Fine Magnetotelluric Inversion And Its Application In The Research Of Crust-mantle Electrical Structure

The paper based on the magnetotelluric sounding (MT) data which is accomplished by the university of Geoscience in November 2008. In the Lab, we systemic used the advanced data processing methods, and completed the work of MT data processing to obtain a reliable MT response. After used the advanced two-dimensional inversion algorithm, and through a series of fine inversion computation, we finally got the high confidence two-dimensional electrical structural model of the studies region.According to the result of magnetotelluric impedance tensor decomposition, the underground electrical structure of research area is approximately a two-dimensional structure, in which electrical axis is nearly south-east direction. Through the work of forward simulation and inverse calculation of the measured data, have indicated that integrate the Seismic velocity model and regional geological structure data to constructs the fine initial model can play a good restraint role to the underground Geological anomaly bottom boundary, and it also can fine depict the underground electrical structural feature, and then enhance the resolution of the result,increases the inversion result's confidence level,make the result have a further improvement, thus achieves the goal of fine inversion.The research results show that: along the study area is distributing many electric gradient and distortion zones, after contrasted with the regional tectonic information found that the position of the electric gradient and distortion zones roughly coincides with the major fault line delineated by regional geological data. These fault zones simultaneously are also to be the each tectonic element's dividing line fault zone in the study area. The electrical structure of the crust for the profile along the north and south direction display as vertical stratification and north,south piecemeal. Within the depth of 100km, the electrical structure of crust can be broadly divided into three electrical layers, contain the upper and lower layer of high resistance, and the middle layer of high conductivity, the surface layer conductive bed possibly is the Cainozoic Era or Cainozoic Era's deposit formation. In the studies region's crust exist an obvious high conductivity layer, located under earth's crust, the high conductivity layer along the profile take on a relatively thickness change, it's thins gradually from south to north, the surface and bottom boundary depth of the high conductivity layer have a bigger change. The earth's crust high conducting layer in the South of Lanzhou assumes the horizontal shape distribution approximately, but in the north of south Lanzhou's the layer seems to be on south dumping patterns.The crustal high conductivity layer within study area may be due to the effect of partial melting , or a combination of partial melting and fluid effect. On one hand,because of the intense structure activity in the study area has broken rock strata serious, maybe caused the impermeable layer be destructed, and lead to the deep fluid migration or release. On the other hand, research has shown that when the temperature rose to 550℃, The rock then may present partial melting, the Qinling fold system and the Qilian fold system have the relatively high heat flow value, so it create the conditions for the dehydration of rocks and minerals due to phase change to partial melting.