Surface wave studies which use only minor arc data do not seem to reliably constrain degree 2 but studies at low angular order but studies which use great circle data give answers which are generally consistent with the modal analyses. These are examples of data sets that could constrain the 3D short-wavelength structure of the new 3D reference model. Such maps are composed at various frequencies and are then inverted for 3D mantle structure. There is general agreement about the location of long-wavelength structure. But the small-scale features still vary considerably from author to author.
Anomalies are percentage phase velocity perturbations.
The signal in the shorter period maps is usually larger than in the longer period ones, indicating that 3D structure is concentrated at shallower depth. The maps are expanded in surface spherical harmonics where the truncation level is indicated in the table below. They are normalized so that the spectrum of a delta-function would be flat. While the long-wavelength patterns generally agree in phase, there is still some disagreement in the spectral amplitude. G. Laske and G. Masters, Constraints on global phase velocity maps from long-period polarization data.