In this paper an important step in the inversion of 3-dimensional velocity structure, the model parameterization, has been discussed and a new parameterization method has been developed. Instead of dividing the model into rectangular blocks, we discretize the model in spatial frequency domain by expanding the perturbation of wave slowness within the area of study into Fourier series and the coefficients of the series can be evaluated by linearized inversion of travel time residuals. A simple form of frequen...