The gradient that optimize the model update in the full waveform routine is mainly obtained by a reverse-time migration of the residuals. However, if we can interpret the whole routine as a combination of seismic processing tools, we believe it is possible to obtain the gradient using any kind of depth migration. In this paper, we are comparing the use of RTM and PSPI migration to estimate the gradient, using the post-stack forward modeling-free approximations of the FWI, the FastWI. The inverted models using RTM and PSPI migrations have similar resolution in a simple velocity model, with the advantage of the PSPI been cheaper, but the RTM may lead to more continuous and high resolution models on more complex geologies.
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