We investigate the upper mantle velocity structure through processing first arrival data from peaceful nuclear explosions. The reported 2D model has been obtained by ray tracing for a spherical Earth, unlike the classical plane-approximation approach with subsequent spherical symmetry corrections, which is not always applicable to a laterally heterogeneous subsurface. The upper mantle velocity highs and lows imaged to 200-220 km depths show obvious correlation with major structures of the craton basement. Namely, low-velocity zones are observed beneath basins, the largest (to 8.0-8.1 km/s) under the Vendian-Early Cambrian Sayan-Yenisei syneclise. A discontinuous high-velocity layer (8.6-8.7 km/s) at depths between 150 and 240 km is underlain by a zone of lower velocity (8.50-8.55 km/s) down to the 410 km discontinuity, where the velocity at the top of the transition zone is 9.4-9.5 km/s.