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`Atlomey Docket No.: 14.0123-PCT-US-CONT-2
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`IN THE UNITED STATES PATENT & TRADEMARK OFFICE
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`United States Patent Application
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`For
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`CONTROL SYSTEM FOR POSITIONING OF MARINE SEISMIC STREAMERS
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`By: Oyvind Hillesund and Simon Hastings Bittleston
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`Attorney Docket No.:
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`l4.0l23-PCT-US-CONT-2
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`CONTROL SYSTEM FOR POSITIONING OF MARINE SEISMIC STREAMERS
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`Cross—reference to Related Applications
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`[0001]
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`Applicant claims priority under 35 U.S.C. § 120 from co-pending serial
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`number 1l:"070,614, filed March 2, 2005, which was a continuation of parent application
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`serial no. 09f78'?,'i'23, filed July 2, 2001, now Patent No. 6,932,017, which was a 35 U.S.C.
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`§
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`371
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`national
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`stage
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`filing from Patent Cooperation Treaty application number
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`PCT!']B99f01590, filed September 28, 1999, which in turn claimed priority from Great
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`Britain patent application number 98212773, filed October 1, 1998, from which Applicant
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`claims foreign priority under 35 U.S.C. § 119, all of which are incorporated herein by
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`reference. This application is also related to co-pending application serial numbers 1l;'__
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`and ll.’
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`; filed simultaneously herewith, which also are both
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`incorporated herein by reference.
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`Background of the Invention
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`[0002]
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`This invention relates generally to systems for controlling seismic data
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`acquisition equipment and particularly to a system for controlling a marine seismic streamer
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`positioning device.
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`[0003]
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`A marine seismic streamer is an elongate cable-like structure, typically up
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`to several
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`thousand meters long, which contains arrays of seismic sensors, known as
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`hydrophones, and associated electronic equipment along its length, and which is used in
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`marine seismic surveying. In order to perform a 3D marine seismic survey, a plurality of
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`such streamers are towed at about 5 knots behind a seismic survey vessel, which also tows
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`one or more seismic sources, typically air guns. Acoustic signals produced by the seismic
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`sources are directed down through the water into the earth beneath, where they are reflected
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`from the various strata. The reflected signals are received by the hydrophones, and then
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`digitized and processed to build up a representation of the subsurface geology.
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`[0004]
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`The horizontal positions of the streamers are typically controlled by a
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`deflector, located at the front end or "head" of the streamer, and a tail buoy, located at the
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`back end or "tail" of the streamer. These devices create tension forces on the streamer which
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`Attorney Docket No.: 14.0123-PCT-US-CONT-2
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`constrain the movement of the streamer and cause it to assume a roughly linear shape. Cross
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`currents and transient forces cause the streamer to bow and undulate, thereby introducing
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`deviations into this desired linear shape.
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`[0005]
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`The streamers are typically towed at a constant depth of approximately
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`ten meters,
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`in order to facilitate the removal of undesired "ghost" reflections from the
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`surface of the water. To keep the streamers at this constant depth, control devices known as
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`_ "birds", are typically attached at various points along each streamer between the deflector
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`and the tail buoy, with the spacing between the birds generally varying between 200 and 400
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`meters. The birds have hydrodynamic deflecting surfaces, referred to as wings, that allow
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`the position of the streamer to be controlled as it is towed through the water. When a bird is
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`used for depth control purposes only, it is possible for the bird to regularly sense its depth
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`using an integrated pressure sensor and for a local controller within the bird to adjust the
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`wing angles to maintain the streamer near the desired depth using only a desired depth value
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`received from a central control system.
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`[{]006}
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`While the majority of birds used thus far have only controlled the depth
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`of the streamers, additional benefits can be obtained by using properly controlled
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`horizontally steerable birds, particularly by using the types of horizontally and vertically
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`steerable birds disclosed in our published PCT International Application No. W0 98.08636.
