1. Method of determining the seismic response to seismic energy propagating through an Earth model, comprising the steps of determining the seismic response within an outer finite-difference grid; storing said first response at grid points forming at least one source injection boundary enclosing a sub space of said model; changing model parameters at grid points within said sub space so as to generate an altered Earth model; subsequently determining a second seismic response within at least one inner finite-difference grid, said inner grid enclosing said at least one source injection boundary and being enclosed in said outer finite-difference grid, using energy propagated from said at least one source injection boundary.