Research On Theory And Application For Advanced Prediction Of Water By Whole Space Transient Electromagnetism

With the construction of high speed railway, highway and submarine tunnel, geologic disasters always play the main part in threatening the safety of tunnel and underground engineering construction. Among these geologic disasters, underground water is one of the main factors. For example, karst cave, underground river, karst sink hole, karst muddy belts, water-saturated fracture zone and so on, all these can bring about difficulty and harm to construction and operation of tunnel and underground engineering. By all appearance, tunnel geological hazard forecasting plays an important part in avoiding these geological hazards. Presently, there are many methods used in the field of tunnel geological hazard forecasting and these methods have their own advantage. But there is an unsolved problem in the exploration of unfavorable geology in tunnel filled with water for the advanced forecast of geology in tunnel construction.Transient Electromagnetic method (TEM) is a new method used to forecast the unfavorable geology filled with water or mud. The new method is sensitive to the unfavorable low resistively geologic bodies filled with water or mud such as faults, caves, zones of rock fracture and water-saturated fracture zones. Because of relative backwardness in explain technology, complex algorithm, a large amount of time and computer memory for the inversion and forward of calculation and other factors, all these severely restricted the development of TEM. It is theoretical and practical demand to study the transient electromagnetic response of two-dimensional and three-dimensional models. At present the numerical simulation of electromagnetic method of finite difference method, finite element method, boundary element law, integral equation method, and so on were commonly used in the exploration. The time domain finite difference and the finite element method are more convenient. This thesis adopts the finite element software ANSYS to analyze transient electromagnetic method for geological prediction. The spreading characteristics of the transient electromagnetic field in tunnels in whole-space and in earth in half-space were compared. The sensitivity characteristics of geoelectric parameters were investigated by changing the depth, radius, and resistivity of an anomalous body and the resistivity of surrounding rock. Influencing factors of the geological anomaly response and the law of the abnormal response changing with the factor were studied.The results of simulating were stored into database as sets of specimens by development of database system. The computed results were transformed into standard samples of transient electromagnetic methods in tunnels using a developed database system, management and maintenance of the database were realized. The results of engineering applications can be transformed into standard samples. The measured data were inputted and compared with the data in database while inversing. Data of the same model on different time were assigned matching weighting, the data in database closed to measured data were selected by comparing the weighing data, and thus the expert system of database for inversion was formed. An improved inversion system of BP neural network had been designed and developed by using MATLAB software. The system avoided complicated calculations of electromagnetic field and can be used to solve some practical problems through learning and training. It can also memorize and make the inversion a continuous process which depends not only on current event but also the previous event. A nonlinear inversion method based on LSSVM network was proposed by analyzing the conventional inversion methods and overcoming their defects. Results of simulating on twenty time were inputted and the geoelectric parameters corresponding the model were outputted. LSSVM expert system was formed by loading function of LSSVM into MATLAB software.Brine and graphite were picked out as the experimental material to simulate rock with high resistivity and water body with low resistivity according to similarity criterion of physical modeling. The response law of transient electromagnetic signal affected by the electrical parameters of receiving loops and transmitting loops were researched. The receiving loops and transmitting loops were designed by spaced winding with lacquered wire according to the displine. The transmitter and receiver in PROTEM47 transient electromagnetic system used countryside were used in experiment. The graphite, receiving loops and transmitting loops were put into a pool filled with brine, the response laws of transient electromagnetic signal were observed by changing the measurement, position and resistivity of a graphite.Three expert systems for inversion were applied into practical engineering. Basing on the formulation of apparent resistivity in whole space, the curves of time vs. response which were gotten from practical exploring were transformed into the curves of depth vs. apparent resistivity by VC program. The data which were gotten from measurement line on working face in tunnel were transformed into the section map of depth vs. apparent resistivity. The result shows that expert system for inversion is feasible.