A promising technique for high resolution geophysical imaging has been proposed which is based on the inversion of an exact solution to the reduced wave equation. The method is known as backpropagation because it propagates a received waveform back into the host formation mapping wavespeed variations. The implementation of backpropagation is complicated by the fact that it requires a complete characterization of the wave source and the wave propagation properties of a homogeneous host medium. The imaging procedure can be simplified by utilizing a zeroth--order phase approximation which allows implementation with first arrival times. Numerical experiments and a field study suggest that this approach is a practical means for high resolution subsurface imaging. ¿ American Geophysical Union 1990