Abstract:
A control device or “bird” for controlling the position of a marine seismic streamer is provided with an elongate, partly flexible body which is designed to be electrically and mechanically connected in series with a streamer. In its preferred form, the bird has two opposed wings which are independently controllable in order to control the streamer&#39;s lateral position as well as its depth.

Description:
Applicant claims priority under 35 U.S.C. §120 from and this is a continuation of application Ser. No. 11/122,389, filed on May 5, 2005, which is a continuation of patent application Ser. No. 10/704,182, filed Nov. 7, 2003 (now U.S. Pat. No. 7,822,552), which was a divisional of Ser. No. 09/893,234, filed Jun. 26, 2001 (now U.S. Pat. No. 6,671,223), which was a continuation of Ser. No. 09/284,030, filed Apr. 6, 1999 (abandoned), which was a 35 U.S.C. §371 national stage filing from Patent Cooperation Treaty application number PCT/GB97/03507, filed Dec. 19, 1997, which in turn claimed priority from Great Britain patent application number 9626442.9, filed Dec. 20, 1996, from which Applicant has claimed foreign priority under 35 U.S.C. §119. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to control devices for controlling the position of a marine seismic streamer. 
     2. Description of the Prior Art 
     A marine seismic streamer is an elongate cable-like structure, typically up to several thousand meters long, which contains arrays of hydrophones and associated electronic equipment along its length, and which is used in marine seismic surveying. In order to perform a 3D marine seismic survey, a plurality of such streamers are towed at about 5 knots behind a seismic survey vessel, which also tows one or more seismic sources, typically air guns. Acoustic signals produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones, and then digitised and processed to build up a representation of the earth strata in the area being surveyed. 
     The streamers are typically towed at a constant depth of about ten meters, in order to facilitate the removal of undesired “ghost” reflections from the surface of the water. To keep the streamers at this constant depth, control devices known as “birds”, attached to each streamer at intervals of 200 to 300 meters, are used. 
     Current designs of birds are battery-powered, and comprise a relatively heavy body which is suspended beneath the streamer, and which has a pair of laterally projecting wings (hence the name “bird”), one on each side. The combination of streamer and birds is arranged to be neutrally buoyant, and the angle of attack of both wings is adjusted in unison from time to time to control the depth of the streamer. 
     Birds in accordance with these current designs suffer from a number of disadvantages. Because they are battery-powered, the batteries can run out before the survey is completed, necessitating either retrieval of the streamer for battery replacement, or deployment of a work boat to replace the battery in the water. The former operation is very time consuming, while the latter can be hazardous. Further, because the birds hang beneath the streamer, they produce considerable noise as they are towed through the water, which noise interferes with the reflected signals detected by the hydrophones in the streamers. The hanging of the birds from the streamers also means that the birds need to be detached each time the streamer is retrieved and re-attached each time it is re-deployed, which is again rather time consuming. 
     During the seismic survey, the streamers are intended to remain straight, parallel to each other and equally spaced. However, after deploying the streamers, it is typically necessary for the vessel to cruise in a straight line for at least three streamer lengths before the streamer distribution approximates to this ideal arrangement and survey can begin. This increases the time taken to carry out the survey, and therefore increases the cost of the survey. But because of sea currents, the streamers frequently fail to accurately follow the path of the seismic survey vessel, sometimes deviating from this path by an angle, known as the feathering angle, of up to 10°. This can adversely affect the coverage of the survey, frequently requiring that certain parts of the survey be repeated. In really bad circumstances, the streamers can actually become entangled, which though rare, causes great damage and considerable financial loss. Current designs of birds can do nothing to alleviate any of these lateral streamer positioning problems. 
     It is therefore an object of the present invention to provide novel streamer control devices which alleviate at least some of the disadvantages of the current designs, and/or which possess more functionality than the current designs. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a control device for controlling the position of a marine seismic streamer, the device comprising a body mechanically connected in series between two adjacent sections of the streamer, sensor means in the body for determining its angular position in a plane perpendicular to the longitudinal axis of the streamer, two opposed control surfaces projecting outwardly from the body, each control surface being rotatable about an axis which in use extends transversely of the streamer, and control means responsive to control signals and the sensor means for independently adjusting the respective angular positions of said two control surfaces so as to control the lateral position of the streamer as well as its depth. 
