Traveltime sensitivity kernels for PKP phases in the mantle

We investigate the finite-frequency effects of perturbations of compressional wave velocity on the traveltimes of PKP phases. Owing to their long paths in the mantle, PKP phases have a large Fresnel zone at the CMB and thus sample a large volume of the D” layer. We compute traveltime Fréchet derivatives of the three PKP phases in the mantle with respect to the ak135 reference model, taking into account the coupling between the different branches. PKP kernels do not have a simple “banana-doughnut” shape, because the different PKP branches interfere with each other, and because of the presence of shadow zones and caustics. In addition, we compute the differential kernels AB–BC, BC–DF and AB–DF. The geometry of PKP differential kernels in D” show significant qualitative differences from those predicted by simple ray tracing. This suggests that the tomographic techniques commonly used to image lower mantle structures could be improved by using 3D Fréchet kernels.