Equation describing a fundamental relationship between seismic velocities (v_p and v_ω‎), the gravitational acceleration (g), and the adiabatic change in density (dρ‎) within the Earth (assuming only hydrostatic pressure) as a function of radium (dr):

This equation is directly applicable to the lower mantle and outer core, but is invalid where the composition is variable, the pressure is not hydrostatic, or the increase in pressure is not adiabatic.