Measuring cables towed by the seismic vessel. Streamers typically comprise lengths of flexible plastic tubing joined end to end, filled with mineral oil to give neutral buoyancy and housing hydrophones and connecting and towing cables at intervals along their lengths. Large seismic vessels operate with up to twelve streamers at a time and some are capable of handling up to twenty. Vanes are used to spread the streamers so that they trail at a set separation, altogether covering an area of several square kilometres.