This paper summarizes the newly research achievements of the underground seismic observation networks and borehole vertical seismic arrays in China, and looks forward to the prospect of underground observation research in the future. There are hundreds of underground observation stations with excellent observation quality currently. The underground seismic observational network pioneers a new technical approach for the physics in the Earth's interior of observation and study in the depths.
This analysis indicates that the underground observation can avoid the surface noise and site effects, and obtain high-quality seismic data. Therefore, it is possible for scientist to construct borehole stations scientifically in the earthquake monitoring areas, even if in the high noise background areas. Then the earthquake epicenter will be determined more accurately based on data observed from the reasonable station layout. The ability monitoring seismic activity is improved greatly. Simultaneously, the underground seismometer can record clear seismic wave near the epicenter because of avoiding the surface noise. The seismic waves observed near the epicenter retain more high-frequency components of waveforms which are essential data for study of the fine structure of the earth. It promotes the development of the seismological science.
It also can reduce the average velocity differences of P and S waves between stations due to different ground station foundations for us to study the spatial heterogeneity of the seismic wave velocity distribution if we use underground observation velocities. The accuracy and reliability of the 3D velocity model can significantly be improved by employing the data that reduced velocity differences mentioned above. The research findings suggested that the Tangshan strong earthquake occurred between the low speed zone and high speed zone too. The study results on the temporal variation of wave velocity indicated that a credible precursor process of wave velocity reduction also appeared before the Wen’an MS5.1 earthquake.
The surface reflected seismic waves near earthquakes have been surveyed through the underground observations. The accurate velocity structure of the crustal sedimentary layer can be established by employing the combination of incident waves and surface reflection waves. The high precision velocity models of the shallow layer are also of great significance to study the fine structure of the Earth’s interior.
The seismic kinematic and dynamic parameters, such as arrival time, component frequency and amplitude of seismic waves can accurately be determined by employing the low noise waves by the underground observation. The reliable high-level research findings are likely achieved based on the accurate parameters. The low noise waves observed by borehole seismometer are actually reasonable constraint for the study on seismic source. It is beneficial for scientist to solve the precise seismic source parameters and to acquire highly reliable results about the source under the strict constraint condition. A large number of excellent results have already been achieved based on the data of underground observation today. The seismic moments and moment magnitudes calculated by employing seismic waveform data from the underground observations are less than that calculated using waveforms by the ground observation. The stress drops and average earthquake dislocations computed using the waveforms from the underground observation are both less than those computed from waveforms observed in the ground bedrock. The corner frequencies calculated by seismic waveforms observed at the underground platforms are also lower than that calculated using data from the ground observations. The high-frequency components of the source spectrum calculated by waveforms from underground observation are weak, and not as abundant as that calculated using waveforms observed on the ground. As mentioned above, the magnitude and moment magnitude of the underground observation are less than those observed in the surface bedrock. The differences between the two kinds of magnitudes may be attributed to the nonlinear amplification effect of the wave in the upper medium of the borehole seismograph and the frequencies of seismic waves. Relatively, the lower corner frequencies of underground observations compared with the surface observations may be attributed to the absorption and amplification effect for different frequency wave components by the upper medium of underground instruments. In addition, the site response of the surface layer also has a significant impact on the source spectral parameters. The majority of the site responses underground platform are greater than 1 at the low-frequency domain and less than 1 at the high frequency domain respectively. The different site response between high and low frequency domain may also cause the magnitude and corner frequency observed by underground stations to be lower than those observed on the ground. The causes of source parameter differences mentioned above may be generally multiple complexities. It is still an important topic of future scientific exploration.
The underground seismometer recorded the surface reflection waves besides of the direct waves. The phenomenon is very valuable. So the nonlinear site effects and amplification characteristics of seismic wave propagation in sediment layers are solved accurately using the two types of data: Direct and reflected waves. Then the uncertainty of theoretical wave field solution can be reduced using the high-precision site effect results. The accuracy of strong ground motion prediction can surely be improved.
The Donghai, Jiangsu Province, borehole vertical seismic array is the first vertical seismic array in China. The array is consisted of one station at surface and four stations established at different deep layers in the borehole over 5000 meters deep and another borehole station with multiple geophysical instruments about 500 m away from the 5000 m deep borehole. The array can observe more clearer waveforms of micro-earthquakes with zero or negative magnitudes and then improve the ability to monitor crustal and seismic activities. The signal-to-noise ratios of waveforms recorded at different depths in the borehole can provide valuable reference for the construction of underground stations currently. The signal-to-noise ratios of waveforms observed in the borehole are also all greater than or equal to 70 dB. The seismic waveforms with high fidelity were obtained in the high noise background areas by means of the vertical seismic array system. The precise three-dimensional seismic wave velocity model can be established using high-quality seismic waveforms, which contributes to comprehensive reveal of the tectonic movement of the earth. The waveforms without site response and noise observed by the vertical array system are more appropriate constraints for the study of seismic sources too. The innovative research achievements on seismic source theory are expectable by the study under above scientific constraint condition.
The borehole seismic observation research not only have made significant contributions to earth science, but also is of great practical significance for the resource assessment, earthquake prediction and disaster prevention and mitigation of earthquakes.
The underground observation net and vertical array observation research are the frontiers of scientific problem in the world currently. More underground observation networks and borehole vertical seismic arrays are being constructed to obtain more high-precision seismic data and research findings, which will continuously innovate the future development of earth science.
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