The Study Of Marine Artificial Source Electromagnetic Method In Frequency Domain Of Three-Dimensional Conjugate Gradients Inversion

With its gradually developed theories and techniques, marine electromagneticmethod finds expanding applications and better effects. Nowdays electronmagneticmethod has become an important tool in prospecting tectonics in oceanic crust andupper mantle, playing a greater and greater role in resources explorations likedetectings in oil, gas, solid minerals et al.When used in detecting offshare oil and gasresources, marine controled-source electromagnetic method in frequency domainshow advantanges of deep sounding, high lateral resolution and senstive response tothe low-resistance zone. While widly used abroad, this method is still pre-mature inour country. This article mainly studies the algorithms dealing with3D forward andinversion problem in marine controlled-source electriomagnetic method in frequencydomain, trying to systematically establish its3D numerical modeling and inversiontheories and techniques, which will lay foundations for its applications.3D forward of marine controlled-source electronmagnetic method in frequencydomain relys on interlaced sampling finite difference algorithms.The totalelectromagnetic field is divided into the primary field and the sencondary field aselectric dipole source exists. Field value distributions of stratified model undergroundderived from Hankel filtering algorithm comprise the primary field. Throughinterlaced sampling grid dissection model, integral form of Maxwell equation isdiscretized to system of linear equations.This system of linear equations under certainboundary conditions are soved by preconditioned conjugate gradient method.Therecomes the secondary field.Under comparision with other algorithms, the accuracy ofthis algorithm is testified.As to inversion of marine controlled-source electromagnetic method infrequency domain, this article adopts conjugate gradient method.Generally, Jacobimatrices need to be soved to get the parameters of models underground. Conjugategradient method transfering caculaltion procedure of Jacobi matrix into two‘quasi-forward’ caculations heps to avoid a Jacobi matrix, which significantly speeds up the inversion and decreases the storage space. Two models are used to testify thereliablity of3D conjugate gradient method involed in this article.Forward caculations are conducted for both sigular emission source and mutipleemission sources.Moreover, this article covers the curves and plane equivalences ofthe total field and the secondary field. Comparied between inversions result underdifferent emmission conditions.