Research On 3d Forward And Inversion Of Frequency Domain Electromagnetic Method Of Well-ground Controlled Source

The well-ground controllable source frequency domain electromagnetic method is an electrical exploration method in which excitation sources are arranged in boreholes and mines for excitation,and the acquisition system is arranged on the ground for reception.This method has the characteristics of large detection depth,strong ability to detect deep targets,and high practicability.Through the boring-terrestrial electromagnetic method’s excitation source and the layout of the observation system,its three-dimensional characteristics have been clearly embodied.In recent years,scholars at home and abroad have conducted many studies on the well-to-earth electromagnetic method.How to improve the calculation accuracy and efficiency of forward and inversion and the inversion technology based on unstructured grid cells are the research hotspots of experts and scholars at home and abroad.First of all,this paper studies the three-dimensional forward modeling of the frequency domain electromagnetic method of the well-ground controllable source based on the unstructured grid.Starting from Maxwell’s equations,the thesis derives the differential equation that the potential function of the electromagnetic field of the underground horizontal electric dipole source satisfies,and adds boundary conditions to obtain the boundary value problem.After that,the unstructured grid elements are used for subdivision,and the interpolation basis function of the unstructured grid elements is derived.By solving the potential function of the grid nodes,the value of the electromagnetic field is further solved and organized into impedance form.The apparent resistivity and phase are solved.When meshing,a method of local densification of anomalous bodies and ground observation systems is used to improve the accuracy of the numerical solution.In order to verify the accuracy of the forward algorithm,a low-resistance cube model and a highresistance cube model were established.The electric dipole was simulated and compared by placing the electric couple source at different depths and at different positions at the same depth for excitation.Forward simulation response when the source is excited at different locations.Secondly,the finite memory quasi Newton method is used for 3D inversion research.Three models are established: low resistance cube model,low resistance body and high resistance body combination model,and low resistance oblique prism model.The method of adding random error into forward data is used to carry out the inversion test of forward model,and good inversion results are obtained.In the inversion of the low resistivity cube model,the electrical sources are set at different positions of the same depth,and the comprehensive inversion test is carried out,and the inversion results are compared with those of single source excitation.