     In a preferred embodiment of the invention, for use with a multi-section streamer which includes an electric power line, the control means is at least partly electrical and arranged in use to receive electric power from said electric power line. 
     When the streamer also includes a control line, the control means is preferably arranged in use to receive control signals from the control line. 
     The control means preferably includes at least one electrical motor, and may also include means for sensing the respective angular positions of the two control surfaces. 
     Conveniently, said two control surfaces rotate about a common axis. 
     Advantageously, each of the two control surfaces comprises a respective wing-like member which is swept back with respect to the direction of tow of the streamer. 
     Preferably, said control surfaces are releasably secured to the body, which may be adapted to be non-rotatably coupled to the streamer. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will now be described, by way of example only, with reference to the accompanying drawings, of which: 
         FIG. 1  is a somewhat schematic representation of a preferred embodiment of a streamer control device in accordance with the present invention; 
         FIG. 1A  depicts a streamer drum around which a streamer may be wound and unwound. 
         FIG. 2  is a simple schematic of a control system forming part of the streamer control device of  FIG. 1 ; and 
         FIGS. 3 to 5  illustrate the operation of the streamer control device of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The bird  10  is provided with two opposed control surfaces, or wings,  24 , typically moulded from a fibre-reinforced plastics material, which project horizontally outwardly from the body  12  and which are independently rotatable about axes x 1 , x 2 , which form a common axis extending substantially perpendicularly through the longitudinal axis of the body. Rotation of the wings  24  is effected under the control of a control system  26  sealingly housed within the body  12 . The wings  24  are generally ogival (i.e. rounded) and swept back with respect to the direction of tow of the streamer  14  (which direction is indicated by the arrow  28 ), in order to reduce the possibility of debris becoming hooked on them. To facilitate their rapid removal and reattachment, the wings  24  are secured to body  12  by a quick-release attachment  30 . 
     As mentioned hereinbefore, the streamer  14  includes hydrophones distributed along its length; it also includes control and conversion circuitry for converting the outputs of the hydrophones into digital data signals, longitudinally extending control and data lines for conducting control and data signals to and from the control and conversion circuitry, and electrical power supply lines for supplying electrical power from the vessel to the circuitry. All these lines are coupled together from the streamer section  14   a  to the streamer section  14   b  via respective corresponding lines  32  which extend through the body  12  of the bird  10  between the connectors  16 ,  18 . Additionally, the control system  26  is connected to receive control signals and electric power from respective ones of the lines  32 . 
     The greater part of the length of the body  12  of the bird  10  is flexible, the only rigid parts being the connectors  20 ,  22 , and a short central section which houses the control system  26  and from which the wings  24  project. This central section, which is made of aluminum or titanium and has holes passing longitudinally therethrough for the passage of Kevlar or other stress members which bear the longitudinal loads on the body  12 , is kept as short as possible, typically around 40 cm, so that once the wings  24  have been detached from the body  12 , the streamer  14  can be wound onto and unwound from the large drum  15 , shown in  FIG. 1A , used for storing the streamer, with the body  12  still connected in the streamer. The quick-release attachment  30  permits the removal and attachment of the wings  24  to be at least partly automated as the streamer  14  is reeled in and out during the survey. 
     The reason for providing the elongate flexible parts of the body  12  is to provide enough length for the inclusion of one or more hydrophones or hydrophone groups, should this be necessary to preserve a desired uniform hydrophone spacing along the length of streamer  14 . If no hydrophones need to be included, the flexible parts of the body  12  can be omitted altogether, along with the aforementioned stress members. 
     The control system  26  is schematically illustrated in  FIG. 2 , and comprises a microprocessor-based control circuit  34  having respective inputs  35  to  39  to receive control signals representative of desired depth, actual depth, desired lateral position, actual lateral position and roll angle of the bird  10  (ie the angular position of the body  12  in a plane perpendicular to the longitudinal axis of the streamer  14 ). The desired depth signal can be either a fixed signal corresponding to the aforementioned 10 meters, or an adjustable signal, while the actual depth signal is typically produced by a depth sensor  40  mounted in or on the bird  10 . The lateral position signals are typically derived from a position determining system of the kind described in our U.S. Pat. No. 4,992,990 or our International Patent Application No WO 9621163. The roll angle signal is produced by an inclinometer  42  mounted within the bird  10 . 
