P-S polarity reversal: is it always at zero offset?

Two numerical seismic modeling experiments have been carried out to investigate whether the polarity reversal on 2D P-S data always occurs at zero-offset. The models included divergent reflectors and a depth-variant Vp/Vs. The modeling results showed that there are situations where the polarity change of P-S events on the in-line geophone will not be at zero-offset. For a Vp/Vs that decreased with depth, the displacement of the normal incidence P-S ray from zero-offset was in the up-dip direction. This displacement was in the range of 50 - 200 m for dips up to 22.5° and reflection depths less than 2500 m, but in rare geological situations where the dip was up to 60°, it was found to be as high as 550 metres for a reflection depth of 2100 m. When the interface was flat, the polarity reversal occurred at zero-offset.