The effects of slope-enhanced shipping noise on the ambient noise field vertical directionality

Recent measurements of the ambient noise field vertical directionality have indicated that the noise level is concentrated within ± 20° of the horizontal. Within this region as many as three separate peaks have been observed. Two peaks near ± 15° have been attributed to distant deep ocean shipping, and a peak near the horizontal has been attributed to slope enhancement of ship noise along the continental shelf edge and in the vicinity of oceanic ridges and seamounts. In addition, ships in near-Arctic waters can also provide noise at shallow angles to receivers located in temperate waters as a result of range-dependent refraction. The effects of these mechanisms on the vertical directionality of the noise from shallow sources are considered using a PE propagation model which provides the vertical directionality at a receiving array. It is concluded that continental slope enhancement is a significant source of low-angle noise in many oceanic regions. Some effects of slope enhancement on the noise field horizontal directionality are also discussed.