Removing tilt noise from the vertical component data of ocean bottom seismograph: A case study on the data from the Pankun test in the South China Sea
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摘要: 由于恶劣的海洋环境，被动源海底地震仪（OBS）的数据中的噪声水平通常远高于陆地台站。海底地震仪与底流相互作用产生的噪声严重影响海底地震仪的数据质量，特别是水平分量数据的信噪比。基于水平分量和垂直分量的噪声数据之间的相关性，利用水平到垂直分量的传递函数方法可以去除垂直分量的倾斜噪声，本文以2019—2020年磐鲲海底地震仪南海测试的数据为例，描述了该方法的原理和过程，并对比了去除倾斜噪声前后的地震波形以及瑞雷面波的频散特征。结果显示，去除倾斜噪声明显提高了海底地震仪数据的地震波形低频段的信噪比，使得地震面波更有利于进行海洋岩石圈深部结构成像；该研究还表明尽管磐鲲海底地震仪的地震计调平系统使地震计的倾角1.0°远小于其容倾角2.5°，在底流作用下，海底地震仪还是产生了明显的倾斜噪声。因此，地震计调平系统的性能对于海底地震仪的数据质量有着非常重要的影响。Abstract: Due to the harsh marine environment, the noise level in the data of the passive source ocean bottom seismograph （OBS） is usually much higher than that of the land station. The noise generated by the interactions between the seismometer and bottom currents strongly affects the data quality of OBS, especially the signal-to-noise ratio of horizontal component data. Based on the correlations between the noise data of the horizontal and the vertical components, we can remove the tilt noise from the vertical component by using the transfer function of the horizontal-to-vertical component. Using the data from the 2019−2020 Pankun OBS South China Sea test, This paper describes the theory and process of this method. It compares the seismograms before and after removing the tilt noise and highlights the improvement of the dispersions of Rayleigh surface waves. The results show that removing tilt noise can improve the signal-to-noise ratio of seismograms in the low-frequency range, resulting in surface waves more conducive to imaging the deep structure of the oceanic lithosphere. This study also shows that although the tilt angle （1.0°） of the seismometer of Pankun OBS is much smaller than the tolerance of the tilt angle of the instrument （2.5°）, the bottom currents still generate a significant level of tilt noise on OBS data. Therefore, the leveling system of the instrument is a crucial component affecting the OBS data quality.
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Key words:
- ocean bottom seismograph （OBS） /
- seismic noise /
- tilt noise /
- removing noise
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图 1 磐鲲OBS在南海的测试位置
红色圆形为台站K02，黑色圆形为其余五个台站
Figure 1. Test sites of Pankun OBSs in the South China Sea
The red circle represents station K02，the black circles represent the other five stations
图 2 K02台站噪声数据筛选过程
（a）垂直分量；（b-c）水平分量频谱；（d）水平分量的噪声时间窗口选择示例数据来自2020年3月18日，（d）中的星号表示该窗口被接受，而没有星号表示该窗口没有通过基于日平均值设置的阈值，在本例中，有一个窗口（箭头所指）被排除。
Figure 2. Spectrogram for the vertical （a） and horizontals （b，c） components. And the noise time windows selection （d）
Example is from March 18，2020. A star in （d） indicates that the window is accepted，while the absence of a star indicates that the window does not pass the threshold set based on the daily average. In this example，one window （indicated by the arrow） is excluded.
图 3 台站K02布设期间水平与垂直分量的最大相关性方向（a）及对应的相关性大小（b）
（a）中的角度表示从水平分量1顺时针转过的角度，得到的方向即为倾斜的方向（H分量的方向）。
Figure 3. Maximum correlation direction （a） and corresponding correlation value （b） of horizontal to vertical component during station K02 deployment
The angle in （a） represents the degrees turned clockwise from the horizontal component 1，and the obtained direction is the direction of tilt （the direction of the H component）.
图 4 水平到垂直分量传递函数的增益系数（a）、相关性（b）和相位（c）
示例数据来自2019年12月25日，紫色背景框表示相关性最大的频段。
Figure 4. Admittance （a），correlation （b） and phase （c） of the horizontal to vertical component transfer function.
Example is from December 25，2019. The purple background box indicates the frequency band with the greatest correlation
图 5 台站K02布设期间的倾斜角度（a）及对应的相关性大小（b）
Figure 5. Tilt angle （a） and corresponding correlation value （b） during station K02 deployment.
图 6 去除倾斜噪声前后的波形及PSD对比
（a）去除倾斜噪声前后的功率密度函数曲线；（b）原始地震波形；（c）去除倾斜噪声后的地震波形。地震波形均滤波至0.1-0.01 Hz。左图为 2020年3月18日巴厘岛南部附近发生的Mw6.2地震；右图为 2019年12月25日东太平洋海隆南部附近发生的Mw6.1地震。
Figure 6. Comparison of waveforms and PSDs before and after removing tilt noise
（a） PSD curves before and after removing tilt noise； （b） The raw seismic waveforms；（c） The seismic waveforms after tilt noise removed. The seismic waveforms were filtered to 0.1-0.01 Hz.The left figure is Mw 6.2 earthquake occurred near the south of Bali on March 18，2020; The right figure is Mw 6.1 earthquake occurred near the Southern east Pacific Rise on December 25，2019.
图 7 去除倾斜噪声后的垂直分量波形与附近陆地台站的比较（滤波至0.1—0.01 Hz）
（a） 地震震中（黄色星形）、海底台站K02（蓝色三角形）和陆地台站（黑色三角形）的位置；（b） 三个台站垂直分量的波形对比，蓝色波形为去除倾斜噪声后的OBS数据，黑色波形为陆地台站的数据，虚线分别表示了P波（红色）和S波（蓝色）的预测到时。
Figure 7. Comparison of vertical component waveforms after tilt noise removed with nearby land stations （filtered to 0.1−0.01 Hz）
（a） Location of earthquake epicenter （yellow star），ocean-bottom station K02 （blue triangle） and land stations （black triangles）；（b） The comparison of waveforms of the vertical component of the three stations. The blue waveform is the OBS data after the tilt noise removed，the black waveforms are the data of the land stations，and the dashed lines represent the predicted arrival time of P wave （red） and S wave （blue），respectively.
图 8 原始地震波形（a）与去除倾斜噪声后的地震波形（b）的频散特征
Figure 8. Dispersion characteristics of the raw seismic waveform （a） and the seismic waveform after tilt noise removed （b）
图 9 台站K03布设期间的倾斜角度（a）及对应的相关性大小（b）
Figure 9. Tilt angle （a） and corresponding correlation value （b） during station K03 deployment
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