Observations and modeling of angular compression and vertical spatial coherence in sea surface forward scattering

Abstract

Measurements and modeling of spatial coherence and related angular spreading associated with forward scattering from the sea surface are presented. The measurements were taken in waters 80 m deep off the New Jersey coast in August 2006. Acoustic signals from a source at depth 40 m were recorded on a vertical line array of length 1.4 m, centered at depth 25 m, and at range 200 m. Measurements in the 14-20-kHz frequency range are reported; the rms waveheight H was 0.16 m, setting kH as approximately 10 where k is acoustic wavenumber. A systematic study of measurements taken over four source-receiver bearing angles separated by 90 degrees suggests a null influence of changing bearing angle or equivalently directional wave effects. Sound speed was characterized by a downward-refracting profile. Refraction modifies the vertical angular spread due to rough sea surface scattering, which can be understood from Snell's law. The Snell mapping is smooth, so an approximation based on the mean grazing angle provides a functional relation between the angular variance near the surface and that at the receiver. The latter is measurably reduced owing to refraction, the effect called angular compression, and a parameter that quantifies this effect is defined.