Mapping structures on the core-mantle boundary using Sdiff postcursors: Part I. Method and Validation

Ultra-low velocity zones (ULVZs) are patches of extremely slow seismic velocities on the core-mantle boundary (CMB). Here, we target them using the postcursors to S core-diffracted phases (Sdiff) caused by ULVZs. We use traveltimes of these postcursors to make probabilistic maps using a reversible-jump Markov chain Monte Carlo inversion setup. For the forward model, we extend 2-D wave front tracking (2DWT) software, previously developed for surface wave multipathing studies, to the CMB. The 2DWT is able to model the full multipathing behaviour of Sdiff postcursors and compute arrival times for a given ULVZ input velocity structure on the order of a few CPU seconds, as opposed to 100s of CPU hours required for 3-D full waveform synthetics. We validate the method using synthetic data sets produced by the 2DWT, as well as 3-D full waveform synthetics, using a parametrisation formed from a collection of ellipses. We also test idealistic data coverage versus a case of more realistic coverage. We show ULVZ size and velocity reduction can typically be well recovered, and our maps show the inherent trade-off between these parameters around the edge of the ULVZ. Our method cannot directly constrain ULVZ height; tests show that it underestimates ULVZ velocity reductions and overestimates ellipticity for thinner ULVZs due to neglecting mantle effects.