The sensitivity of SS precursors to the presence of topography on the 660-km discontinuity is addressed using an axisymmetric finite difference approximation to the SH wave propagation in the Earth mantle. Numerical experiments are lead to quantify the bias in both wavelength and amplitude committed in estimating depth information on the upper-mantle discontinuities from ray inversion of SdS arrival times. It is further shown that long period S660S arrival-times do not provide information on the deepening of the '660' in presence of a velocity increase expected as the thermal signature of a subducting slab.¿ 1997 American Geophysical Union