A method of obtaining information about the positions of sources in a marine seismic source array that has N seismic sources, including at least Nunique types of sources that are nominally non-identical to one another, where 1≦Nunique≦N−1. The method comprises measuring the pressure field at M independent locations of the N sources, where M>Nunique, and obtaining information about the positions of sources from the M pressure measurements and from the constraint that at least two of the sources are nominally identical to one another.