Analysis of Ambient Noise Spectrum Level Correlation Characteristics in the China Sea

Jianbo Zhou

doi:10.1109/access.2019.2963331

Abstract

The ambient noise field in the ocean is formed by a mixture of many source types, such as seismic noise, marine noise, shipping noise, and wind-generated noise. The correlation matrices of the ambient noise spectrum levels offer a promising approach for identifying and isolating the active source mechanisms in ambient noise datasets. This study attempts to use frequency correlation matrices to examine inter-frequency relationships and identify frequency bands that are dominated by specific sources present in samples of ambient noise. Deep-ocean ambient noise recorded by hydroacoustic monitoring systems in the East China Sea and South China Sea was used to analyze the frequency correlation characteristics over a broadband frequency range, with a focus on the contribution of wind-related and shipping-related source mechanisms to the overall ambient noise field at various frequencies. The primary sources observed in both the South China Sea and East China Sea were shipping sources and wind sources. During the observation window, wind was responsible for a significant amount of the ambient noise energy above 477 Hz in the East China Sea, while the corresponding frequency threshold was shifted up to 575 Hz in the South China Sea. Another interesting feature was also observed in the East China Sea: a higher degree of correlation was found between the levels at the wind noise dominated frequencies and mixture noise (mainly consisting of wind noise and shipping noise) dominated frequencies at the upper receiver than at the lower receiver. Data analysis results show that the cause of this effect is that shipping activity contributes more to the soundscape at the lower receiver than at the upper receiver.

Highlights

The spectral characteristics of ambient noise in the deep ocean are continually changing due to changes in the nature and positions of the many sources that contribute to it [1] as well as the fluctuations in transmission associated with varying oceanographic conditions [2]–[5]
The ability to use correlation matrices to identify active sound sources was further exploited by Miksis-Olds and Nichols in 2016 [36], by analyzing the primary drivers behind multiyear changes in ambient noise levels, these authors provided a reasonable explanation for the decreasing trend in low-frequency noise (5-115 Hz) in the Equatorial Pacific and South Atlantic Oceans observed in recent years
Frequency correlation matrix analysis was performed on ambient noise data recorded in the South China Sea and the East China Sea to quantify the predominant sources driving the noise levels in different frequency regions

Summary

INTRODUCTION

The spectral characteristics of ambient noise in the deep ocean are continually changing due to changes in the nature and positions of the many sources that contribute to it [1] as well as the fluctuations in transmission associated with varying oceanographic conditions [2]–[5]. In 2013, they [32] applied this technique to identify and isolate sources present in large low-frequency ambient noise datasets and successfully identified microseism, earthquake, airgun and whale noise from deep ocean data They [33] investigated the relationship between the wind speed and the sound levels in different frequency bands to determine the prominence of wind-related noise in the combined ambient noise spectrum. The ability to use correlation matrices to identify active sound sources was further exploited by Miksis-Olds and Nichols in 2016 [36], by analyzing the primary drivers behind multiyear changes in ambient noise levels, these authors provided a reasonable explanation for the decreasing trend in low-frequency noise (5-115 Hz) in the Equatorial Pacific and South Atlantic Oceans observed in recent years.

THEORY OF NOISE FREQUENCY CORRELATION

CONCLUSION