2.5-Dimensional Inversion Of REBOCC Using Controllable Source Audio Magnetotelluric Sounding Data

Controlled source audio magnetotelluric(CSAMT)methoduses artificial field sources to emit electromagnetic wave fields,and by transmitting and receiving electromagnetic signals at different frequencies,it can achieve the purpose of detecting different depths of electric media structures in the underground.This method has the advantages of strong anti-interference ability,high collection efficiency,and is widely used in various geological surveys.Due to the influence of “field source effect”,but also due to the lack of further research on the two-dimensional inversion algorithm of electromagnetic detection data with field source,there is still a lack of practical twodimensional inversion computer program with field-sourced electromagnetic detection data,Therefore,CSAMT data processing can only select far-field data,and use the twodimensional inversion technique of magnetotelluric sounding(MT)data to perform inversion,so as to obtain an underground electric structure model in the study area.However,it is difficult to guarantee that the data of different frequencies can satisfy the far-field condition;that is,it is difficult to guarantee that the current CSAMT detection result is completely true.Moreover,the artificially launched electromagnetic field is in the near field and transition field,the spherical wave properties do not satisfy the theoretical foundation of the data processing of the magnetotelluric field.So that is impossible to use MT data algorithm for inversion.As a result,the CSAMT probe data was greatly wasted and the detection capability was reduced.Based on the above reasons,this paper uses the calculated electromagnetic field value to carry out the inversion of the whole area resistivity,and calls the two-dimensional model of the three-dimensional source as a 2.5-dimensional problem.The two-dimensional inversion of the model is realized by considering the three-dimensionality of the source.In the forward part,the method of calculating the secondary field is used to reduce the influence of the artificial source,and the total field is calculated by superposing the primary field and the secondary field.The advantage of this method is to reduce the meshing requirements near the field source,while avoiding the accumulation of errors in the calculation.In this paper,the finite element method is used to establish electromagnetic coupling equations and finite element equations for the first type of boundary conditions.The forward sparse matrix is solved in the frequency wave number domain,and then the inverse Fourier transform is performed to obtain the values of the electromagnetic field components in the spatial domain.After a series of comparative experiments,the accuracy of the forward performance was verified.In the inversion,the data subspace(REBOCC)method is used to inverse.The sensitivity calculation part uses the adjoint matrix method to calculate the sensitivity matrix in the frequency wavenumber domain,and uses the inverse Fourier transform to obtain the results in the frequency domain.Appropriate selection of subspace parameters can greatly reduce the calculation time without affecting the inversion effect,and we selected the most suitable subspace parameter by comparing different subspace selections.The inversion tests were performed on the single low-resistance model,the high-resistance model,the combined model and the transitionarea data.The results show that the inversion model is in good fitting with the theoretical model.