1. A method for smoothing an original velocity model of a given subsurface such that a travel-time through the given subsurface is preserved, the method comprising: receiving the original velocity model, wherein the original velocity model has a set of model parameters that determine a given parameterization and the original velocity model has velocity discontinuities at various depths; selecting a non-zero sub-set of model parameters (v, δ, ε) from the set of model parameters of the given parameterization; converting the selected sub-set of model parameters (v, δ, ε) to composite parameters (n −1 , n 1 , n 3 ); smoothing with a processor the composite parameters (n −1 , n 1 , n 3 ) by applying a convolutional filter (g) such that velocity moments (m −1 , m 1 , m 3 ) are preserved at the velocity discontinuities; and generating a smooth velocity model by converting the smoothed composite parameters into a smoothed sub-set of model parameters.