Note that Snell's law is also valid for the reflection angle if V2 on the right hand side of the equation is replaced by V1. Figure 2 is incomplete for the general elastic case because it neglects the phenomenon of seismic mode conversions. The angles of reflection and refraction for the converted raypaths can also be calculated using Snell's law. As was the case in Figure 2, all changes in density and acoustic velocity in a layer-cake model are confined to the layer interfaces. When a seismic wave encounters an interface, it is partitioned into a reflected wave that bounces off the interface and a refracted wave that crosses the interface, but may change its propagation direction.
Seismic velocities vary greatly with the type of rock or medium.
These materials, together with shales and carbonates, are the main components of the world's sedimentary basins. Using seismic methods, it is usually possible to derive estimates of subsurface seismic velocities. Unfortunately, even perfect knowledge of the seismic velocity does not provide a unique identification of rock type.