P- & S-wave seismics

Comparison of P-wave & S-wave seismic

Longitudinal and transverse elastic vibrations can propagate within an elastic medium and are called seismic body waves. The propagation behaviour of these waves is determined by the elastic properties of the medium and their spatial distribution. Observing and analysing the propagation of these waves  therefore enables conclusions to be drawn about the elastic properties and their distribution within the medium. This made it possible, for instance, to use observations of the propagation of waves from the Earth's surface to reveal the layered internal structure of the earth for the first time with its solid core, liquid mantle, and solid crust.

Longitudinal seismic waves (compression waves, also known as P-waves) are usually used to explore geological structures, e.g. for economic deposits. These waves can be relatively easily generated by explosions or percussion, and provide information on the underground structure to depths down to crustal scales. The fact that they react sensitively to the presence of liquids and gases in underground formations made them interesting early on for the exploration of oil and gas, which also meant that a great deal of investment has been made in research into this wave type.

The use of transversal waves (shear waves, also known as S-waves) tends to be of more subordinate importance for underground exploration activities − with the exception of marine applications. Seismic shear waves can be polarised - similar to light waves − and their propagation properties depend on the orientation of their vibrations to the bedding. This requires much more complex and therefore much more expensive measuring technology − particularly for the required stimulation of the shear waves. However, shear waves have been gaining increasingly in importance for some time now because their slower propagation velocity compared to P-waves means they can improve the structural resolution. And their non-sensitivity to liquid and gas concentrations in the pore spaces, in particular, proves to be an advantage. Shear-wave velocities are also increasingly used as an important indicator when investigating shallow geotechnical aspects.