An elastic body wave in which particle motion is in the direction of propagation. The type of seismic wave assumed in conventional seismic exploration. Also called primary wave (undae primae), compressional wave, longitudinal wave, push-pull wave, pressure wave, dilatational wave, rarefaction wave, and irrotational wave. In an isotropic homogeneous solid, the P-wave velocity VP can be expressed in terms of the elastic constants and the density (ρ):
where λ and μ are Lamé’s constants, E is Young’s modulus, and σ is Poisson’s ratio. See also wave notation and Sheriff and Geldart (1995, 44–45).