Mid-frequency measurements of array signal and noise characteristics

Experiments using seismic-type arrays with lengths in terms of wavelengths λ, from 20λ at 50 Hz to 143λ at 340 Hz have been conducted in the Mediterranean Sea and Northwest Atlantic Basin to ranges of 750 km. Signal-gain cumulative distribution functions (CDFs) were experimentally determined as a function of acoustic aperture and integration time. We found that for an array 143λ long that when the combined effects of array shape and multipath vertical arrival angle structure were contained in an off-broadside beam; when the coherent integration times were O(10 s); when peak tracking was used; and when incoherent averaging was O(35 min); then array signal gain degradations 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 differences in vertical arrival angle of the paths, array shape deformation, and beam wander due to system motion. After compensation for array shape and motion, the major environmental cause of signal gain degradation, 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 small, phase randomness due to volume scattering appears to be the limiting factor. Beam noise levels (BNLs) forward of broadside were found to be dominated by coherent arrivals from the bottom-reflected tow-ship noise. Consequently, aft beams were utilized to measure the CDFs for the ambient BNLs. Ambiguous BNL results at different headings yielded an average directional response consistent with the shipping distributions for moderate aperture lengths (50λ) with BNLs decreasing with 3 dB per aperture doubling between 25λ and 50λ. Different and more varied results were found for apertures between 50λ and 100λ, showing that beam-noise statistics change as the system resolves individual ships