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`The benefits that can be obtained by using properly controlled horizontally steerable birds
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`can include reducing horizontal out-of-position conditions that necessitate reacquiring
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`seismic data in a particular area (i.e.
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`in-fill shooting), reducing the chance of tangling
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`adjacent streamers, and reducing the time required totum the seismic acquisition vessel
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`when ending one pass and beginning another pass during a 3D seismic survey.
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`[0007]
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`It
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`is estimated that horizontal out-of-position conditions reduce the
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`efficiency of current 313 seismic survey operations by between 5 and 10%, depending on
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`weather and current conditions. While incidents of tangling adjacent streamers are relatively
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`rare, when they do occur they invariably result in prolonged vessel downtime. The loss of
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`efficiency’ associated with turning the seismic survey vessel will depend in large part on the
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`seismic survey layout, but
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`typical estimates range from 5 to 10%. Simulations have
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`concluded that properly controlled horizontally steerable birds can be expected to reduce
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`these types of costs by approximately 30%.
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`[00018]
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`One system for controlling a horizontally steerable bird, as disclosed in
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`UK Patent GB 2093610 B,
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`is to utilize a manually-operated central control system to
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`transmit the magnitudes and directions of any required wing angle changes to the birds.
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`While this method greatly simplifies the circuitry needed within the bird itself, it is virtually
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`impossible for this type of system to closely regulate the horizontal positions of the birds
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`because it requires manual input and supervision. This becomes a particularly significant
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`issue when a substantial number of streamers are deployed simultaneously and the number
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`of birds that must be controlled goes up accordingly.
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`[0009]
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`Another system for controlling a horizontally steerable bird is disclosed
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`in our published PCT Intemational Application No. WO 98208636. Using this type of
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`control system,
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`the desired horizontal positions and the actual horizontal positions are
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`received from a remote control system and are then used by a local control system within the
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`birds to adjust
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`the wing angles. The actual horizontal positions of the birds may be
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`determined every 5 to 10 seconds and there may be a 5 second delay between the taking of
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`measurements and the determination of actual streamer positions. While this type of system
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`allows for more automatic adjustment of the bird wing angles, the delay period and the
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`relatively long cycle time between position measurements prevents this type of control
`system from rapidly and efficiently controllingxthe horizontal position of the bird. A more
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`deterministic system for controlling this type of streamer positioning device is therefore
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`desired.
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`[0010]
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`It
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`is therefore an object of the present
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`invention to provide for an
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`improved method and apparatus for controlling a streamer positioning device.
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`[0011]
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`An advantage of the present invention is that the position of the streamer
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`may be better controlled, thereby reducing the need for in-fill shooting, reducing the chance
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`of streamer tangling, and reducing the time needed to turn the seismic survey vessel.
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`[0012]
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`Another advantage of the present invention is that noise in marine seismic
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`data associated with streamer position over-correction and streamer positioning errors can be
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`significantly reduced.
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`Summary of the Invention
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`[0013]
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`The present invention provides methods and apparatus for controlling the
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`positions of marine seismic streamers in an array of such streamers being towed by a seismic
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`survey vessel,
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`the streamers having respective streamer positioning devices disposed
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`therealong and each streamer positioning device having a wing and a wing motor for
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`changing the orientation of the wing so as to steer the streamer positioning device laterally,
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`said methods and apparatus involving (3) obtaining an estimated velocity of the streamer
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`positioning devices, (b) for at least some of the streamer positioning devices, calculating
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`desired changes in the orientation of their wings using said estimated velocity, and (c)
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`actuating the wing motors to produce said desired changes in wing orientation.
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`[0014]
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`The invention and its benefits will be better understood with reference to
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`the detailed description below and the accompanying figures.
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`Brief Description of the Drawings
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`[0015]
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`FIG.
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`1 is a schematic diagram of a seismic survey vessel and associated
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`seismic data acquisition equipment;
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`[0016]
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`FIG. 2 is a schematic horizontal cross-sectional view through a marine
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`seismic streamer and an attached streamer positioning device;
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`[0017]
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`FIG. 3 is a schematic vertical cross-sectional view through the streamer
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`positioning device from FIG. 2; and
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`[0018]
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`FIG. 4 is a schematic diagram of the local control system architecture of
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`the streamer positioning device from FIG. 2.
