Abstract:
Apparatus, systems and methods for connecting two seismic streamers are disclosed that enable two streamers to be towed in a desired arrangement. One apparatus comprises an elongate member having a first portion and a second portion, and an orientation member connected to the elongate member between the first and second portions, the orientation member functioning, when the streamers are connected by the apparatus and towed, to maintain orientation of the streamers. It is emphasized that this abstract is provided to comply with the rules requiring an abstract, which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates to the field of marine seismic instrumentation and methods of using same. More specifically, the invention relates to apparatus and methods for improving seismic images obtained using seismic instrumentation, as well as related systems, methods, and devices. 
   2. Related Art 
   Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. For large survey areas, seismic vessels tow one or more seismic sources and multiple seismic streamer cables through the water. The seismic sources typically comprise compressed air guns for generating acoustic pulses in the water. The energy from these pulses propagates downwardly into the geological formations and is reflected upwardly from the interfaces between subsurface geological formations. The reflected energy is sensed with hydrophones attached to the seismic streamers, and data representing such energy is recorded and processed to provide information about the underlying geological features. 
   Previous attempts have not provided optimal de-ghosting of marine seismic images. While these techniques are improvements in the art, further improvement is desired. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, apparatus, systems and methods are described for positioning seismic streamers, as well as seismic streamers positioned in a desired orientation employing the apparatus and systems, and methods of controlling position of streamers so connected. The apparatus, systems and methods of the invention reduce or overcome problems with previous apparatus and methods. Apparatus, systems and methods of the invention may be used to collect data that can be de-ghosted using mathematical filters that are valid only when the streamers are separated at a constant vertical separation. 
   A first aspect of the invention is an apparatus comprising:
         (a) an elongate member having a first portion and a second portion; and   (b) an orientation member, the orientation member functioning, when two streamers are connected by the elongate member and towed, to maintain a desired orientation of the streamers.       

   The elongate member may comprise a front part of a hydrofoil, while the orientation member may comprise first and second hydrodynamic flaps attached to the elongate member, each flap adapted to move independently during a seismic data acquisition run. In another embodiment the elongate member may be an elongate rod, and the orientation member comprises an even number of hydrofoils rotatably mounted to the elongate rod and able to move independently. In yet another embodiment, the elongate member is an elongate rod, and the orientation member comprises one or more remotely controllable birds mounted on the first and second streamers. An alternative to the latter embodiment is mounting the birds inline in the streamers. The elongate rod may exist as one or more than one member. In all embodiments the orientation member may be remotely controllable. 
   The first portion of the elongate member may be releasably secured to the first streamer via a first mount. The second portion of the elongate member may be securely fastened to the second streamer. The first and second portions may be first and second ends of the elongate member. The first portion of the elongate member may be mounted to a first streamer employing a clamp, and the clamp may be adjacent an inductor inside the first streamer for supplying electricity to the apparatus. Alternatively, a battery may be operatively connected to the clamp. The second portion of the elongate member may be releasably mounted to the second streamer via a second mount. The first mount may connect either the first portion or the second portion of the elongate member to a streamer in a fashion allowing electrical power to flow to the apparatus other than by induction. The first mount may include a hold and release mechanism, which allows easier take-up and roll-out of the pair of streamers. 
   A second aspect of the invention is a system comprising:
         (a) a first seismic streamer;   (b) a second seismic streamer; and   (c) a connecting element that connects the first and second steamers, the connecting element comprising an elongate member having a first portion connected to the first streamer, a second portion connected to the second streamer, and an orientation member, the orientation member functioning, when the system is towed, to maintain orientation of the first and second seismic streamers.
 
Systems of the invention include those systems wherein the first streamer is positioned at a shallower depth than the second streamer, and systems wherein the first streamer is positioned over the second streamer in over/under configuration.
       

   Another aspect of the invention comprises methods of controlling orientation of a pair of seismic streamers, one method comprising:
         (a) connecting a first and a second streamer with a connector; and   (b) adjusting an orientation member to control a desired relative position between the first and second streamers.       

