LAUSR

ABSTRACT Full waveform inversion in transversely isotropic media with a vertical symmetry axis provides an opportunity to better match the data at the near and far offsets. However, multi‐parameter full waveform inversion, in general, suffers from serious cycle‐skipping and trade‐off problems. Reflection waveform inversion can help us recover a background model by projecting the residuals of the reflected wavefield along the reflection wavepath. Thus, we extend reflection waveform inversion to acoustic transversely isotropic media with a vertical symmetry axis utilizing the proper parameterization for reduced parameter trade‐off. From a radiation patterns analysis, an acoustic transversely isotropic media with a vertical symmetry axis is better described by a combination of the normal‐moveout velocity vn and the anisotropic parameters η and δ for reflection waveform inversion applications. We design a three‐stage inversion strategy to construct the optimal resulting model. In the first stage, we only invert for the background vn by matching the simulated reflected wavefield from the perturbations of vn and δ with the observed reflected wavefield. In the second stage, the background vn and η are optimized simultaneously and the far‐offset reflected wavefield mainly contribute to their updates. We perform Born modelling to compute the reflected wavefield for the two stages of reflection waveform inversion. In the third stage, we perform full waveform inversion for the acoustic transversely isotropic media with a vertical symmetry axis to delineate the high‐wavenumber structures. For this stage, the medium is described by a combination of the horizontal velocity vh, η and e instead of vn, η and δ. The acoustic multi‐parameter full waveform inversion utilizes the diving waves to improve the background as well as utilizes reflection for high‐resolution information. Finally, we test our inversion algorithm on the modified Sigsbee 2A model (a salt free part) and a two‐dimensional line from a three‐dimensional ocean bottom cable dataset. The results demonstrate that the proposed reflection waveform inversion approach can recover the background model for acoustic transversely isotropic media with a vertical symmetry axis starting from an isotropic model. This recovered background model can mitigate the cycle skipping of full waveform inversion and help the inversion recover higher resolution structures.