Observations of acoustic noise bursts accompanying nonlinear internal gravity waves on the continental shelf off New Jersey

Abstract

Anomalously large, transient fluctuations of acoustical noise intensity, up to 4–5 orders of magnitude above the background, were observed with single-hydrophone receiver units (SHRUs) and on the L-shaped horizontal and vertical line array of hydrophones (HVLA) in the Shallow Water 2006 experiment on the continental shelf off New Jersey. Here, temporal and spatial properties of these noise bursts are investigated. As tidally generated nonlinear internal waves (NIWs) move across the site of the experiment from the shelf break toward the coast, they form trains of localized, soliton-like waves with up to 25–35 m displacement of isopycnal surfaces. The NIW trains consecutively cross the positions of five SHRUs and HVLA that are located about 5–8 km from each other along a line perpendicular to the coast. The noise bursts were observed when a NIW train passed through locations of the corresponding acoustic receivers. Turbulence of the water flow, saltation, and bedload of marine sediments were the dominant causes of the acoustic noise bursts caused by NIWs at different frequency bands. On near-bottom hydrophones, the most energetic part of the observed noise bursts is attributed to collisions of suspended sediment particles with each other, the sensor, and the seafloor.

Noise of Internal Solitary Waves Measured by Mooring-Mounted Hydrophone Array in the South China Sea

“…Sound propagation is the most effective method of information transmission in the ocean, and ISWs have a great impact on underwater sound propagation because they cause time-dependent spatial variations in the water temperature and sound speed [11][12][13][14]. The drastic variations in the sound propagation characteristics under ISWs can cause changes in arrival time, propagation path, transmission loss, horizontal refraction and other factors of acoustic waves [15,16].…”

“…In addition to sound propagation characteristics, ISWs have been observed to contribute substantially to ambient noise by breaking waves on the sea surface, increasing the internal velocity of currents, and stirring marine sediments [12]. There are three types of noise induced by ISWs, which have been widely studied.…”

“…Rip-band noise can increase the ambient noise by 18 dB at frequencies of 5-15 kHz [17,18]. Near-bottom currents induced by ISWs generate the second type of noise by moving sediments [12,19]. After the sediment particles have been moved away from the seabed, they produce noise by colliding with each other and with the shells of near-bottom hydrophones.…”

“…When ISWs flow past a hydrophone and an entire mooring system, pressure fluctuations occur in the turbulence [12]. This noise is referred to as flow noise (the third type), usually below 100 Hz [23].…”

“…There have been a number of works on the effects that internal waves have on underwater noise [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. However, noise induced by ISWs in the South China Sea has rarely been studied directly-which is also true of the mechanism of the noise.…”