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:

\[V_p = 1.16 V_s + 1.36\]

Where \(V_p\) and \(V_s\) 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 BP.


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  2. "University of Houston directory"