The Research Of 2.5-D Nonlinear Conjugate Gradient Inversion Of CSAMT

Controlled source audio magnetotelluric sounding(CSAMT)is developed on the basis of MT and AMT.This method is classified as frequency-domain electromagnetic sounding exploration method and it can achieve the purpose of exploration of underground electrical structures in the target area,through the emission of eletro-magnetc waves of different frequencies.Since the 1970 s,CSAMT has become a major exploration tool for non-seismic technology,and have a wide range of applications in the field of oil and gas,mineral exploration,geothermal,groundwater,and environmental research.However,as the artificial source electromagnetic wave field propagates in the lossy medium,it is divided into “near area”,“transition area” and “distant area” according to the different propagation characteristics and laws of the wave field;among them,only in the “distant area” observation Only in this way can the observation data be protected from field sources.Therefore,when the two-dimensional inversion algorithm of electromagnetic sounding data with source is not yet mature,the inversion of CSAMT data often uses “distant area” field data,and the MT inversion is used for processing and interpretation..However,the classification of “distant zones” is not only related to the “succeeding-distance” distance,but also determined by the frequency of the observed signals and the conductivity of the underground media.Therefore,as the signal frequency and the conductivity of the underground media change,the “distant zone” range It is also changing.This will inevitably result in the deviation of the CSAMT inversion result obtained by MT two-dimensional inversion,and the “transition area” data cannot be used.Therefore,the CSAMT two-dimensional inversion method with artificial field source must be studied.This paper proposes a 2.5D CSAMT nonlinear conjugate gradient two-dimensional inversion method.The forward modeling based on the finite element method is used to accurately calculate the two-dimensional electromagnetic field value with the field source through the secondary field algorithm so as to eliminate the near field source.The singularity;the inversion uses a two-dimensional non-linear conjugate gradient method to avoid storing large sparse matrices by means of quasi-orthogonalization,reducing the requirement for computer memory and improving the speed of single iteration iteration.The paper deduces the frequency domain finite element calculation formula,handles the boundary problem,completes the derivation of the pseudo-forward formula,indirectly obtains the Jacobian matrix,and then implements the 2.5-dimensional CSAMT inversion algorithm.