The Research And Application Of Magnetotelluric Impedance Decomposition

Magnetotelluric Sounding(MT)is widely used to study the regional electrical structures.Due to the complexity of the Earth's electrical structure,the Magnetotelluric data generally present three dimensional characteristics,but most geological bodies can be approximated to two dimensional geological structures extending from different scales.However,the existence of small scale three-dimensional electrical anomalies on the surface will lead to the distortion of the observed MT impedance tensors.As a result,the distorted impedance tensor cannot well reflect the actual underground electrical structure.It affects the Magnetotelluric data processing and the inversion interpretation.The distortion characteristics of the impedance tensor can be studied by the impedance tensor decomposition,and the distortion correction of the impedance tensor can be used to restore the true regional impedance.Based on the theory of Magnetotelluric distortion,the basic characteristics of the Magnetotelluric impedance tensor distortion are discussed,and the basic principles and physical assumptions of the physical decomposition methods of the impedance tensor are summarized,as well as the corresponding applicable conditions.After realizing these decomposition methods,the effectiveness of each method is verified by theoretical synthesis data.The properties of the phase tensor and the calculation of the phase tensor decomposition distortion parameters are introduced in detail,and the optimization method for the nonlinear overdetermined equations of the GB decomposition is presented,The GB decomposition of single frequency single point is realized by using phase tensor decomposition constraint and nonlinear least squares generalized inverse matrix algorithm.At the same time,the stability of the decomposition methods under noise interference is studied,and an algorithm for estimating the reasonable strike direction under noise interference and the corresponding correction method of the apparent resistivity curve are introduced,which has a satisfactory effect on the analysis of the strike direction orientation.Through the three-dimensional Magnetotelluric forward mtheod,the 3D/2D electrical structure model is designed to simulate the response of the actual application of the geological structure,and the parameters of the impedance tensor distortion are calculated by different methods,and a reasonable and effective physical decomposition method of impedance tensor is summed up by comparison and analysis.And then those methods are applied to the processing and interpretation of two MT profile data in Western Yunan.Combining the phase tensor decomposition,GB decomposition and stable electrical strike direction,we construct the dimensionality characteristics of the MT data in the study area.And the regional structure electrical strike direction is analyzed.The structure of the section is divided into different blocks,and the distribution of the resistivity of the data is analyzed with emphasis on the distribution of the phase tensor ellipse.Through the two-dimensional nonlinear conjugate gradient inversion of Magnetotelluric,the electrical structure model of the section is obtained,and the geological and electrical structure model of the Gaoligongshan oblique collision zone and adjacent area is established,and the cause of the large range low resistivity anomaly body in this area is discussed,It is considered that may be formed by the joint action of brine fluid and partial melting.