  An explicit expression for P-wave velocity is proposed to develop a novel
tomographic technique in a spherically symmetric model of the Earth (MZY). The
distribution of the P velocity structure in the mantle is determined using only
34 P- and 2 PcP-observed traveltimes. By applying a non-linear inversion, the
P-residuals in the range between 0 and 100 degrees are minimised up to a
maximum value of 0.015 s. Furthermore, from the high quality computation of PcP
traveltimes, with residuals much better than 0.13 s, it is possible to infer
the existence of a brief low velocity layer in the D" region. This is then
followed by a gradual increasing in the velocity profile towards the core,
which begins at a depth of 2893.9 km.
