Study on the characteristics of magnetotelluric source effects and its correction

Magnetotelluric generally follows the source for the plane wave assumption, which is generally applicable to the high frequency magnetotelluric study, but in the area with significant non-plane wave field characteristics or in long period magnetotelluric study, bring challenges to the theoretical basis of the plane wave source, if the theoretical hypothesis is not satisfied, the reliability of deep exploration will be reduced and the risk of exploration target positioning will be increased. This paper selects linear, sheet and irregular field sources model to simulate the auroral electric current, equatorial ring current and transverse wave number field sources, carries out simulation study of source effects of geodetic electromagnetic field, and analyzes the differences between non-plane wave mode and plane wave mode. The simulation has shown that the source effects strength of the electromagnetic field is positively correlated with the average resistivity of the model, negatively correlated with frequency and the horizontal distance between the measured point and the source, and is related to the height, width and transverse wavelength of the field source. Difference between the forward response of the non-plane wave mode and the plane wave mode is over 10 % in many cases. Based on source effects characteristics, this paper summarized and put forward the five effect correction methods. First one is a computing model limit of truncation effect source spectrum frequency domain truncation method. The second one is the average method for multiple simultaneous observation of adjacent points. The third one is to prolong the time of the measuring point observation time domain extension method. The fourth one is to eliminate the stronger the magnetic field component perpendicular to the corresponding source effects strong piece of data of time domain method. The fifth one is directly consider the inversion of field source. All of 5 methods, to a certain extent, can reduce the effect of electromagnetic field source to the earth. For the magnetotelluric data affected by the field source effects, if the inversion is carried out directly in the way of plane wave, there may be great distortion in the deep part. If the relevant parameters of the field source are obtained, the magnetotelluric inversion considering the field source can be carried out to eliminate the influence of the field source effects.