ELF(extremely low frequency) electromagnetic method which is combined withgeophysics and radio physics is a kind of exploration technology.The signal of ELF isemerged by high-power artificial source so that data can be collected far from thesource.Compared with traditional MT and CSAMT methods,ELF has advantages suchas stable frequency signal,antijamming capability and deep investigation depth.Though ELF is in the developing stage,it is supported by well-developed theoryand there are broad prospects for development.It is necessary to build a forward andinversion procedure for ELF.In this paper,2D forward and inversion methods whichare now usually used in the frequency-domain electromagnetic technology arestudied.And theory and programs used in ELF are built finally.Because of the long-range and stronger signal,the effect of fields caused byionosphere should be considered in numerical modeling.In this paper,recursionformulas of vector potential in stratified model are adopted to settle the problem.This paper will use the secondary field method to avoid the singularity of thesource.Using rectangular grid subdivision,system of equations is formed by elementforward modeling.It is necessary to use the compressed method to storage matrix andGaussian elimination to solve the equations for getting the results of the wave numberdomain.Then the inverse Fourier transform will show results of the spatial domain.Data-subspace inversion method is adopted in the inversion algorithm.Theprocess is introduced in detail. The Jacobian matrix of different components of thefrequency-wave domain can be deduced by adjoint equation method.The reliability ofthe inversion method is proved by computation of several simple models.Comparingthe results from electric field component and both apparent resistivity and apparentphase components,the advantages and disadvantages of each way has been found.TheMT and CSAMT inversion programs are also used in processing the same data,but theresults are worse than those got from the inversion method developed in this paper. |