The parameter ν = ∂ ln υs/∂ ln υp, the ratio of relative S to P velocity heterogeneity, can be measured seismically and is important in determining the mineral physics of the deep mantle. However, seismic results are inconclusive because they have generally been based on P and S velocity models which are similar, but not proportional, and which are derived from seismic data sets in different frequency ranges and sample the mantle in different ways. Hence current values of νare subject to inco...