Is the lower mantle homogeneous?

Comparison of some array dt/dΔ studies with the global travel times of Dziewonski and Anderson (1983) leads to the conclusion that a discontinuity in the P travel times between 80° and 85° is consistent with both sets of data. This discontinuity in dt/dΔ corresponds to an increase in velocity of about 0.1 km/sec between depths of 2400 and 2600 km. Models of the P velocity distribution which match the Dziewonski and Anderson travel times reasonable well have the shadow zone for short-period “diffracted” P beginning at about 110° arc distance.