# Dictionary:Raypath parameter

1. For horizontal velocity layering, the quantity ${\displaystyle p={\frac {dt}{dx}}={\frac {1}{V}}\sin(\alpha )}$, where ${\displaystyle {\frac {dt}{dx}}}$ is the reciprocal of apparent velocity, ${\displaystyle V}$ is instantaneous velocity, and ${\displaystyle \alpha }$ is the angle a raypath makes with the vertical. Used to specify a raypath. See tau-p mapping.
2. For a spherical velocity layering model (as used when considering long earthquake paths), the raypath parameter is ${\displaystyle p={\frac {r}{V}}\sin(\alpha )}$, where ${\displaystyle r}$ is the Earth’s radius.