| Magnetotelluric (MT) and Transient Electromagnetic (TEM) have been widelyused in geophysical exploration sphere nowadays.The numerical modeling/inverseimaging for topography magnetotelluric and direct time-domain transientelectromagnetic, are not only very interesting topics, but also difficult problems ingeophysical electromagnetic. For the former is within the frequency-domainelectromagnetic sounding method, and study on modeling for electromagneticresponse characteristics of various geophysical models under conditions offrequency-domain, topography, natural source, and inverse imaging forinteterpretation of measured data. For the latter is within the time-domainelectromagnetic sounding method, and research in modeling for electromagneticresponse characteristics of various geophysical models under conditions oftime-domain, artificial source, and inverse imaging for inteterpretation of measureddata. This thesis is precisely on these two issues.1. In the numerical modeling for magnetotelluric, the thesis puts forward anumerical modeling of finite element method (FEM) using topography, triangularelement grid, and continuous variation of electric parameters. To model arbitrarilyshaped two-dimensional topography and structures in field work, triangular elementgrid was used in FEM. In view of the fact of continuous variation of the subterraneanrock-mineral electric parameters, the electromagnetic field and some electricparameters of models are designed to bilinear variation within each triangularelement in our numeric modeling method, and which is developed for modelingtwo-dimensional MT under the field topography condition. Calculation of theauxiliary field and definition of the apparent resistivity, and the impedance phase are deduced according to the relationship between the main fields of the three nodes andthe linear shape function within each element. By calculating a continuous mediummodel and two topography models of other scholars set up to test our method, theresult of our method show a high accuracy (the mean square error is less than1%),and the results of modeling two topography models accord with other scholar’s, too.Through modeling for a slopeing interface abnormity body, we find that our methodcan model arbitrarily complicated terrain and geoelectric bodies preferably.2. Based on the iterated asymptotic Zohdy method, a new direct inverse method,so-called the improved Zohdy-Oldenburg direct inverse method, is puted forward inthe sense of the least square method, which has characteristics of both Zohdy’s ratioand Oldenburg’s difference, can modify the parameters of the model throughcomparison. The method is successfully applied to two-dimensional inversion of MT,and proposed based on topography, with phase information, the two-dimensional MTimproved Zohdy-Oldenburg direct inverse method. Through calculation and analysison the model data and measured data, the method shows that the method has highconvergence speed and fitting accuracy. Compared with conventional linearedinversion method, the calculation speed of our method can be increased at least10times.3. In the numerical modeling for time-domain transient electromagnetic, toavoid oddity electromagnetic field response for the total-field solution effectively,the thesis puts forward a numerical modeling method on two-dimensionaltime-domain transient electromagnetic secondary-field for line source withDuFort-Frankel finite-difference. Based on study works of Oristaglio (1984) andAdhidjaja (1985), some improved and advanced technologies were designedincluding the treatment of earth-air boundary conductivity, the calculation of thenormalized partial derivative of the induced electromotive force (Emf), anddetermination of time step forward. Through adopting previous achievements onsecondary-field solution such as the technology of upward continuation from theearth-air interface field to air grid nodes and condition of zero-value boundary, themethod is not only more efficient but is also simpler than the total-field solution. Wecompute and analyze the homogeneous half-space model and the flat layered model,the results of our method show a high computation precision (the maximum relativeerror is less than0.01%between our method and analytical method),and the solutionspeed is roughly three times faster than total-field solution. Lastly, modeling andanalysis for a model which consists of a thin body embedded in a homogeneoushalf-space at different delay times, we depicted the physical downward and upward spread characteristics of the induced eddy current, and the physical interactionprocesses between electromagnetic field and the underground anomalous body.These geophysical abnormous characteristics can be applied actual production, andthe curve of forward modeling combined with the measured data curve analysis, canwell determine the level of abnormous body positions.4. According to the cuve of the all-time transient electromagneticnon-monotonic character, this thesis puts forward a “phased search method” tocalculate the all-time apparent resistivity. And the all-time apparent resistivity iscombined rationally with the “Smoke ring” inverse method. A improved TEMinverse method is proposed to be called the all-time resistivity “Smoke ring”inversion. Trough calculation of the spread features about the transientelectromagnetic field induced eddy currents in the underground; the method canmodify the parameters of the model directly. Though calculation and analysis for themodel data and measured data, the results show that the mothod has a rapid speed(the50mesaured points of inversion, expended time is less than10s) andsatisfactory precision.In the thesis, magnetotelluric method, transient electromagnetic method of dataprocessing and interpretation, modeling accuracy, calculate the actual problems ofslow as the main research. Based on the previous achievements, forward numericalmodeling and inverse imaging method will been done a thorough research. On theforward numerical modeling, this thesis used the finite element method and the finitedifference method for numerical modeling. Field of MT complex terrain, the body ofarbitrary shape to power the analog model proposed block electrical parameterschange continuously MT finite element two-dimensional numerical simulation; totalfield TEM method for numerical simulation of the singularity near the sourcemidfield issue, the line source two-dimensional time domain finite difference TEMsecondary field simulation method. On the inversion imaging, proposed atwo-dimensional MT improved Zohdy-Oldenburg direct inversion, based on theregion’s apparent resistivity TEM "smoke ring" direct inversion method. Bytheoretical models, the calculations show that the measured data to study thecontents of the improved modeling accuracy and computational speed, as the theoryof electromagnetic geophysical research, data processing and interpretation hashelped. |
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