Pseudo-Two-Dimensional Inversion Of Controlled Source Electromagnetic

Controlled source audio-frequency magnetotelluric method(CSAMT) is an important means of electromagnetic exploration, and it’s widely used in the field of ground and marine electromagnetic exploration. In recent years, the technology of 2D and 3D inversion has a rapid development. But in the interpretation of the practical field data, 2D and 3D is difficult to obtain accurate inversion result in a relatively short period of time due to their complexity and large amount of calculation. Although 1D inversion has lower precision than multi-dimensional inversion, but it’s fast and has low memory requirements, it is beneficial for preliminary interpretation. 1D inversion makes the inversion results prone to mutation which like noodle shape among adjacent measuring points. In this regard, this paper adopts the pseudo-2D inversion that adds lateral constraint for model on the basis of the 1D inversion to ensure the lateral continuity and stability of the inversion results and retain the advantages of 1D inversion.Firstly, this paper introduced the forward theory. Adopting the method of the virtual interface based on the work of Das et al. to calculate electromagnetic field produced by limited long wire source in homogeneous layered conductive surface, and using digital filtering methods on the numerical calculation. We used limited memory quasi-Newton method(L-BFGS) in the inversion, this method has fast convergence and needn’t to calculate complex second order partial derivatives and its inverse. Then, it introduced the basic principles and advantages of the limited memory quasi-Newton method in detail, and gives the numerical simulation of 1D inversion for controlled source electromagnetic sounding data. Analyzing impacts of the inversion results from the initial model, data type, initial regularization factors, termination conditions, noise and frequency selection. This paper adopts the pseudo-2D inversion that adds lateral constraint for model on the basis of the 1D inversion to ensure both vertical and horizontal continuity and stability. Through the pseudo-2D inversion of forward modeling for two 2D theoretical models and a 3D theoretical model to verify the inversion method can be realized. Finally, illustrate the applicability of pseudo-2D inversion method further by comparing with 1D, SCS2 D inversion results and geological interpretation profile map of the field measurement data.We can obtain conclusions through theoretical analysis, numerical simulation and the study of inversion for measured data as follows: 1. It’s feasible to use limited memory quasi-Newton method in 1D inversion for controlled source magnetotelluric sounding data. 2. The method of 1D inversion by using limited memory quasi-Newton has little dependence on the initial model and data types; Selecting the appropriate regularization factor and the constraints of objective functions can make the results more desirable; Selecting the key frequency for inversion based on geological data can improve efficiency; It has the better effect to set the termination condition of inversion as that the target function of the relative change is smaller 3 to 4 orders of magnitude than the initial value; It also demonstrates the limited memory quasi-newton method has certain anti-interference ability. 3. This paper realized pseudo-2D inversion for controlled source electromagnetic sounding data by limited memory quasi-Newton method. Compared with the results of 1D inversion, the method ensured both vertical and horizontal continuity and stability, and it’s suitable for the inversion of layer or lateral stratigraphic structure with small changes. 4. Pseudo-2D inversion isn’t as good as 2D and 3D inversion in accuracy, however, it can reflect the formation of structure generally, save the time of inversion and the fitting is relatively good between inversion response and the observed data. The results can provide initial model for 2D and 3D inversion.