| A large number of microseismic events are usually generated in the process of rock fracture.Effective recording and analysis of microseismic signals can obtain a variety of information about rock fracture.This method has played an active role in tunnel construction,safety monitoring of underground powerhouse,mining safety monitoring and other fields,and also proved the effectiveness of microseismic monitoring method.The effect of microseismic monitoring directly depends on the accurate location of microseismic events.Accurate P-wave time acquisition is very important for the above work.However,due to the influence of the on-site monitoring environment,the acquisition of the micro seismic signal will inevitably stack various kinds of noise signals,which makes the original micro seismic record with the whole process of the event appear to be submerged by noise in the time domain,which makes it difficult to pick up the P-wave in time.Based on the above problems,this paper mainly focuses on two aspects: noise suppression and P-wave time acquisition of microseismic signalsRandom noise suppression is the key step of microseismic data processing.The accuracy of picking up P-wave in time of microseismic signal in the field largely depends on the appropriate filtering algorithm.To meet this demand,this paper proposes a method of microseismic signal noise reduction based on ensemble empirical mode decomposition and sample entropy.The steps of this method are as follows: firstly,EEMD decomposition of microseismic signal is carried out,Then,calculating the sample entropy of each IMFs,Combined with the characteristics of microseismic signal,the threshold value is set based on the sample entropy of each IMFs component.The microseismic signal is reconstructed by extracting the IMFs component which accords with the threshold value of sample entropy,so as to achieve the purpose of denoising.The proposed method is applied to the simulation data and the actual micro seismic data,which shows that the method has an ideal noise reduction effect.In view of the weak anti-interference ability and bad pick-up effect of the common P-wave pick-up method,based on the traditional method,using the polarization characteristics of the microseismic signal,the Rec_Dip method is adopted and the P-wave pick-up is realized by setting the threshold value of the function.Combined with the selected real microseismic signal,the threshold value and the time window for obtaining the polarization characteristics of microseismic signal are studied.Finally,the time window is set as 80 sampling points,and the threshold value is 0.7,100 microseismic data of a hydropower station under construction are used to verify the method,and compared with STA / LTA,kurtosis method and AIC method.The experimental results show that the method has strong anti noise,certain accuracy and robustness.Based on the research of this paper,a reliable joint method of P-wave time-to-time acquisition is proposed for the microseismic signal with high noise,which effectively solves the problem that it is difficult to obtain reliable acquisition results for low SNR microseismic data.The method consists of four steps: noise reduction of microseismic signal,rough estimation of Pwave arrival time,selection of data segments,and accurate pick-up of P-wave arrival time.Firstly,the method of EEMD and sample entropy is used to suppress the noise,Secondly,we use Rec_Dip method to pick up the P-wave arrival time of the noise reduced microseismic signal and determine the P-wave arrival time initial.Then,data segments are selected before and after the arrival time of P-wave of microseismic signal.According to the determined range of initial arrival time,the exact arrival time of P-wave is deduced by AIC algorithm.The method is used to pick up the P-wave arrival time of the simulated three-component micro seismic signal with high noise and the actual low SNR three component micro seismic signal,which prove that the method can pick up the P-wave arrival time effectively. |
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