| For a long time,the magnetotelluric sounding is based on two basic assumptions:one is that the source of magnetotelluric field is approximate to plane electromagnetic wave mode,and the other is that the earth curvature is ignored to approximate the earth's horizontal medium model.These two kinds of approximations are applicable to the study of magnetotelluric sounding in small area,middle and high frequency and middle and low latitude.However,when the study of magnetotelluric sounding is carried out in the area with non plane wave field characteristics(such as Aurora area),or in a large area and in a large depth,the theoretical basis of the field source plane wave and the earth's horizontal medium model will be challenged.Because the theoretical assumptions are not satisfied,The reliability of inversion results of deep part is reduced and the uncertainty of exploration target location is increased.The main work of this paper is as follows: firstly,by selecting linear,flaky and irregular flaky field source models,the simulation of the source effects of magnetotelluric field is carried out,and the difference between non plane wave model and plane wave model is compared;secondly,by studying the two-dimensional forward and reverse algorithm of magnetotelluric sounding in cylindrical coordinate system and spherical coordinate system,the two-dimensional forward and reverse simulation considering the curvature of the earth is realized and differences from horizontal stratigraphic model is compared.The main purpose is to provide scientific basis for when considering the field source effects of magnetotelluric sounding and the earth model,and for what method can be adapted to decrease or reduce the risk of magnetotelluric research when the two basic assumptions are not met.In the field source research,considering that the magnetotelluric field source may be composed of plane wave source,auroral current collector,equatorial ring current,transverse wave current and other non plane wave fields in different periods and regions,the characteristics of field source effects are obviously different.In this paper,firstly,through the forward study of several typical non plane wave fields,the characteristics of field source effects of magnetotelluric are simulated and analyzed.It is found that the larger the resistivity is,the lower the height of the line source is,orthe closer the measuring point is to the line source,the stronger the field source effects of the line source is,the smaller the half width is,the stronger the field source effects of the sheet source is,the smaller the transverse wavelength is,or the greater the model resistivity is,the stronger the field source effects of the wave number field is.Then,according to the characteristics of the field source effects,the correction methods of the field source effects in the frequency domain,space domain and time domain are summarized.In the frequency domain,the influence frequency band of field source effects in different latitude or different homologous parameters is given,which has certain reference value for the data processing and interpretation of magnetotelluric;in the space domain,two correction methods of normalization and measuring point average are introduced,and their limitations are pointed out;in the time domain,the extension method and elimination method are summarized,and the analysis have shown that the time-domain correction method can reduce the field source effects significantly,but the extension method needs long-term observation,and the elimination method needs to effectively identify the non plane wave response data in the time series.Finally,considering the limitations of the current correction methods,a one-dimensional magnetotelluric inversion method of the non-plane wave field source is proposed and implemented,and the distortion characteristics of the non-plane wave field source's magnetotelluric response are given through the simulation test.In the study of the earth curvature,the traditional three-dimensional simulation of magnetotelluric is the simulation method which ignores the influence of the earth curvature in the rectangular coordinate system,and which will bring the error of detection,positioning and inversion interpretation.In this paper,the three-dimensional forward simulation of magnetotelluric based on the earth curvature model in the spherical coordinate system is completed firstly,and the staggered grid finite-difference separation and dispersion is carried out in the spherical coordinate system;Secondly,based on the solution of three-dimensional lateral boundary problem,a two-dimensional forward simulation of finite difference in cylindrical coordinate system is proposed and realized.The model calculation has shown that the difference of 3D forward response value based on spherical coordinate and rectangular coordinate is more obvious as the frequency decreases,and the more complex the model construction is,the greater the forward difference is;at the same time,the paper uses the finite memory quasi Newton method to realize the magnetotelluric two-dimensional and three-dimensional inversion based oncylindrical coordinate system and spherical coordinate system.The results of theoretical and practical inversion have shown that the two-dimensional and three-dimensional inversion considering the curvature of the earth can reflect the shape or model of the curvature of the earth more intuitively.Compared with the traditional inversion algorithm based on rectangular coordinate system,it can avoid the error after the model projection,and the inversion method considering the curvature of the earth is more suitable for large-scale magnetotelluric research.The above research has shown that the difference between the non plane wave field source model and the plane wave field source model,and the problem of considering or not considering the influence of the curvature of the earth,are mainly reflected in the low-frequency or ultra-low-frequency exploration,that is,the difference between the exploration of large depth and special area or large area is large.Using the original hypothesis or approximate exploration ideas and inversion interpretation will lead to higher exploration risk,inaccurate interpretation and positioning.Therefore,it is necessary to adopt certain field source correction method and simulation interpretation scheme.The research results also have shown that the significant difference between the influence of nonplanar wave field source model and earth curvature and the original theoretical hypothesis is mainly on the research of low-frequency electromagnetic wave field,that is,researches on the depth of deep crust and mantle;the preliminary quantitative analysis results have shown that the relative curvature of magnetotelluric field source problem may be more prominent.The main work of this paper focuses on the research of magnetotelluric forward and inverse modeling considering the field source and the curvature of the earth.However,due to the lack of relevant research results and the complexity of the field source and curvature,the influence of curvature is ignored in the research of the field source,and only a simple assumption is made for the field source.However,from the perspective of deep crust mantle structure research and cross regional global scale,the combination of the field source and curvature has more realistic significance,which can further explore the complex mechanism of magnetotelluric response and propose more reasonable and scientific solutions,and which is the focus of the author's next research work. |
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