So far the studies have been based on the assumption that the ambient background noise consists of `random plane waves incident from below'. Whether or not this is a valid assumption is an open question; however, people have observed relatively strong seismic events coming from the reservoir interval due to micro-fracturing during hydrocarbon production. It is also likely that the surface noise will propagate into the subsurface, and in turn be reflected and recorded. So if the dominant source of ambient noise is not plane waves incident from below, but rather point sources within the zone of interest, is the conjecture still valid?
Figure 3 shown a ray diagram for a point source. As with the case with the velocity anomaly, the kinematics of the reflection event are not affected by the location of the source; although the kinematics of the direct wave are. Again this suggests that although there may be problems with the kinematics of the direct wave, the reflection event may be correct.