Scattering in the deep mantle
Even though the resolution of tomographic studies increases there is a fundamental limit below which structure cannot imaged with tomographic inversions. Especially small-scale heterogeneities can influence the seismic wave field and contribute to the attenuation of seismic waves. Moreover, these small-scale inhomogeneities provide information on dynamic processes and mantle mixing and may be related amongst others to microstructures, melt pockets, or topography of boundary layers.
While scattering in the crust and from shallow structures manifests itself as coda to seismic waves and crustal scattering may overprint scattering from deeper layers, one region in the Earth can be investigates without crustal influence of scattering: precursors to the core phases such as PKPdf. Since the scattered waves travel along different paths, their travel time is such that they arrive as precursors to PKPdf.