*3.4. Spectrum Comparison of ISW Noise at Different Depths*

To illustrate the depth dependence of ISW noise, Figure 7 shows the power spectra comparison as a function of the frequency for five hydrophones at depths of 200 m, 300 m, 500 m, 700 m, and 950 m. As shown in Figure 7, the distant noise component below 100 Hz was derived from the flow noise induced by ISWs. The fundamental frequencies of the noise differed substantially because of the velocity difference of internal waves at different depths. *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 10 of 14

**Figure 7.** Power spectra comparison of ISW noise from five hydrophones at different depths from 18:06 to 18:08 UTC+8 on 14 March 2017. 18:06 to 18:08 UTC+8 on 14 March 2017.

The noise spectrum levels of the hydrophones at 200 m, 500 m, and 700 m were less than those at 300 m and 700 m. The key was the structure of the entire mooring system. The vibration amplitude of the cable decreased with the increase of tension [38,39]. The hydrophone, C050, at 200 m, was close to the main floater at the top of the cable system. quency of low frequency noise was 6.15 Hz. **Figure 7.** Power spectra comparison of ISW noise from five hydrophones at different depths from The noise spectrum levels of the hydrophones at 200 m, 500 m, and 700 m were less than those at 300 m and 700 m. The key was the structure of the entire mooring system. The vibration amplitude of the cable decreased with the increase of tension [38,39]. The hydrophone, C050, at 200 m, was close to the main floater at the top of the cable system.

Hydrophone C061, at 500 m, approached the central floater where the ADCPs were lo-

floater at 500 m divided the cable into two sections, shorter at the top and longer at the bottom. The longer the vibration length of the cable, the greater the vibration amplitude

This section compares noise measurements from the 2019 Internal Solitary Wave Observation Experiment. The experiment was located on the continental shelf of the South

Figure 8 shows the temperature data and the noise of the same internal wave received by two hydrophones. The maximum amplitude of this ISW was 91.74 m which arrived at 19:42 UTC+8. Figure 8b shows the time–frequency spectrum below 100 Hz recorded by a hydrophone located at depth of 200 m, which was the middle hydrophone of the mooringmounted linear array. The length of the cable in this experiment was about 300 m. The fundamental frequency of low frequency noise was 10.20 Hz, with uniform harmonic

The results confirmed that the flow noise induced by ISWs was indeed caused by the VIV of the cable system. The noise in figure 8c was from the near-bottom hydrophone at a depth of 340 m, 17.8 km west of the mooring-mounted linear array. The length of the cable that held the hydrophone was 30 m. When the same ISW flowed past the near-bottom hydrophone, the noise intensity was substantially reduced. The fundamental fre-

China Sea with an average water depth of 360 m.

**4. Comparison of ISW Noise in 2019 ISW Observation Experiment** 

[39].

components.

Hydrophone C061, at 500 m, approached the central floater where the ADCPs were located. Hydrophone C010, at 950 m, was located near the fixed object at the bottom of the cable. Hydrophone C058 had a larger spectrum level than hydrophone C051, because the floater at 500 m divided the cable into two sections, shorter at the top and longer at the bottom. The longer the vibration length of the cable, the greater the vibration amplitude [39].
