Processing data representing a wavefield propagating through a medium by defining, based on one or more properties of a region of the medium and of the wavefield therein, a desired timestep and a desired spatial step for a discrete operator. Discrete seed operators having an initial timestep and an initial spatial step less than the desired timestep and the desired spatial step are then defined, and these seed operators are compounded to obtain an operator having a greater timestep and upscaled to obtain an operator having a greater spatial step. The compounding and upscaling are repeated until an operator having the desired timestep and the desired spatial step is obtained. The operator having the desired timestep and the desired spatial step may be applied to the data representing the wavefield propagating through a medium to propagate the data backwards in time to recreate the wavefield at earlier times.