Presently very little is known about the characteristic length scales of the structures in the upper mantle. In generating synthetic seismograms and in large-scale inversions, one usually assumes that the upper mantle is smooth, so that ray theory can be used. However, it is known that structures in the mantle exist on a scale of a few hundred kilometers, notably near subduction zones and other tectonic features. In this paper, a model of all the subduction zones and spreading ridges is used to investigate the effect of small-scale structures on normal mode frequency shifts and on global inversions. The normal mode frequency shifts for this model have the same characteristics as observed normal mode frequency shifts. Furthermore, the global models obtained from inversions of synthetic data for the model of subduction zones and spreading ridges are similar in character to the models obtained from global inversions of real data. For a realistic set of wave paths, the model obtained from the synthetic inversions differs appreciably from a smoothed version of the true model. This is due to an uneven path coverage and to a lesser extent due to neglecting scattering effects. This means that the models obtained from global inversions of seismological data may contain appreciable artifacts. ¿1991 American Geophysical Union