   Methods of the invention may comprise wherein said orientation member is connected to the connector and the adjusting is performed by communicating with the orientation member. Communicating with the orientation member may be performed by telemetry selected from hard wire, wireless, and optical telemetry. Other methods of the invention comprise adjusting one or more of the orientation members to move the pair of seismic streamers to a desired position, which may be any direction in 3-dimensions, for example lateral (horizontal), vertical, or any direction in between these extremes. The desired position may be relative to another pair of streamers. The other pair of streamers may employ apparatus of the invention. 
   Apparatus, systems and methods of the invention will become more apparent upon review of the brief description of the drawings, the detailed description of the invention, and the claims which follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The manner in which the objectives of the invention and other desirable characteristics can be obtained is explained in the following description and attached drawings in which: 
       FIG. 1  is a perspective view of a first apparatus of the invention; 
       FIG. 2  is a cross-sectional view, taken along the line A-A of  FIG. 1 ; 
       FIGS. 3A and 3B  are schematic rear views of the apparatus of  FIG. 1  where two orientation members are displayed in two alternative orientations; 
       FIG. 4  illustrates a control scheme that may be utilized to control orientation members in apparatus of the invention; 
       FIG. 5  illustrates schematically an acoustic ranging method for sensing tilt of a pair of streamers connected using an apparatus of the invention; 
       FIGS. 6A and 6B  illustrate perspective and cross-sectional views, respectively, of a second apparatus of the invention; 
       FIG. 7  illustrates one towing arrangement employing apparatus or systems and methods of the invention; 
       FIG. 8  illustrates a perspective view of a third apparatus of the invention; 
       FIG. 9  is a cross-sectional view, taken along the line C-C of  FIG. 8 ; 
       FIGS. 10 to 12  illustrate operation of the orientation member of the system of  FIGS. 8-9 ; 
       FIG. 13  illustrates a control scheme that may be utilized to control orientation members in the apparatus of  FIGS. 8-9 ; 
       FIGS. 14A  and B illustrate perspective views, with portions in phantom, of two embodiments of the invention; and 
       FIGS. 15A-C  illustrate schematically drive arrangements for moving flap type orientation members. 
   

   It is to be noted, however, that the appended drawings are not to scale and illustrate only typical embodiments of this invention, and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
   DETAILED DESCRIPTION 
   In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
   The present invention relates to various apparatus, systems and methods for controlling position of one or more marine seismic components. One aspect of the present invention relates to apparatus for positioning seismic streamers. Another aspect of the invention is a combination of two streamers connected using an apparatus of the invention and comprising a system. Other aspects of the present invention, which are further explained below, relate to methods for remotely controlling position of marine seismic streamers. The terms “orientation member”, “hydrodynamic flap”, and “flap” are generally used interchangeably herein, although it will be recognized by those of skill in the art that a flap is a specialized device used in aviation to control lift of an airplane. In this sense, “orientation member” is deemed broader than “flap” in that the orientation members described herein are capable of movements that may result in any one or multiple straight line or curved path movements of the apparatus of the invention in 3-dimensions, such as lateral, vertical up, vertical down, horizontal, and combinations thereof. The terms “controlling position”, “position controllable”, “remotely controlling position” and “steering” are generally used interchangeably herein, although it will be recognized by those of skill in the art that “steering” usually refers to following a defined path, while “controlling position”, “position controllable”, and “remotely controlling position” could mean steering, but also could mean merely maintaining a relative position, for example relative to one or more reference points, such as natural or man-made objects, or merely deflecting an object. As “position controllable” and “controlling position” are somewhat broader terms than “steering”, these terms are used herein, except when specific instances demand using more specific words. 
   As an example,  FIG. 1  illustrates a perspective view of one apparatus embodiment  100  of the invention. Identical reference numerals are used throughout the drawing figures when the same component or element is referred to in different figures. Streamers  2  and  2 ′ are illustrated in over/under arrangement connected together by an elongate rigid or semi-rigid member  110  having first and second ends  112  and  114  and a central portion  113 , end  112  being connected with a first streamer coupler  140 , and second end  114  connected via a second streamer coupler  145 . The large arrow labeled “F” denotes the direction of travel of water past apparatus  100  when apparatus  100  is in use, being towed by a tow vessel (not illustrated). Streamers  2  and  2 ′ may be positioned a desired distance apart, generally ranging from about 1 to about 50 meters, about 5 meters being typical, although the upper bound for the separation distance is only limited by the materials of construction and the surrounding environment, for example, depth of water, obstruction in the water, and the like. Two independently moveable orientation members  130  and  135 , sometimes referred to herein as flaps, are illustrated mounted to and aft of elongate member  110  (referenced to a flow direction, indicated by arrow “F” in  FIG. 1 ). Alternatively, or in addition thereto, orientation members  130  and  135  may be mounted to streamer couplers  140  and  145 , as discussed in more detailed herein. Orientation members may number more or less than two. An even or odd number may be employed, although with an odd number certain other parameters may need adjustment. (For example, with three flaps, the size (surface area) of one flap might be twice the size of the two remaining flaps in order to achieve balanced forces.) Also shown in phantom is an optional bird  116 , which may be employed in certain embodiments of the invention, as explained further below. 
