Near surface velocity irregularities distort the seismic wavefield and cause focusing anomalies in the data. These anomalies can be detected by examining plots of trace power in midpoint-offset coordinates.
I generate synthetic data by upward continuing a synthetic wavefield through a v(x,z) model. The location of the velocity anomaly is determined by analyzing a plot of trace power in midpoint-offset coordinates. By performing iterative slant stacks of the power plots, I can image the velocity anomaly. Finally, I downward continue the wavefield through the velocity anomaly in order to remove distortions from the data.