| As an important geophysical method gradually developed in recent years,marine controlled source electromagnetic method has been widely used in marine exploration activities.Based on its unique advantages,it makes up for the shortcomings of other marine exploration methods.Marine controlled source electromagnetic method mainly transmits low-frequency electromagnetic signals through artificial transmitting sources,and receives electromagnetic signals with target information by receivers set on the seabed or seawater,thereby detecting the required seabed geological information.At present,the one-dimensional and two-dimensional numerical simulation of marine controlled source electromagnetic method has been very in-depth,and good results have been achieved in inversion research.This paper mainly focuses on the numerical simulation of three-dimensional MCSEM with multiple field sources.In this paper,the unstructured finite element method combined with MPI parallel calculation is used to realize the numerical simulation of 3D MCSEM.Firstly,the research results of domestic and foreign experts and scholars on the numerical simulation of marine controlled source electromagnetic method are summarized.Secondly,the Helmholtz equation is derived based on the Coulomb gauge potential,and the finite element equation system is obtained by the unstructured node finite element method.Considering the characteristics of multiple field sources,the paper adopts the method of directly solving the total field to avoid repeated solutions to the background field.Finally,the finite element equations are solved by PARDISO direct solver.The numerical simulation algorithm of three-dimensional MCSEM is written and realized in FORTRAN programming language.Unstructured tetrahedral elements are used to segment and refine the model,which improves the accuracy and effectiveness of the algorithm.The mesh refinement strategy is to refine the element where the receiver and the transmitting source are located,combined with refining the mesh element of the target area according to the skin depth.At the same time,the MPI parallel computing program is used to calculate the model data,which greatly shortens the calculation time of the algorithm.The calculation accuracy and applicability of the algorithm are verified by the model.First,the calculation results of the one-dimensional ocean layered reservoir model are compared with the one-dimensional analytical solution to verify the accuracy of the algorithm.At the same time,the algorithm is applied to multi-field sources,and the influence of the number of field sources on the calculation time requirement of the algorithm is explored to verify the availability of the algorithm for multi-field sources.Secondly,the effects of model parameters such as different source location and different emission frequency on the forward response of the algorithm are analyzed.Finally,the algorithm is applied to the marine oil and gas reservoir model with complex seabed topography.The results show that the topography factors will lead to the distortion of forward response,and the seabed factors such as topography are important factors that cannot be ignored in the marine controlled source electromagnetic exploration. |
