Are we exceeding the limits of the great circle approximation in global surface wave tomography?

The theoretical great circle approximation surface wave tomography is found to be limited to Earth models with a maximum degree 1 less than or equal t o 30 for surface waves at 40 s and 1 less than or equal to 20 for surface w aves at 150 s. This result holds for both phase velocity and group velocity maps. The highest resolution in present-day global surface wave tomography is close to these limits of ray theory. In order to obtain higher degree r esolution models of the Earth in future surface wave tomography, it is nece ssary to take the scattering of surface waves into account. Increasing the data coverage in seismological networks will not improve the details of tom ographic images if ray theory is still applied. It is essential to include the finite-frequency effects as well.