Computing images in reverse time migration and model parameter gradients from adjoint wavefields in full-waveform inversion (FWI) requires the correlation of a forward-propagated wavefield with another reverse-propagated wavefield. Although in theory only two wavefield propagations are required, one forward propagation and one reverse propagation, it requires storing the forward-propagated wavefield as a function of time to carry out the correlations, which is associated with significant input/output (I/O) cost. Alternatively, three wavefield propagations can be carried out to reverse propagate the forward-propagated wavefield in tandem with the reverse-propagated wavefield. We have determined how highly accurate reverse time migrated images and FWI model parameter gradients for anisotropic elastic FWI can be efficiently computed without significant disk I/O using two wavefield propagations by means of the principle of superposition.

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