Multidimensional Inversion Of Resistivity And Magnetic Susceptibility Parameters From Magnetotelluric Data

Magnetotelluric （also called MT）method, which has a lot of advantagessuch as no need of artificial field source, simple construction, large penetrationdepth, not affected by the high-impedance shield, high-resolution for the lowerresistivity layer, is a natural source electromagnetic method proposed byformer Soviet scholars А.Н.Тихонов and French scholars L.Cagniard. After60years of further research and development, MT has become one of the mostsignificant and mature approach in electromagnetic exploration, which haswidespread applications and many branches.Both resistivity and magnetic susceptibility are very important parametersof magnetotelluric, but Conventional magnetotelluric data processing methodis generally assumed that the subsurface is non-magnetic, and only selects theresistivity as an inversion object. Usually this hypothesis is right, except for MTexploration in igneous areas and other places have a lot of ferromagneticmineral, because it will cause distortion of the resistivity parameterinversion.In this paper, we choose two parameters resistivity and magneticsusceptibility as the object of study, use two-dimensional Occam inversionalgorithm to simultaneously inverse the distributions of resistivity and magneticsusceptibility from a single magnetotelluric data set. This paper is completedfrom the following three aspects:The first part is the introduction of the electrical conductivity and magneticpermeability of rocks and minerals, also give the range of values.Then,describing the basic electromagnetic theory and formulas briefly.Second, in forward side, the MT two-dimensional forward formula of theresistivity and magnetic susceptibility has been deduced. And then using the finite element method (FEM) to achieve the MT two-dimensional forwardsimulation, Calculations results show that the magnetic susceptibilityparameters has effect on the magnetotelluric forward response, whenk=0.01,the forward response can distinguish the influence of the magneticsusceptibility, the forward response are more obviously with the increase ofmagnetic susceptibility. Meanwhile, for the same geological model, whenmagnetic susceptibility changes, the effect on TM Polarization response aremore obvious than TE Polarization response, the effect on high resistivitymodel is more obvious than the low resistivity model.In inversion side, the partial derivative matrix of TE and TM polarizationobservational data on the resistivity and magnetic susceptibility modelparameters is deduced. Establishing the resistivity and magnetic susceptibilitytwo-dimensional Occam inversion objective function partly, then gain the finalinversion objective function which can inverse resistivity and magneticsusceptibility parameters simultaneously by weighting the two objectivefunctions. Improving the new inversion code, establishing different theorygeological models, inverse both resistivity and magnetic susceptibilityparameters by MT two-dimensional Occam inversion method. The resultsshow that for the layered model and prism model, both resistivity and magneticsusceptibility are well agreed with theoretical models. For the combinationprism model, the magnetic susceptibility inversion results somewhat distorted.Inversion by the measured data shows that the inversion of the magneticsusceptibility has great reference value to the interpretation of the undergroundsection. According to the resistivity distribution, supplemented by theinformation of the magnetic susceptibility distribution will be able to betterreflect the geological structure, more accurate delineation and identification ofregional lithologic features. Finally, draw conclusions and recommendations.