Research And Application Of Magnetotelluric Forward Modeling And Inversion Considering Earth Curvature

The Magnetotelluric Sounding(MT)is a frequency sounding method,which is based on the electric difference of the underground medium and using the natural electromagnetic field as the source to study the underground electrical structure.In this paper,it mainly studies the forward modeling and inversion of two-dimensional MT considering curvature of the earth,and apply to the data processing of the Fugong-Luliang profile in Yunnan.In the case of two-dimensional media,the long profile shows as a curved arc in the earth scale,then the earth curvature needs to be taken into account when turning to the forward modeling and inversion.Firstly,we introduce the derivation of the MT partial differential and variation equations.In forward modeling,using the finite element method based on the sector subdivision,the biquadratic interpolation,gauss numerical integration and the preconditioned nonlinear conjugate gradient method(NLCG)to solve equation.By comparing with the numerical solutions in the case of homogeneous half-space and H-shaped geo-electrical model,we proved the correctness of the program.After contrasting high and low resistivity electrical model in floor magnetotelluric method,we found that magnetotelluric has a better reponse to the low resistivirty body.Besides,TE has better vertical resolution and TM has good lateral resolution.So considering the curvature of the earth is more reasonable.We adopting the NLCG method for the inversion of high and low resistivity model,it is found that the magnetotelluric inversion of the low resistivity abnormal body is better.Otherwise,the lateral resolution of anomalous body is higher than vertical resolution and the upper interface resolution of the anomalous body is higher than that of the bottom interface in the longitudinal direction.The length of the profile has less affection on small-scale anomaly body,but larger for large-scale.In the case of the long profiles,the longer length of abnormity body,the greater influence of the earth curvature.The response shows that the result matches the actual curved ground.We introduced the tectonic setting,development and the sense of Fugong-Luliang,MT profile in Yunnan Province.Describe the profile layout,data aquisition and data processing.Especialy,the application of ARIMA model to deal with missing data in MT;then we analysis of Niblett-Bostick penetration,the result shows that the TM penetration depth is deeper than TE,And the average is about 80 km.Then We calculate the strike direction and 2D skew with Swift method,The Strike direction is consistent with structural trend,the 2D skew e of the low-frequency part is much high suggesting the three-dimensionality is strong.We get the electrical structure of Fugong-Luliang profile after inversion and divided it into 5 part from the west to the East: the high-low-high resistivity layered crust-mantle structure of Lanping?Low resistivity banded crust-mantle structure of the Jinsha River?high resistivity Blocky crust-mantle structure in southwestern margin of Yangtze block?The high-low-medium resistivity layered crust-mantle structure of Lanping in central Yunnan and The high resistivity band crust-mantle structure in East Yunnan.Two high resistivity bodies were found below the Jinding and BeiYa ore concentrated areas.The initial inference is that the ultrabasic magma invaded the shallow part of the crust.Most earthquakes occur where is the resistivity boundary region or the high resistivity region near low resistivity area,and the study area can be divided into five seismically active regions:The earthquake area of the Jinsha River fault zone,The earthquake area of Binchuan-Chenghe fault zone,The earthquake zone of the Yaoan fault zone,The earthquake area of the Lvzhijiang fault zone,The earthquake area of Xiaojiang Fault Zone.