This is the cable that is towed directly behind a survey vessel (2D, 3D). The cable is the listening device and registered signals are fed through the cables and routed to the instrument room of the vessel, where the signals are converted and stored on data tapes for further analyses.

A cable towed behind the vessel conducting the seismic survey and containing hydrophones that record the pressure changes. 2D surveys use a single cable and 3D surveys use up to 12 cables.

In parallel with the registration and data storage, an onboard quality control and preliminary data view is carried out locally onboard by the onboard Geophysicist.

Seismic cables have a slight positive buoyancy and will float to the surface if they are left with no tension in water. The cable has to be balanced to operate at a constant certain depth of 7m below the sea surface; this is achieved by pre-survey adjustments (adding/removing ballast or streamer fluid) and maintained during operations by means of cable control device (cable levelers or birds) positioned typically every 300m along the cables. Ideally, the cables should be towed straight behind the vessel. If this is not possible due to currents and influence by the environment, the streamer may be expected to ’feather’ up to 10 degrees.

Continuous information about how the shape of the cables changes and where they are located are achieved through cable mounted compasses, and GPS receivers on the tail buoys of the cables. A total picture of positions and behavior of the cables are at all times presented on various displays onboard the vessel. Should there be an emergency situation, the crew have the possibility to quickly (10 minutes) dive the cables to safe depth > 20m using the depth controlling birds, This usually has to be done only if there are ships traveling on a collision course with the cables and the crew and the assistant vessel are not able to get in contact with this ship to ask for a change of course.