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研制的光纤干涉式海底地震海啸监测系统于2017年布放在我国南海。具体地讲,从2017年10月29日到2017年11月15日,包含地震计和水位计的密封舱被布放在国家海洋局乌场海洋环境监测站西南2 km的海底,主机被安装在岸边监测站内,如图1所示,使用 12 km长的12芯铠装海底光缆连接海底密封舱内的传感器和岸边主机。
在安装之前,光纤干涉系统的性能和密封舱的密封性能均经过严格测试,两组地震计和水位计固定在密封舱内互为备份。选取的布放海域为沙质海底,且沙层厚度超过50 cm,没有合适的基岩将密封舱固定在海底。因此,将密封舱固定在5 kg重的水泥墩上,再沉放于海底,如图2(a)所示。为了避免电化学腐蚀,用于固定密封舱的紧固螺丝钉和扣环均采用钛合金材质,如图2(b)所示。
图 2 (a)固定在水泥墩上的密封舱;(b)密封舱表面部件
Figure 2. (a) Sealed capsule fixed on the bed of a concrete block; (b) Fastening screw and the fixed button on the sealed capsule
由于传感器在海底的工作环境远比陆地环境恶劣很多,传感器必须被密封以防止海水腐蚀。密封舱使用钛合金制造,具有抗腐蚀、强度高、轻质、长寿命等优点。钛合金的海水腐蚀速率大约为每年0.16 μm,密封舱的设计预期寿命为25年,对应大约4 μm直径的腐蚀损失量。密封舱包含圆柱体身、顶盖、底盖、隔水板、穿仓连接器、固定海底光缆的钳位部件、以及抗弯部件,如图3所示。圆柱体身、顶盖和底盖用于实现密封舱的整体密封性能。隔水板在密封舱的两个内部舱体之间,用于当一个舱体被海水渗透以后,防止另外一个舱体受到影响。穿仓连接器本身具有良好的密封性能,与钳位部件、抗弯部件一起用于海底光缆与传感器的连接。另外,密封舱表面标记有红色指示箭头,如图2(b)所示,有利于潜水员在海底布放密封舱时保证三分量地震计具有正确的方向性。
密封舱的安装深度小于500 m,且海底光缆长度仅为2 km,因此在密封舱和岸边基站之间的海底光缆不需要额外的中继设备保证传输性能。为了满足浅海布放过程中的张力需求,海底光缆设计有多层保护结构,以及较好的张力和环境适应性,重要技术指标如表1所示。在密封舱和海底光缆布放完成之后,系统主机被安装在岸边基站内,并进行系统校准和调试工作。测量数据通过专有网络上传到SOA的中央服务器上。
表 1 海底光缆重要技术指标
Table 1. Important specifications of submarine optical cable
Parameters Values Minimum breaking load 577000 N Instantaneous tensile strength normal load 403000 N Operating normal load 201000 N Permanent strength normal load 134000 N Minimum bending radius 0.9 m Operating temperature −10-50 ℃ DC resistance in metallic conductors < 2.0 Ω/km -
研制的海底地震海啸监测系统是基于光纤干涉技术。具体地讲,地震计是基于迈克尔逊干涉仪,水位计是基于马赫-曾德尔干涉仪,如图4所示。系统包含2个窄线宽外腔激光器(ECL),用来产生波长变化的连续相干光波。地震计中,激光器的波长为余弦调制;水位计中,激光器的波长为线性调制。地震计激光器发出的光波被多路选择器分为3个通道,然后通过光纤耦合器进入3路迈克尔逊干涉仪(振动探头)。水位计激光器发出的光波直接通过光纤耦合器进入马赫-曾德尔干涉仪(水位探头)。地震计使用相位生成载波技术(PGC)解调干涉相位,实现环境振动的测量;水位计使用线性调频和相位解调来测量实时水位。系统中的通用信号发生器产生余弦调制和线性调制的相位调制信号,处理器实现数字相位解调。具体的调制参数,以及模数转换器(ADC)采样速率和数据采集速率,如表2所示。
图 4 光纤干涉海底地震海啸监测系统结构图
Figure 4. System block diagram of optical fiber interferometric seafloor seismic tsunami monitoring
表 2 系统关键数字化指标
Table 2. Key digital specifications of system
Parameters Values Cosine modulation frequency (seismometer) 16 kHz ADC sampling rate (seismometer) 500 kHz Measurements sampling rate (seismometer) 200 Sps Linear modulation frequency (water level monitor) 10 kHz ADC sampling rate (water level monitor) 10 MHz Measurements sampling rate (water level monitor) 100 Sps 在三分量地震计中,连续光波与振动探头感受到环境振动的相互作用。振动探头采用由一对光纤缠绕的顺变柱体组成的推挽结构,谐振频率大约为250 Hz,如图5所示。振动探头由惯性质量块和2个顺变柱体组成。光纤的终端带有法拉第旋光镜,用来抑制偏振衰落现象。质量块由黄铜组成,顺变柱体由硅胶制成,光纤使用单模通信光纤[20]。三分量地震计探头柱体直径100 mm,高180 mm,质量块重520 g。
PGC调制常用的解调方法有两种:微分交叉相乘(DCM)解调和反正切解调。在设计的地震计中,DCM解调用于获取干涉相位测量数据[19]。为了测试地震计性能指标,建立了包含三分量振动探头的地震计实验平台。三分量振动探头被固定在超低频震动台上,并使用MEMS加速度计的测试结果作为参考测量数据。测试结果表明,光纤干涉式地震计的灵敏度达到57 dB (0 dB代表1 rad/g),三分量探头的灵敏度偏差小于0.5 dB,工作频率范围0.005~50 Hz,最低可测加速度值 43.4 ng/
$\sqrt{{\rm{Hz}}} $ [20-21]。在水位计中,为了获得大量程测量范围和高测量精度,使用了基于双准直器的光纤干涉结构。水位计由2个耦合器、2个光纤准直器、进水口、水软管和传感光纤组成,探头整体长度约400 mm,外径约200 mm,如图6所示。在传统的光纤干涉仪中,传感臂的光纤同时发挥着敏感单元和光信号传输的作用。然而,光纤的应变拉伸范围太小,无法实现较大的水位变化量程。为了满足水位大量程测量的需求,水位计中马赫-曾德尔干涉仪的传感臂必须有非常长的传感光纤,但这种结构会使得干涉仪非常容易受到环境干扰影响,从而导致测量精度较差。因此,研发的水位计采用双准直器替代传感光纤作为敏感单元,解决大量程测量的问题。
水位计中,2个准直器之间安置1个水软管,水软管的底部密封,另一端通过进水口连接到外部水位。水软管的内部和外部水压力是平衡的,水位变化就转换为2个准直器之间的距离变化,从而引起干涉臂长差的改变。
