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Study Of Three-dimensional TEM Forward Modeling And Non-linear Conjugate Gradients Inversion

Posted on:2017-01-17Degree:DoctorType:Dissertation
Country:ChinaCandidate:X LiuFull Text:PDF
GTID:1220330482983951Subject:Geophysics
Abstract/Summary:PDF Full Text Request
After entering the 1980 s transient electromagnetic method(TEM) was developed rapidly. TEM is widely used in metal, oil and geothermal exploration of mineral resources, engineering prospecting et. al. But the 2D and 3D forward and inversion technology is just in the stage of exploring research and lack of practical value. So we study the TEM 3D forward and inversion technology in the paper.We use pseudo-spectral method(PSM) to TEM modeling in whole-space and in seafloor with electric dipole source. The example shows the advantage of pseudo-spectral method. It has high computational efficiency and high precision. The modeling can show the diffusion of electromagnetic field and the response of low resistivity continuously and give a wealth of information in time domain. The potential applications of PSM are mine and seafloor TEM modeling. It also can used to TEM molding in surface if joining the air-earth condition properly.In this paper, we deduce the staggered-grid sampling finite-difference iterative equation starting from Maxwell’s equation. We use a staggered-grid sampling finite-difference time-domain method(FDTD) to TEM forward modeling of loop source. The results are matching the results in frequency domain. The forward modeling algorithms is correct. Then the responses of some low resistance body model, high resistance body model and the combination model are calculated. They illustrate the response characteristic and advantages of TEM in time domain.Because of the huge amount of observed data of TEM, the general inverse method is of very low efficiency. From the methods of seismic migration imaging, we study EM migration imaging. We use the back propagated fields to compute the gradient of error function by imaging methods to avoid of calculating the Jacobian matrix. The calculation of gradient is the key part of inversion. Then we apply non-linear conjugate-gradient(NLCG) to find the minimum of objective function and get an appropriate conductivity model.Last we show how the residual fields reflect underground anomalous body. And we inverse the synthesis of the electromagnetic field data using NLCG. The inverse result of low resistance and high resistance prism model is good. This proves that TEM distinguish low resistance object better then high resistance. The result is better and inverse location is close to the real location when increase more data with more source.
Keywords/Search Tags:the three-dimensional forward modeling and inversion of TEM, pseudo-spectral method, staggered-grid sampling finite-difference time method, non-linear conjugate-gradient inversion
PDF Full Text Request
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