Short Duration Events on OBS recordings in the Northwestern 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，利用长短时窗均值算法在单个OBS台站拾取了多达12万个短时事件。根据信号特征以及可能的成因机制，可将事件分为连续性短时事件（C-SDE）和随机短时事件（R-SDE）。C-SDE的主要特征是在一段时间内连续且有规律发生的短时事件；包含多个事件组，事件组间隔约100—300 s，每个事件组包括3—10个持续约1 s的单波；其主频介于20—44 Hz之间，不同台站信号主频的不同可能是因信号源到台站距离不同而衰减的结果；C-SDE能量较强，在间距约105 km的三个OBS台站的三个分量上均有记录。R-SDE的主要特征是随机发生，持续时间约0.3—2 s；主频集中于8—25 Hz，在频谱上常呈现双波峰；由于其事件能量较小，很难在多个台站上拾取到同一个事件。本文探讨了C-SDE和R-SDE的信号源可能的产生机制，推断该时间段内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所产生的破裂过程所导致，表明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 OBSs deployed in the northwestern sub-basin of the South China Sea （SCS） from October 2019 to May 2020, we detected more than 120 000 Short Duration Events （SDEs） at one station by using STA/LTA algorithm. These SDEs are classified as consecutive short duration events （C-SDEs） and random short duration events （R-SDEs） according to their characteristics and generation mechanism. C-SDEs occur over several hours or days consecutively and regularly. The interval between the two Group Events （GEs） is about 100-300s and contains 3−10 single-wave （SW） with a duration of about 1s. The dominant frequencies of C-SDEs range from 20 Hz to 44 Hz, likely depending on the distance from the sources. C-SDE has strong energy and record on three OBS stations spaced about 105 km apart. The R-SDEs have the dominant frequency ranging from 8 Hz to 25 Hz with double frequency peaks on the spectrum and a duration of 0.3−2 s. Due to the limited energy of the R-SDEs, it is not easy to detect the same event on multiple stations. We present the possible generation mechanisms for C-SDEs and R-SDEs, respectively. We infer there are active C-SDE sources in the northwestern sub-basin of the SCS. They may result from 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 leakages from the sedimentary layer in the northwestern sub-basin of the SCS.
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Key words:
- OBS /
- short duration events /
- STA/LTA /
- gas escape /
- Whale Song
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图 1 典型SDE波形及幅度频谱
（a—b）马尔马拉海2011年布放的OBS（OBS-01台站）记录的海底沉积层中气泡逃逸产生的SDE的三分量波形和幅度频谱（ Tsang Hin Sun et al，2019）； （c—d）东北太平洋布放的OBS台站记录的2012-11-03 19:00:00的鲸鱼叫声产生的SDE的三分量波形和幅度频谱（数据来源：IRIS ，网络号：X9 ，台站号：BS080） （ Kuna，Nábělek，2021）。
Figure 1. Waveforms and amplitude spectra of typical SDEs。
（a，b） Three-component waveforms and amplitude spectra of SDEs from bubble escape in seafloor sediment recorded by OBS （OBS-01 station） in the Sea of Marmara （Tsang Hin Sun et al，2019）； （c，d） Three-component waveforms and amplitude spectra of SDEs produced by whale calls on OBS stations （IRIS data. Network：X9，Station：BS080，Begin time：2012-11-03 21:16:52） in the Northeast Pacific.） （Kuna ，Nábělek，2021）.
图 3 STA/LTA方法拾取SDE示意
（a）单个SDE波形序列；（b）连续SDE波形序列；（c）单个SDE的STA/LTA的特征曲线；（d）连续SDE的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. Example of SDEs detected by the STA/LTA method
（a，b）. The waveform of a single SDE and consecutive SDEs. The pink box is noise data and the light green box is the SDE signal data. （c，d）. Character curves of a single SDE and the consecutive SDEs in detections using STA/LTA method.
图 4 SDE数量随时间的变化
（a） 2019年10月—2020年5月间，K02，K03，K08数量变化（日统计）的时序图； （b） 2020年5月3日—10日间，SDE在K02，K03，K08台站拾取的数量变化（小时统计）时序图； （c） 2020年1月3日—10日间，SDE在K02，K03，K08台站拾取的数量变化（小时统计）时序图
Figure 4. Temporal distribution of the SDE number
（a）. Temporal distribution of the SDE number in OBS stations （K02，K03，K08） from October 2019 to May 2020；（b）. Temporal distribution of the SDE number in OBS stations （K02，K03，K08） from May 3 to 10， 2020；（c）. Temporal distribution of the SDE number in OBS stations （K02，K03，K08） from January 3 to 10，2020.
图 6 C-SDE波形图
（a）相距约52 km的K02，K03，K08台站上垂直分量的C-SDE信号，R1，R2，R3，R4分别表示R-SDE， E1表示地震事件；（b） 相距约52 km的K02，K03，K08台站上垂直分量的单一GE波形；（c） K08台站上三个分量（E，N，Z）的波形
Figure 6. Waveform of C-SDEs
（a） The vertical components of C-SDE signals on OBS stations （K02， K03， K08） about 52 km apart，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）.（b） The waveform of the three components （E，N，Z） on K08 station.
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，K03，K08）
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
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，台站号：BS080，时间：2012-11-03 19:00:00） （Kuna ，Nábělek， 2021）、（c，d）短周期OBS的人工震源（气枪）信号和（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 records at OBS stations Network：X9， Station：BS080， Begin time：2012-11-03 19:00:00） （Kuna ，Nábělek， 2021），（c，d） Artificial sources （air guns） signal recorded by short-period OBS and this paper studied（e，f）.
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 经度/°E 纬度/°N 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 
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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 LTA 时间 开始触发值 结束触发值 持续时间 信噪比 设置值 0.1（s） 50 （s） 8 6 0.05—2 （s） 3
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表 3 SDE拾取结果
Table 3. The pick-up quantity of SDEs on each station
Station HHE HHN HHZ Total 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，C-SDE信号发生及持续时间
Table 4. Occurrence and duration of C-SDE signal 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年5月3日 0:54:15 7:43:10 6:48:55 2 2020年5月3日 16:30:35 1:12:50（+1天） 8:42:15 3 2020年5月5日 0:33:30 7:55:30 7:22:00 4 2020年5月6日 3:58:25 13:50:25 9:52:00 5 2020年5月8日 3:28:40 8:24:10 4:55:30 6 2020年5月9日 1:30:12 8:38:55 7:08:43 7 2020年5月9日 18:42:07 22:20:10 3:38:03
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