Seismic Velocities
In environmental geophysics seismic energy sources are typically a small explosion or a hammer blow, which generate three fundamental types of elastic waves: P (primary, compressional) waves; S (secondary, shear) waves, and surface waves. The P and S waves propagate through the body of the earth, while the surface waves can exist only close to the free surface. Since P and S waves are predominately used in environmental geophysics, surface waves are not included in this discussion.
P waves have a particle motion in the direction of propagation, while particles under the influence of S waves move transverse to the direction of propagation. S waves cannot propagate through liquids because liquids have no shear rigidity.
P waves velocities are typically 50% higher than S waves. Vp and Vs are governed by the elastic moduli (Young's modulus (E), Poisson's ratio (ν), and Bulk modulus (κ)) and the density (ρ) of geologic material.
P-wave velocity = Vp = √ [(κ + 4/3μ)/ρ]
S-wave velocity = Vs = √ (μ/ρ)
where, ρ = density; k = bulk modulus; and μ = shear modulus
Seismic velocities are usually expressed in SI units of meters per second. Occasionally, seismic velocities are expressed as kilometers per second or meters per millisecond.