Multistage elastic full-waveform inversion for tilted transverse isotropic media

Seismic anisotropy is an important physical phenomenon that significantly affects wave propagation in complex sedimentary basins. When geological structures exhibit steep dips or severe folding, the symmetry axis of the transversely isotropic (TI) representation of the region can be rotated, leading to tilted transversely isotropic (TTI) media. We seek to find the optimal full-waveform inversion (FWI) strategy to estimate both the seismic velocities and the anisotropic parameters, including the tilt angle, in the presence of elastic TTI media. We first formulate the forward and inverse problems for elastic TTI media and analyse the radiation patterns of the model parameters. Based on the analyses of the radiation patterns, we propose two similar multistage FWI strategies that add inversion parameters over three stages, beginning with the isotropic parameters (horizontal P- and vertical S-wave velocity) and moving to the anisotropic parameters; the tilt angle is directly inverted in the last stage. Since diving waves, which are useful for providing long-wavelength updates, are mainly controlled by horizontal motion in anisotropic media, it is reasonable to choose the horizontal P-wave velocity rather than the vertical P-wave velocity. Then, the anisotropic parameters are inverted mainly using the reflected waves based on the isotropic background model built in the first stage. The main difference between the two multistage FWI strategies is whether the anisotropic parameter η is inverted. Comparing the two multistage FWI strategies with the simultaneous inversion strategy for a downsized version of the synthetic BP TTI model, we confirm that the multistage FWI strategies yield better inversion results than the simultaneous inversion strategy. When we compare the two multistage FWI strategies with each other for surface seismic data, ignoring η during the FWI process (focused multistage FWI) yields better inversion results for the tilt angle than those obtained with the inversion of η because η has less influence on the FWI than the other parameters and is not recovered well, which plays a role in degrading the tilt angle. Numerical examples support our conclusions that the focused multistage FWI strategy (neglecting η) is the optimal FWI strategy for TTI media and achieves computational efficiency for surface seismic data.