Research On One-dimensional Forward And Inversion Method Of Grounded Electrical-source Transient Electromagnetic In Anisotropic Medium

Transient ElectroMagnetic method(TEM)is a time domain artificial source electromagnetic exploration method established based on the principle of electromagnetic induction.At present,the research on the forward and inversion of transient electromagnetic exploration is mostly based on the isotropic medium model.However,with the advance of the research,the changes of the observation data caused by the anisotropy of the underground medium have gradually attracted researchers' attention.More and more theoretical studies and measured data show that the anisotropy of various physical properties(such as elasticity,electrical conductivity,susceptibility,density,etc.)of the subsurface medium can cause significant changes in the response of the relevant geophysical field.Among them,the electromagnetic impedance response function is very sensitive to electrical anisotropy.Correspondingly,the electromagnetic data observed on the earth surface is also affected by the anisotropic characteristics of the subsurface medium.Therefore,it is necessary to conduct an in-depth study on the electromagnetic forward response and inversion methods in anisotropic media to improve the reliability and accuracy of the Earth's electromagnetic detection results.This paper studies the transient electromagnetic forward modeling and inversion methods using electrical current sources with underground electrical anisotropy media based on the theory of electrical anisotropy of subsurface media.In terms of forward modeling,the one-dimensional forward frequency domain expression of the transient electrical source in the anisotropic medium is solved based on the conventional transient electromagnetic forward modeling theory and anisotropy theory which is introduced in the electromagnetic wave impedance recursive process.On this basis,the time domain expression of the transient electromagnetic field in anisotropic media is obtained by using the Guptasarma algorithm.In terms of inversion,the inversion procedure for electrical properties anisotropic medium is realized based on the Occam inversion theory and anisotropic forward modeling theory which is introduced in the important steps of inversion gradient calculation and roughness matrix composition.The related program implementation was completed and a large number of inversion experiments have been carried out on the electrical anisotropy model on the basis of the above research on the forward and inversion methods.The experimental results show that the anisotropy in the model medium will have an impact on the forward response.The impact of anisotropic medium on transient electromagnetic field is as follows: the higher the degree of anisotropy of the model,the more severe the distortion caused by the transient electromagnetic field observation data,which leads to a larger difference between the conventional inversion result and the real model parameter.deviation.In the term of inversion,the Occam inversion based on the electrical anisotropy forward has achieved good inversion imaging results.The verification of the theoretical model shows that the transient electromagnetic inversion and inversion based on the anisotropy theory can ensure the correctness and reliability of the inversion results.And furthermore,the underground medium will be more accurately identified based on the anisotropic forward and inverse algorithm.