Research Of 2D ZTEM Forward Modeling And Inversion With Uneven Topography

The ZTEM is a frequency-domain airborne electromagnetic sounding method that results from natural sources. The ZTEM tipper data are transfer functions that relate the local vertical field that measured from a moving helicopter platform to orthogonal horizontal fields measured at a reference station on the ground. ZTEM data can be used to map large-scale structures that are difficult to survey with ground based surveys. This also needs to consider the impact of undulating terrain. The method also has the advantage of high lateral resolution. The foreign scholars have studied the ZTEM method from the beginning of 2008, they have carried on the careful research to the ZTEM 2D and the 3D forward and inversion method, and also considered the influence of the undulating terrain. But, in China, the research on ZTEM is still blank, and it is only a little researches on the information of the MT tipper.Based on the analysis of ZTEM tipper response and the conjugate gradient algorithm and the impact of undulating terrain, we develop a two-dimensional(2D) finite difference method forward modeling and conjugate gradient inversion algorithm for ZTEM tipper data with uneven topography.Based on the Maxwell equations of electromagnetic field, the finite difference method is adopted, and the "step like" grid is used to approximate the relief of the terrain, i study the 2D finite difference method forward modeling of ZTEM with uneven topography. I also analyze the effects of various undulating terrain on the response of ZTEM tipper. ZTEM tipper 2D nonlinear conjugate gradient inversion algorithm is based on the MT 2D nonlinear conjugate gradient inversion algorithm. The inversion algorithm includes the calculation of the objective function and the "quasi forward" problem. The synthetic example demonstrates the validity and stability of the 2D conjugate gradient inversion algorithm for inverting ZTEM tipper data with uneven topography and compared with the results of two dimensional inversion of electromagnetic impedance with uneven topography.On the basis of the above, this paper realizes the ZTEM two dimensional forward and inverse modeling with uneven topography. The results show that the ZTEM 2D NLCG inversion method with uneven topography can get the inversion model which is close to the underground real conductivity structure, and basically recovered the size, shape and location of the underground abnormal body. Compared with the results of the inversion of MT impedance data, The inversion results of the low resistivity anomaly body with ZTEM tipper data show slightly higher resistivity, but in terms of the lateral boundary of the restricted anomaly body, the inversion results of ZTEM tipper data are better than that of the impedance data.