     FIG. 2  illustrates a cross-sectional view along the section A-A of  FIG. 1 , illustrating the relative position of elongate member  110  and orientation member  130  when orientation member  130  is mounted on a hollow or solid shaft  131 , as more fully discussed in reference to  FIGS. 14 and 15 . Double-headed arrow “S” illustrates how orientation member  130  might swivel or pivot on shaft  131  in accordance with the invention. Orientation member  135  is similarly moveable. Orientation members  130  and  135  may either be retractable and extendable in the direction indicated by the double-headed arrow “R”, as in retractable airplane flaps, or may be in a fixed position, as illustrated. In any case, the movement S is allowed in at least one direction. In as much as the functions of elongate member  110  are primarily to connect streamers  2  and  2 ′, and serve in controlling distance between streamers  2  and  2 ′, member  110  may be any shape, cross-section, or material of construction as desired. For example, the cross-section of elongate member  110  could be oval or rectangular; its material of construction may be metal, plastic, composite, and the like. One or more parallel, closely spaced elongate members are possible. As well, more than one elongate member may be employed, fit together or joined to form one elongate member, and shaft  131  may comprise more than one shaft, as illustrated in  FIG. 14 . Elongate member  110  could comprise any number of alternative arrangements, including pipe-in-pipe, solid rod-in-pipe, solid rod-in-box arrangements, and the like, allowing sensors, transmitters, receivers, and the like to be carried by elongate member  110 . 
   Although orientation members  130 ,  135 , and  116  are illustrated in  FIGS. 1 and 2  positioned aft of elongate member  110  connecting streamers  2  and  2 ′, it will be understood by those of ordinary skill in the art that the orientation member or members may be positioned forward of elongate member  110 , as is known in the aerodynamics art. Moreover, the use of both fore and aft flaps are deemed variants within the present invention. It is also considered within the invention for the orientation member to comprise one or more birds, for example, a combination of an elongate connection member and a bird attached to each streamer near the connecting points between the streamers and the elongate member. These embodiments may or may not include flaps  130  and  135  as illustrated in  FIG. 1 . An embodiment with no flaps is discussed in reference to  FIG. 8  herein. Birds may be positioned fore or aft of elongate member  110 . 
   Very often water currents vary significantly with depth and the two streamers in a pair are easily brought out of the ideal position, which may be directly on top of each other in an “over and under” configuration. To correct for that, apparatus of the invention are adept at enforcing a moment on the streamer pair, as illustrated in  FIG. 3A . As illustrated in  FIG. 3A , a moment (denoted by arrow “M”) may be accomplished by moving orientation members  130  and  135  in opposing directions. In other situations currents induce so-called “feathering” to the streamer pair, or the current may vary along the length of the streamer pair causing the streamer pair to “snake.” In such situations, it may be desirable to induce a net force on the streamer pair, as illustrated by the arrow “L” in  FIG. 3B , by moving all orientation members, in this case  130  and  135 , in the same direction. It is within the invention to provide for both movements exemplified by  FIGS. 3A and 3B , that is, both a moment and a translational force simultaneously. 
