Seismic interferometry recovers the Green's function between two receivers by cross-correlating the field measured from sources that surround the receivers. In the seismic literature, it has been widely reported that this processing can produce artifacts in the Green's function estimate called "spurious multiples" or the "virtual refracted wave." The spurious multiples are attributed to the head wave and its multiples and travels in the seabed. The head wave phenomenon is shown to be observable

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... from both controlled active sources and from ocean ambient noise and for both vertical and horizontal arrays. The processing used is a generalization of the passive fathometer to produce cross-beam correlations. This passive fathometer is equivalent to the seismic interferometry techniques for delay and sum beamforming but not for adaptive beamforming. Modeling and experimental data show the head wave is observed in ocean noise and can be used to estimate the seabed sound speed.