Two-dimensional Magnetotelluric Inversion Of Interlaced Model

The "interlaced model" refers as a geological model combined by series of linear structures(pseudo 2D) which have different spatial characters both on horizontal and vertical direction. Taking the geo-structure distribution into account, the primary feature of interlaced model will be marked by geo-electrical strike azimuth variations caused by lateral and longitudinal structures beneath the surface. From the structure dimensionality analyzing point of view, an interlaced model can be treated as a peculiar 3D model assembled by many rotated 2D models, which makes big difficulties in getting reliable recovered resistivity distribution by directly inverting magnetotelluric data in 2D assumptions. By taking advantage of pseudo 2D character of interlaced model, we promote a brand new sub-frequency-band and sub-section inversion scheme.In this paper, which first recurs to impedance tensor imaging technology to study 3D theoretical model and measuring profile of Tan-lu fault zone, completely shows the whole process of the sub-frequency-band and sub-section inversion scheme as follows.First of all, we design a simple 3D longitudinal interlaced model, and then calculate it by 3D forward program as well as compute its 2D model of the target section by 2D forward program to obtain a 3D, 2D response,respectively. To compare the two we conclude that the 3D model should be in the TM polarization mode to conduct 2D inversion. Based on impedance tensor decomposition(CCZ method), by analyzing the regional dimensionality and the impedance tensor imaging of the cross section to determine the electrical strikes distribution and their corresponding frequency ranges in the frequency domain. Thus, for this model, we propose to do 2D inversion of a frequency band first, and its result as the initial model to inverse another frequency band. By comparing the result of the new program with conventional single-axis inversion and 2D model inversion, we draw that inverse the low frequency band first,then take its result as initial model for the high frequency band inversion will better reflect the actual model information.Secondly, the above model's result which takes conventional inversion under the average strike azimuth(22.5 degree) relatively reliable. In order to verify the applicable angle range of the average azimuth strike inversion, so we design another model, only the strike of shallow structure different from the above-mentioned model(in this case the average strike azimuth is 35 degree). After calculating the model by 3D and 2D forward program as the same with the above model, we uniquely identify the frequency bands to different electric strike by impedance tensor imaging combined with real induction vector analysis. Then, we discover that the result of average azimuth strike inversion which compare to the 2D model inversion result for the model cause severely structure distorted and no longer reliable. It follows that in order to ensure the accuracy of interpretation of the inversion, inversion selected strike azimuth should less than 30 degree relative to the actual strike of the structure. In addition, the model is also used to carry out the new sub-frequency-band inversion and the final inversion results support the reliability of this new inversion scheme.Once more, for the study of the applicability and effectiveness of the sub-frequency-band inversion scheme under other interlaced model, we design horizontal interlaced models,that is, select the target cross section across two or more different structures with different strikes. By choosing three different survey lines demonstrate and summarizes the practicability of impedance tensor imaging in the detection of linear structures. Based on this, we found different electrical strikes in frequency domain overlap, can not determine the corresponding frequency ranges to different strikes in the frequency domain, namely sub-frequency-band inversion scheme no longer applicable. Whereas, according to the characteristics of the site based cloud diagram, we propose to do 2D inversion of a site segment first, and its result as the initial model to inverse another site segment. By comparing the result of the new program with conventional single-axis inversion and 2D model inversion as well, we obtain that inverse the strike remarkably low site segment first,then take its result as initial model for the strike remarkably high site segment inversion will better match the actual electrical structure.Finally, according to the research ideas of theoretical model, the paper through further researching a measuring profile of Tan-lu fault zone to verify the new sub-frequency-band and sub-section 2D inversion scheme which proposed from theoretical model. That is: Based on the full profile, sub-section and sub-frequency-band impedance tensor imaging, including the dimensionality of the structure and electrical strike statistical imaging analysis to identify the presence of complex interlaced structure, therefore the profile need to combined sub-frequency-band inversion with sub-section inversion. And then by taking the real induction vector and the relationship between electrical strike with the profile distribution orientation into consideration, select the optimal strike azimuths for inversion. At the same time, the conventional single strike inversion results under the same parameters as a reference. After analyzing and discussing the results obtained by new sub-frequency-band?sub-section inversion, we found its resolution higher than conventional inversion, especially the major tectonic fault zone of survey area-Tan-lu fault zone was reflected clearly. What's more, in various aspects, including the inversion itself, theoretical study appearing the same high resistivity abnormalities, the high conductivity anomalies identified by the impedance tensor imaging, and previous geological?geophysical achievements in the same area, have verified the reliability of the new sub-frequency-band ? sub-section inversion result. Thus indicating that sub-frequency-band?sub-section 2D inversion have better model resolution and reliability in complicate structure area.Through this study, we propose a new sub-frequency-band?sub-section 2D inversion scheme for interlace mode. Comparative analysis of the inversion results from theoretical models and measured data of the southern section of the Tanlu fault zone, display and demonstrate the new inversion methods' implementation process, the reliability of the inversion results and resolution. It's a new inversion method which can be applied in complicate structure area.