Research On Partial Extrapolation BCGS-FFT Algorithm In Electromagnetic Scattering Calculation Of Coal Seams

The electromagnetic scattering models of non-uniform objects in free space,half space and multi-layer backgrounds have many practical applications in remote sensing and underground detection.Simulation of large scale electromagnetic scattering of inhomogeneous objects in layered media is a major aspect in computational electromagnetism.In the process of underground coal mining,the existence of geological structures such as goaf,coal seam fault and pinch-out brings great difficulties and hidden dangers to underground coal mining.Therefore,it is of great significance to study the electromagnetic scattering problems of underground coal seam and its abnormal conditions for the realization of fine detection of underground coal.The underground exploration of coal seam is a electromagnetic scattering problem under the plane stratification model.A common method to solve electromagnetic scattering problems in layered models is the stabilized biconjugate gradient fast Fourier transform(BCGS-FFT)method.Based on the plane stratification model,the electric field integral equation(EFIE)of the traditional BCGS-FFT algorithm is given,and the roof basis function is used to discretize the integral equation weakly.The accuracy and effectiveness of BCGS-FFT is verified by numerical example simulation.In the process of solving scattering electric field and scattering magnetic field in traditionalBCGS-FFT algorithm,the solution of the dyadic Green’s function is an important process of the method,and the accurate solution of the spatial Green’s function is carried out by introducing Sommerfeld integral.This method has the characteristics of oscillation,singularity and slow convergence when solving large-scale electromagnetic scattering problems rapidly.In this thesis,the types of singularities and their physical meanings in Sommerfeld integral are given.A convenient semi-elliptic integral path is introduced to minimize the oscillations and singularities of Sommerfeld integral.Parameterization of semi-ellipse and interpolation methods are used to ensure the accuracy of integral.A partial extrapolation BCGS-FFT algorithm is proposed to solve the electromagnetic scattering problem in layered media by combining BCGS-FFT method.The concrete derivation process and pseudocode are given for reference.In the numerical experiment part,two algorithms are realized by FORTRAN programming language.The accuracy of the partial extrapolation BCGS-FFT algorithm is verified by taking the surrounding rock structure of underground shallow coal seam as an example,and the computational efficiency of the partial extrapolation BCGS-FFT method is compared with that of the traditional BCGS-FFT method.Then three typical abnormal structures of fault,gangue inclusion and pinch-out in underground coal seam are simulated.The partial extrapolation BCGS-FFT method can save 21～32%computation time while maintaining the same computational accuracy.It is proved that this method can be used to calculate the electromagnetic scattering of large complex coal seam scatterers embedded in layered medium space,which provides a new analytical method for rapidly solving the electromagnetic scattering field in a specific area of target coal seam.