Seismic waveforms and their signal-to-noise ratios of borehole observation in Donghai station，Jiangsu Province

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 M_L0.8 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 M_L0.8 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.