Magnetotelluric Data Inversion Resistivity And Magnetic Susceptibility Parameters Were Studied

The MT has been a powerful geophysical tool in exploring the interior of the earth and widely applied in the oil and gas prospecting, geothermal prospecting and deep survey. It is assumed in traditional MT data processing that underground media is not magnetism, in which only the resistivity parameter. Sponsored by NSFC, we calculated and analyzed the effect of rock susceptibility on geoelectric and geomagnetic observations and proposed a new method to invert resistivity and susceptibility distribution simultaneously from the MT data. The application of this method in observed data processing is successful. The main contents and achievements of the thesis are as follows:(1) The 2-D forward modeling formula of the resistivity and susceptibility as model parameters is deduced. By use of the finite difference method, which is based on transmission network proposed by Madden and Mackie, we completed forward calculation and demonstrated in detail the effect of resistivity and susceptibility on MT response. It is conformed that the susceptibility mainly affects the in-phase components of magnetic signal, but a little effect on the quadrature components of magnetic and electrical signal. When the susceptibility is k≥0.01SI, the apparent resistivity amplitude may increase about 5Ωm at some low frequency, and the orders of change increase observably as increase of magnetic susceptibility. But, impedance phase is quite small. So in the MT survey, the resistivity of rock is the decisive factor. However in the region with high susceptibility, the susceptibility must be taken into account.(2) A new method to inverse the resistivity and susceptibility simultaneously from the MT data is proposed. In this inversion method, both resistivity and susceptibility are parameters. The relationship between the observed data and physical parameters of rock and physical parameters of rock themselves are nonlinear. We use a linear function to depict the nonlinear relationship between resistivity and susceptibility, and use the nonlinear conjugate gradient method proposed by Rodi and Mackie to deal with the nonlinear relationship between observed data and physical parameters. We accomplished the program codes of the new method.(3) The expression of observed data with model parameters under the condition of TM and TE polarization in 2-D MT sounding is given. Using the idea of calculation sensitivity proposed by Mora and the algorithm developed by Rodi, Mackie and Madden, we gave the expression of calculating sensitivity and the resolving procedures.(4) By numerical experiments and observation data experiments, the results show that it is feasible to inverse the resistivity ant susceptibility distribution simultaneously and that the program is right. Using this program, We processed the observed data from DaYangShu basin, and the results show that the output of resistivity and susceptibility can gave more information about the distribution of the lava and then can gave new information about sediment. Using this method we can solve the negative in-phase problem as well so that the negative in-phase component can also be used as valuable information.