Bruce K. Newhall
Appl. Phys. Lab., Johns Hopkins Univ., Johns Hopkins Rd., Laurel, MD 20723-6099
An extremely long horizontal towed array was used to perform high-resolution noise measurements at very low frequencies. The directional measurements in one shallow site are compared with more extensive measurements in deep water. The very low-frequency noise at all sites is highly anisotropic due to the high azimuthal resolution of the discrete merchant shipping sources that dominate underwater noise at these frequencies. The deep water measurements were performed at sites in three oceans, covering an extreme range of shipping density conditions, however, the shapes of the azimuthal distributions of the noise levels are relatively insensitive to shipping density. The shallow water noise exhibits azimuthal spreading not apparent in the deep water. From the loudest sources this spreading may partly obscure the spatially quiet regions that are otherwise present in directions between loud ships. Although they have different shape parameters, both the shallow and deep water data fall between Weibull and gamma distributions, and are well fit by generalized gamma distributions. The effects of multipath fading are diminished by the spatial averaging across the long aperture. Low-resolution noise distributions, obtained from subapertures of the array, are dominated more by multipath fading, and approach gamma distributions.