A method to tomographically image the spatial variation of group velocity across a region of geologic interest has been developed. The tomographic algorithm used represents a development of the Simultaneous Iterative Reconstruction Techniques (SIRT). It includes a ray tracing capability. The principal advantage of this method over previously described methods is that it permits the dispersion function to vary continuously over the region. Computer simulations and the results from some field tests in Texas demonstrate the utility of this method.