P-wave and S-Wave Differences

a.  The S-wave velocities are slower than P-wave velocities for the same geology.

b.  As fluids and gasses have shear moduli of 0, S-waves are not affected by the high compressibility of gas or fluid in pores, unlike P-waves.

Clean sand is much more sensitive to gas saturation than shaly sand.

As shallow, less-consolidated rocks have significantly lower S-wave velocities, the Vp/Vs ratio will increase for shallower rocks and decrease for deeper, more consolidated rocks. As the Vp/Vs ratio increases, the amplitude increases more with offset than for more consolidated rock (Castagna and Swan, 1997)

What this means is that P-waves are slowed down in porous rock, while the S-waves are not affected by the porosity. Since the S-waves are already slower, if the P-waves are slowed, they may coincide with S-waves, resulting in an increase in return signal amplitude (a "bright spot") that has nothing to do with the actual reflectivity of the reflectors. 

c.  P-waves are not affected by polarization but S-waves are. S-waves therefore can only be recorded if the recorder matches their polarity. Also, S-waves will not propagate through a polarized medium (such as aligned fractures). Instead, an S-wave will be split into complex modes, one with particle motion aligned to the polarized trend and the other perpendicular to that trend. This second portion is the slowest of the two new waves.