| Magnetotelluric(MT) Method has been widely used in the domains of geological exploration and mine exploration. The characteristics of apparent resistivity and phase curves are mainly basis of analyzing the structure of the earth and the distribute of conduction, therefore, Analyzing, calculating and processing of impedance are one of the most basis and importance aspect, and processing study of noise to affect impedance characteristic is very important especially. In generally noise may be categorized four mainly : Source noise, geologic noise, man-made noise and random noise.0 In measuring magnetic and electric field ,noise will make impedance severely biases and affect correctness, accuracy and reliability of explanation. Then it is the first task of electromagnetic(EM) prospecting to remove noise effectually and achieve stationary impedance estimation.Having reviewed the developments of magnetotelluric method, this paper has analyzed characteristic of natural signal in detail, and analyzed their excitation mechanism and affecting behavior to impedance estimation by type of noise. The study result show:(l)noise make impedance estimation severely non-stationary, the more noise in some time, the more non-stationary; (2) noise processing may be made by difference of noise and signal characteristic.Having compared features of Fourier transform with wavelet transform, the paper submits that Fourier transform is only suited to stationary signal and for wavelet transform to non-stationary. After Characteristic of noise is studied in the wavelet domain, it is proposed that corrective technique of noise is based on wavelet transform and adaptation. Results of synthetic data test show that corrected data is of high correlation coefficient (>0.91) and less energy loss (<10%) compared with origin data and processing is spend a few time, in general computer it takes two seconds only for 512 datum, when the origin data is added 5-10 times noise. Results of real data indicate that the new method can achieve stationary and reliable impedance tensor estimations whichare of less error bars and more smooth reasonable compared with traditional methods.In the marine environment (low signal-noise ratio), the paper has carried out the propagating theoretical study of nature source in marine horizontal strata earth, numerically calculated effects of seawater on nature source and EM responses in depth and conduction contrast. The results indicate (1) the effect of seawater on electric and magnetic field is different, stronger on magnetic field and weaker on electric field; (2)the more conduction contrast ,the more effect on EM field;(3) EM field up to 10-1 Hz can be measured in the deep water(> 1000m), and may be measured up to 100Hz in the shallow water(<100m). Characteristics of marine nature source by real data show in fellow: (1) marine nature source is affected not only by high conduction seawater but also by seawater moving(noisy frequency of this measurement: 0.084hz, 0.75hz,1.4Hz ); (2) strength of nature signal is weak in 10~2_io~3 level; (3)S/N (signal to noise ratio) is low about from 1.315 to 1.668 (average S/N).The paper proposed theory and calculating method of Robust estimation of marine MT impedance based on correlative coefficient for low S/N environment. Results of synthetic data test compared with other methods (traditional and common robust method) show: when S/N is better than 10:1, results of all methods have same high accuracy, but with S/N being low, the new method possesses better robustness, even S/N up to 1:1. Result of real data indicates the curve by new method can be used to invert directly, and result of inversion is identical with that of logging-well, but results of other methods can not be convergent in inversion.Finally, the paper has analyzed physical mechanism of source and static effect, ideals on separation of source and near-reference correct of static effect have proposed, and diagram of impedance robustness estimation in noise environment is studied and plotted.
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