Ongoing research on the Earth's core and lowermost mantle investigates topics such as anisotropy and structure of the inner core, structure of the outer core, and the characterization of the core-mantle and inner-core boundaries. Inadequate spatial sampling of the central inner core by PKP waves in all directions makes further advances in understanding anisotropic properties (especially anisotropy's radial dependence and hemispherical pattern) very difficult. One of the reasons for this incomplete sampling lies in the fact that, in order to pass through the central regions of the inner core, PKP waves must be nearly antipodal. With the spatial distribution of large earthquakes and current configuration of the seismographic stations worldwide, this is difficult to achieve, except for the paths nearly parallel to the equatorial plane.
We are, however, working toward improving the spatial sampling of the core and the lowermost mantle by exploiting other core phases, such as PKPPKP waves.
There are three major approaches that could be pursued to achieve a better spatial samling of the deep Earth:
- Observation and analysis of seismic phases with more complex geometry, such as PKPPKP or PnKP, which must be employed as a necessary supplement to PKP measurements, when using seismic travel times to study deep Earth structure (because of their unique sampling of the core that cannot be achieved by PKP waves only);
- Installation of seismic stations at extreme geographic latitudes and ocean islands, in order to increase the coverage of the inner core by polar paths. The Australian deployments of the seismographic stations across Australia and in Antarctica greatly help in achieving this objective;
- Development and application of new techniques (e.g. array signal processing techniques, or travel time measurements by non-linear inversion instead of direct time picking), which will allow us to use PKP and other data that were previously discarded.