### 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
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*Appl. Phys. Lab., Univ. of Washington, Seattle, WA 98195
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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.]