Research On Magnetotelluric Time-domain Calibration, Impedance Estimation And Denoising Technology

The natural electromagnetic field observed by the magnetotelluric sounding method has the characteristics of wide frequency bandwidth,small amplitude intensity,strong randomness,and uneven energy distribution with frequency.The magnetotelluric sounding instrument will suppress or gain the original signal.At the same time,the built-in filter of the instrument will filter the original signal.Most of these built-in filters are not zero-phase filters,resulting in amplitude and phase differences between the signal recorded by the instrument and the real electromagnetic field signal.In order to obtain real signals,the system needs to be calibrated.The traditional magnetotelluric sounding calibration is carried out in the frequency domain,which leaves the system's effects on the time series uncorrected.It is difficult to achieve the expected results in time domain filtering and transform domain filtering based on noise pattern recognition in the time series.Therefore,on the basis of traditional frequency-domain calibration,this paper proposes a time-domain calibration method for magnetotelluric sounding.The thesis systematically studies the time-domain calibration method and its application in magnetotelluric sounding.The corresponding unit impulse response in time domain is caculated according to the system's frequency-domain calibration file,and the unit impulse response and the system response recorded by the instrument are used to perform convolution calculation to realize the time-domain calibration of MT.Based on the filter theory,this paper obtains the effect of the system on different types of filtering methods and time-domain impedance estimation by analyzing the system's effect on the waveform of effective signal and noise.Finally,the effectiveness of the time-domain calibration proposed in this paper is tested through the processing of the measured data forom Baiyinnuoer deposit of Inner Mongolia.It is found that the denoising results in the time domain and the transform domain is affected by the acquisition system to varying degrees.Specifically,the error bars and dispersion of theapparent resistivity-phase curve increase,and a false anomaly appears.The denoising method depends more on the waveform of noise the more complex the noise type is,and the more significant the impact of the acquisition system will be.In addition,the effects of the gain and phase shift of the data acquisition system will cause distortion of the electromagnetic field time series,resulting in frequency dispersion and increased error bars in the time-domain impedance estimation results.And the greater the difference between the influence of the system on the track and the track,the more obvious the influence on the time-domain impedance estimation.The type of noise in the mining area is complex,and the effect of the MT denoising process is obviously affected by the system,especially in the "dead band" where the effective signal is weak.Traditional methods are difficult to achieve the ideal denoising effect,so this paper proposes a time-domain calibration method to correct the system impact,and implement the MT denoising method based on the CEEMD-HHT(Hilbert-Huang Transform based on Complete Ensemble Empirical Mode Decomposition)for the purpose of suppressing the earth's electromagnetic "dead band" noise in the mining area.the idea of this algorithm is to separate the abrupt noise part from the large-scale gentle part,leaving the large-scale gentle noise affecting individual frequency points,and filtering the abrupt noise.Since this method is not affected by the waveform of the noise,its denoising effect on the "dead band" signal in the mining area is better than other denoising methods.Besides the time-domain calibration method can eliminate the influence of the system on the signal to a certain extent.Improve the denoising effect of other methods.Finally,in order to verify the effect of this method on the measured data,this paper apply the time-domain calibration method and CEEMD-HHT method to the measured data in the Baiyinnuoer mining area in Inner Mongolia.The denoising results prove the superiority of CEEMD-HHT suppression in the "dead band" noise.Besides the denoising effects before and after the time-domain calibration show that this method can eliminate the system deviation caused by acquisition system in magnetotelluric time series,after the calibration processing,we can achieve better denoising effect based on morphological recognition and transform-domain denoising methods,at thesame time the convergence of the time-domain impedance estimation results is enhanced and the error bars are reduced.Therefore,this paper proposes that the time-domain calibration of the instrument system should become an important link in the processing of magnetotelluric data.