Abstract: A method and apparatus are disclosed for processing seismic trace data to distinguish Class II sand formations sandwiched between low and high porosity layers, where the intermediate sand layer causes a polarity reversal with offset in seismic reflections. A velocity analysis of data from a vertical seismic section is generated to include coherency contours corresponding to polarity reversals in the seismic traces by using an extended crosscorrelation algorithm for multi-trace coherency. The velocity analysis thus indicates a stacking velocity that will maximize coherency of the seismic traces exhibiting the polarity reversal.