Research On Extraction Technology Of Ambient Noise Body Wave In Ultra-high Density Urban Underground Space

With the acceleration of the urbanization process in my country,the urban construction has expanded from the surface to the underground space,and the precision of urban exploration has become higher and higher.Due to dense population and safety issues,active source experiments are not suitable for studying shallow crustal structures in urban areas.Therefore,it is essential to seek green and non-destructive exploration methods.Ambient noise imaging is a low-cost,green and environmentally friendly exploration method.In the exploration of urban underground space,the Green function between the pair of stations can be used to study the high-resolution 2D /3D crustal structure.Shallow body waves are difficult to be extracted due to the problems that the shallow body waves have noise sources from the surface,which is difficult to meet the seismic interference hypothesis,uneven noise distribution,and excessive suppression of body waves in the data by excessive coherent noise.Therefore,extracting body waves from ambient noise can not only improve the longitudinal resolution of the deep part and make up for the problem of insufficient depth of surface wave exploration,but also greatly reduce the exploration cost and damage to the environment.Based on the ambient noise data collected in Wanqingsha Town,Nansha District,Guangzhou City,Guangdong Province,and Huangzhuang-Gaoliying Fault(Beiqijia Section),Changping District,Beijing City,Guangdong Province,the characteristics of ambient noise distribution and regional environmental characteristics are studied respectively The feasibility of using ambient noise to extract body waves in the region.In this study,a set of ambient noise preprocessing based on seismic interference theory,coherent noise suppression,class-like autocorrelation,auto-correlation,cross correlation,linear superposition,time-frequency phase weighted superposition,virtual shot set extraction,quality Controlling(correlation coefficient curve,RMS convergence curve,mean curve,etc.)body wave extraction complete program,fully use the frequency and phase information,improve the extraction accuracy of the weak body wave signal and the stability of the Green function,obtain a more reliable and stable Body wave signal;the effects of de-mean,de-linear trend,waveform tipping,resampling,whitening,superimposition time,related bandpass filtering before and after bandpass filteringon body wave extraction are discussed respectively.Found the balance point between the calculation efficiency and the extraction effect of body waves.On the premise of ensuring the extraction of body waves,a clear surface wave(mainly sound waves)is obtained by selecting a suitable re-sampling frequency,calculation duration,and superposition method.Refracted and reflected waves.Finally,through the comparison of the autocorrelation profile,ambient noise shear wave velocity profile,and active source post-stack profile,the location of the hidden active fault in the city is determined.The results of this study prove that short-time ambient noise(1d)can also effectively extract body waves,which is of great significance for urban shallow exploration,especially in urban areas where the environment is complex and not suitable for active source exploration.