Precursors to the seismic core phases PKP and PKKP result from scattering off small-scale heterogeneity in the deep mantle or core. We apply stacking techniques to high-frequency data from the global seismic networks to image details of the time and range dependence of the precursor wavefields. Results for PKP suggest that small-scale velocity perturbations with 8-km scale length and 1% rms amplitude are distributed throughout much of the lower mantle, with no evidence for a concentration of heterogeneity near the core-mantle boundary. This supports geochemical evidence and numerical convection experiments that suggest incomplete mixing and small-scale chemical heterogeneity are present in the mantle. The PKKP precursor observations can be modeled by short wavelength topography on the core-mantle boundary (about 300 m rms height), but exhibit a range dependence that is difficult to explain with simple models. The outer core appears free of scattering, but the inner core is a possible contributor of scattered energy to the PKKP precursors.