Studies Of Three Dimensional Stagged-grid Finite Difference For Mobile Magnetotelluric Numerical Simulation

Mobile MT(MMT)is a new type of natural source airborne electromagnetic method which combines magnetotelluric method and airborne electromagnetic method.In this method,a three-component magnetic field is measured by an aerial platform,and the horizontal electric field is measured by a reference station on the ground.Similar to MT,the apparent resistivity can be used to infer the absolute geoelectric structure.Compared with the ZTEM system,this method has the characteristics of higher working efficiency and larger exploration depth and can distinguish the electrical property differences of different geological bodies in any direction.Due to a large amount of data acquisition of natural source airborne electromagnetic method,data processing,and inversion interpretation mainly focus on two-dimensional,but the actual geodetic model is mostly three-dimensional geoelectric model.Therefore,it is urgent to carry out Mobile MT 3D inversion research,but efficient and stable 3D forward modeling is the key to 3D inversion.In this paper,a three-dimensional numerical simulation of Mobile MT based on a staggered grid finite-difference algorithm was carried out.Starting from Maxwell's equations,the staggered sampling finite difference scheme is used for discretization.Combined with mixed boundary conditions,the three-dimensional numerical simulation of Mobile MT is transformed into the solution of large-scale linear equations with a single component of the magnetic field.In order to speed up the solving speed of 3D forward modeling,the agglomerative algebraic grid algorithm is introduced.Starting from the coarsening strategy and nested iteration technology of agglomerative algebraic multigrid algorithm,the Mobile MT 3D forward modeling algorithm is developed by agglomerative multigrid method.After the implementation of the algorithm,the correctness of the algorithm was verified by comparing the calculated results of the layered model with the analytical solution.Besides,the quasi-minimal residual method,and the aggregate algebraic multigrid method are compared and analyzed.The results show that the aggregate algebraic multigrid algorithm has more efficient in large grid and high precision.A typical geoelectric model is designed to carry out 3D numerical simulation,and the 3D response characteristics of Mobile MT are analyzed.Combined with ZTEM and MT 3D response characteristics,the merit and demerit of Mobile MT method are demonstrated.Starting from the analysis of response characteristics of typical ore body model,the feasibility of Mobile MT in mineral resources exploration is demonstrated.Finally,the effects of topography,buried depth of abnormal body,flight height,and location of ground base station on the response of Mobile MT are analyzed by model test,which laid a foundation for the working method and 3D inversion of Mobile MT.