A metal halide fill for forming an ionizable fill comprises at least one inert gas, mercury and metal halides, the fill comprising the constituents Hg halide, Na halide, Tl halide and halides of the rare earths. Compositions containing reinforcing agents, especially fibrous or platy reinforcing agents are particularly useful. The frame has an open area closed by a net or web to capture the vegetation or debris as the rake is pulled through the water. The umbilical cable is surrounded by umbilical pipe downhole, and the logging head also provides for mechanical and hydraulic connection between the umbilical pipe and the wireline tool. In a preferred aspect, each position device is integrally formed with the substrate, or can be formed on the substrate by stamp forming technique, that each is used to confine the corresponding press key to move following only the specific direction, such that the wobbling of the press keys is prevented. The said propulsion resources include a first electric motor capable of driving the left wheel-train, a second electric motor capable of driving the right wheel-train, and synchronisation resources to synchronise the two electric motors as a function of the control signal.