1. A computer-implemented method for obtaining a numerical approximation of a physical system to be simulated for assessing wave propagation arising in that physical system, the method utilising a computational domain comprising a coarse-grid, having a plurality of coarse-grid cells, and a primary level fine-grid region comprising two or more neighbouring primary level fine-grids having a plurality of primary level fine-grid cells of refinement integer k, wherein each of the coarse-grid cells and fine-grid cells has one or more solution points at which values representing a physical quantity of the physical system to be simulated may be obtained, wherein a calculation procedure is performed in order to obtain a value for at least one solution point of every cell at a given stage in time and wherein, during the calculation procedure, new values at primary level fine-grid cell solution points at an interface between the coarse-grid and the primary level fine-grid which is common to neighbouring fine-grids is obtained from previous values at primary level fine-grid cell solution points adjacent to that common interface.