Controlled Source Audio-frequency Magnetotelluric Inversion For Minimum Structure

The least squares inversion is the most traditional and effective way in most of the linear inversion methods. Although the least squares inversion of 1D CSAMT can simulate the electromagnetic field accurately, but it is only limited to simple horizontal layered model and rely on the initial model heavily, which is easy to produce multiple solutions, and can not apply in the actual data inversion. In this paper, we used the method that one dimension CSAMT minimum structure inversion based on the frequency to invert the actual datas, which is based on the inversion of minimum structure, and in this method, we made the number of the frequency equal to the number of the layers, and used the frequency and the surface resistivity to get the thickness of every layer in order to construct the initial model. As a result, on the one hand, it not only takes into account the actual distribution of subsurface electrical properties, but also it can inhibit the unreasonable ground structure, and eliminate redundant structure; on the other hand, it can make the the initial model of inversion built more simple, reasonable, and tends to the true model easily, so as to invert more suitable for the actual data.In the three-dimensional forwarding numerical simulation of CSAMT, the most important things are how to deal with boundary conditions and how to load the source. In this paper, we used the infinite element method to handling the out boundary, which can not only make the electromagnetic value is zero as distance increasing in the infinity in the true sense, but also can greatly reduce the NO. of the meshes and the excess calculation region, which are generated by using the traditional finite element method to handle the boundary. On the basement of ensuring the accuracy, using the finite element - infinite element coupling method can reduce computational domain, greatly increased computing speed, in order to achieve three-dimensional inversion of CSAMTIn the 3D CSAMT inversion, because of the grid cells and the parameters massive, the inversion always make the sensitivity matrix computation time.consuming, equation seriously ill, and with poor convergence and stability,which can not meet the actual requirements. In this paper, we take the method that making the three dimensional model of the measured data equivalent to one dimensional model on the base of the thought of the equivalent one-dimensional model, and use the analytical sensitivity matrix of one-dimensional model instead of three-dimensional the sensitivity matrix to achieve the three-dimensional inversion fast and simply. The inversion results shows that the spatial and electrical characteristics of the true 3-D models can be well reflected by the CSAMT minimum structure inversion based on the frequency,however, it can not eliminate the boundary effect of the target body which comes from the 3D forwarding.