Survey And Research Of Ultra-Long Period Magnetotelluric Sounding In Longmen Mountain Fault Zone

In this paper, combining a new type of ultra-long period magnetotelluric sounding instrument LEMT417 with multifunction instrument V8 to carry out the broad-band ultra-long period for magnetotelluric observation in outlying Longmen mountain fault zone, and use of advanced data processing and interpretation of electromagnetic technology to analyses outlying Longmen mountain fault profile, which has gained very credible outlying Longmen mountain deep fault electrical structure, and summed up the results of previous work. At the same time, the Longmen mountain fracture with a number of hot issues were discussed and researched according to the advantage of magnetotelluric sounding.The paper firstly introduces the work of the outlying Longmen mountain thrust belt profile. And the profile is from the south to the north that is from Tianquan, Ya'an to Guangyuan, which total length is 500km.Then reviewing the study history of Longmen mountain and the development course of magnetotelluric sounding, combining with ultra– long period instrument characteristics, lay outing the work point the location of along the fault zone, and the paper briefly introduces the process of ultra– long period data processing.Qualitative analysis and interpretation of profile information is one of the key points of this paper. The best direction of electrical axis are achieved by rotating tensor impedance, the best main axis direction is North West - South East in the northern section and the middle section ,but is deflected to North East - West in the southern section; using of Mohr circle analysis and polarization diagrams analysis for all 12 measuring points, to get the deviation angle and anisotropic two-dimensional angle, and the conclusion is that the overall performance of the profile is two-dimensional , and anisotropy and three-dimensional is very strong in sub-region of the fracture; similar conclusion is also reached in two-dimensional impedance deviation; The electrical characteristics of the main structure is understood provides the basis for interpretation by the analysis and the study of the measured apparent resistivity, the type of phase curves and characteristics of cross-section.Based on studying characteristics of two instruments, comparing the measured apparent resistivity curve of two instruments from the same measuring point, and carrying out convergence and adjustment of data, is to be credible broadband magnetotelluric sounding curve; to the choice of model in the inversion, summed up the valuable experience of those predecessors, the paper introduces the development of inversion and the basic principle of one-dimensional inversion, briefly presents MTsoft2D V2.1 software processes .At last inversion method OCCAM1D and TE + TM joint inversion model inversion were chose, which has gained the inversion results of wide-band profile.Finally, in the background of the history of Longmen Mountain, the paper introduces the distribution of major stratigraphic, geological faults, etc. in the study area, and combining the results of qualitative analysis, reveals the existence of the Longmen mountain thrust and analyses the structure of electrical section.