Measurements and modeling of vertical spatial coherence and related angular spreading associated with forward scattering from the sea surface are presented. The van Cittert-Zernike theorem is used in the modeling. The measurements were conducted in waters 80 m deep off the North American continental shelf (Shallow Water 06, Aug. 2006). Acoustic signals were recorded on a moored array system (MORAY) consisting of three arrays (each 1.4 m): two vertical line arrays at depth 25 m and 50 m, and one horizontal array at depth 27 m. The source (1-20 kHz) was deployed at depth 40 m from the R/V Knorr, at ranges 50-500 m from the MORAY. Conditions were characterized by a downward-refracting profile, e.g., sound speed going from 1530 m/s at the surface to 1490 m/s at depth 25 m. Refraction modifies the vertical angular vertical spread due to scattering, which can be understood from Snell's law. The Snell mapping is smooth, and a Taylor expansion around the mean grazing angle provides a functional relation between the angular variance near the surface and that at the receiver associated with scattering. The latter is measurably compressed owing to refraction, an effect mirrored in the spatial coherence data. Research support from ONR.

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