1. A computer-implemented method for discrete element modelling of a plurality of discrete elements corresponding to particles and physical geometry elements, wherein said modelling performs a simulation through time of physical interactions of the particles with each other and with the physical geometry elements in a three-dimensional space, the method comprising: providing a virtual geometry object comprising a user-defined shape, wherein the virtual geometry object does not undergo physical interaction with the particles or physical geometry elements during the simulation; receiving user-defined parameters for determining the position, orientation and any movement of the virtual geometry object with respect to the three-dimensional space, locating the virtual geometry object in the three-dimensional space during the simulation in accordance with the user-defined parameters; using an analysis tool to select a list of particles by identifying particles that have a specified relationship with respect to the virtual geometry object; and a user specifying movement of the virtual geometry object through time with respect to the three-dimensional space, wherein the virtual geometry object is linked to a physical geometry element such that the virtual geometry object has the same movement through the three-dimensional space as the linked physical geometry element.