| The airborne electromagnetics(AEM)is one of widely used techniques in geophysical prospecting.The forward and inverse problems of AEM have been hot field in recent years.In the research of AEM survey system,increasing the resolution of prospecting,accelerating the speed of calculation and reducing the usage of memory of computer are the most important aspects in the research direction.The AEM survey systems are divided into time domain systems and frequency domain systems.In the current research of airborne electromagnetics,the time domain systems are interpreted by using time domain method.The data reconstruction of frequency domain systems are performed in the frequency domain.In the time domain system,the data reconstruction can be benefit from the broadband signal acquisition.However,it will lose the information of shallow ground structure since the distortion of transmitter signal exists in the cut-off time.The inversion of time domain systems is usually performed after the source current cutting off.On the other hand,the frequency domain systems usually collect a few number of frequencies to reconstruct the structure underground,which will encounter the lower resolution and less information in deeper underground structure.Based on the feature of different survey systems,the computation cost in the frequency domain system can be reduced efficiently and resolution in time domain system can be enhanced.In this research,we proposed a three-dimensional(3D)inversion algorithm in the frequency domain to interpret the data from time domain system for airborne transient electromagnetics(ATEM)survey system.The proposed algorithm will focus on the efficiency of computation and resolution of reconstruction.The forward calculation is based on the volume electric field integral equation(EFIE)in the 3D inversion.The volume EFIE can avoid the low frequency breakdown in solving the EM problem in the frequency regime for geophysical prospecting.To calculate the scattered field excitated by the electrical line source,the multilayered medium Green's function generated by electrical dipole is adopted to calculate the field in space.By dividing the line source into a number of electrical dipoles,the electric flux density in the calculation domain can be obtained by the summation of each electrical dipole.In the calculation of Green's function,the Sommerfeld integral is calculated by using numerical integration method.To solve the EFIE efficiently,the stabilized bi-conjugate gradient(BCGS)method is applied to produce the electric flux density iteratively in the computation domain.The iteration solver can be accelerated by fast Fourier transform(FFT)due to the convolution and correlation of the multilayered medium Green's function.The BCGS-FFT method can solve the large-scale scattering problems,and shows a significant improvement in efficiency over traditional or other CG-type methods.The cost function in the frequency domain inversion is formulated by using Born iterative method(BIM)with Tikhonov regularization.The layered medium Green's function of the background does not need to be updated for the BIM during the iteration.They can be stored in the hard disk before the inversion and loaded for each iteration,which can accelerate the speed of inversion efficiently.The theoretical model of semi-airborne TEM and helicopter TEM(HTEM)system are designed to validate the proposed 3D algorithm.We also test the robustness,selection of frequency range and local inversion of the proposed algorithm.The results of numerical experiments show that:(1)The BIM algorithm can reconstruct the location,size and conductivity of the scattered object underground.(2)The BIM method show stability and tolerance of noise during the iteration.(3)The frequency domain inversion can reduce the computation cost in the inversion,which is benefit for interpreting a large number of data.A series of data pre-processing techniques are applied to the experimental data before interpretation of ATEM survey system in the frequency domain.(1)To produce the induced electromotive force in the theoretical coordinate with a reasonable signal noise ratio,attitude correction and data stacking are applied to the experimental data in survey system.(2)Based on the differential relationship between induced electromotive force and magnetic field,the measured magnetic field is obtained in the frequency domain by using FFT.(3)The ground responses with specific conductivity are calculated by using the multilayered medium Green's function.The real and imaginary parts of scattered field will be extracted by subtracting the ground response.The data processing techniques and inversion algorithm are applied to the semi-airborne TEM experiments in Changyi,Shandong and Taohemu,Inner Mongolia.Then an experimental data of HTEM system in XilinGol League in Inner Mongolia are also reconstructed to validate the proposed inverse algorithm.The reconstructed results have been compared with the geological and drilling data provided by the Institute of Electronics,Chinese Academy of Sciences.The good agreements show that the 3D BIM inversion algorithm can be used to reconstruct the underground structure for ATEM survey system.For the first time,the frequency domain 3-D interpretation of the semi-airborne TEM data is performed by using BCGS-FFT-BIM with Tikhonov regularization.By extending the algorithm to HTEM survey system,the reconstructed results of this area present a promising engineering application for AEM survey system... |
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