1. n. [Geophysics]

An elastic constant that is a measure of the compressibility of material perpendicular to applied stress, or the ratio of latitudinal to longitudinal strain. This elastic constant is named for Simeon Poisson (1781 to 1840), a French mathematician. Poisson's ratio (σ) can be expressed in terms of properties that can be measured in the field, including velocities of P-waves (V_P) and S-waves (V_S) as shown below.

σ = ½ (V_P² − 2V_S²) / (V_P² − V_S²)

Note that if V_S = 0, then Poisson's ratio equals 0.5, indicating either a fluid, because shear waves do not pass through fluids, or a material that maintains constant volume regardless of stress, also known as an ideal incompressible material. Poisson's ratio for carbonate rocks is ~0.3, for sandstones ~0.2, and greater than 0.3 for shale. The Poisson's ratio of coal is ~0.4.

See: elastic constants,  P-wave,  S-wave,  velocity