The cross-correlation of acoustic wave fields at two receivers yields the exact Green's function between these receivers, provided the receivers are surrounded by sources on a closed surface. In most practical situations the sources are located on an open surface and as a consequence the illumination of the receivers is one-sided. In this Letter we discuss the conditions for accurate Green's function retrieval for the situation of one-sided illumination. It appears that the Green's function retrieval method benefits from the fact that the earth is inhomogeneous, without relying on assumptions about disorder.