ABSTRACT

Wavefield reconstruction inversion (WRI) extends the search space of full-waveform inversion (FWI) by allowing for wave-equation errors during wavefield reconstruction to match the data from the first iteration. Then, the wavespeeds are updated from the wavefields by minimizing the source residuals. Performing these two tasks in an alternating mode breaks down the nonlinear FWI as a sequence of two linear subproblems, relying on the bilinearity of the wave equation. We have solved this biconvex optimization with the alternating-direction method of multipliers (ADMM) to efficiently cancel out the data and source residuals in iterations and stabilize the parameter estimation with appropriate regularizations. Here, we extended WRI to viscoacoustic media for attenuation imaging. Attenuation reconstruction is challenging because of the small imprint of attenuation in the data and the crosstalk with the velocities. To address these issues, we recast the multivariate viscoacoustic WRI as a triconvex optimization and update wavefields, squared slowness, and attenuation factor in alternating mode at each WRI iteration. This requires linearization of the attenuation-estimation subproblem via an approximated trilinear viscoacoustic wave equation. The iterative defect correction embedded in ADMM corrects the errors generated by this linearization, whereas the operator splitting allows us to tailor 1 regularization to each parameter class. A toy numerical example shows that these strategies mitigate crosstalk artifacts and noise from the attenuation reconstruction. A more realistic example representative of the North Sea further indicates the ability of the method to jointly reconstruct the wavespeed and the attenuation starting from a very crude attenuation-free initial model, the moderate strength of the crosstalk artifacts, and the sensitivity of the multiparameter reconstruction to noise.

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