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`Detailed Description of the Invention
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`[0019]
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`In FIG. 1, a seismic survey vessel 10 is shown towing eight marine
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`seismic streamers 12 that may, for instance, each be 3000 meters in length. The outermost
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`streamers 12 in the array could be 700 meters apart, resulting in a horizontal separation
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`between the streamers of 100 meters in the regular horizontal spacing configuration shown.
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`A seismic source 14, typically an airgun or an array of airguns, is also shown being towed
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`by the seismic survey vessel 10. At the front of each streamer 12 is shown a deflector 16 and
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`at the rear of every streamer is shown a tail buoy 20. The deflector 16 is used to horizontally
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`position the end of the streamer nearest the seismic survey vessel 10 and the tail buoy 20
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`10
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`creates drag at the end of the streamer farthest from the seismic survey vessel 10. The
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`tension created on the seismic streamer by the deflector 16 and the tail buoy 20 results in the
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`roughly linear shape of the seismic streamer 12 shown in FIG. 1.
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`[0020]
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`Located between the deflector 16 and the tail buoy 20 are a plurality of
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`streamer positioning devices known as birds 18. Preferably the birds 18 are both vertically
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`and horizontally steerable. These birds 18 may, for instance, be located at regular intervals
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`along the streamer, such as every 200 to 400 meters. The vertically and horizontally
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`steerable birds 18 can be used to constrain the shape of the seismic streamer 12 between the
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`deflector 16 and the tail buoy 20 in both the vertical (depth) and horizontal directions.
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`[0021]
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`In the preferred embodiment of the present invention, the control system
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`20
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`for the birds 18 is distributed between a global control system 22 located on or near the
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`seismic survey vessel 10 and a local control system located within or near the birds 18. The
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`global control system 22 is typically connected to the seismic survey vessel's navigation
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`system and obtains estimates of system wide parameters, such as the vessel's towing
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`direction and velocity and current direction and velocity, from the vessel's navigation
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`system.
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`[0022]
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`The most important requirement for the control system is to prevent the
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`streamers 12 from tangling. This requirement becomes more and more important as the
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`complexity and the total value of the towed equipment increases. The trend in the industry is
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`to put more streamers 12 on each seismic survey vessel 10 and to decrease the horizontal
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`separation between them. To get better control of the streamers 12, horizontal steering
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`becomes necessary. If the birds 18 are not properly controlled, horizontal steering can
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`increase, rather than decrease,
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`the likelihood of tangling adjacent streamers. Localized
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`current fluctuations can dramatically influence the magnitude of the side control required to
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`property position the streamers- To compensate for these localized current fluctuations, the
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`inventive control system utilizes a distributed processing control architecture and behavior-
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`predictive model-based control logic to properly control the streamer positioning devices.
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`[0023]
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`In the preferred embodiment of the present invention, the global control
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`system 22 monitors the actual positions of each of the birds 18 and is programmed with the
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`desired positions of or the desired minimum separations between the seismic streamers 12.
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`The horizontal positions of the birds 18 can be derived, for instance, using the types of
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`acoustic positioning systems described in our U.S. Pat. No. 4,992,990 or in our PCT
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`International Patent Application No. W0 98:'21163. Alternatively, or additionally, satellite-
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`based global positioning system equipment can be used to determine the positions of the
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`equipment. The vertical positions of the birds 18 are typically monitored using pressure
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`sensors attached to the birds, as discussed below.
