1. A method for removing internal multiple reflections from seismic data, or performing local inversion or local imaging of seismic data, comprising defining a conceptual subsurface box having sources and receivers located on its edges or faces and estimating Green's functions by estimating direct arrival times from source locations to receiver locations, then updating the Green's functions by iterative optimization that minimizes error in source field reconstruction, or by another numerical solution technique, then using the optimized Green's functions to determine simulated internal multiple reflections within the box and subtracting them from the seismic data, or using the optimized Green's functions to do local imaging of the seismic data within the box or build a local velocity model for the box, or for any other seismic data processing purpose requiring redatuming the seismic data to a different subsurface location, wherein at least said iterative optimization is performed using a computer.