Vertical directionality of the noise field at a deep ocean site off Cape Hatteras

Abstract

The vertical ambient noise field was measured during April 1978 at a deep ocean (5400 m) site approximately 250 nm southeast of Cape Hatteras using a 31 element array (10 m hydrophone separation). Data has been analyzed using FFT narrowband processing and beamforming over the frequency range 45–100 Hz. The mean noise level due to surface shipping was observed to decrease rapidly beyond ±20° in elevation angle. Beyond ±20° multiple bottom-reflected arrivals were observed from nearby (<30 nm) ships. The long-range arivals were poorly defined in angle with the exception of those near 0°. Propagation models suggest that the near 0° arrivals represent noise from coastal shipping which has propagated through the oceanic front defined by the Gulf Stream and been channeled to near broadside angles. Noise between 10° and 20° off broadside is shown to differ spectrally from that in the near broadside region suggesting propagation from a different section of the ocean. The results are compared to theoretical models and the differences discussed.