Many of these models, which are still debated, can be tested and refined as the distribution of seismicstations around the globe continues to improve; however, the uneven distribution of earthquakes placesfundamental limits on the geographical sampling of the inner core. Moreover, in order to mitigate the effectsof mantle heterogeneity and source mislocation, many body wave studies of the inner core measure thearrival time of PKIKP (PKPdf) with respect to a reference phase that only traverses the mantle and outer core(PKPcd, PKPbc, and PKPab), which tends to limit coverage mostly to the top ~300 km of the inner core, i.e., tothe outer parts of the inner core (OIC).