MID-FREQUENCY MEASUREMENTS OF ARRAY SIGNAL AND NOISE CHARACTERISTICS

Experiments using seismic-type arrays with lengths in terms of wavelen gths lambda, from 20 lambda at 50 Hz to 143 lambda at 340 Hz have been conducted in the Mediterranean Sea and Northwest Atlantic Basin to ra nges of 750 km. Signal-gain cumulative distribution functions (CDF's) were experimentally determined as a function of acoustic aperture and integration time. We found that for an array 143 lambda long that when the combined effects of array shape and multipath vertical arrival an gle structure were contained in an off-broadside beam; when the cohere nt integration times were O(10 s); when peak tracking was used; and wh en incoherent averaging was O(3-5 min); then array signal gain degrada tions were O(1 dB). However, when incoherent averaging O(3-5 min) was used without peak tracking the average signal-gain degradation was O(3 dB). Degradations in signal gain were found to be caused by the diffe rences in vertical arrival angle of the paths, array shape deformation , and beam wander due to system motion. After compensation for array s hape and motion, the major environmental cause of signal gain degradat ion, for off-broadside arrivals, was the vertical arrival structure of the paths, a characteristic of the sound channel. Broadside arrivals are less sensitive to these effects and, when the deformations are sma ll, phase randomness due to volume scattering appears to be the limiti ng factor. Beam noise levels (BNL's) forward of broadside were found t o be dominated by coherent arrivals from the bottom-reflected tow-ship noise. Consequently, aft beams were utilized to measure the CDF's for the ambient BNL's. Ambiguous BNL results at different headings yielde d an average directional response consistent with the shipping distrib utions for moderate aperture lengths (50 lambda) with BNL's decreasing with 3 dB per aperture doubling between 25 lambda and 50 lambda. Diff erent and more varied results were found for apertures between 50 lamb da and 100 lambda, showing that beam-noise statistics change as the sy stem resolves individual ships.