Spatial coherence of signals forward scattered from the sea surface in the East China Sea

Measurements of sea surface forward scattering, wind speed, and directional wave spectra made in 100 m of water in the East China Sea are discussed. The experiment was part of the Asian Seas International Acoustics Experiment (ASIAEX) conducted in the spring of 2001. Signals were received at ranges near 500 m on 2 vertical line arrays that were co-located but separated in depth by 25 m. Estimates of the vertical spatial coherence along these arrays as a function of frequency, path geometry, and sea surface environmental conditions are compared with a model for spatial coherence. The model is based on identifying the probability density function that describes vertical angular spread at the receiver position. An alternative approach utilizing the van Cittert–Zernike theorem from statistical optics is shown to give equivalent results. Both approaches require computation of the sea surface bistatic cross section, done here with the small slope approximation. Forward scattering from the sea surface represents an important channel through which sound energy is transmitted, and spatial coherence determines in part the performance of imaging and communication systems that utilize the sea surface bounce path.