Images courtesy of Tom Davis
The three major wavefields are shown: compressional (P-P), converted (P-S) and pure shear waves, which all travel at different velocities -- hence, time scales are different. The p-wave is the fastest, and the shear wave travels at roughly half the speed of the p-wave. The shear wave is sensitive to anisotropy in the subsurface and splits into two waves, making shear waves a tool to characterize, measure and monitor anisotropy, which aids in evaluating fracture regimes. In the example above, the reservoir interval being studied (5,000-7,500 feet in depth) is the Mesaverde gas reservoir at Rulison Field, Piceance Basin, Colo.