| The earth’s interior medium will change under the influence of external factors such as natural and human activities.At present,scholars generally believe that the velocity of underground medium will change with the change of its nature or stress state.Seismic wave is a "beacon" illuminating the earth.In the use of seismic wave for deep exploration of the earth,the velocity of seismic wave is the most accurate and reliable parameter.However,natural earthquakes have some limitations in time and space and are not suitable for long-term dynamic monitoring of underground media.Artificial source can compensate for the shortage of natural earthquake to some extent.The large-capacity air gun source developed in recent years in inland areas has the advantages of good controllability,high signal repeatability and rich low-frequency components.It is an ideal source for dynamic monitoring of regional underground medium changes.Earthquake background noise is widely distributed and independent of specific sources.It is a method developed in recent years to detect the structure of underground media and its changes.It is an important direction of Seismology development to measure the change of underground medium by using seismic wave signals generated by artificial source and background noise.This paper briefly introduces the development history of background noise and artificial sources and their progress at home and abroad.On the basis of the previous studies on the velocity variation of Hutubi M6.2 earthquake on Dec.8,2016 by using the data of fixed seismic stations,the velocity variation of underground media before and after Hutubi M6.2 earthquake is studied by using the large capacity air gun source and background noise signals observed by Hutubi mobile observation network.The following understandings are obtained:(1)The airgun signal body wave recorded by the mobile station 60 km away from Hutubi M6.2 earthquake does not show any change in wave velocity,nor does it have obvious periodic change.Before the M6.2 earthquake,the variation of wave velocity at Shiladezi station,77 km away from the station,presents a low anomalous variation,which may be related to whether the signal propagation path passes through the focal area.The wave velocity obtained from surface wave signal of air gun changes periodically,and the trend of change is positively correlated with the periodic change of gas pressure in Hutubi gas storage.This periodic change may be caused by the process of gas storage filling and discharging.(2)The variation of wave velocity obtained from background noise signals of most stations shows that the co-seismic wave velocity increases when Hutubi M6.2 earthquake occurs.Previous studies have shown that the variation of background noise wave velocity is positively correlated with the variation of pressure.Before the Hutubi M6.2 earthquake,the trend of wave velocity decline is different from that of pressure change.The change of wave velocity after the earthquake is positively correlated with the trend of pressure change,which may be the change of regional underground stress state caused by the earthquake incubation process.(3)No co-seismic response of M6.2 Hutubi earthquake was found by using the velocity variation of air gun source body wave and surface wave signals recorded by Hutubi mobile station.The co-seismic response of Hutubi M6.2 earthquake was found by using the variation of wave velocity obtained from background noise signal.The analysis shows that the difference of focal characteristics between the two earthquakes may be due to the fact that the air gun source is a pulse source and the background noise is similar to the continuous source,which may be the main reason for the different variation characteristics of wave velocity. |
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