Seismic Velocity-Density Relationships

Definition

Seismic velocity is defined as the speed with which a seismic wave travels through a medium and is measured in km/s. The density is the mass per unit volume and is expressed in g/cc.

Types of seismic waves

The body wave and the surface wave are the main seismic waves. The body waves are the longitudinal (P) and the shear (S) waves in which the particles vibrate parallel and perpendicular to the direction of wave propagation respectively. The velocities of P-wave (V _P) and S-wave (V _S) are related to the density, ρ as

$$ {V_{\text{P}}} = \sqrt {{\frac{{\kappa + \frac{4}{3} \mu }}{\rho }}}$$

(1)

$$ {V_{\text{S}}} = \sqrt {{\frac{\mu }{\rho }}}$$

(2)

where κ and μ are the bulk and rigidity moduli respectively. It appears that the seismic velocity and density are inversely related, but the denser rocks are characterized by higher velocities compared to lighter rocks because the density also depends on κ and μ. The halite with low density (1.8 g/cc) and high V _P(4.5 km/s) shows...