2-D CSAMT Forward Research

Now most of CSAMT data processing methods are developed in one dimension. that is thatsupposing the earthmedia is horizontal layers, this will cause some wrong results for an earthwhose conductivity varies in two or three dimensions. For some case's, the survey lines areparallel to the geologic structure's dip direction. In such situation, to consider three dimensionssource and two dimensions earth is enough. In order to reasonably Simulate the three dimensionsource and two dimensions earth, this paper introduces a 2. 5D finite element forward modelmethod for CSAMT data, which bases on the Maxwell's equation. The main principle of thisforward modeling method is to synthesize data for three dimensional signal source with a seriesof different wave numbers in geologic structure's strike direction (direction x in Cartesioncoordinate system). One typical three layers model is used for 2.5D forward modeling, Becausefor such typical model the EM caused with a 3D source has an analytical integralsolution,through numerical method, The EM field values is easy to determined.Thoughcomparison with these two methods, the reliability of 2. 5D modeling method and software isverified. Then we introduce another model outlined from known geological structure andcaculated it's electro-magnetic field. The results show that the 2.5D forward modeling methodand correspond software in this paper provides a good tool for CSAMT data modeling becausethe finite element method can be used for the models with lateral variation of earth electricproperties, it will be useful to model 3D source and 2D earth media.Firstly, this paper investigated the researchment of CSAMT in the world, and then consider-ed that the new work should be work out the electro-magnetic field ten kilometers away from themain section and then make compareration of 1-D forward solutions, to kown the method isvalidity. In chapter two, this paper worked out the formulas of electic dipole on a layeredmedium, and made 1-D farward by the Hankel Transformation Method. The compareration withthe other work showed that the 1-D farward is correct, so the I-D farward solution can be usedto vadidate the 2-D forward solution. In chapter three, using the Maxwetl's equentions and theboundary conditions, this paper worked out the actual differential equations, and then workedout the finite element method equations. In chapter four, this paper paticularly introduced thefinite element method, including triangle element analysis, linear equations foundation, triangleelement storage and linear equations predigestion, others variables calculation. Because of thefinite element method using second field, the first field is needed to solve the linear equations,and in chapter five, the first field formulas are worked out and can be calculated by chavemethod. When the elecromagnetic field is worked out from the linear equention, the athwartfourier transformation is needed to give the actual elecromagnetic field, and in chapter six, thetheory and method of athwart fourier transformation is particularly gaven out. In order to checkup the correctness of the finite element method solution, the comparation of 1-D solutionbetween the finite element method solution shoud be show out, and in chapter seven, this papershow out three kinds of different earth model structure comparation between the finite elementmethod solution and 1-D solution, the comparation including Ex, Hy and Hz field, and theprofile is from 1 kilometer, 2 kilometer, 4 kilometer, 6 kilometer, 8 kilometer, 10 kilometer and 12 kilometer. For more validation, in chapter eight, this paper show out the solution of twodifferent model structure which one is low resistivity in the center and the other one is highresistivity in the center, which including resistivity and phasic pictures both.Because of the CSAMT method using manpower, the signal is much better to incept, eventhough in the industrial interferential area. In these years, the CSAMT method having been usedin many areas, such as, groundwater reconnaissance, terrestrial heat reconnaissance,engineering exploration, metal mine exploration, coalfield disaster forecast and so on.Using the finite element method, this paper carded out the electic dipole source 2-D modelstructure forward program. The innovation point is that the source is in x direction, the earthmodel structure is 2-D, and the solution is 12 kilometers away form the source. Through thecomparation between 1-D solution and 2-D solution, we believe that the finite element method iscorrect. What is more, this paper show out the solution of two different model structure whichone is low resistivity in the center and the other one is high resistivity in the center, whichvalidates the correctness also.The comparation between 1-D solution and 2-D solution shows that the solution in 10kilometers is acceptable, and the 2-D model structure forward shows the finite element methodis availability. So, the program can be used to near field emendation and 2-D reverse.This program has shortages also, when the frequency is low(lower than 1 Hz), thecomparation between 1-D solution and 2-D solution is unacceptable, it needs theory and methodimprovement, when the distance of the profile y becoming larger the error is larger, this issimply because of the athwart fourier transformation is not enough, at the edge of the finiteelement gridding the electromagnetic field forward is also unacceptable, so it need proper finiteelement gridding, what is more, the electromagnetic field near the source can not be calculatedcorrectly, because the electromagnetic field near the source is tend to infinite.