Elastic reverse-time migration in irregular tunnel environment based on polar coordinates*
Qu Ying-Ming ♦1, Zhou Chang 1, Worral Qurmet 1, Li Zhen-Chun 1, Wang Chang-Bo 2, and Sun Jun-Zhi 1
1. Department of Geophysics, School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China;
2. Geophysical Research Institute, Shengli Oilfi eld Branch Co., SINOPEC, Dongying 257022, China.
Abstract：
When seismic exploration is conducted in a special geological environment such as a tunnel space, the traditional imaging method in the Cartesian coordinate system cannot accurately discretize the air column in that environment. Thus, obtaining Thus, obtaining high quality imaging results is difficult. Therefore, an elastic-wave reverse-time migration method based on the polar coordinate system is proposed. In this method, three boundary conditions exist: outer, inner, and corner boundaries. In the outer boundary, the polar-coordinated absorbing boundary in the radial direction is used to suppress the artificial-boundary reflection. The free-surface boundary condition is adopted in the tunnel space at the inner boundary. In the angular boundaries, we use two different boundary conditions for two cases. The air column in the tunnel space is usually not an irregular circle. Therefore, the irregular tunnelspace geological body in the polar coordinate system is meshed into curvilinear grids and transformed into a regular one in an auxiliary polar coordinate system using the mapping method. Finally, elastic reverse-time migration technology is applied into the auxiliary polar coordinate system. In the numerical examples, two typical models are used to test the proposed method, which verify that the proposed method can obtain accurate images from the datasets in the tunnel space.
2020, Vol. 17 
