The Study Of Simulation And Inversion Of 1D Controlled Source Audio-frequenc Magnetotellurics Method

Controlled Source Audio-Frequency Magnetotellurics (referred to as CSAMT) is a frequency-domain electromagnetic methods for measuring the depth. The principle is to stimulate the alternating electromagnetic field by the use of artificial sources, and then calculate the wave impedance and apparent resistivity response by observing the electromagnetic responses. Frequency domain CSAMT follows the AMT method. The artificial source takes the form of the magnetic source and electrical source. As that the CSAMT of electrical source can detect greater depth, it is often applied for exploring coal, geothermal, oil and gas and solid minerals'deep-prospecting, etc., This thesis is centered only on the electrical dipole source.In this paper, we derived the equations of each component on the basis of the former research. The source we used is horizontal eclectical dipole on the layered earth. Particularly the equations are in frequency domain. When came to solving the Hankel integral, we introduced two linear filter coefficients in forward modeling, and worked out the FORTRAN programs for computing. On the choice of the inversion method we adopted the previous proposal and made some change. Futher more, based on the numerical simulation of models we accomplished the inversion of full 1D CSAMT data.Forward theory: First, assume that the layered strata are horizontal. For the electric fields generated by electric dipole with the homogeneous anisotropic dielectric, the Maxwell's equations will be transformed into the frequency domain forms; then in line with the case of the physical meaning, transform it into components under the Cartesian coordinate system with mathematical method, and ultimately get the forward calculation formula under the layered media, whose most critical problem is to solve Hankel transform. For the Hankel transform, the author takes two sets of linear digital filter coefficients, compares their calculation precision and calculation speed, and arrives at some valuable conclusions. For the first time, the second set of linear digital filter coefficient is applied in one-dimensional forward models in CSAMT. After the derivation of the Forward formula, the author writes the one-dimensional forward modeling program of CSAMT, and conducts a large number of model simulations which include H, K, A, Q and other typical geoelectric models, and analyzes the forward curve with many comparison tests, which holds instructing meanings for the treatment interpretation of Controlled Source Magnetotellurics.Inversion theory: currently CSAMT mostly uses linear inversion. This thesis takes the results of Bostick as the initial model, and the optimal inversion uses the generalized inverse linear inversion method. The selection of the initial model in the optimal inversion has a direct impact on the inversion process. As a fast imaging method, Bostick is widely used in electromagnetic theory. Although the result is not precise enough, the calculation is simple and a direct underground resistivity variation with depth is showed. By using this result as the initial model, a comparatively reasonable initial value can be obtained. Lately some good result is acquired by the inversion of some typical geoelectric models with generalized inverse linear inversion method. Through comparative analysis, the author integrates the above two methods and makes a qualitative explanation on the base of the Bostick fast inversion, and then, taking the result as the initial model for generalized inverse inversion, does fine inversions. The test proves that the combination of the two methods can not only improve the efficiency of the inversion calculation, the results are more close to the real model.