The interpretation of refraction profiles that traverse laterally varying velocity structures has been hindered by lack of a practical algorithm for computing synthetic seismograms to compare with observations. However, a modification of zero-order asymptotic ray theory to incorporate the amplitudes of rays that turn in a velocity gradient, as well as reflected waves, allows the computation of high-frequency synthetic seismograms for laterally varying velocity structures. The method is general in that synthetic seismograms may be computed for any structure through which rays can be traced.
We have modeled seismic refraction data from the Imperial Valley, California, by applying this method. A major feature of our model is a sedimentary column that thickens from ∼4 km at the Salton Sea to ∼5.5 km at the United States-Mexico border. To approximate the observed amplitude behavior, a dipping velocity discontinuity was modeled near 13-km depth, but velocity gradients were found to be more appropriate than sharp boundaries in the rest of the model.