In the first section of this document it was noted a portion of acoustic wave traveling down through the earth was reflected any boundary between two layers where a contrast in acoustic impedance existed. The discussion to this point has ignored the fact that the same is true for waves traveling back to the surface, that is very time a contrast of acoustic impedance is encountered a some of the energy continues upwards, but some of the energy is back down. The result is that there are many paths a signal may take in traveling between a source and a receiver reflecting off different boundaries. The signal via the path that involves a single reflection between the source and receiver is called the primary reflection. The received signals that have experienced more than one reflection are called multiples. Mutliples are unwanted reflections that make it more difficult to determine what the subsurface looks like. The following figure depicts a variety of the types of multiples that can be received.
Figure 10 - Multiples