什么是 Reference Earth model？

A reference Earth model is a table of the speed of P waves, the speed of S waves and the density against depth, the same in every direction, chosen to agree with the observed travel times of seismic waves and with the mass of the Earth.

A reference Earth model is a spherically symmetric Earth, given as the seismic speeds $v_P(r)$, $v_S(r)$, the density $\rho(r)$ and, in the fullest models, the attenuation as functions of the radius $r$ alone, that fits the observed travel times, free-oscillation periods, the Earth’s mass and its moment of inertia. The most widely used is PREM, the Preliminary Reference Earth Model (1981).