In a marine seismic air-gun array, each gun location does not necessarily coincide with a node in a finite-difference grid. Especially for coarse-grid, finite-difference modeling, this problem must be handled with care since there might be up to three or four air guns between adjacent grid points. The real sources are represented by fictitious monopole- and dipole source functions located at grid nodes. The effective sources are estimated from the extrapolated pressure field at a horizontal surface located below the sources. We find that an array consisting of eight guns separated by a distance of 3 m and located at 7.5 m depth can be approximated by six monopole- and dipole source functions distributed on a finite-difference grid with 10 m spatial sampling. The residual error energy norm between the actual wavefield and the corresponding finite-difference wavefield observed on a fictitious streamer placed at 95 m depth is less than 0.5 percent.