When a wavefront from a conventional seismic source strikes an interface, the reflected energy is partitioned into P (pressure) and S (shear) waves.

This diagram shows an incoming P-wave *P*_{inc} travelling at velocity
*V*_{1} striking the interface between the upper (yellow) material and the
lower (blue) material. Two reflected waves and two transmitted waves are produced as a result:

- Reflected shear:
*S*_{reft} - Reflected pressure:
*P*_{reft} - Transmitted shear:
*S*_{trans} - Transmitted pressure:
*P*_{trans}

The figure on the right
is from an animation of P-wave to S-wave conversion. When viewing the animation, Note the velocity
difference between the P and S waves. Typical sedimentary rocks have a *V*_{P}/*V*_{S}
ratio of between 1.5-2.5 .

Also note the difference in the direction of particle motion relative to the direction of wave propagation.
P-waves have particle motion in the **same direction** as wave propagation, while shear waves have particle
motion in a direction **perpendicular** to wave propagation.

This is, of course, a very simple animation, where wave propogation is represented by a one-dimensional 'string of beads' model.

The *P-P* and *P-S* reflectivity
varies as a function of the media parameters and the incident angle.

The reflection and transmission functions are described by the *Zoeppritz* equations.

Here is a plot of the P-P reflectivity and the P-S reflectivity as a function of incident angle and the ratio
between the *V*_{P} and *V*_{S} in the top layer.

This can lead to very significant Amplitude Versus Offset (AVO) affects.