As in the previous studies, the models are parametrized in terms of 2 by 2 cells and 18 layers in depth through the mantle and the ak135 model is used as a reference. A partitioned inversion algorithm allows the tomographic inversion to be performed iteratively so that 290,000 equations need to be solved at a time rather than the full set of 580,000. Different heterogeneity regimes can be recognised in the upper mantle and transition zone, in the bulk of the lower mantle and in the lowermost mantle through the relative variation of the two wavespeeds. In the upper mantle many features can be explained by thermal effects but there is a suggestion of a compositional component in shield structures. In some orogenic zones, e.g. western North America, the opposite sense of the bulk-sound and shear wavespeed variation requires compositional effects or volatiles to outweigh any thermal effects.
Subduction zones have a clear shear signature but somewhat variable character in bulk-sound.
In the lower mantle, the pronounced narrow structures which may represent remnant subduction are most marked in shear waves. In general, the level of bulk-sound speed variation relative to the shear wavespeeds diminishes with depth in the lower mantle reaching a minimum near 2000 km. Below this depth the amplitude of variation of both wavespeeds increases, but as the core-mantle boundary is approached there is very little concordance in the variations of the two wavespeeds which suggests the presence of widespread chemical heterogeneity.