In marine seismic exploration, a vessel tows acoustic sources, such as airguns, and one or more hydrophones arrayed in a streamer. It is important that both sources and hydrophones arc positioned at an exactly predetermined depth beneath the sea surface. This is often a difficult task with the hydrophone streamer.
During towing, the streamer should not deviate significantly from the predetermined depth. To maintain the streamer within tolerances of the predetermined depth, a depth control device, commonly referred to as a "bird", is placed at intervals along each streamer in the array. The bird usually includes one or more control surfaces, such as wings, to move the bird up and down in the water. In order to adjust the pitch of the wings and thereby control depth, a control signal may be transmitted from a control system to order the bird up or down. The control signal causes a drive motor to turn in one direction or the other and, through a system of linkages, the drive motor drives an axle on which the wings are mounted.
Unfortunately, the drive mechanism between the drive motor and the wings commonly presents a penetration of the skin of the bird through which seawater can leak. Such a penetration usually embodies a dynamic seal which eventually wears and starts to leak. Any such leakage of seawater into the interior of the bird can cause catastrophic failure of the depth control provided by the bird and halt seismic operations. Thus, there remains a need for a water tight means of coupling the drive motor to the wings that eliminates this source of seawater inleakage into the bird.
Further, the bird usually includes a heading sensor to determine the precise heading of the bird, and by extrapolation, the seismic streamer. Such heading sensor includes sensitive instruments affected by the earth's magnetic field to ascertain the three axis heading of the bird. Any stray magnetic signature of the bird can interfere with this heading determination. Thus, there remains a need for a drive mechanism for coupling the drive motor to the wings of the bird which minimizes the interference with the bird's heading sensor.