Correlation between Underwater Noise and Sea Level at Ieodo Ocean Research Station

Byoung-Nam Kim,Seong Hyeon Kim,Bok Kyoung Choi

doi:10.3390/jmse9010001

Abstract

To investigate the correlation between underwater noise and sea level, data collected at Ieodo Ocean Research Station (from 15 May to 27 August 2013) were analyzed. Temporal variations in the overall level of underwater noise were similar to those in sea level. The average underwater noise levels at frequencies below 50 Hz were 99.0–106.6 dB during spring tides and 78.4–90.2 dB during neap tides, with a difference of 12.6–20.6 dB. When low-pass filtering at six hours was applied to the absolute value of sea level, a high correlation coefficient of 0.7 for its association with underwater noise was obtained. This strong correlation could indicate flow noise caused by currents. Underwater noise levels around the frequency of 16 Hz were relatively strongly correlated with sea level (correlation coefficient of 0.71). Wavelet analysis of the overall level of underwater noise showed strong periodicity at 0.25, 0.50, 1.0, 16.0, and 30.0 days, as well as characteristics of a tidal constituent. In wavelet coherence analysis, the overall level at a period of 0.25 days exhibited a phase difference of −90 degrees relative to sea level. This finding indicates that the noise level peaks at 1.5 h before the high tide or low tide, which was confirmed in the correlation analysis of noise level and sea height with various time lags. This study shows that the overall level of underwater noise from ebb currents is higher than that from flood currents due to differences in current velocity.

Highlights

The northern area of the East China Sea has complicated flow patterns due to the coexistence of a variety of currents (Kuroshio, Jeju Warm Current, and others) and tides [1]
Low-pass filtering at 6 h w applied to overall levels of underwater noise for long-period analysis of the correlatio between noise level and sea level
The continuous wavelet analysis was performed to a alyze the coherence between the overall level of underwater noise and sea level

Summary

Introduction

The northern area of the East China Sea has complicated flow patterns due to the coexistence of a variety of currents (Kuroshio, Jeju Warm Current, and others) and tides [1]. Previous studies on flow noise in the ocean caused by tides have mostly investigated the relationship between sea level and flow velocity [16,17,18]. Sea level was measured every 10 min from 15 May to 27 August 2013 using a wave radar device (RangeFinder, Miros, Norway) that was installed on a steel frame at IORS and located at a height of 24 m above the sea surface These data were rearranged in 30-min intervals for comparison with the overall underwater noise level measurements. A recording Doppler current meter (RDCP 600, Aanderaa) was installed on the sea bottom a few tens of meters away from IORS As this instrument was frequently lost due to fishing operations around IORS, no current velocity data were collected in 2013. We used current velocity data obtained every 30 min from 17 October 2009 to 20 February 2010

Correlation and Continous Wavelet Transform Analyses

Findings

Discussion

Conclusions