Contacts: akios@japex.co.jp (Akio Sakai)
Nonlinear elastic wavefield inversion is challenging since it provides high resolution with good accuracy and the full estimate of elastic wave velocity. Velocity information is separated into longer wavelengths and shorter wavelengths, and there has been the practical difficulty in searching relatively longer wavelengths by the local iterative methods. To overcome its difficulty, frequency-cascade scheme combined with the multigrid approach is examined. Elastic wavefield inversion was conducted under several approached 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. Frequency-cascade scheme on the simultaneous mode proved to be effective in practice for the models over a wide value range of elastic wave velocities and Poisson's ratios.