It is of particular interest to accurately estimate the effects of complex structure in the deep mantle and D'' on PKP differential travel times in order to reach reliable conclusions about the physical and chemical properties of the Earth's inner and outer core. It is important to assess how much of the data can be explained by mantle structure alone. By using differential travel times, we assume that effects of upper-mantle and source mislocation are minimized, and that the dataset is mostly sensitive to the deep mantle and core where the paths of the two phases differ the most. The model derived without polar paths is shown in Figure 29.2 . Our results demonstrate that, with only about 1500 PKP differential travel time data, we are able to retrieve a D'' map of quality comparable to other maps derived using by far larger number of ISC data.
Unsampled regions are black.
The robustness of this model should be assessed by comparing it with figure 1 which shows sampling per each particular cell. Acknowledgements We thank Ken Creager, Tom McSweeney, Annie Souriau and Michael Wysession for generously sharing their data, and also to Roger Hensen, Kent Lindquist, Doug Christensen and personnel of UAF Geophysical Institute, Bill Shannon, Sylvia Lehman and Luc Saumure from CNSN, for helping one of us, Hrvoje Tkalcic, collect data from the Alaska Network. Creager, K. C., Large-scale variations in inner core anisotropy, J. Geophys. Engdahl, E. R., R. D. van der Hilst and R. P. Buland, Global teleseismic earthquake relocation with improved travel times and procedures for depth determination, Bull. Kennett, B. L. N., E. R. Engdahl and R. Buland, Constrains on seismic velocities in the Earth from traveltimes, Geophys.