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 brine-saturated siliciclastic rocks (i.e. sandstones and shales).

The equation reads:

${\displaystyle V_{p}=1.16V_{s}+1.36}$

Where ${\displaystyle V_{p}}$ and ${\displaystyle V_{s}}$ refer to P-wave velocity and S-wave velocity, respectively.^[1] Velocities are given in kilometers per second (km/s).

The name of the equation refers to John Castagna, professor of exploration geophysics at the University of Houston,^[2] who discovered the relation while working for ARCO, an oil company that is now a subsidiary of Marathon Petroleum.