DEEP-OCEAN VERTICAL NOISE DIRECTIONALITY

Measurements of vertical noise intensity versus angle with arrays near the depth of the sound channel axis in the deep ocean have shown that the low-frequency distribution (< 200 Hz) is broadly peaked about the horizontal (0°), whereas the higher frequency (~ 400 Hz) distribution is peaked at the sound fixing and ranging (SOFAR) angles (~ ±15°), with a minimum at the horizontal. This 0° effect has been attributed to the noise from ships over the basin margins by Wagstaff [1]. However, because the spectral variation of the 0° effect is smooth and has also been observed in areas of the world where shipping is sparse and, furthermore, because the low-frequency noise from ships is known to include tonals, broadband wind noise must also be considered. In 1986, Bannister [2] attributed this behavior to high-latitude winds and the shallowing sound channel found in the Southern Hemisphere waters. The structure of the beam noise measured at the output of a vertical array in a range-dependent ocean basin was investigated using the modified wide-angle parabolic equation (PE). Noise sources were distributed throughout the basin, and the field due to each noise source at an array located in mid-basin was calculated. The response of the array to the superposition of these noise sources was found by beamforming. An efficient and direct approach was developed that superimposes the noise sources on the PE field as the field is marched toward the array. Thus, only one PE run is needed in a cylindrically symmetric ocean basin. Downslope calculations of the mid-basin vertical directionality were made between 50 and 400 Hz with this technique. Use of a geoacoustic model shows that the bottom behaves as a low-pass filter. The low-frequency energy at higher angles interacts with the bottom and is converted to low-angle energy in the deep sound channel, while at the higher frequencies energy is absorbed. The computed vertical directionality distributions are shown to compare favorably with previous observations made by others and then summarized by Carey and Wagstaff [3]. The shallowing sound-channel calculations were performed for a northern Pacific 2000-km track which features a shallowing of the sound-channel axis as one proceeds to higher latitudes. Calculations between 50 and 400 Hz of the mid-basin directionality for an array near the sound-channel axis were found to exhibit a dual-“horned” distribution, even at the lower frequencies. Absolute levels imply that wind noise may be an important component of the vertical noise directionality. © 1990 IEEE