What is a converted wave?

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 V₁ 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

Energy conversion in action

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.

View the animation in Quicktime format (recommended) (143k) or MPEG format (135k).

Reflectivity variation with offset

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.

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