oa Frequency-cascade Scheme to the Elastic Wavefield Inversion: A strategy for low Wavenumber Velocity Estimate and Several Applications

The importance of the elastic wave velocity is known such as in the seismic exploration of gas hydrates, but it has been rarely analysed in practice. While nonlinear elastic wavefield inversion is promising to estimate elastic wave velocity compared with other conventional methods, there has been practical difficulty in searching relatively longer wavelengths by the local iterative methods. To overcome its difficulty, frequency-cascade scheme is examined in space-time domain formulation. Elastic wavefield inversion is conducted under several approaches such that Vp or Vs is constrained when inverting the other, that Vp and Vs is alternately inverted at each step, and that Vp and Vs are simultaneously inverted. Simultaneous mode proved to be the most effective both in efficiency and accuracy of the estimate. It is proved that frequency-cascade scheme on the simultaneous mode is effective in practice for the models over wide value ranges of elastic wave velocities and Poisson’s ratios.