Research On Methods For Monitoring The Performance Of Broadband Seismic Instruments Based On Seismic Ambient Noise

In recent years,research on the seismic ambient noise has made great progress in both theory and practice,and has yielded fruitful results.The ambient noise cross-correlation method is not limited by the spatial and temporal distribution of earthquakes that has long affected seismological research.Coupled with the deep and extensive research into the internal structure of the earth occurring all over the world,it has prompted the establishment of broadband seismic networks throughout the world and promoted the development of broadband seismic instruments.Although modern broadband seismic instrument systems are generally considered to have very stable instrument response parameters,some studies have shown that instrument responses may have subtle problems that gradually change over time,and these errors are often difficult to identify directly from conventional waveform inspections.During the long-term recording of ambient noise data at broadband seismic stations,especially as new seismic networks and instruments continue to emerge,systematic monitoring of the performance of the instrument will help enhance the operation of the seismic network and improve the quality of observations.In order to monitor these instrument response errors that change over time,the conventional calibration process is encouraged but the input calibration signal interferes with normal recording of ground motion data.At present,methods for evaluating instrument performece in situ using seismic waveforms,earth tides,and teleseisms are primarily focused on long-period bands of instrument response and are limited by the occurrence of events and the establishment of an earth model.The method of monitoring the power spectral density data of seismic ambient noise at a single station based on the mid-period band is affected by the noise level of the instrument itself,and the measurement accuracy is low.In response to these limitations,the main methods for monitoring temporal variations in broadband seismic performance were studied based on the mid-period band ambient noise data and their cross-correlation methods.The research was supported through a national public welfare industry research project,entitled “Deep Exploration Technology and Experimental Research Special Project—Project 9: Development and Experiment of Key Instrument Equipment for Deep Exploration—Topic 4: Development of the Cable-less Self-locating Seismic Exploration System”.The specific research areas are as follows:(1)From theoretical study of the ambient noise cross-correlation function,it is clear that amplitude information is mainly related to the energy distribution of noise sources and the attenuation of the medium.Therefore,in order to analyze the influence of noise source on the amplitude of the cross-correlation function,a numerical experiment was conducted to simulate the distribution of the noise source.The influence of different distributions of the noise source on surface wave and coda wave amplitude was analyzed under the same medium structure.In addition,from the experiment of ambient noise data recorded by the broadband seismic network,arrays at different positions were selected to analyze the variation of the surface wave,coda wave,and tailing wave,as extracted from the cross-correlation function at different distances,azimuths,and frequency.Combining the results of the cross-correlation functions of the two experiments,it is believed that the coda wave has better stability and higher symmetry,and is suitable for long-term monitoring applications.In addition,more information about noise sources was obtained from the amplitude study of the full cross-correlation function.(2)After understanding the configuration of broadband seismic stations,instrument response calculation,the description of instrument parameters in the metadata,and instrument response correction,the reasons for the instrument response phase error in the process of instrument response record and instrument response correction were identified.These included poles-zeros changes,the causality difference of the instrument response correction method,and the error of the filter transfer function coefficient records.Using instrument response information of the broadband seismic station,the data time offset caused by the phase error was measured from one sampling point to several seconds.For the ambient noise data with different delay times,the phase shift linear fitting method was used to calculate the time offset of the cross-correlation function in the two microseismic bands.From the results of the cross-correlation functions of all station pairs,their time offset range is 74%-99% of the data delay time,clarifying the necessity of monitoring instrument performance.(3)A method for monitoring temporal variations in instrument response using cross-correlation function is proposed.The amplitude and phase monitoring are implemented by using amplitude information of the coda wave and traveltime of the surface wave,extracted through the ambient noise cross-correlation function.By observing the daily changes of amplitude and phase parameters of the paired cross-correlation calculations of the three stations over the two-year observation period,not only can the station with the instrument response error be identified in real time,but mutual reference and constraint for error estimation can also be provided.The analysis of the amplitude monitoring method affected by the noise source shows that the coda wave helps reduce the effect of non-isotropic fields,and it is considered that the accuracy of the amplitude monitoring is an amplitude change of instrument response magnitude of 0.01%.In addition,using the initial instrument response in data processing instead of the available instrument response simulates the instrument response error to verify the accuracy of the results.(4)Based on the establishment of a stable reference cross-correlation function and daily cross-correlation functions,the time difference between station pairs is calculated using the movwing window cross-spectrum method,this method overcomes the limitation of the asymmetry of the noise cross-correlation function in time.Long-term monitoring results show that the average time difference between each pair of stations does not exceed the order of magnitude of 0.01 s,and the verification principle of the time deviation closure relationship between three stations is satisfied.An over-determined equation is established by the time offset between the station pairs and the time error standard value of a single station obtained by least squares inversion is the order of magnitude of 0.01 s,and from the analysis of the monitoring results from multiple stations to determine the time error station.(5)It is an important application of the ambient noise method to monitor temporal variation in the structure of Earth's crust.We use observation data with instrument response phase error,and the method of ambient noise seismic interferometry,to analyze the influence of the instrument's phase error on the relative variation of wave velocity.The results show that when monitoring the relative changes in seismic wave velocity of the subsurface medium structure,disturbances significantly larger than standard wave velocity occur during instrument response errors that experience zero-pole changes.Combined with the analysis of the results of three stations in the regional network,it is proved that the performance of monitoring seismic instrument can help to eliminate the error before the seismological research begins,and to show the importance of the monitoring of the instrument performance.