Zoeppritz equations
In seismology, the Zoeppritz equations describe how seismic waves are transmitted and reflected at media boundaries, which are boundaries between two different layers of earth.
The Zoeppritz equations, formulated by the German geophysicist Karl Bernhard Zoeppritz, relate the amplitudes of P-waves and S-waves at each side of an interface.
For instance, a blast of dynamite will create a seismic wave (P-wave) that will travel through the earth, be reflected off of various layers, and return to the surface where it can be detected. The time it takes to return is related to the depth of the various layers. By detecting the seismic waves at various points on the surface, one can see how the reflections change with angle of incidence. One can then use this information along with the Zoeppritz equations to learn more about the density and velocity of each layer. This is helpful for instance in locating underground reservoirs and deposits.
For practical purposes, various forms of approximations are usually used.