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 as:
for some function a(x).
Birch's Law was discovered by Francis Birch and allows for the seismic speed of a P-wave to be represented as a function of with regard to the atomic weight of the substance the wave is moving through.
Birch's law can be used in the discussion of geophysical data. The law is used in forming compositional and mineralogical models of the mantle by using the change in the velocity of the seismic wave and its relationship with a change in density of the material the wave is moving in. Birch's law is used in determining chemical similarities in the mantle as well as the discontinuities of the transition zones. Birch's Law can also be employed in the calculation of an increase of velocity due to an increase in the density of material.
It had been previously assumed that the velocity density relationship is constant. That is, that Birch's Law will hold true any case, but as you look deeper into the mantle, the relationship does not hold true for the increased pressure that would be reached as you look deeper into the mantle near the Transition zone (Earth). In such cases where the assumption was made past the Transition zone (Earth), the solutions may need to be revised. In future cases, other Laws may be needed to determine the velocities at high pressures.
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