The word migration as it applies to seismic imaging is definitely a misnomer. It is believed to have arisen because oil migrates up dip since it is less dense than water. This knowledge proved to be exploration dynamite. Once understood, explorationists exploited it by looking for anticlines rather than synclines—and the California fields around the Brea tar pits became history. Analogously, dipping events on unmigrated seismic sections move up-dip on the final imaged or migrated section, so using the term migration in place of the more accurate imaging terms was quite natural.
It is also quite natural to think of seismic migration as being somewhat akin to photographic imagery. An image is captured, either digitally or on film, by recording the result of passing a reflected source of light (the sun or artificial light) through a properly focused lens on a photographic plate, film, or charge coupled device (CCD). This works because light travels in a straight line at a known constant speed and the lens, when focused, refracts the light to collect it in the proper place on the plate or CCD. We can think of this process in three steps. First, the light wavefield travels out from the source in all directions until it strikes a non-transparent reflector. Second, the reflected wavefield passes through the lens to form the image. Third, the camera's shutter captures an instant in time to record the final image. It is safe to say that radar imagery operates in much the same way and the only real difference lies in the construction of the "lens."
However, seismic migration differs from the photographic process in many ways. Sound replaces light (or radar or electro-magnetic sources) as the imaging source, and the speed of sound in subsurface rocks is definitely not constant, and it cannot be assumed to travel in a straight line. Moreover, as we will see later, each and every sound source, regardless of type, may generate three different, but coupled, wavefields as the energy spreads. As far as the author knows, there is no simple seismic analogy to the photographic lens.
Perhaps a better way to say this is that the lens for each seismic imaging effort is essentially unique to that effort. In a sense, this observation is the most crucial difference between imaging with sound and imaging with light. In the former case, we must somehow estimate the lens during the seismic imaging process. This lens is called the Earth model. In its simplest form, an Earth model is a three-dimensional velocity field that describes the subsurface speed of a compressional sound wavefield. In simple terms, a compressional wave is one wherein the particle motion occurs along the direction of propagation and represents a compression followed by a rarefraction of the particles. In its most complex form, an Earth model also includes the sound speeds of two additional waves called shear waves because the particle motion is perpendicular to the direction of propagation. An Earth model may also include other rock properties that influence the way in which sound propagates through the earth, but those will be of little interest here.
Seismic imaging can be considered to be a data-processing technique that creates an image of the earth's structure from the data recorded by a seismic reflection survey.
