# Mudrock line

In rock physics and petrophysics, the mudrock line, also called Castagna's equation or Castagna's relation, is an empirical linear relation between seismic P-wave velocity and S-wave velocity in siliciclastic rocks (i.e. sandstones and shales).

${\displaystyle V_{\mathrm {P} }=1.16V_{\mathrm {S} }+1.36\ }$
Where ${\displaystyle V_{\mathrm {P} }}$ and ${\displaystyle V_{\mathrm {P} }}$ refer to P-wave velocity and S-wave velocity, respectively. Velocities are given in kilometers per second (km/s).