Temporal Seismic Velocity Changes Studied With Ambient Noise Of Borehole Seismic Array

In the past ten years,the rapid development of ambient noise studies has provided a new method for detecting time-varying information of underground media.Most previous studies focused on the influence of volcanic activity or major earthquakes on the temporal seismic velocity of underground media.However,in some areas or periods when no earthquake or volcanic activity occured,the seasonal seismic velocity changes can still be observed.These changes may come from "non-tectonic" influences around the stations,such as precipitation,temperature,atmospheric pressure and other environmental factors.Sometimes the seismic velocity changes related to strong environmental factors may even cover up the weak seismic velocity change signals related to tectonic events.Therefore,exploring the influence of environmental factors on temporal seismic velocity can better understand the physical mechanism of seismic velocity changes,and by removing the seismic velocity changes caused by environmental factors,the accuracy of the ambient noise cross-correlation technology for detecting time-varying information related to the underground tectonic process can be improved.In this study,we used the continuous seismic records of the small aperture borehole array in Yangjiang,Guangdong.With the advantage that the borehole array may be less affected by surface factors,the three-component ambient noise interferometry technology was used in multiple frequency bands(2-4 Hz,1-2 Hz,0.5-1 Hz),and the long-term temporal seismic velocity results of layered media in different depth ranges were obtained.We analyzed the main compositions of the ambient noise of the array based on the characteristics of the ambient noise source in the study area,and discussed the influence of the noise amplitude of different frequency bands on the seismic velocity change results.In view of the regional and environmental characteristics of the study area,we quantitatively calculated the correlation coefficients between various environmental factors and temporal seismic velocity,and then determined the main influencing factors of seismic velocity changes at different depths.Finally,the transfer function was established to reasonably correct the environmental influence factors in the temporal seismic velocity at different depths(frequency bands).In this study,it was shown that there are certain differences in the amplitude of temporal seismic velocity measured at different frequency bands(depths),and the results of different frequency bands showed different characteristics of seasonal changes.The characteristic analysis of the ambient noise source in the study area showed that the main compositions of the ambient noise of the stations are natural noise and microseismic noise,the temporal change of noise level has no obvious correlation with the temporal seismic velocity,indicating that the noise level was not the dominant factor of seismic velocity.By comparing the results of temporal seismic velocity with surface meteorological data(precipitation,temperature,atmospheric pressure,etc.),hydrological data(groundwater level)and marine meteorological data(height of wind wave,etc.),we find that there is a strong correlation between temporal seismic velocity and the height of wind wave and rainfall / groundwater level.The seismic velocity changes(0.5-1 Hz frequency band)of deep media was significantly negatively correlated with the ocean wave height data.Combined the delay estimation of the time series of the height of wind wave and the temporal seismic velocity,we conclude that the height of wind wave has an immediate response on seismic velocity.The variation of seismic velocity in surface media(2-4 Hz frequency band)has a strong correlation with rainfall data and groundwater data.Combined with the time series of pore pressure of precipitation transformation,we speculated that the mechanism of rainfall on seismic velocity changes was that the direct water load instantaneously affects the pore pressure of media,resulting in the change of seismic velocity;the velocity changes residual corrected by environmental factors weakens the original seasonal characteristics to a certain extent,and can obtain more high-precision seismic velocity changes of underground media reflecting tectonic information.