑制镜像伪影。最后，研究了勘测布局（一个和两个侧壁）对隧道成像结果的影响。针对具有倾斜界面的简单隧道模型获得的RTM结果表明，基于非结构化网格的方法可以有效地抑制低频噪声和镜像伪影，从而获得清晰的成像结果。此外，可使用两侧壁隧道勘测布局来准确获得隧道界面前方倾斜界面的实际位置。复杂的隧道数值模拟和实际数据迁移结果表明了使用非结构网格的有限元方法的有效性。