D"

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 is 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.

References

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