The stiffness of soils depends on the state of stress.  Therefore, the velocity of wave propagation can be used to assess the state of stress and its changes.  Indeed, given sufficient data, measurements can be inverted to produce a tomographic image of the stress field.  A preliminary study of this approach was conducted with simulated data and is summarized briefly.  Results of an experimental study are then presented.  Three common geotechnical systems were tested: a footing, a moving surface load, and a retaining wall.  Calculated velocity changes are compared with predicted velocity changes, with stress changes being estimated by classical analytical solutions.  Good agreement is found in three cases.  The need for accuracy in travel-time measurements, difficulties in tomographic imaging, and the propagation of uncertainty from inverted velocities to stresses are discussed.