Frequency‐domain waveform inversion using the unwrapped phase

Abstract

Phase wrapping in the frequency‐domain (or cycle skipping in the time‐domain) is the major cause of the local minima problem in the waveform inversion. The unwrapped phase has the potential to provide us with a robust and reliable waveform inversion, with reduced local minima. We propose a waveform inversion algorithm using the unwrapped phase objective function in the frequency‐domain. The unwrapped phase, or what we call the instantaneous traveltime, is given by the imaginary part of dividing the derivative of the wavefield with respect to the angular frequency by the wavefield itself. As a result, the objective function is given a traveltime‐like function, which allows us to smooth it and reduce its nonlinearity. The gradient of the objective function is computed using the back‐propagation algorithm based on the adjoint‐state technique. We apply both our waveform inversion algorithm using the unwrapped phase and the conventional waveform inversion and show that our inversion algorithm gives better convergence to the true model than the conventional waveform inversion.