     FIG. 4  is a schematic diagram of a control scheme useful with the apparatus of  FIGS. 1-3 . In  FIG. 4 , “N” refers to the Nth apparatus, while N 1 , N 2 , on so on refer to an N 1  apparatus, an N 2  apparatus, and so forth. A positioning unit  16 , mounted on a float  8  (not illustrated) attached to apparatus  100 N ( FIG. 1 ) transmits position of apparatus  100 N to a navigation system  17  located on the tow vessel (not illustrated). Navigation system  17  provides the location information received from positioning unit  16  to an on-board supervisory controller  32 . On-board supervisory controller  32  may be a computer, a distributed control system, an analog control system or other control device known to those having ordinary skill in the art. On-board supervisory controller  32  may communicate with a local controller  29 N mounted in or on elongate member  110 N through a separate umbilical  27 N, or through a combination of an umbilical and streamer  2  or  2 ′, or may alternatively communicate through a wireless or optical transmission. Local controller  29 N may also be positioned within or on one of the streamer couplers,  140 ,  145  ( FIG. 1 ). Umbilical  27 N contains conductors for providing power and control signals to and from streamer  2  or  2 ′. Local controller  29 N may send a signal to an electric motor  31 N that moves an actuator  21 N, which in turn moves orientation member  130 N. When orientation member  130 N moves, the lateral force imparted against it by the water directs streamers  2  and  2 ′ to the desired position. Sensors  28 N may detect the angular position of orientation member  130 N and feedback information to local controller  29 N and, optionally, to on-board supervisory controller  32  where it may be displayed for an operator to read. Sensor  28 N may also be used as a tilt sensor to sense the tilt angle between pairs of steamers. This is one of at least two alternatives of determining the tilt. An alternative method is illustrated in  FIG. 5 . Difference signals, along with any feed-forward information received through an input  32 N, any information about other apparatus N 1 , N 2 , etc., through input  33 N, and any supervisory control signals received from supervisory controller  32  through input  45 N may be used by local controller  29 N to calculate the roll angle of orientation member N and, optionally of any birds, which together will produce the necessary combination of vertical force (upwardly or downwardly) and lateral force (left or right) required to move apparatus N to a desired depth and lateral position. Local controller  29 N then adjusts each orientation member N independently by means of the motor  31 N, so as to start to achieve the calculated roll angle and wing angular positions. Information may also be sent to other apparatus N 1 , N 2 , etc., through output  43 N, and information may be sent to on-board supervisory controller  32 , if any, through an output  41 N. Numerous variations in the control scheme are possible. Supervisory controllers, feed-forward controllers, and the like may be cascaded with local controller  29 N. Other control schemes are possible, either alone, or cascaded with the feedback control. A control scheme may comprise a so-called feed-forward controller utilizing information about currents, wind, and other environmental conditions, in order to counteract for any deviations relative to the nominal that is predicted to take place, and do so before the deviation actually takes place or to do so in an early stage of the deviation. An adaptive control scheme may also be used. 
     FIG. 5  illustrates schematically methods for sensing tilt of a pair of streamers connected using an apparatus of the invention. Streamers  2  and  2 ′ are illustrated connected via an apparatus of the invention  100 , while streamers  22  and  22 ′ are illustrated as connected using a second apparatus  100 ′ of the invention. Apparatus  100  and  100 ′ may be identical, similar, or different in construction. For example, they may be of the same length (same distance between streamers) but have differing numbers or styles of orientation members, or they may have identical number and style of orientation members, but be of different lengths (different separation distance between streamers). One orientation member could be like embodiment  100  described in reference to  FIGS. 1-3 , while the other might be like embodiment  200  of  FIG. 6  or embodiment  300  of  FIGS. 8-9 . Real time tilt angles, α and α′, relative to vertical (indicated by dashed lines marked “V”) may be sensed using one or more tilt sensors operatively coupled to one or more local controllers. The controllers and sensors are not illustrated for clarity.  FIG. 5  shows that tilt angles could also be sensed using acoustic ranges (indicated by dotted lines) between streamer  2  and steamer  22 ′, and between streamer  2 ′ and streamer  22 . The acoustic ranges may be used to calculate the real time tilt angles, α and α′, of the streamer pairs. In either case, a non-zero tilt value indicates that the streamers are not located directly on top of each other. A human or computer may then make corrective action through a control scheme as discussed in reference to  FIG. 4 , by movement of one or more orientation members (such as illustrated in  FIG. 3 ). Using the orientation members to maintain the apparatus in the desired position and orientation may minimize tilt. 