     The control circuit  34  has two control outputs  44 ,  46 , connected to control respective electrical stepper motors  48 ,  50 , each of which is drivingly connected to a respective one of the wings  24 . The stepper motors  48 ,  50  have respective outputs at which they produce signals representative of the their respective current angular positions (and therefore of the current angular positions of the wings  24 ), which outputs are connected to respective control inputs  52 ,  54  of the control circuit  34 . 
     In operation, the control circuit  34  receives between its inputs  35  and  36  a signal indicative of the difference between the actual and desired depths of the bird  10 , and receives between its inputs  37  and  38  a signal indicative of the difference between the actual and desired lateral positions of the bird  10 . These two difference signals are used by the control circuit  34  to calculate the roll angle of the bird  10  and the respective angular positions of the wings  24  which together will produce the necessary combination of vertical force (upwardly or downwardly) and lateral force (left or right) required to move the bird  10  to the desired depth and lateral position. The control circuit  34  then adjusts each of the wings  24  independently by means of the stepper motors  48 ,  50  so as to start to achieve the calculated bird roll angle and wing angular positions. 
       FIGS. 3 to 5  illustrate the operation of the bird  10  in the case where the streamer  14  is slightly heavy (slightly negative buoyancy), and the bird  10  thus needs to produce lift to maintain the streamer at the desired depth. This lift is produced by the flow of the water over the wings  24  of the bird  10 , resulting from the 5 knot towing speed of the streamer  14  through the water, and can be changed by changing the angle of attack of the wings with respect to the flow. The magnitude of the lift required for the situation envisaged by  FIG. 3  is indicated by the length of the arrows  60 . 
     If the streamer  14  now needs to be moved laterally to the right (as viewed in  FIGS. 3 to 5 ), the angular position of the left wing  24  of the bird  10  is first adjusted to increase its lift, while the angular position of the right wing is adjusted to decrease its lift, as represented by the length of the arrows  64  in  FIG. 4 , thus causing the bird  10  to roll clockwise from the position shown in  FIG. 3  to the position shown in  FIG. 4 . This clockwise roll continues until the bird  10  reaches the steady state condition shown in  FIG. 5 , where it can be seen that the vertical component of the lift produced by the wings  24 , indicated by the arrows  66 , is equal to the lift represented by the arrows  60  of  FIG. 3  required to maintain the streamer  14  at the desired depth, while the much larger horizontal component, represented by the arrows  68 , moves the streamer  14  to the right. 
     While adjusting the angular positions of the wings  24  of the bird  10 , the control circuit  34  continuously receives signals representative of the actual angular positions of the wings from the stepper motors  48 ,  50 , as well as a signal representative of the actual roll angle of the bird from the inclinometer  42 , to enable it to determine when the calculated wing angular positions and bird roll angle have been reached. And as the aforementioned difference signals at the inputs  35  to  38  of the control circuit  34  reduce, the control circuit repeatedly recalculates the progressively changing values of the roll angle of the bird  10  and angular positions of the wings  24  required for the bird and streamer reach the desired depth and lateral position, until the bird and streamer actually reach the desired depth and lateral position. 
     The body of the bird  10  does not rotate with respect to the streamer  14 , and thus twists the streamer as its rolls. The streamer  14  resists this twisting motion, so acting as a kind of torsion spring which tends to return the bird  10  to its normal position (ie with the wings  24  extending horizontally). However, this spring returning action, though beneficial is not essential, and the bird  10  can if desired be designed to rotate to a certain extent with respect to the axis of the streamer  14 . 
     It will be appreciated that the bird  10  has several important advantages with respect to prior art birds. Its in-line connection in the streamer  14  not only reduces the noise it generates as the streamer is towed through the water, but also enables it to derive power and control signals via the streamer and so obiviates the need for batteries (although they may still be provided if desired for back-up purposes). But most importantly, it enables the horizontal or lateral position of the streamer  14  to be controlled, and not just its depth. 
     Another significant advantage of the bird  10  is that by virtue of the shortness of the stiff parts of the respective body  12  and easily detachable wings  24 , it does not need to be removed from the streamer  14  during winding and unwinding. This saves a considerable amount of time when carrying out the seismic survey. 
     Many modifications can be made to the bird  10 . For example, the wings  24  can be staggered slightly along the length of the body  12 , in order to provide slightly more room for their respective drive trains. Additionally, the electric motors  48 ,  50  can be replaced by hydraulic actuators.