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`[0024]
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`The global control system 22 preferably maintains a dynamic model of
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`each of the seismic streamers 12 and utilizes the desired and actual positions of the birds 18
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`to regularly calculate updated desired vertical and horizontal forces the birds should impart
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`on the seismic streamers 12 to move them from their actual positions to their desired
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`positions. Because the movement of the seismic streamer 12 causes acoustic noise (both
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`from seawater flow past the bird wing structures as well as cross current flow across the
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`streamer skin itself), it is important that the streamer movements be restrained and kept to
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`the minimum correction required to properly position the streamers. Any streamer
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`positioning device control system that consistently overestimates the type of correction
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`required and causes the bird to overshoot its intended position introduces undesirable noise
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`into the seismic data being acquired by the streamer. In current systems, this type of over—
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`correction noise is often balanced against the "noise" or "smearing" caused when the seismic
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`sensors in the streamers 12 are displaced from their desired positions.
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`[0025]
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`The global control system 22 preferably calculates the desired vertical
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`and horizontal forces based on the behavior of each streamer and also takes into account the
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`behavior of the complete streamer array. Due to the relatively low sample rate and time
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`delay associated with the horizontal position determination system,
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`the global control
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`system 22 runs position predictor software to estimate the actual locations of each of the
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`birds 18. The global control system 22 also checks the data received from the vessel's
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`navigation system and the data will be filled in if it is missing. The interface between the
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`global control system 22 and the local control system will typically operate with a sampling
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`frequency of at
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`least 0.1 Hz. The global control system 22 will typically acquire the
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`following parameters from the vessel's navigation system: vessel speed (this), vessel heading
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`(degrees), current speed (mfs), current heading (degrees), and the location of each of the
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`birds in the horizontal plane in a vessel fixed coordinate system. Current speed and heading
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`can also be estimated based on the average forces acting on the streamers 12 by the birds 18.
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`The global control system 22 will preferably send the following values to the local bird
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`controller: demanded vertical
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`force, demanded horizontal
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`force,
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`towing velocity, and
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`crosscurrent velocity.
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`[0026]
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`The towing velocity and crosscurrent velocity are preferably "water-
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`referenced" values that are calculated from the vessel speed and heading values and the
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`current speed and heading values, as well as any relative movement between the seismic
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`survey vessel 10 and the bird 18 (such as while the vessel is turning), to produce relative
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`velocities of the bird 18 with respect to the water in both the "in-line" and the "cross-line"
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`directions. Alternatively, the global control system 22 could provide the local control system
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`with the horizontal velocity and water in-flow angle. The force and velocity values are
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`delivered by the global control system 22 as separate values for each bird 18 on each
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`streamer 12 continuously during operation of the control system.
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`[0027]
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`The "water-referenced" towing velocity and crosscurrent velocity could
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`alternatively be determined using flowmeters or other types of water velocity sensors
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`attached directly to the birds 18. Although these types of sensors are typically quite
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`expensive, one advantage of this type of velocity determination system is that the sensed in—
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`line and cross-line velocities will be inherently compensated for the speed and heading of
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`marine currents acting on said streamer positioning device and for relative movements
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`between the vessel 10 and the bird 18.
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`[0028]
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`FIG. 2 shows a type of bird 18 that is capable of controlling the position
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`of seismic streamers 12 in both the vertical and horizontal directions. A bird 18 of this type
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`is also disclosed in our PCT International Application No. WO 98328636. While a number
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`of alternative designs for the vertically and horizontally steerable birds 18 are possible,
`including those utilizing one full-moving. wing with ailerons, three full-moving wings, and
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`four full-moving wings, the independent two-wing principal is, conceptually, the simplest
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`and most robust design.
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`[0029]
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`in FIG. 2, a portion of the seismic streamer I2 is shown with an attached
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`bird 18. A communication line 24, which may consist of a bundle of fiber optic data
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`transmission cables and power transmission wires, passes along the length of the seismic
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`streamer 12 and is connected to the seismic sensors, hydrophones 26, that are distributed
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`along the length of the streamer, and to the bird 13. The bird 18 preferably has a pair of
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`independently moveable wings 28 that are connected to rotatable shafts 32 that are rotated
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`by wing motors 34 and that allow the orientation of the wings 28 with respect to the bird
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`body 30 to be changed. When the shafts 32 of the bird 18 are not horizontal, this rotation
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`causes the horizontal orientation of the wings 28 to change and thereby changes the
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`horizontal forces that are applied to the streamer 12 by the bird.
