Two-dimensional Inversion Of CSAMT Tensor Data With Topography

The most important feature of the CSAMT method is the use of an artificial source, which greatly increases the electromagnetic signal strength and makes up for some shortcomings, such as the weak signal of natural field source, difficult to observe and so on. The CSAMT method has been widely applied to oil and gas exploration, mineral prospecting, hydrology environment investigation, etc., and played a huge role. In addition, it has a high efficiency, small high resistance shielding effect, the horizontal and vertical high resolution and other advantages.However, the traditional CSAMT method mostly uses single dipole device of scalar measurements,which is suitable for simple geological environment rather than complex geological exploration. The second deficiency is the observation limit of single dipole transmitter.The position of source is needed to be adjusted in long section observation and area observation. If then source is disposed on faults, the source effect will cause the difficulty in processing those data. So the research of tensor CSAMT data inversion has practical value and is very necessary.In forward, two mutually perpendicular sources which oblique 45 ° to the survey line are divided into parallel X-axis and parallel Y-axis sub-field source, then we separately calculate the electromagnetic field components produced under the role of sub-field source and calculate the tensor impedance elements through tensor impedance formula. Finally, we compute the apparent resistivity and phase by Cagniard resistivity formula. We calculate the electromagnetic field componentsgenerated by individual electric dipole using the secondary field method to reduce the impact on the results of the source and improve accuracy. The forward matrix equation is formed by the finite element derivation, solving the equation to get the results in wavenumber domain, and finally we get the spatial domain results by reverse Fourier transform.In the inversion, the focus is the calculation of sensitivity matrix. In this paper,we use adjoint equation method to calculate the sensitivity matrix, and the sensitivity matrix of the different electromagnetic field components can be obtained by selecting a different source. In the calculation of the sensitivity matrix of apparent and phase,the method proposed by Rodi in 2001 is used. The data-space inversion based on the improvement of Occam inversion is employed to inverse the synthetic data, and good results have been obtained byinverting different models, which verifies the correctness and reliability of thealgorithm. The trial inversion with real data shows that the inversion algorithm can be used for processing and interpretation of the measured data.Past is inversion research is under the environment of uniform half space, but in actual production application, terrain factors cannot be ignored, different terrain response to the underground resistivity have very important influence, in view of this,this paper adopts the method of "quasi homogeneous half space" to simulate the different terrain conditions, reflect the situation of the underground resistivity, from the point of inversion results, terrain factors plays a very important role in the inversion, even lead to the deformation of inversion results mutations.In the inversion,Because involves the sensitivity matrix to calculate, Based on this point, we decide to use MPI, then we canmobilize the strongest function of the computer. The result is the speed of the inversion can get promoted. For the efficiency is be promoted, we can make it pragmatic.