Finite Element Numerical Simulate Of2.5-D CSAMT

CSAMT (Controlled Source Audio-frequency Magnetotelluric) is widly used and studied in many domains of today’s geophysical exploration, as a kind of high efficiency method.Currently,FEM(finite element method) is a kind of common and usefull method in Electromagnetic field numerical simulation.Because of the limit of the computer memory and computation speed, the practical application of3-D CSAMT inversion method is impossible.This paper mainly studies2.5-D CSAMT finite element forward modeling.Because that in the simulation process each wave number solution is independent,so we can use parallel arithmetic method.Thus,we can improve computation speed, so2.5-D reversion is possible.In the end,the effect of2.5-D CSAMT reversion is nearly the same that3-D.Currently,there are a few faults in FEM numerical simulation of CSAMT:The traditional boundary conditions,Dirichlet boundary conditions has been proved to be an effective and easy method for truncating the infinite domain, but the boundaries must be far away from the source and anomalous bodies, usually tens of thousands meters, node number is too many,drop calculate efficency. The souce lode is also a problem,the fields of electric and magnetic dipole souces are formally singular at the source locations,we cannot get well results,even using well mesh generation and the high-order interpolation function.Aiming at solving the above problems, first,this paper use Galerkin finite element method to derive governing differential equations, employing rectangle mesh to discretizate computational domain.Loading traditional boundary conditions, the boundaries are tens of thousands meters,at the boundary condition,the fields of electric and magnetic are forced0.In order to modle3D source,we apply3-D pseudo-delta function.we test calculation accuracy and error of CSAMT2.5-D forward process by comparison with the theoretical solution of the controlled-source,it indicates that transition fields and far fields are reliability and steaty.On this basis, this paper designs earth current models with a resistor and a conductor in a homogeneous half-space.The results show that no matter it is a resistor or a conductor,TE and TM modes can reflect abnormal.In order to explain the reliability better,this paper designs two abnornal bodies,TE and TM modes can reflect the position of the two bodies.In order to improve the speed and accuracy of program,this paper leads into infinite element method,the method replaced the raditional boundary conditions.By using it,this paper solves the big error at the truncation boundary conditions and too many nodes.this paper designs earth current models with a resistor and a conductor in a homogeneous half-space.The results show that by comparison between using the infinite element and the raditional finite element,we improve the speed and guarantee the accuracy.