The Research Of Three Dimensional Finite Difference-fictitious Wave Domain Method In The Transient Electromagnetic Simulation

The transient electromagnetic method or time domain electromagnetic method(TEM)has great application prospect in the fields of geological prospecting,environmental monitoring,groundwater exploration and mine water hazard investigation.It has the advantages of high construction efficiency and can provide more practical information.It is sensitive to low resistance targets.Moreover,simultaneous measurement and sounding work can be completed at the same time.This method obviously has more advantages than other geophysical exploration methods,and is popular around the researchers from all of world.In China,the research of the TEM method starts late and is lack of theoretical innovation.The numerical simulation and data interpretation of the TEM method are only carried out around simple one-dimensional and two-dimensional earth model.At present,the research of three-dimensional TEM method,especially for the numerical simulation and data interpretation of complex terrain,is not enough and has a huge gap compared with foreign research.Mineral exploration has been basically completed in the flat area in China.However,the complex terrain area,which accounts for more than half of our country's land area,is rich in mineral resources.Due to the rugged terrain and poor environment,the exploration degree is low.Therefore,it is very urgent and necessary to find out an efficient method for three dimensional TEM numerical simulation,which can provide a technical guidance for TEM detection in complex terrain environment.In view of the above problems,this paper was supported by National Natural Science Foundation of China(No.41174095),the Development of Key Instruments of Deep Exploration(No.ZDYZ2012-1-03)and the China Scholarship Council(201406170119).The finite difference fictitious wave domain method and its application in the Transient Electromagnetic method are studied.The main contents are as follows:(1)In order to study the fictitious wave domain method in numerical simulation of TEM exploration,the physical correspondence principle is used to deduce the numerical simulation.This technique covers correspondence between the fictitious wave domain and diffusive domain,the transformation of diffusive Maxwell's equations,the construction of the governing equations in the fictitious wave domain,the method of recovering the diffusive field response.We develop and improve the fictitious wave domain method in the TEM exploration.(2)The finite-difference method of time domain is used to study the solution of the governing equations in the fictitious wave domain.Based on the differential discretization method,the governing equations are discretized in space and fictitious time.After the model is subdivided,the iterative solution is carried out.The process of solution follows the Courant-Friedrichs-Lewy stability condition.In addition,the air layer is processed and introduced into the calculation area.In the simulation of the transmitting source,the derivative of the first order Gaussian function is chosen as the transmitting signal.The analytic formula of the full space in the fictitious wave domain is deduced as a reference to verify the correctness of the Finite difference-Fictitious wave domain(FD-FWD)method's solution.(3)In the fictitious wave domain,the Complex Frequency Shifted Perfectly Matched Layer(CFS-PML)boundary condition is used to solve the boundary condition problem of the FD-FWD method.The theoretical formula of boundary condition is deduced in the time domain of fictitious wave field,and the setting method of boundary condition is given.The performance of the absorbing parameters is analyzed by using the uniform half space and the three dimensional anomaly model.The absorption effect is compared with the Perfect Matching Layer(PML)boundary condition.(4)Using the FD-FWD method,the electromagnetic response of the three dimensional anomaly model of the magnetic source and electrical source is simulated separately.The correctness of the method is verified by comparing with the Spectral Lanczos Decomposition method.The electromagnetic response characteristics of different TEM detection devices and different models are studied.(5)The numerical simulation of the electromagnetic field in the mountain area,the valley and the undulating terrain is carried out.The relative influence and relative response of the terrain and the abnormal body are analyzed,and the influence of the terrain on the electromagnetic response is summarized.(6)Based on the influence of terrain in the TEM response,the FD-FWD method is used combined with the apparent resistivity ratio between the adjoining time window to simulate the terrain model and study the terrain correction of the interpretation of field data.After the terrain correction in the interpretation of field data,the influences of terrain eliminates are not so obvious.