Passive seismic imaging is based on the relation between the reflection and the transmission responses of the subsurface. Let one have the transmission responses measured at surface points A and B of a 3D inhomogeneous medium in the presence of white noise sources in the subsurface. When these transmission responses are cross-correlated, one obtains the reflection response of the same medium as if measured at point A in the presence of an impulsive source at point B. The quality of the simulated reflection response strongly depends on the whiteness and the distribution of the noise sources. Reflectors present below the sources cause the appearance of some ghost events. Random distribution of the noise sources will, however, weaken these ghost events.