D"

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The notations D' and D" are the sole survivors of a labelling of the internal layering of the Earth from A to G introduced by Keith Edward Bullen in 1947. A was the crust; B the mantle down to 400km depth; C the mantle from 400 to 1000km depth; D the mantle from 1000km depth to the core; E and F the outer core; and G the inner core.

D" is the lower most 150-120 km of the mantle. It's believed that the velocity is reduced in the D" due to the chemical heterogeneity and interaction between the outer core and mantle. There is a weak P-wave diffraction in the D" into the Shadow Zone at a distance between 103 and 130 degree.

[edit] References

Fowler, C.M.R. (2005) The Solid Earth: An Introduction to Global Geophysics (2nd ed.). Cambridge University Press. ISBN 978-0521584098