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`[0030]
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`The motors 34 can consist of any type of device that is capable of
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`changing the orientation of the wings 28, and they are preferably either electric motors or
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`hydraulic actuators. The local control system 36 controls the movement of the wings 28 by
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`calculating a desired change in the angle of the wings and then selectively driving the
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`motors 34 to effectuate this change. While the preferred embodiment depicted utilizes a
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`separate motor 34 for each wing 28, it would be also be possible to independently move the
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`wings 28 using a single motor 34 and a selectively actuatable transmission mechanism.
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`[0031]
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`When the bird 18 uses two wings 28 to produce the horizontal and
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`vertical forces on the streamer 12, the required outputs of the local control system 36 are
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`the wings 28, or equivalently the magnitude and direction the motors'34 need to be driven to
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`produce this wing movement. While the required outputs of the local control system 36 for
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`such a two full moving wing design is quite simple, the structure and operation of the overall
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`system required to coordinate control of the device is relatively complicated.
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`[0032]
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`FIG. 3 shows a schematic vertical cross-sectional View through the
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`streamer positioning device shown in FIG. 2 that will allow the operation of the inventive
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`control system to be described in more detail. The components of the bird 18 shown in FIG.
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`3 include the wings 28 and the body 30. Also shown in FIG. 3 are a horizontal coordinate
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`axis 38 and a vertical coordinate axis 40. During operation of the streamer positioning
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`control system, the global control system 22 preferably transmits, at regular intervals (such
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`as every five seconds) a desired horizontal force 42 and a desired vertical force 44 to the
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`local control system 36.
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`[0033]
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`The desired horizontal force 42 and the desired vertical force 44 are
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`combined within the local control system 36 to calculate the magnitude and direction of the
`desired total force 46 that the global control system 22 has instructed the local control
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`system to apply to the streamer 12. The global control system 22 could alternatively provide
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`the magnitude and direction of the desired total force 46 to the local control system 36
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`instead of the desired horizontal force 42 and the desired vertical force 44.
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`[0034]
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`While the desired horizontal force 42 and the desired vertical force 44 are
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`preferably calculated by the global control system 22, it is also possible for the local control
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`system 36 in the inventive control system to calculate one or both of these forces using a
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`localized displacement/force conversion program. This type of localized conversion
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`program may, for instance, use a look-up table or conversion routine that associates certain
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`magnitudes and directions of vertical or horizontal displacements with certain magnitudes
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`and directions of changes in the vertical or horizontal forces required. Using this type of
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`embodiment, the global control system 22 can transmit location information to the local
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`control system 36 instead of force information. Instead of the desired vertical force 44, the
`global control system 22 can transmit a desired vertical depth and the local control system
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`36 can calculate the magnitude and direction of the deviation between the desired depth and
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`the actual depth. Similarly, instead of transmitting a desired horizontal force 42, the global
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`control system 22 can transmit the magnitude and direction of the displacement between the
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`actual horizontal position and the desired horizontal position of the bird 18. One advantage
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`to this alternative type of system is that the required vertical force can be rapidly updated as
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`the local control system receives updated depth information from the integrated pressure
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`sensor. Other advantages of this type of alternative system include reducing communication
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`traffic on the communication line 24 and simplifying the programming needed to convert the
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`measured vertical andfor horizontal displacements into corresponding forces to be applied
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`by the birds 13.
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`[0035]
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`When the local control system 36 has a new desired horizontal force 42
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`and desired vertical force 44 to be applied, the wings 28 will typically not be in the proper
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`orientation to provide the direction of the desired total force 46 required. As can be seen in
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`FIG. 3, the wings 28 introduce a force into the streamer 12 along an axis perpendicular to
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`the rotational axis of the wings 28 and perpendicular to the streamer. This force axis 48 is
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`typically not properly aligned with the desired total force 46 when new desired horizontal
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`and vertical force values are received from the global control system 22 or determined by
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`the local control system 36 and some rotation of the bird 18 is required before the bird can
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`produce this desired total force 46. As can be seen, the force axis 48 is directly related to the
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`bird roll angle, designated in FIG. 3 as (p.
