Mudrock line

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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).

The equation reads:

Where and refer to P-wave velocity and S-wave velocity, respectively. Velocities are given in kilometers per second (km/s).

The name of the equation refers to John Castagna, a professor of exploration geophysics at the University of Houston, who discovered the relation while working with ARCO, an oil company that is now a subsidiary of BP.


  • Castagna, J. P., M.L Batzle, R.L. Eastwood, 1985, Relationships between compressional-wave and shear-wave velocities in clastic silicate rocks: Geophysics, 50, 571–581.