Abstract
For the past 13 years, Defence Research Establishment Atlantic has been actively engaged in measuring properties of the low‐frequency ambient noise field in the North Atlantic Ocean and Canadian coastal waters. Concurrently, there has been a strong emphasis on developing surface‐suspended arrays that can reliably and inexpensively extend such measurements down to the infrasonic range. Special attention has been paid to hydrophone design and shrouding, and to minimizing the effects of sea‐surface motion and current shear. The success of this program has led to the construction of small superdirective research arrays, and more recently, quiet large‐aperture vertical and horizontal line arrays. Near‐surface noise directionality measurements made in midocean with these arrays complement reported results from bottom‐moored research arrays. The measurements of vertical directionality, in particular, show distinct trends, with arriving low‐frequency noise becoming more concentrated towards the horizontal in the northern ocean, and more diffuse with decreasing frequency. Azimuthal anisotropies are less pronounced, with some bias towards busy shipping lanes and fishing grounds.
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