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摘要:
天然地震探测是提取地球深部结构信息的最有效手段之一.而海域约占地表总面积的70%, 所以对海域开展地震探测十分重要.但海域有水体覆盖，海底地貌条件复杂，导致海域天然地震资料采集难度非常大，探测程度远低于陆域.针对海域地震采集环境的特殊性，人们先后发展了不同类别的海域天然地震资料采集技术.本文对埋入式、沉底式和漂浮式等传统的和新兴的非常规海域天然地震采集方法、仪器技术参数及其所采集的地震信号质量进行了介绍.然后总结了不同采集方法的优缺点.总体来看，埋入式永久海底地震台网建设成本高，不适宜大规模部署；沉底式海底地震仪(OBS)投放成本低、效率高，适合于密集台阵部署；漂浮式海域潜标地震仪(MERMAID)可以在深海区域采集强震P波信号，对全海域三维成像探测具有独特优势；新兴的海底光缆地震仪对应变敏感，适宜于海啸预警.以上信息有助于推动天然地震探测技术在海域更广泛的应用.
Abstract:Seismological exploration is one of the most effective tools to extract deep earth structural information. The sea area accounts for about 70% of the total surface area of the earth, so it is very important to carry out seismic exploration in sea area. However, the coverage of sea water and the complex seafloor geomorphic conditions hinder seismic data acquisition in marine area, which leads to a much lower exploration level in marine areas than in continental areas. Several seismic data acquisition methods have been successively developed in view of the particularity of the marine environment. The present study tries to firstly introduce features, instruments and signal qualities for traditional buried-style, sinking-style and floating-style and new unconventional marine seismic data acquisition methods. Then, the advantages and disadvantages of different methods are summarized: Permanent Ocean Seismic Network composed of buried-style seismometers is expensive, inefficient and unsuitable for large-scale deployment; OBS is cheaper, more efficient, and suitable for dense array deployment; Mobile Earthquake Recording in Marine Areas by Independent Divers(MERMAID)can collect P-wave signals of strong earthquakes in deep sea areas, which makes three-dimensional imaging of the whole sea area possible; submarine cables are sensitive to strain and suitable for tsunami early warning. The summary information will help promoting the wider application of seismological explorations in marine areas.
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图 1
海底钻孔埋入式地震采集系统示意图
Figure 1.
Schematic diagram of ocean bottom borehole buried seismic acquisition system(modified from Stephen et al., 2003)
图 2
沉底式OBS地震采集系统示意图
Figure 2.
Schematic diagram of bottom sinking OBS seismic acquisition system(modified from Nedimović, 2019)
图 3
漂浮式海域潜标地震仪(MERMAID) 运行图(Hello et al., 2011)
Figure 3.
Operation diagram of floating marine mobile seismograph (MERMAID) (Hello et al., 2011)
图 4
海底光纤地震仪采集原理示意图
Figure 4.
Schematic diagram of acquisition principle of submarine optical fiber seismograph(modified from Marra et al., 2018)
图 5
钻孔深埋永久地震台站与海底OBS地震台站波形对比(波形修改自Stephen et al., 2003)
Figure 5.
Comparison of waveforms between deep borehole permanent seismic station and submarine OBS seismic station(waveforms are modified from Stephen et al., 2003)
图 6
海底OBS地震台与MERMAID记录的波形对比
Figure 6.
Comparison of waveforms recorded by submarine OBS seismic station and MERMAID
图 7
海底光纤地震仪与两端陆基地震仪记录的同一个地震的南北分量波形对比(Marra et al., 2018)
Figure 7.
Comparison of the north-south component waveforms of the same earthquake recorded by the submarine optical fiber seismograph and the land-based seismographs at its both ends (Marra et al., 2018)
表 1
陆域海域宽频地震仪主要技术参数
Table 1.
Main technical parameters of land and marine broadband seismographs
采集方式/代表性地震台站 频带范围 动态范围/dB 灵敏度 置放深度 陆基/陆地地震台 0.0028~100 Hz >167 1, 500 V/ms-1 陆域浅埋 埋入式/永久地震台 0.003~5 Hz (KS-54000) >143 2, 400 V/ms-1 < 100 m (井下) 沉底式/海底地震仪OBS 0.008~100 Hz >142 -162 dB re: 1 V/μPa < 6000 m (水下) 漂浮式/潜标地震仪MERMAID 0.1 Hz~50 kHz (OSEAN) - -198 dB re: 1 V/μPa < 3000 m (水下) 非常规/海底光纤地震仪 3~800 Hz < 94 39 dB re: rad/g 不限 注: 陆地地震仪数据主要根据https://www.guralp.com/products/surface中列举的主流摆式地震仪参数综合得出；埋入式永久地震台数据来自http://www.geoinstr.com/ks54000c.htm；OBS数据来自https://www.guralp.com/products/ocean-bottom-seismometers和https://www.passcal.nmt.edu/content/instrumentation/sensors/broadband-sensors；潜标地震仪数据来自http://www.osean.fr/pdf/osean_oceanographic_profiler_V1.pdf和 丁巍伟等(2019) ；光纤地震仪数据参考自陈瑛和宋俊磊(2013) 和Marra等(2018) .
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表 2
不同海域天然地震采集方法主要指标对比
Table 2.
Comparison of main indicators of different marine seismological data acquisition methods
采集方式/代表性地震台站 适用海域 布设方式 布设成本 信噪比 运行时间 震相 探测维度 埋入式/永久地震台 浅海 散点 高 优 不限 完整 1D, 3D 沉底式/海底地震仪OBS 浅海 阵列 低 适中 短 完整 2D, 3D 漂浮式/潜标地震仪MERMAID 深海+浅海 散点 较低 较低 不限 强震P波 3D 非常规/海底光纤地震仪 深海+浅海 — 极低 良好 不限 应变(率) 1D
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