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

M. Gratziella Kirtland Grech, Donald C. Lawton, Robert R. Stewart, Saúl Ernesto Guevara, Gary F. Margrave

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 degrees and reflection depths less than 2500 m, but in rare geological situations where the dip was up to 60 degrees, 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.