Comparison of P- & S-wave seismics

Within an solid, elastic body, vibrations can propagate using longitudinal and transversal kinds of particle motions, which are called seismic body waves. The propagation behaviour of these waves depends on the elastic parameters of the solid body and their distribution in space. The observation and analysis of the wave propagation within the solid body allows deducing to the elastic parameters and their distribution. Thus, e.g. the observation of the seismic waves at the earth surface had not until allowed an insight into the earth interior of the spheroid construction of solid inner core, liquid mantel, and solid crust.

During the exploration of the subsurface e.g. searching for deposits longitudinal seismic waves (preasure waves, also called P-waves) are commonly in use. These can be generated relatively easy by explosives or hammer strike and provide findings about the subsurface structure down to nearly 50 km depth. Especially their sensitivity for fluids and gas in the subsurface made them attractive in the early stages of hydrocarbone exploration, which led to a significant cash flow for the research of this wave type also.

The utilisation of transversal waves (shear waves, also called S-waves) is more subordinated in subsurface exploration, except of some marine applications. Seismic shear waves are polarized e.g. like light waves and their propagation behaviour depends on the wave orientation to the layering. This requires – particularly with regard proper defined shear wave generation – a more sophisticated and therefore more cost intensive technique. Since a couple of time shear waves gain in importance because they enable higher resolution compared to P-waves due to their lower propagation velocity. In doing so there is an advantage due to their non-sensitivity for fluid and gas in the pore volume. Furthermore the shear wave velocity also gain in importance as an indicator for shallow geotechnical responsibilities.