Birch's law

Birch's law, discovered by the geophysicist Francis Birch, establishes a linear relation between compressional wave velocity v_p and density ${\displaystyle \rho }$ of rocks and minerals:

${\displaystyle v_{\mathrm {p} }=a({\bar {M}})+b\rho }$

where ${\displaystyle \,{\bar {M}}\,}$ is the mean atomic mass in formula units and ${\displaystyle \,a(x)\,}$ is an empirical function determined by experiment.