Abstract: A method (1000) of compositing a group of graphical objects (B). The method (1000) forms the group of objects (B) in a temporary buffer (W). For each object (C, D) in the group of objects (B), a corresponding clipping path is provided. A resultant clipping path (Z) defined by a union of the clipping paths is calculated. The group of objects from the temporary buffer (W) is composited to a main buffer (V) through the resultant clipping path (Z).

Abstract: A method of monitoring the path of a borehole comprises acquiring drill bit seismic data while a borehole is being drilled. The acquired drill bit seismic data is used to determine whether the drilling path of the borehole is correct, for example by using the acquiring drill bit seismic data to update the geological model used to determine the drilling path. The drilling path of the borehole is updated using seismic data acquired as the borehole is being drilled, so that it is not necessary to interrupt the drilling process in order to update the drilling path. The invention thus makes possible a real-time, or near real-time, method of progressively updating the drilling path. The invention also provides a method of determining the properties of a surface or near-surface layer (7). The source of seismic energy for this method is acoustic energy, generated by the impact of the drill bit (9), that is transmitted up the drill string (10) and re-radiated into the earth at the top of the borehole (6).