1. A method for deriving the far-field signature of a marine seismic source array from near-field measurements of acoustic signals, comprising the steps of: (a) deploying, in a body of water, a marine seismic source array that includes a plurality of source elements; (b) deploying, in said body of water, a signature streamer including a plurality of sensors, said sensors being spaced apart by less than one-half the length of the shortest wavelength of interest, the length of said signature streamer being greater than the length of said source array by an amount such that the signal amplitude contributions by the sensors positioned at the extremities of the signature streamer are less than a desired percentage of a summation of all signals from said sensors; (c) disposing said signature streamer beneath said source array at a desired depth such that the change in the sensor signal envelopes, due to lateral offset of the signature streamer relative to said source array, in terms of phase shift and amplitude due to a ghost effect is small; (d) triggering the elements of said source array to produce an acoustic wave; (e) recording the output signals from said sensors, due to said acoustic waves, on an archival storage medium; (f) processing said recorded signals by wavefield extrapolation to define a far-field source signature of said array; and (g) utilizing said far-field source signature for enhancing the quality of processed reflection seismic data.