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`[0036]
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`The local control system 36 optimizes the control process by projecting
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`the desired total force 46 onto the force axis 48 (i.e. multiplying the magnitude of the
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`desired total force by the cosine of the deviation angle 50) to produce an intermediate
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`desired force 52 and then adjusting the wing common angle ct (the angle of the wings with
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`respect to the bird body 30, or the average angle if there is a non-zero splay angle) to
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`produce this magnitude of force along the force axis. The calculated desired common wing
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`angle is compared to the current common wing angle to calculate a desired change in the
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`common wing angle and the wing motors 34 are actuated to produce this desired change in
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`the orientation of the wings.
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`[0037]
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`A splay angle is then introduced into the wings 28 to produce a rotational
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`movement in the bird body 30 (i.e. to rotate the force axis 48 to be aligned with the desired
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`total force 46). The splay angle is the difference between the angles of the wings 28 with
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`respect to the bird body 30. As the bird body 30 rotates and the force axis 48 becomes more
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`closely aligned with the desired total force 46, the bird roll angle and the bird roll angular
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`velocity are monitored, the splay angle is incrementally reduced, and the common angle is
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`incrementally increased until the intermediate desired force 52 is in the same direction and
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`of the same magnitude as the desired total force. The local control system 36 carefully
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`regulates the splay angle to ensure that the streamer is stable in roll degree of freedom. The
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`calculated common wing angle and the splay angle are also regulated by the local control
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`system 36 to prevent
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`the wings 28 from stalling and to ensure that the splay angle is
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`prioritized.
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`[0038] When using the type of birds described in our published PCT
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`International Application No. WO 98/28636, where the bird 18 is rigidly attached, and
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`cannot rotate with respect, to the streamer 12, it is important for the control system to take
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`the streamer twist into account. If this is not taken into account, the bird 18 can use all of its
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`available splay angle to counter the twist in the streamer 12. The bird 18 will then be unable
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`to reach the demanded roll angle and the generated force will decrease. The inventive
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`control system incorporates two functions for addressing this situation;
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`the anti-twist
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`function and the untwist function.
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`[0039]
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`In the
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`anti-twist
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`fimction,
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`the
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`streamer
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`twist
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`is
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`estimated by
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`weightfunction filtering the splay angle measurements instead of simply averaging the splay
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`angle measurements to improve the bandwidth of the estimation. The anti-twist fimction
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`engages when the estimated twist has reached a critical value and it then overrides the
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`normal shortest path control of the calculated roll angle. The anti-twist function forces the
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`bird 18 to rotate in the opposite direction of the twist by adding +!—l80 degrees to the
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`demanded roll angle. Once the twist has been reduced to an acceptable value, the anti-twist
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`function disengages and the nonnal shortest path calculation is continued.
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`[0040]
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`The untwist function is implemented by the global control system 22
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`which monitors the splay angle for all of the birds l8 in each streamer 12. At regular
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`intervals or when the splay angle has reached a critical value, the global control system 22
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`instructs each local control system 36 to rotate each bird 18 in the opposite direction of the
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`twist. The number of revolutions done by each bird 18 is monitored and the untwist function
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`is disengaged once the twist has reached an acceptable level. FIG. 4 is a schematic diagram
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`EX. PGS 1008
`
`Ex. PGS 1008
`
`

`
`Attorney Docket No.:
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`l4.0l 23-PCT-US-CONT-2
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`of the architecture of the local control system 36 for the bird 18. The local control system 36
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`consists of a central processor unit 54, having EEPROM S6 and RAM 58 memory, an
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`inputtoutput subsystem 60 that is connected to a pair of motor drivers 62, and an analog to
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`digital conversion unit 66. The motor drivers 62 are connected to and actuate the wing
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`motors 34 to produce the desired change the orientation of the wings 28 with respect to the
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`bird body 30.
