Non‐linear waveform inversion for surface waves with a neighbourhood algorithm—application to multimode dispersion measurements

Summary A new technique for multimode dispersion measurement has been developed by employing fully non‐linear waveform inversion for a path‐specific 1‐D profile using a neighbourhood algorithm (NA). One‐dimensional models derived from waveform inversion are quite sensitive to the model parametrization and the reference model used to start the inversion. With different approaches to the parametrization of the shear wave speed profile, we can find models with significant differences in velocity variation with depth, which provide similar levels of fit to the observed waveforms. Although the models differ, the calculated phase dispersion for the first few modes of the surface waves are very close indeed. We therefore regard the 1‐D models derived from the multimode waveform inversion as an implicit description of the path‐specific dispersion for each of the modes. The inversion procedure using NA samples a substantial number of models and we select the 1‐D wave speed model that achieves minimum misfit. Phase speeds for each mode branch are then calculated from the model parameters with the minimum misfit. Because we use the 1‐D models as a summary of the average multimode dispersion along the path, we do not need to make assumptions concerning the nature of anisotropy. We can employ simple isotropic models and the method can be applied to both Rayleigh and Love waves independently. The new method is applied to sets of paths in the Australian region and provides stable measurements of multimode dispersion from a single seismogram. The multimode phase speeds measured using the NA inversion can be used to retrieve phase velocity maps as a function of frequency for higher‐mode branches as well as the fundamental mode, which will be crucial constraints on 3‐D Earth models.