# Birch's law

Birch's law establishes a linear relation of the compressional wave velocity vp of rocks and minerals of a constant average atomic weight Mavg with density $\rho$ as:

$v_p = a(M_\mathrm{avg}) + b \rho,$

for some function a(x).[1]

Francis Birch published two papers[2][3] in 1961, which later became known as Birch's Law.