Deep-earth diffracted-wave tomography: A multiple-frequency high-resolution approach

We present a method to address the seismic forward and inverse problem at the global scale, with a specific focus on waves diffracted in the lowermost mantle. This spectral-element approach is developed in spherically symmetric background models, offering full-wave sensitivity up to 1 Hz. In order to get a direct view of the interconnection between surface displacements and earth structure, we examine, for a given source-receiver pair, the time-dependent sensitivity of the seismic signal to perturbations in the target region of interest. That analysis suggests data selection criteria to incorporate into future inversions. It is furthermore conceivable to assess different 1D background models for a specific D" region in question which may be undertaken with very few preliminary test simulations and comparison to data. We measure and model our static observables (traveltimes, amplitudes) in multiple-frequency passbands, which together span the usable broadband range. This increases robustness and resolution by multiplying the number of constraints on the inverse problem. It also allows us to selectively draw only upon frequency bands with high signal-to-noise ratio (e.g., to exclude the microseismic noise bands for some locations). We discuss coverage maps and target regions for which diffracted-wave tomography promises the greatest improvements in resolution and potential geodynamic interpretations.