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`[0041]
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`The wing motor 34i’wing 28 units are also connected to wing position
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`indicators 64 that sense the relative positions of the wings and provide measurements to the
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`analog to digital conversion unit 66 which converts the analog wing position indicator 64
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`measurements into digital format and conveys these digital values to the central processor
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`unit 54. Various types of wing position indicators 64 can be used, including resistive angle
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`or displacement sensors inductive sensors, capacitive sensors, hall sensors, or magneto-
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`restrictive sensors.
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`[0042]
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`A horizontal accelerometer 68 and a vertical accelerometer 70, placed at
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`right angles with respect
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`to one another, are also connected to the analog to digital
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`conversion unit 66 and these accelerometers convey measurements that allow the central
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`processor unit 54 to deten-nine the roll angle and roll rate of the bird 18. An angular velocity
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`vibrating rate gyro (rategyro) can also be used to measure the roll rate of the bird 18. A
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`temperature sensor 72 is connected to the analog to digital conversion unit 66 to provide
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`temperature measurements that allow the horizontal accelerometer 68 and the vertical
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`accelerometer 70 to be calibrated.
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`[0043]
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`A pressure sensor 74 is also connected to the analog to digital conversion
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`unit 66 to provide the central processor unit 54 with measurements of the water pressure at
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`the bird 18. To calculate an appropriate depth value, the measured pressure values must be
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`filtered to limit the disturbance from waves. This is done in the inventive control system
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`with a weightfunction filter that avoids the large phase displacements caused by mean value
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`filters. Instead of using an instantaneous depth value or simply calculating an average depth
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`value over a given period of time (and thereby incorporating a large phase displacement into
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`the depth value),
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`the inventive control system uses a differentially weighted pressure
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`filtering scheme. First the pressure values are transformed into depth values by dividing the
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`EX. PGS 1008
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`Ex. PGS 1008
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`

`
`Attorney Docket No; 14.0123-PCT-US-CONT-2
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`pressure sensor reading by the seawater density and gravitational acceleration. These depth
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`values are then filtered using a weight function filter. Typical
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`incremental weighting
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`functions values range from 0.96 to 0.90 (sample weights of 1.0, 0.9, 0.81, 0.729, etc.) and
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`the filter will typically process depth values received over a period of at least 100 seconds.
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`[0044]
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`The central processor unit
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`54 is
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`also connected to a RS485
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`communications unit 76 that allows information to be exchanged between the local control
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`system 36 and the global control system 22 over the communication line 24 that passes
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`through the streakier 12. The RS485 bus may, for instance, utilize Neuron chips that
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`communicate using a Local Operating Network protocol to control the data transfer.
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`[0045]
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`Preferably,
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`the central processor unit 54 and associated components
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`comprise a MicroChip l';'C756 processor. This type of microprocessor has very low power
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`requirements, a dual UART on-chip, 12-channel, 10 bit ADC on-chip, 908x8 RAM, 16kxl6
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`ROM, and 50 digital U0 channels. The software running on the central processor unit 54
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`will typically consist of two units, the local control unit and the hardware control unit. It is
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`typically not possible to pre-load both of these program units into the EEPROM 56 and it is
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`possible to update these program units without having to open the bird 18. The on-chip
`memory may thus only initially contain a boot-routine that enables the loading of ‘software
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`units into the external memory via the RS485 communication unit 76. The external program
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`memory (EEPROM 56) will typically be a non-volatile memory so that these program units
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`do not have to be re-loaded after every power down.
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`[0046]
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`The central processor unit 54 must be able to run the local control system
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`software fast enough to secure the sampling fiequency needed for effective local bird
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`control. This may mean, for instance, a sample rate of 10 Hz, which may be 10 to 100 times
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`faster than the sample rate of the communications between the global control system