1. A method of determining a scattered wave particle velocity for a formation, comprising: defining an embedded grid of the formation; defining a contrast grid of the formation that includes a contrast feature of the formation; calculating a Green's function over the embedded grid; calculating a first scattering vector of the contrast feature for a first offset between the embedded grid and the contrast grid; determining the scattered wave particle velocity for the contrast feature at the first offset using the calculated Green's function and the first scattering vector; calculating a second scattering vector of the contrast feature for a second offset between the embedded grid and the contrast grid; and determining, using a computer processor, the scattered wave particle velocity for the contrast feature at the second offset using the calculated Green's function and the second scattering vector.