Localized waveform inversion applied to the D' region in teleseismic imaging

A localized waveform inversion technique based on hybrid modeling was developed to investigate the shear wave velocity structure of the lowermost mantle. Utilizing ray theory and the Kirchhoff integral, the source wavefield from the hypocenter and the receiver wavefield recorded at the Earth's surface can be extrapolated to the subsurface near the core-mantle boundary (CMB). Ray theory solutions for a smooth background Earth model are interfaced to a finite difference solution to the wave equation applied in a local heterogeneous region near the CMB. The velocity structure in the finite difference region is updated iteratively by zero-lag cross-correlation of the forward and backward propagated wavefields that represents the direction of minimizing the data misfit. As the finite difference method is only applied in a small region, the hybrid method requires much less computer memory when it is implemented to invert localized structures. This method is applied to study an event recorded by the RISTRA Array and the depth and amplitude of the D" discontinuity is recovered. This demonstrates that the hybrid waveform inversion is feasible in teleseismic imaging.