| Traffic construction plays aleading role in the development of national economy,and the proportion of tunnel engineering in railway traffic is increasing gradually.Because of the complexity of geological conditions around the tunnel, once the geological conditions change, it will seriously affect the construction progress, and even cause casualties. So finding out the disease situationafter tunnel lining is an important link of the tunnel construction.At present,the transient electromagnetic method with advantages of high efficiency, economy and nondestructive is widely used in resource exploration, hydrogeology survey, and other fields. In addition, the sensitivity of the water bearing structure makes this method obtain the good results.The transient electromagnetic method(TEM for short) is a kind of time domain electromagnetic detection method based on the principle of electromagnetic induction.This method uses the grounding line or line source to transmit pulse magnetic field.Under its excitation, induced eddy currentexcited by geologic bodies will produce secondary field varies with time. Because the secondary field contains the geological information related to the geological bodies, extracting the secondary field response and analyzing its characteristics can achieve the purpose ofdetecting the space distribution of underground medium.This thesis based on the theory of transient electromagnetic method, starting from the maxwell’s equations, derived the time domain expression of the vertical magnetic field of the center line in horizontal layer. The the linear digital filtering method is used to calculate the hankel integral, sine and cosine integral. By comparing the accuracy of different filter coefficients, we choose 140 points of hankel J1 transform filter coefficients and 250 points of sine transform filter coefficients as a combination of one dimensional numerical simulation. Use one dimensional forward process written by C language to calculate the transient electromagnetic response of different parameters and different layered geoelectric model. Comparing and analyzing the response curve, we summarize the characteristics of the response curve in different condistions.Weachieve the two-dimensional finite difference time domain simulation by using DuFort-Frankel difference equation and dealing with two line source and the boundary conditions.We achieve the different transient fields response calculated in uniform half space model, plate model, cave model and fault model. Comparing and analyzing the electrical field contour and induced electromotive force curve in different delay, we can understand the law of electromagnetic wavepropagation and the impactof geologic bodies on response.It shows that the transient electromagnetic method is sensitive to low resistivity.geologic bodies.Two kinds definition of all-time apparent resistivity that one is based on magnetic field intensity and another is based on induced electromotive force is introduced in this thesis.The induced electromotive force can be converted to magnetic field intensity by using outward extension and interpolation, and theprecision of calculation results is high. We use binary search algorithm to solve inverse function, and calculate two kinds of all-time apparent resistivity that can reflect the continuous changes of apparent resistivity in whole time. Comparing and analyzing the smoke ring inversion of many geoelectric models, research shows that the smoke ring inversion of low resistivity is better than that of high resistivity. The inversion result calculated by the induced electromotive force is better than that calculated by magnetic field intensity, but it contains false anomaly. So we should pay attention to the false anomaly in the interpretation process.We ues one dimensional forward modeling, smoke ringinversion and two dimensional forward modeling to calculate the response in geoelectric models containlow resistivity shielding layer.Research shows that the low resistivity layer can reduce the diffusion velocity of transient electromagnetic field and it aggregates the induced eddy current, weaken the geological anomaly reflection under the low resistivity layer. It need longer time to detect the geologic abnormal body in same depth. The smoke ring inversion can not reflect the high resistivity layer that is under the low resistivity layer. If the resistivity of underlying strata is lower than the low resistivity layer or the thickness is greater, we can identify the anomaly from the inversion result.A series of preprocessing can improve the signal to noise ratio. We can draw the depth-apparent resistivity profiles, and apply the interpretation key to interpretation. The rebar in the lining will increase the induced electromotive force. The apparent resistivity is lower and the the maximum depth is smaller. This results verify the validity of the numerical simulation results. The longer time is needed to detecting the same depth where conductive overburden exists, and it needs to increase the transmission power or use power equipment. In addition, we need to analyze the information which combines the geological data and geophysical data and it will improve the detectionaccuracy. The results of this thesis can be applied to diseases detection after tunnel lining and also applied to surface or water survey. |
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