Investigating small-scale deep-mantle structure, the stories told by high frequency scattered waves

The structure of the Earth's deep mantle is a result of complex processes that are influenced by surface tectonics through the subduction of oceanic lithosphere and by core dynamics through the heat flow across the core-mantle-boundary. The other way around the structures in the deep mantle affect the Earth's surface by feeding mantle plumes that sustain volcanism. By modulating the heat flow at the CMB the mantle also affects the dynamics of the core and the magnetic field.

These processes focus in the D'' layer that marks the mysterious few hundred kilometers directly above the core-mantle-boundary which contain dominant features like the Large Low Shear Velocity Provinces and features with rather extreme properties like the Ultra Low Velocity Zones. Knowledge of the structural features in the D'' layer is of importance for the understanding of long- and short-term processes in our direct environment at the surface of the Earth.

The remoteness of D'' layer more than 2,500 kilometers below the surface poses challenges for geophysical investigations and limits the resolution of seismological imaging. Seismic tomography with surface waves and normal modes therefor locate the large scale features, only. Detailed wavefield analysis and modeling of particular seismic phases, often based on array observations provide more detailed information about locally dominating structures and their contrasts. For the characterization of distributed small scale structures that can be referred to as heterogeneity even wavefield analysis fails due to the superposition of waves scattered at different locations of the heterogeneous material. Such heterogeneity can for instance represent remnants of oceanic crust that has been subducted down to the CMB.

Despite the complexity of signals generated by distributed heterogeneity the analysis of high frequency scattered waves provides constraints on the presence structures at short length scales of a few kilometers in the deep mantle. I review the theoretical basics of scattering theory and the observational evidence for deep Earth distributed heterogeneity. I discuss new observations of high frequency seismic waves scattered in the deep mantle together with limitations in the interpretation imposed by the nature of the scattered wavefield.