Electromagnetic Numerical Simulation For Irregular 3D Abnormal Body Based On Octree Grids

The transient electromagnetic method has a good resolution for the underground low resistivity abnormal body,which is widely used in many fields,such as metal resources,groundwater,and underground unexploded ordnance.Numerical simulation is one of the important methods to obtain electromagnetic response data,and it is also the basis of data interpretation,which plays an important role in the study of response law.Compared with the finite volume method,the finite element method,the integral equation method,the meshless method and other numerical simulation methods,the finite difference method has attracted more and more attention because of its intuitive principle and reliable calculation.Compared with the FDTD method,the FDTD method can directly add the excitation source,avoid the calculation of the initial field and the constraints of stability conditions,and is more suitable for the calculation of electromagnetic response of shallow abnormal bodies.However,the frequency-domain finite-difference method needs to solve large-scale linear equations in the calculation process,which is slow,especially in the numerical simulation of electromagnetic response of three-dimensional irregular abnormal bodies.In this paper,the octree grid is combined with the finite difference method based on Yee grid in frequency domain.Ensuring enough calculation area,the area of irregular abnormal body is subdivided.For the normal abnormal body,the fine grid is used to subdivide the whole area,and for the abnormal body with slope,only the irregular part.In the process of numerical simulation,the storage coordinates of grid step,electric field component and magnetic field component are solved by using the method of space zero compensation.The missing magnetic field components and electric field components at the boundary of coarse and fine grid are obtained by using the method of averaged value and the method of binary paraboloid interpolation,which solve the problem of transmission of electromagnetic field component.The correctness of the algorithm is verified by calculating and comparing the electromagnetic response of the uniform geodetic model,the layered geodetic model,and the geodetic model with three-dimensional abnormal bodies.By calculating the electromagnetic response of irregular abnormal bodies such as thin plate,L-shaped and trapezoid,the calculation time of Yee grid and octree grid are compared.It is found that octree grid can improve the calculation efficiency to some extent.At last,the electromagnetic response characteristics of the abnormal body under different buried depth,conductivity and size are analyzed,and the variation law is summarized.