A modal approach to Arctic noise modeling with implied vertical coherence

Abstract

Analytic formulas of the modal attenuation coefficients appropriate for deep Arctic under-ice conditions, and modal energy partitioning of ambient noise sources are proposed. These functions can be used, together with various assumed spatial and temporal distribution functions to calculate various statistical ambient noise functions, such as vertical coherence. Vertical noise data between 15 and 50 Hz have been analyzed for model comparison, using a bandwidth of about 5 Hz. These data show that the vertical coherence usually has a sin x/x dependence, with the first zero crossing at one wavelength. Sometimes, however, zero crossing lengths of more than two wavelengths are observed. Longer correlation lengths are consistent with much of the noise energy radiating from a small region, although other interpretations are possible. Initial simple model calculations show agreement with the data. A strong nearby source of noise will cause the correlation to lengthen, while several more distant sources support the one wavelength zero crossing result. Initial sensitivity calculations, however, show that the vertical coherence function by itself is not a good validator of the model functions. Other statistics will be sought to further examine model validity. [Work supported by NORDA and NOSC 6.2 Arctic programs.]