Transient Electromagnetic 3D Forward Modeling Based On CNCSU-FDTD And Complex Model 3D Visualization Modeling Software Development

Transient Electromagnetic Method(TEM)is a time-domain electromagnetic exploration method based on the principle of electromagnetic induction,which has been widely used in the fields of metal mineral exploration,groundwater investigation,coal mining area detection,and karst development area survey due to its sensitivity to low resistance bodies.With the development of the forward modeling algorithm and computer technology,3D forward modeling has become an effective mean to study the TEM response of different targets.The finite-difference time-domain(FDTD)method is one of the commonly used TEM 3D forward modeling algorithms,but the traditional explicit FDTD algorithm is strictly limited by the CFL condition,resulting in an excessive number of time steps in the simulation process and low computational efficiency.The Crank-Nicolson FDTD(CN-FDTD)method is an unconditionally stable implicit difference method,and its time step is no longer limited by the CFL condition.But it requires solving large sparse matrix in the simulation process,which leads to low computational efficiency.With the application of TEM method in engineering and geological investigations,the computational speed and the model complexity of the 3D forward modeling algorithm are required to be higher.The traditional time-domain FDTD algorithm usually adopts the modeling method of model file,which can build some simple and regular models with a small number of parameters,and lacks the means to build complex terrain or complex anomalous geodynamic models.In terms of model meshing,it meshes by manual input parameters which cannot visualize the meshing results and is not conducive to checking the mesh dissection and mesh quality.In the aspect of calculation result display,the simulation results of the specified points are extracted according to the pre-design,and the calculation results are post-processed by third-party software.Therefore,in the field of TEM FDTD forward modeling simulation,there is a lack of a software that integrates visual modeling,meshing,fast forward modeling and visualization of calculation results for complex models.To address the above problems,this paper firstly adopts the Crank Nicolson cycle sweep uniform(CNCSU)method to approximate the CN-FDTD equation in the forward algorithm.By introducing a higher-order error term into the single time-step iterative equation,the iterative equation is split into two sub-steps,thus splitting the large sparse matrix in the iterative process into multiple diagonally dominant tridiagonal matrices that can be solved quickly using Thomas algorithm,which speeding up the forward computation.Subsequently,a problemtype suitable to the FDTD algorithm is developed based on the GID platform.And an interface between the CNCSU-FDTD algorithm and GID is written to form a software package integrating pre-processing,forward modeling and post-processing for TEM forward modleing of complex models.Finally,the developed software is used to perform 3D forward simulation of several complex models and analyze the response characteristics respectively.