Research And Application Analysis Of Two-dimensional AMT Inversion Based On Unstructured Finite Element

As a branch of magnetotelluric sounding,audio-frequency magnetotelluric(AMT)method is a very promising method that has been applied in metal ore exploration,engineering geology,oil and gas resource exploration,geological mapping,etc.Compared with other electrical methods,the audio magnetotelluric method uses natural field sources,which makes it more convenient to collect data.By combining unstructured triangular grids,the modeling and inversion of the AMT data can adapt to various complex terrains,reduce the impact of terrain on data,and improve the accuracy and usefulness of the method.In this paper,a two-dimensional fast OCCAM inversion based on unstructured finite element method for AMT method is studied.In terms of forward modeling algorithm,we use the finite element method and introduces the boundary conditions of the two-dimensional geoelectric model,this paper uses the finite element method and introduces the posterior error estimation based on the gradient recovery technology to realize the adaptive finite element forward modeling.Through a horizontal layered model example,the unstructured finite element modeling results are compared with the one-dimensional analytical solution,which proves that the forward program employed in this paper is correct.In terms of inversion algorithm,the fast Occam method is used,which can reduce the time spent in searching the Regularized factors under the minimum error,and accelerate the computational speed of Jacobian matrix by using gauss-newton method.In the inversion,by adding bound constraints to different model parameters and choosing different Regularized factors for different model parameters,the multi-solution of the inversion is reduced and the stability of the inversion is increased.Then experiments were carried out on the inversion effect of the proposed algorithm,and the efficiency and effect of inversion initial grids with different density were studied,as well as the analysis of inversion results of spherical abnormal bodies under different terrains and complex models.In order to further verify the practicability of the algorithm,the algorithm is used to invert the measured data of Karamay area in Xinjiang Province,and the inversion result of survey line 10 is compared with the known geological section and the inversion result obtained from the notable Winglink software,which strongly proves the practicability of the algorithm.Finally,the application effect of this method in detection of fault structure in Karamay region in Xinjiang Province is summarized,which reveals the formation of the Darbut fault and intrusive ganites in the area.The applicaiton of the algorithm can be used for reference in geological exploration of other similar structural areas.