A method and apparatus for providing a remote user with a selected data resource, such as a picture or video clip, via a low bandwidth link, which is capable of carrying only basic data. A wheel speed sensor has a sensor head in which a sensor is accommodated, and a tube-shaped metallic cover that covers a portion of the sensor head and protects the sensor head. The system preferably maintains a lossy hierarchical representation of the documents indexed in such a way that allows for fast queries for similar but not necessarily equivalent documents. from 45 to 70 wt % hydrocarbons boiling at 482\xb0 C. The coil underlayer lies under the specific coil element, and has a front end face. The method comprises urging a tool through the layer of material by fluidising the material adjacent the tool such that the tool may be positioned at or adjacent the position of the buried cable or pipe.