Model Study For Phase Rolling Out Of Quadrant Magnetotelluric Data At The Northern Tibetan Plateau

In observed magnetotelluric(MT) data, most of the off-diagonal impedance phases lie in the 1st and 3rd or 2nd and 4th quadrants, but a few MT sites' phases might exceed the normal quadrants, we call this phenomenon as Phase rolling out of quadrant(PROQ). In recent years, some PROQ MT sites were recorded when we were doing MT observations in the Northern Tibetan Plateau. We observed the phases roll out of quadrants at low frequencies; the frequency is usually lower than 0.1Hz. The PROQ was first mention at the beginning of the eighties in last century, and could not be explained by the inversion technology at that time, the possible reason was speculated as a kind of distortion effect or some effect caused by three dimensional conductivity structures. The PROQ data can not be explained with one-dimensional or two-dimensional isotropic methods, and always be recognized as an effect of distortion and would be excluded during two-dimensional inversion. As the rapid growth of three-dimensional forward and inversion methods, it is possible to explain the PROQ data by three-dimensional modeling. The studies for the basic theories of the PROQ effect and the practical three-dimensional inversions of the PROQ data are still very few.The former studies of PROQ are mainly focus on the synthetic three-dimensional modeling, and the researchers didn't reach a consensus of the features of the models which can cause PROQ data. In this thesis, four synthetic three-dimensional models are reconstructed according to the former study and their structure features are analyzed. Then the theory of current channeling is introduced to study the mechanism of the formation of PROQ, the approximate analytical formulas are used to analyze the influence of the current channeling on electric and magnetic fields. Meanwhile, three-dimensional isotropic inversion program Wsinv3 dMT and ModEM are used to calculate the models that fit the observed PROQ data which are collected in the north to the Hexi Corridor area at the Northern Tibetan Plateau. Finally, the three-dimensional inversions are used to explain the PROQ data at the northern Qaidam Basin. The main research contents and results of this thesis are summarized below.(?). Four synthetic three-dimensional models which can produce the PROQ data are rebuilt in a uniform format. Then the forward responses are calculated by the subroutine of the Wsinv3 dMT program. The three-dimensional models have several similar features, such as the high-conductors are embedded in the high-resistivity background, the high-conductors are comprised by a regional conductor and a local conductor which are connected with each other, the PROQ data appear in the low frequencies which is similar to our observed data. In the responses analysis of the model, the phases of the electric fields of the PROQ part change dramatically, while the corresponding phases of the magnetic fields do not change at all. As for the location of the PROQ effect, the PROQ data appear above the special sections of a S-shaped high-conductivity structure in one kind of models and appear above the high-resistivity background near the local conductor in the other kind of models. We consider that there must be two mechanisms which trigger phases exceed the normal quadrants.(?). The current channeling theory, which always be considered as a kind of distortion effect, is introduced in this thesis. The current channeling effect is caused by the electric or electromagnetic perturbation of the concentrated current flow induced in three-dimensional conductors. The current channeling effect contains two parts, the surface charge part and the induced current part(they are correspond to the galvanic distortion and induced distortion); these two parts can explain those two different PROQ phenomena in the synthetic models. The mechanism of current channeling can be described in two forms. First, current induced in the channeled-like three-dimensional conductors flow from the regional conductor to the local conductor, and surface charge is cumulated on the electrical conductivity discontinuities; when the skin depth is larger than the length of region conductors, the surface charge part will affect the regional induced electric field and cause phases roll out of the normal quadrants. Second, the high-concentrated induced current flowing in the local conductor may change direction according to the shape of the conductor, as the current flowing in a direction heavily deviates from the regional polarization direction, then the phases may roll out the normal quadrants.(?). Three-dimensional isotropic inversions were applied to PROQ data at north to the Hexi Corridor, the response fits the observed data very well. With three-dimensional forward modeling, we tested the contribution of the high-conductivity anomalies in the upper crust. The regional conductor northeast to the PROQ MT sites is corresponding to the Badain Jaran Desert, it connects a local conductor on the east side of the PROQ MT sites; these two conductors, forming a L-shaped high-conductivity structure in plane view, are the main contributors to the PROQ data north to the Hexi Corridor. Beside that, there is another isolated conductor west to the PROQ sites that can influence the distribution of abnormal phases.(?). In the Qaidam basin, there are almost half MT data whose phases roll out of quadrants at the low frequencies. In order to study the model features of this area, we use two inversion strategies. One is using all MT data during the three-dimensional inversion and we can get a general structure of the model in this area, the other is inversing 13 lines by three-dimensional program one by one and we can get the detail structures along the line, in this way the inversion models' responses can fit the PROQ data better. Finally, we get two kinds of models which fit the PROQ data in difference scales. In the electrical model of the regional three-dimensional inversion, about 15 high-conductivity blocks are distinguished and some of the conductors are intersected in high-resistivity background. Forward modeling is applied to test the influence of these conductors to the PROQ data. As the model can not fit the PROQ data very well, further work is still needed in the future.