Short duration events on OBS recordings in the Northwest Sub-basin of the South China Sea
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摘要: 本文对2019年10月—2020年5月在中国南海西北次海盆布放的宽频带海底地震仪的数据进行了分析，利用长短时窗均值比算法在单个海底地震仪台站拾取了多达25万个短时事件（SDE）。对拾取到的SDE信号特征进行了分析，并根据信号特征将事件分为连续短时事件（C-SDE）和随机短时事件（R-SDE）。探讨C-SDE和R-SDE的信号源，可能的产生机制，结果表明：在布放时间段内中国南海西北次海盆存在着活跃的C-SDE信号源可能来自人类活动，如气枪放炮或者低频水声通信；R-SDE则来源于离台站很近的源，很可能是海底沉积层中的气体逃逸过程造成的沉积层破裂所导致，表明中国南海西北次海盆海底沉积层中存在气体泄露。Abstract: Based on the analysis of the data on broadband ocean bottom seismographs deployed in the Northwestern sub-basin of the South China Sea from October 2019 to May 2020. We detected more than 150 000 short duration events （SDEs） at each OBS station by using STA/LTA algorithm. The characteristic of SDE signals is analyzed. According to different signal characteristics, these SDEs are classified as consecutive short duration events （C-SDEs） and random short duration events （R-SDEs）. The possible generation mechanisms for C-SDEs and R-SDEs are analyzed, respectively. We infer there are active sources of C-SDE signal in the Northwest Sub-basin of the SCS. The source may be caused by human activities, such as air gun firing or low frequency hydroacoustic communication signals. The R-SDE sources appear close to the OBS stations, likely generated by the ruptures due to gas escape from seafloor sediments. R-SDEs on OBS recordings indicate persistent gas escape from the sedimentary layer in the northwestern sub-basin of the SCS.
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Keywords:
- ocean bottom seismographs （OBS） /
- short duration events /
- STA/LTA /
- gas escape
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图 1 典型短时事件的三分量波形（左）和幅度频谱（右）
（a） 东北太平洋布放的OBS080台站记录的鲸鱼叫声产生的短时事件（数据来源：IRIS ，网络号：X9）（ Kuna，Nábělek，2021）；（b） 马尔马拉海布放的OBS01台站记录的海底沉积层中气泡逃逸产生的短时事件（ Tsang-Hin-Sun et al，2019）
Figure 1. Waveforms （left） and amplitude （right） spectra of typical SDEs
（a） Three-component waveforms and amplitude spectra of SDEs produced by whale calls on OBS080 stations （IRIS data. Network：X9） in the Northeast Pacific （Kuna，Nábělek，2021）；（b） Three-component waveforms and amplitude spectra of SDEs from bubble escape in seafloor sediment recorded by OBS01 in the Sea of Marmara （Tsang-Hin-Sun et al，2019）
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图 2 海底地震仪在中国南海西北次海盆布放位置
Figure 2. Sites of OBSs in the Northwest Sub-basin of the South China Sea
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图 3 单个（a）和连续（b）SDE波形序列及其STA/LTA的特征曲线
粉色区域为噪音数据，浅绿色区域为SDE信号数据
Figure 3. The waveform and the character curves of a single SDE （a） and the consecutive SDEs （b） in detections using STA/LTA method
The pink box is noise data and the light green box is the SDE signal data
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图 4 台站K02，K03和K08不同时段拾取到SDE的数量分布
Figure 4. Temporal distribution of the SDE number in OBS stations K02，K03and K08
（a） 2019−10—2020−05；（b） 2020−05−03—10；（c） 2020−01−03—10
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图 5 C-SDE波形图
（a） K02，K03和K08台站垂直分量上的C-SDE信号，R1，R2，R3，R4分别表示R-SDE， E1表示地震事件；（b） K02，K03和K08台站上垂直分量的单一GE事件组波形；（c） K08台站上三个分量的波形
Figure 5. Waveform of C-SDEs
（a） The vertical components of C-SDE signals on OBS stations K02， K03， K08 and R1，R2，R3，R4 are the R-SDEs，and E1 is an earthquake event；（b） The GEs waveforms of vertical components on OBS stations K02，K03，K08；（c） The waveforms of the three components on K08 station
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图 7 K02 （a），K03 （b）和 K08 （c）台站观测到的GE信号的波形及频谱图
Figure 7. The waveforms and spectra of GE signals on OBS stations K02 （a），K03 （b），K08 （c）
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图 8 K08台站地震信号（a）与典型R-SDE信号（b-f）波形及频谱图
Figure 8. The waveforms and spectra of earthquake signals （a） and typical R-SDE signals （b-f） on the K08 station
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图 9 不同时间尺度下各种机制导致的C-SDE的波形对比
（a，b） OBS台站上的鲸鱼叫声记录（数据来源：IRIS data。网络号：X9） （Kuna，Nábělek，2021）；（c，d） 短周期OBS的人工震源（气枪）信号；（e，f）本文研究
Figure 9. Comparison of C-SDEs caused by various mechanisms on different time scales
（a，b） Whale songs recorded by OBS station （IRIS data recorded at OBS stations，Network：X9） （Kuna，Nábělek，2021）；Artificial sources （air guns） signal recorded by short-period OBS （c，d）；（e，f） this paper studied
表 1 宽频带海底地震仪实验信息
Table 1 Experimental information of broadband OBS
台站编号 东经/° 北纬/° 深度/m 投放日期
年-月-日回收日期
年-月-日K08 115.499 17.799 3 780.01 2019-10-20 2020-05-15 K03 115.999 17.799 3 835.82 2019-10-20 2020-05-15 K02 116.499 17.799 3 896.38 2019-10-21 2020-05-14 
下载: 导出CSV
表 2 STA/LTA方法拾取SDE参数设置表
Table 2 The parameter set of SDE pickup with the STA/LTA method
参数名称 STA 时间/s LTA 时间/s 开始触发值 结束触发值 持续时间/s 信噪比 设置值 0.1 50 8 6 0.05—2 3
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表 3 SDE拾取结果
Table 3 The pick-up quantity of SDEs on each station
台站 各方向上的拾取数量 拾取总量 HHE HHN HHZ K02 28020 85721 40010 153751 K03 41549 122051 18829 182429 K08 77693 83942 93849 255484
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表 4 2020年5月3—9日间C-SDE信号发生及持续时间
Table 4 Occurrence and duration of C-SDE signals from May 3 to 9，2020
序号 日期
年-月-日开始时间
时:分:秒结束时间
时:分:秒持续时间
时:分:秒1 2020-05-03 0:54:15 7:43:10 6:48:55 2 2020-05-03 16:30:35 1:12:50 （5月4日） 8:42:15 3 2020-05-05 0:33:30 7:55:30 7:22:00 4 2020-05-06 3:58:25 13:50:25 9:52:00 5 2020-05-08 3:28:40 8:24:10 4:55:30 6 2020-05-09 1:30:12 8:38:55 7:08:43 7 2020-05-09 18:42:07 22:20:10 3:38:03
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