For most sedimentary interfaces, the PP and PS reflection coefficients, R PP and R PS , have opposite sign. In such cases, the corresponding PP and PS events have the same display polarity on their respective seismic sections. In processing and interpreting PS seismic sections, one normally relies on correlation of corresponding PS and PP events in the so-called registration process. Getting the polarity wrong on an event on the PS section can lead to a mistie of at least half a cycle between correlated events on the two sections and, consequently, over- or underestimated V P /V S ratios for the affected intervals. An unexpected polarity switch could conceivably even inhibit the correct correlation altogether and lead to a totally spurious correlation.
We examine under just what conditions an event will be recorded with opposite display polarities on PP and PS and show that the sign of R PS depends on the sign of ΔU , where U = ρβ h and h is a function of P and S velocities as well as densities. In this analysis we derived an accurate small-angle R PS approximation, one that we show is better than the Aki-Richards expression in many situations, not just for small angles.
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