Abstract: This paper uses limited memory quasi-Newton method to solve 1-D inversion for impedance response of long wire source frequency sounding. Impedance response of a long wire source in horizontal layered earth model is calculated from the ground level orthogonal electromagnetic fields, and the fields are obtained by the virtual interface method. To solve optimization problems of one dimensional inversion, limited memory quasi-Newton method is adopted. Furthermore, it uses smoothness constrains for model and inverses impedance response directly. Adaptive method of target function is used for adjusting regularization parameter in the process of inversion. The inversion model is divided into many layers that scale up the thickness. It's inversed from homogeneous half-space model until the fitting error is less than 10^-4.We inverse theoretical model and actual data respectively, and then the data with 10% random noise are tested in order to investigate the stability. The numerical results show that the method of limited memory quasi-Newton is feasible for 1-D inversion for impedance response of long wire source frequency sounding; The method has little dependence on initial model, and inversion from homogeneous half-space model can still restore the true model; The objective function converges quickly at first and the speed slows down gradually later, and it takes about 40 iterations generally; The inversion results of noise data show that random noise has little effect, it also demonstrates the limited memory quasi-Newton method has certain anti-interference ability. This paper presents a new inversion method for controlled source electromagnetic data, meanwhile, the results can provide initial model for 2-D and 3-D inversion.

Key words: electromagnetic sounding, long wire source, limited memory quasi-Newton method, 1-D inversion, adaptive regularization