In seismic experiments, waves that bounce back and forth between layers are called multiple reflections. Since these waves interact repeatedly with the heterogeneities in the earth, the relation between the multiple reflections and the earth structure is nonlinear. Present-day seismic processing is geared toward suppressing these multiples rather than extracting the information that is contained in this part of the wavefield. The main motivation for this is that the retrieval of earth structure from multiply-scattered waves is assumed to be an inherently unstable process so that random noise or unwarranted assumptions magnify under the nonlinear inversion steps needed to map the multiple reflected waves onto a model for the earth's interior. It is for these reasons that Claerbout (1985, 363) notes that: “Multiple reflections is a good subject for nuclear physicists, astrophysicists, and mathematicians who enter our field. Those who are willing to take up the challenge of trying to carry theory through to industrial practice are rewarded by some humility.”

Scales, J. A., & Snieder, R. (1997). Humility and nonlinearity. GEOPHYSICS, 62(5), 1355–1358.