ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

2pUW3. On spatial coherence and angular spread of sound forward scattered from the sea surface: Measurements and interpretive model.

Peter H. Dahl

Appl. Phys. Lab., Univ. of Washington, Seattle, WA 98195

Measurements of the spatial coherence of high-frequency sound forward scattered from the sea surface are discussed, along with a simple interpretive model. The data originate from an experiment off the southern California coastline using the research platform FLIP. Measurements were made using omnidirectional sources suspended from a spar buoy (tethered to FLIP) and a line array mounted on FLIP's hull, with range varying between 500 and 1000 m. The frequency was 20--40 kHz and the roughness parameter (chi)=2kh sin (theta) was >>1 (where k is the acoustic wave number, h is the rms waveheight, and (theta) is the grazing angle); thus the measurements represent high-frequency, incoherent scattering in a single surface bounce channel. The coherence was measured across a line array oriented transverse to the direction of propagation, thereby giving an estimate of the horizontal coherence. Horizontal coherence to the degree of horizontal angular spreading and to the performance of beam forming arrays can be related. The simplest model with which to interpret these results is derived from the high-frequency, large-roughness limit of the Kirchhoff approximation. This model satisfactorily explains the data, and provides basis for a predictive model for horizontal coherence and angular spreading in surface forward scattering. [Work supported by ONR.]