The Forward And Inversion Method Research About Magnetotelluric In The Condition Of Sector Boundary

Based on the natural alternating electromagnetic field as the field sources, themagnetotelluric sounding method–a frequency domain sounding method–is usuallyused to study the crust and upper mantle electrical structure. The forward and inversionmethod of magnetotelluric in cartesian coordinates have been comparatively mature sofar, but it is rare to see the forward and inversion method in other coordinate.Considering the spherical shape of the earth, this article main research the forwardand inversion method of2-D magnetotelluric under the condition of sector boundary.To provide a new and more reasonable numerical simulation method for the longperiod magnetotelluric.The earth`s curvature should be taken into consideration in magnetotelluricsounding methods when the survey line is more than hundreds kilometers long orlocated at high latitudes. In this paper, we get magnetotelluric partial differentialequations in spherical coordinates and the two-dimensional magnetotelluric partialdifferential equations in the condition of the sector boundary. The trapezoidal elementsubdivision and the bilinear interpolation finite element method(FEM) are used in theforward modeling. We validate the results by comparing numerical solutions of FEMwith analytical solutions of1-D modeling. And then study the nonlinear conjugategradient inversion algorithm of magnetotelluric under the condition of the sectorboundary. After the analysis of the theoretical model, we find There must be longenough profile to get better inversion results for LMT. With the increase of latitude orprofile length, The earth curvature will have a greater impact. So we think it is moreintuitive and more reasonable using the sector boundary model than the rectangularboundary model.A long peroid magnetotelluric profile along the Luqu-Longmenshan-Hechuan hasbeen finished which helps to study the crust-mantle electrical structure in Longmenshan and its adjacent area, Institutive and reasonable2-D inversion algorithmunder sector boundary was adopted. The electrical results of this section has shown thecrust-mantle high resistance belt in Ruergai, Longmenshan, central Sichuan and lowresistance belt in Songpan from northwest to southeast. Longmenshan crust-mantlehigh resistance block shows up as wedge-shaped high resistance block stretching tonorthwest under the Longmenshan thrust belt. Due to bi-directional compression ofEast margin of Qinghai-Tibet plateau and upper Yangtze plate, Crust materials ofSongpan-Ganzi massifs thrust to Longmenshan, and middle-lower crust and uppermantle slides toward southwest, presenting a trend that west margin of upper Yangtzeplate in shape of wedge inserting into east brim of Qinghai-Tibet Plateau. It wasinitially thought that Songpan crust-mantle ductile shear belt is regarded as the westmargin deep boundary of upper Yangtze plate since Mesozoic-Cenozoic Era. Thefound of Longmenshan crust-mantle high resistance block and the greaterunderstanding of the west boundary of Yangtze block are helpful for the study of deepstructure of Longmenshan and oil and gas prospective evaluation and disastersprediction such as earthquake.