为了测试水位计的测量范围、精度和响应时间,使用高精度压力计模拟水位变化。压力计的压力变化范围是2 MPa,精度是0.02%。当压力变化范围是0.10~1.07 MPa时,对应的水位深度是0~110 m,干涉相位与压力,也就是水位,之间的关系具有非常好的线性特性。测试结果表明,水位计测量精度达到±1 cm,测量范围高达110 m,响应时间为0.01 m[23]。
地震计和水位计的探头布放在海底,解调主机安装在岸边基站内。解调主机接收探头传输的光信号,经过光电转换、模数转换得到数字量干涉信号,进一步使用数字解调方式获得地震波加速度测量值和水位测量值,将测量数据存储在本地计算机的同时,上传至SOA的中央服务器。
Development and installation of fiber optic interferometric seafloor seismic and tsunami monitoring system
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摘要: 研制了基于光纤干涉技术的海底地震海啸监测系统,并布放在海南省万宁市的国家海洋局乌场海洋环境监测站附近海域。研制的三分量光纤干涉地震计谐振频率大约250 Hz,灵敏度达到57 dB(0 dB代表1 rad/g),工作频率范围0.005~50 Hz,最小可探测振动加速度43.4 ng/
$\sqrt{{\rm{Hz}}} $ ;光纤干涉水位计测量精度达到±1 cm,测量范围0~110 m。系统自2017年10月完成布放,已经连续工作超过4年,并在长期运行过程中记录地震和实时水位数据。从2018年1月到2021年12月,在系统运行期间,东北亚地区发生里氏6.0级以上地震共计60余次,其中39次被系统成功监测,例如,给出了印度尼西亚苏拉威西附近海域发生的里氏6.8级以上地震的详细测量结果。文中报告了与这项工作相关的规划、设计、制造、实验室测试、海上安装、观测、数据采集和分析。介绍了系统布放过程,描述了地震计和水位计的工作原理,给出并分析了典型的地震记录和同期的水位监测记录。-
关键词:
- 光纤干涉仪 /
- 光纤干涉水位计 /
- 海底地震海啸监测系统 /
- 三分量光纤干涉地震计
Abstract: A seafloor seismic and tsunami monitoring system based on fiber optic interferometric technology was developed and installed in the nearby waters at China's State Oceanic Administration’s (SOA’s) Wuchang (Hainan Province, China) monitoring site. The developed tri-component version of fiber optic interferometric seismometer had a resonance frequency of about 250 Hz, a sensitivity of 57 dB (0 dB represents 1 rad/g), with the operation frequency range of 0.005-50 Hz and the minimum detectable vibration acceleration of 43.4 ng/$\sqrt{{\rm{Hz}}} $ , while the fiber optic interferometric water level monitor has an accuracy of ±1 cm, a measurement range of 0-110 m. The system had been operating well more than 4 years since October, 2017, and earthquakes as well as real-time water levels had been recorded during the long-term field trial. From January in 2018 to December in 2021, there were over 60 earthquakes with ML6.0+ in Northeast Asia area, and 39 earthquakes were recorded by our system. As an example, the detailed measurement results about the ML6.8+ earthquake in adjacent waters of the Sulawesi, Indonesia were given. In this paper, we reported on the planning, design, fabrication, laboratory tests, sea installation, observation, data acquisition, and analysis associated with this effort. The system installa-tion process was introduced, the working principle of the seismometer and water level monitor was described, and the typical recorded earthquake and real-time water level monitoring data were presented and analyzed. -
表 1 海底光缆重要技术指标
Table 1. Important specifications of submarine optical cable
Parameters Values Minimum breaking load 577000 N Instantaneous tensile strength normal load 403000 N Operating normal load 201000 N Permanent strength normal load 134000 N Minimum bending radius 0.9 m Operating temperature −10-50 ℃ DC resistance in metallic conductors < 2.0 Ω/km 表 2 系统关键数字化指标
Table 2. Key digital specifications of system
Parameters Values Cosine modulation frequency (seismometer) 16 kHz ADC sampling rate (seismometer) 500 kHz Measurements sampling rate (seismometer) 200 Sps Linear modulation frequency (water level monitor) 10 kHz ADC sampling rate (water level monitor) 10 MHz Measurements sampling rate (water level monitor) 100 Sps -
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