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The exact interferometric equation gives the relation between the retrieved Green’s function and signals arriving from an enclosed surface integral of sources (monopole and dipole sources). This relation is exact, even for complicated media. If only monopole sources are assumed as needs to be done for all practical cases, retrieval can be far from perfect in heterogeneous media. We show that this is due to intermodal cross terms that will appear even if the source distribution is perfect. Especially when sources are present at the free surface only, cross terms have the potential to overwhelm higher modes completely. Luckily one-bit correlation overcomes this problem by the amplifying higher modes.