P-waves and S-waves

The Second Wave

Every synthetic so far has been acoustic: it modelled only the compressional P-wave, in which rock particles oscillate back and forth along the direction the wave travels, squeezing and stretching the medium. Real rock is elastic, and it carries a second wave: the shear S-wave, in which particles move across the direction of travel, distorting the shape of the rock without changing its volume.

Slower, and Fluid-Blind

Two properties of the S-wave, both on show above, run the rest of this part. First, it is slower. Rock resists a change of shape less stiffly than a change of volume, so shear travels more slowly than compression, and the ratio $V_p/V_s$ is typically between 1.5 and 2. That is why the P-wave in the top row runs ahead of the S-wave below it.

Second, and far more useful, the S-wave needs rigidity to exist. A fluid, gas, oil, or water in the pores, has no shear strength, so it cannot carry a shear wave at all. Shear velocity therefore responds to the rock frame and barely to the fluid, while P velocity responds to both. The difference between how P and S see a rock is a direct readout of its fluid, and that difference expresses itself in how a reflection's amplitude changes with angle. This is why an acoustic model cannot see fluids the way an elastic one can, and it is the physical foundation of AVO, amplitude versus offset, which the rest of Part 6 builds. To get there, the next section watches P and S energy split at a boundary.