Seismic waveforms and their signal-to-noise ratios of borehole observation in Donghai station，Jiangsu Province
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摘要: 为了探索在高噪声干扰地区获取高信噪比地震信息的有效途径，本文分析了我国第一个超深井地震观测站江苏东海地壳活动国家野外科学观测研究站设置在地面与井下三个不同深度处的地震仪所记录的波形及其信噪比特征。结果显示：由于观测研究站周围强烈的噪声扰动，地面地震仪记录中无法识别ML0.8地方震波形，而深井地震仪可清晰地记录到该小震波形，且深井地震仪可观测到较地面地震仪更多的零级或负震级地震；井下三组不同深度地震仪所记录波形的信噪比均远高于地面地震仪，且不同深度地震波形信噪比的平均值随仪器深度的增大而增加。井下1 559.5 m处的地震仪的波形平均信噪比为69.20 dB，2 545.5 m处的信噪比达到74.15 dB，均达到高保真波形的信噪比值，这说明1 500 m深处地震仪所观测到的波形可以有效地避免地面干扰，因此深井地震观测能够提供高信噪比的波形资料，为研究震源过程和场地效应等提供真实可靠的基础资料，这也预示着深井观测将促进深井地震学的研究与发展。Abstract: The Jiangsu Donghai Crustal Activity in Continental Deep Hole National Observation and Research Station （Donghai station for short） is the first deep borehole seismic observation station in China. In order to explore an effective way to observe seismic information with high signal-to-noise ratio in high noise interference areas and to carry out the study on the borehole seismology, this paper analyzed the waveform characteristics and signal-to-noise ratios of waveforms recorded by a seismometer on the ground and three borehole seismometers at different depths underground in Donghai station. The results show that the waveform of a ML0.8 local event cannot be recognized on the seismogram of the seismograph on the ground due to the strong non-seismic noise disturbance around the Donghai station. However, the waveforms of the ML0.8 event can clearly be recognized on the seismograms of the three deep borehole seismographs. Furthermore, the earthquakes with zero magnitude or weaker negative magnitude recorded by the deep borehole seismographs are also more than those recorded by the seismograph on the ground. Meanwhile, the signal-to-noise ratios of waveforms recorded by borehole seismometers at three different depths are much higher than those recorded by the seismometer on the ground; moreover, the average signal-to-noise ratios of waveforms increase with the depths of observation instruments in the borehole. The average signal-to-noise ratio of the waveforms recorded by the seismograph at 1 559.5 m depth underground is 69.20 dB, and that of the seismograph at 2 545.5 m depth is 74.15 dB. Both of the two ratios reached the signal-to-noise ratio value of high fidelity waveform. Therefore, the observation data of seismograph at about 1 500 m depth can effectively avoid the interference on ground and record waveform with high signal-to-noise ratio. Such waveforms provide real and reliable data for the study on source process and site effect, promoting the development of deep borehole seismology.
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图 5 东海站地面L0与深井下L1，L2和L3地震仪观测地震信噪比计算结果
Figure 5. Signal-to-noise ratio curves of seismic waveforms observed by seismometers L0 on the surface and L1，L2 and L3 in the borehole underground in Donghai station
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图 1 江苏东海地壳活动国家野外科学观测研究站周缘地质构造图（引自徐纪人等，2016）
Figure 1. Geological structure around the Jiangsu Donghai Crustal Activity in Continental Deep Hole National Observation and Research Station in China （after Xu et al，2016）
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图 2 东海观测站地面与井下地震仪记录的2013年2月3日江苏灌云ML0.8地震（震中距97.2 km，方位角80°）的三分量波形图
（a） 地面宽频地震仪L0记录波形；（b） 图（a）波形经中通滤波后的波形；（c） 井下地震仪L3记录波形
Figure 2. Three-component waveforms of the ML0.8 earthquake （epicentral distance is 97 km，azimuth is 80°） in Guanyun，Jiangsu on February 3，2013 recorded by the seismographs on the ground and in the borehole of the Donghai station
（a） Waveforms recorded by the broadband seismograph L0 on the ground；（b） Waveforms after medium-pass filtering to the waveforms in Fig.（a）；（c） Waveforms recorded by the seismograph L3 in the borehole underground
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图 3 东海观测站地面与井下不同深度地震仪记录的2013年5月24日安徽利辛ML2.7地震（震中距271.5 km，方位角56°）的三分量波形图
（a） 地面L0地震仪记录波形；（b） 图（a）中波形经中通滤波后所得波形；（c） 井下544.5 m深处地震仪L1记录波形；（d） 井下2 545.5 m深处地震仪L3记录波形
Figure 3. Three-component seismic waveforms of the ML2.7 event in Lixin，Anhui on May 24，2013 recorded by seismometers on the ground and at different depths in the borehole underground of the Donghai station
（a） Waveforms recorded by seismograph L0 on the ground；（b） Waveforms after medium-pass filtering to the waveforms in Fig.（a）；（c） Waveforms recorded by seismograph L1 at 544.5 m depth in the borehole；（d） Waveforms recorded by seismograph L3 at 2 545.5 m depth in the borehole
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图 4 东海观测站深井地震仪L3观测的微小地震三分量波形
Figure 4. Three-component waveforms of the earthquakes recorded by seismograph L3 in the borehole underground of the Donghai station
表 1 深井中的地震仪参数及工作环境岩性和地球物理参数
Table 1 Seismograph parameters and lithologic and geophysical parameters around the seismographs in the borehole
地震仪编号 深度/m 地震仪主频/Hz 岩性 围岩vP/（km·s−1） 围岩vS/（km·s−1） 温度/℃ L0 0 宽频3ESP L1 544.5 4.5 斜长角闪岩和退变榴辉岩 6.65 3.70 29.1 L2 1559.5 4.5 花岗质片麻岩（正片麻岩） 5.50 2.94 51.0 L3 2545.5 4.5 斜长片麻岩 5.98 75.0 
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