Figure 10 . Depth maps of the 410-km, 660-km discontinuities,
and MTZ thickness using the CRP imaging method for 8° 8° (a, b, c) , 5°
5° (d, e, f), and 2° 2° (g, h, l) input topography models.
Figure 11 explores how undulations on the 410-km discontinuity would be
resolved if they are embedded in 3-D velocity heterogeneity. We analyze
waveform simulations for model T5-410-S40, described above. Figure 11
shows the resolved depth variation of the 410-km and the 660-km
discontinuities and the MTZ thickness before (in a, c, and e) and after
(in b, d, and f) ray-theoretical traveltime corrections have been
applied to remove the effects of shear-velocity heterogeneity on the
traveltimes. Figure 11a and 11b show that without traveltime corrections
for 3-D heterogeneity, the resolved harmonic undulations of the 410 are
overprinted by an east-to-west deepening. The ray-theoretical traveltime
corrections remove the regional trend but the traveltime corrections do
not accurately remove the effects of shear-velocity heterogeneity
because the depth variations of the 410-km discontinuity are not
perfectly resolved (compare Figures 11b and 10d). The 660-km
discontinuity. a horizontal boundary in T5-410-S40, is resolved with the
same east-to-west deepening without traveltime correction. After
traveltime corrections, the 660-km discontinuity does not have a
distinct harmonic or tectonic structure but the topography indicates
that the corrections are imperfect or that the effects of the 410-km
discontinuity on the traveltimes may be projected as spurious 660-km
discontinuity structures. As shown also in Figure 9, the ray-theoretical
corrections are too strong, but smaller than a factor of two (Figure 9).
The map of MTZ thickness, with a distorted checkerboard pattern, remains
largely unchanged before and after the traveltime correction, though the
checkerboard depth ranges become larger. The synthetic test of the joint
effect of 3D mantle structure and undulations on the 410-km
discontinuity suggests that the MTZ thickness can be determined more
robustly than the absolute depths of 410-km and 660-km discontinuities
using the CRP imaging approach.