FIG. 3. A synthetic example of imaging lateral aquifer heterogeneity: (A) radar velocity model, where the velocity decreases gradually from the surface to the water table, then drops sharply, simulating full water saturation (a clay layer is present at 16 m; 3- by 3-m-high velocity anomalies are placed in the vadose and saturated zones); (B) simulated common midpoint normal moveout (NMO) stack comparable to a conventional fixed offset ground-penetrating radar survey; (C) unconstrained reflection tomogram and (D) results of prestack depth migration (PSDM) using (C); (E) tomogram with linear gradient and boundary discontinuity constraints and (F) results of PSDM using (E). Unconstrained inversion has good lateral resolution but poor vertical resolution of the anomalies. Allowing velocity discontinuities across horizons and applying a linear gradient constraint produces excellent resolution of the anomalies.