| Electromagnetic detection technology is a kind of geophysical exploration method to solve engineering,environment or disaster geological problems,study the geological structure,and look for useful mineral resources by observing and studying the temporal and spatial distribution of natural or artificial electromagnetic fields according to the electromagnetic properties of various rocks and orebodies in the earth's crust.Numerical simulation,instrument equipment and data processing constitute the three branches of geophysical electromagnetic method detection technology.With the great demand for resources and energy,and the deep development of geophysical technology,electromagnetic detection technology is developing towards in detail,large depth and high efficiency,the numerical simulation and measurement of electromagnetic detection in complex media and complex models becomes an urgent problem.The now available numerical simulation techniques are based on mesh,they inevitably relate to the mesh generation and need requirement for the shape of the grid.When dealing with complex models and complex media,it has problems such as low precision,mesh generation and difficulty in adaptive analysis.The existing electromagnetic detecting instruments also have the problems of single function and weak ability of anti-interference in the field environment,it is becoming more and more difficult to adapt to the rapid development of electromagnetic method.The thesis is co-sponsored by the National Instrument Developing Project “Research and development of core equipment for deep resource exploration”and National Natural Science Foundation of China project “3-D numerical simulation of time domain airborne electromagnetic detection based on meshfree implicit difference method”.In order to overcome the shortcoming that the existing numerical simulation technology relies too much on the grid.In view of the problem of electromagnetic detection in complex media and complex model,we roundly apply the meshfree method to the numerical simulation of time-frequency electromagnetic prospecting.The basic theory of meshfree method and its application in electromagnetic exploration are systematically studied.Different meshfree methods which are suitable for different models and different detection methods are developed.The idea of timefrequency joint electromagnetic measurement is proposed based on the analysis of the response characteristics of frequency domain electromagnetic method and time domain electromagnetic method.The corresponding acquisition device is developed,and the acquisition system test and model verification are carried out.The abnormal body in the near region using time domain measurement and far zone using frequency domain measurement are realized,and a relatively complete research system is formed.The main research contents are as follows:1.The basic theory and realization of meshfree method are systematically studied.In the weak meshfree method,the two kinds of shape function construction methods,moving least square method and radial base point interpolation method are mainly studied.We take the Possion equation in electromagnetic problems as an example,the key problems in the global weak meshfree method are studied,the influence of the size of support domain,weight function,different basis functions and Gauss number on the computational accuracy is also researched.The meshfree collocation method based on radial basis function is introduced into the solution of partial differential equations.The basic principle and implementation process of RBF interpolation and partial differential equations are also introduced.We also provide specific simulation examples to verify the validity of the RBF collocation method,which shows that the meshfree method has the advantages of convenient implementation and high accuracy.It lays a foundation for the application of electromagnetic prospecting.2.The numerical simulation of electromagnetic detection in frequency domain based on global weak meshfree method is studied.In view of the problem that it is difficult to accurately simulate the complex geoelectric structures in the frequency domain electromagnetic exploration by using the classical electromagnetic calculation method.The weak forms equivalent equation of boundary value problem of natural field source magnetotelluric method and artificially controlled source electromagnetic method are derived in detail.We respectively introduce the basic principle of the global weak meshfree method.Algorithm verification and parameter optimization are carried out under the uniform half space model.The correctness and accuracy of the algorithm are demonstrated by comparing the results with the finite element method.The complex models of continuous conductivity,undulating terrain,anisotropy and inclined fault are calculated and analyzed.The electromagnetic response differences under different models are discussed,which opens up a new idea for high-precision numerical simulation of frequency domain electromagnetic detection in complex electric and distribution models.3.The meshfree method based on radial basis function for electromagnetic exploration in time domain is studied.In view of the problem that traditional numerical methods need fine mesh analysis and can not describe the boundary of abnormal bodies accurately in the time domain electromagnetic numerical simulation,a meshfree coupled finite difference method is proposed.We derive the diffusion equation for the electric field along the strike direction,the radial basis function is used to discretize the spatial domain,and the Crank-Nilcoson difference scheme is used to discretize the time,then the iterative control equation is obtained.The ground-air boundary condition is obtained by using upward continuation,and the stability conditions are also given.The calculation results are compared with the finite difference method,complex models with abnormal bodies are calculated and the electromagnetic response characteristics in time domain are also studied.The advantages and disadvantages of electromagnetic detection in time domain are pointed out,which lays the foundation of the development of the electromagnetic acquisition system with time frequency integration.4.On the basis of systematically studying the response characteristics and advantages and disadvantages of frequency domain electromagnetic method and time domain electromagnetic method,we use the idea of time-frequency joint electromagnetic measurement combined with the advantages of high efficiency,high resolution in the time domain electromagnetic detection and high stability,great depth in the frequency domain electromagnetic detection.A time-frequency integrated acquisition device with multi-channel,broadband and high precision is developed.The overall design scheme and performance design index of the acquisition system are given.We have designed the cooperative anti-surge protection circuit,low noise analog signal conditioning circuit,signal acquisition circuit,large capacity high-speed data transmission circuit and so on.The background noise and dynamic range are tested in the room,and consistency tests are carried out for different channels.The time-frequency joint abnormal ring test is conducted in Changchun Culture Square,which verifies the effectiveness of the acquisition system and the abnormal body in the near region using time domain measurement and far zone using frequency domain measurement.The thesis focuses on the problem of electromagnetic exploration in complex models,along with the direction of "question-basic theory research-theoretical verification-calculation and analysis of complex models-time frequency integrated electromagnetic detection system-model test and verification ".We follow the principle of "from easy to difficult,from general to individual,from theory to practice" in the research of theory and the practice.The thesis forms a complete research system,which has opened up new ideas for numerical calculation and instrument research in electromagnetic exploration. |
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