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 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.


  1. ^ Castagna, J. P.; Batzle, M. L.; Eastwood, R. L. (1985). "Relationships between compressional-wave and shear-wave velocities in clastic silicate rocks". Geophysics 50: 571–581. doi:10.1190/1.1441933. 
  2. ^ "University of Houston directory"