The propagation of energy via waves is a familiar phenomenon in our everyday life. The particular waves to be studied here are seismic waves which are intentionally created to image the interior of the earth ,  and .
Our three dimensional earth consists of more than the geological structures we are accustomed to thinking about, much of the earth is fluid or fluid-like. Here the principal fluids of interest are hydrocarbons. The other essential fluid to consider is water. The fluid-like materials include the many gases trapped in earth, gases like carbon dioxide, helium, and natural gas. Actually, we may not be aware of the less visible subsurface structure, but all of the surface geology you can observe, and more, exists in some form under the surface.
To find accumulations of petroleum requires an intimate knowledge of the subsurface geology, the history of the material source and the structure of the subsurface. A reservoir requires porosity, a sealing mechanism, and a hydrocarbon source. The storage capacity is dependent on the porosity, the seal prevents leakage of the hydrocarbons, and the source generates the hydrocarbons. Note, the source rocks are not always the same as the reservoir rocks.