     FIGS. 6A  and B illustrate perspective and cross-sectional views, respectively, of a second apparatus  200  of the invention. Streamers  2  and  2 ′ are illustrated connected together via an elongate member  202  comprising an elongate, cylindrical rod having ends  208  and  210  connected to steamers  2  and  2 ′, respectively, using couplers  140  and  145 . In as much as the functions of elongate member  202  are primarily to connect streamers  2  and  2 ′, and serve as an attachment or support for orientation members  204  and  206 , member  202  may be any shape, cross-section, or material of construction as desired. For example, the cross-section of elongate member  202  could be oval or rectangular; its material of construction may be metal, plastic, composite, and the like. In apparatus  200 , orientation members  204  and  206  may be allowed to pivot freely, or they may be coupled to elongate member  202  and/or steamers  2  and  2 ′ and their movements controlled remotely. One or more orientation members are possible in this configuration. As well, more than one elongate member may be employed, attached together end-to-end. Elongate member  202  could comprise a pipe-in-pipe arrangement, where orientation members  204  and  206  are mounted on their own respective outer pipes or conduits, which are allowed to move about an inner pipe or solid rod.  FIG. 6B , which is taken along the cross-section indicated as B-B in  FIG. 6A , illustrates schematically one way of moving orientation member  204  by remote control. A hollow, cylindrical rod  202  has a section  203  having teeth that mesh with teeth  207  on a wheel or gear  205  having a diameter smaller than that of rod  202 . Wheel or gear  205  may be mounted on a shaft  209  that is in turn connected to a motor or other prime mover, not shown, housed inside hollow rod  202  or elsewhere inside orientation member  204 . A local controller, power supply, sensors, and the like, may also be housed inside hollow rod  202 . 
   Apparatus of the invention may connect to at least one streamer in such a way that it is able to communicate with the outside world, which may be a vessel, satellite, or land-based device. The way this may be accomplished varies in accordance with the amount of energy the apparatus requires and the amount of energy the apparatus is able to store locally in terms of batteries, fuel cells, and the like. If the local storage capacity for batteries, fuels cells, and the like is sufficient, the mount or coupling to the master streamer (the streamer used for communication) can be similar to the methods used to power so-called “birds” used for steering streamers. These birds may be clamped onto the streamer skin at locations where there is located an inductor inside the streamer skin. Similarly, streamer couplers  140  and  145  ( FIG. 1 ) may be clamped onto the streamer skin at such locations. Then the apparatus and the streamer can communicate through the skin with electrical impulses. If, on the other hand, the apparatus needs charging power from the streamer a different approach is required. In this case the apparatus may be mounted between two streamer sections and as such comprise an insert between two streamer sections, as described below. 
   Depending on the handling procedure, apparatus of the invention may require the ability to release one of the streamers in the sense that one streamer is, for some time, allowed to slide inside one of the streamer couplers or mounts  140 ,  145  ( FIG. 1 ). This may be the streamer that is not the master streamer. This could be the scenario, for example, if for some reason it is not possible to operate the streamers in the desired parallel position, such as over-under position. This may be due to weather, obstructions, and the like, or because of desire to position streamers further fore or aft relative to the other or because the two streamers stretch differently under tension. In these cases the two streamers may have the capability to slide past each other inline relative to each other. An actuator allowing grasp and release of the streamer may be included in mounts  140  or  145  for this function. 
   It is also within the invention to combine apparatus comprising elongate members, orientation members, and streamers as described with one or more other control devices, such as “birds.” One type of bird useful in the invention is described in commonly assigned U.S. Pat. No. 6,671,223, describing a bird that is designed to be electrically and mechanically connected in series with a streamer. One embodiment of this bird, known under the trade designation “Q-FIN”, available from WesternGeco L.L.C., Houston, Tex., has two opposed wings that are independently controllable in order to control a streamer&#39;s lateral position as well as its depth. Other birds useful in the invention include battery-powered birds suspended beneath the streamer and including a pair of laterally projecting wings, the combination of streamers, elongate member, orientation member, and birds being arranged to be neutrally buoyant. Clamp-on birds, as discussed previously, may also be employed. Birds useful in the invention, including suspended birds, in-line birds, and clamp-on birds may include on-board controllers and/or communications devices, which may be microprocessor-based, to receive control signals representative of desired depth, actual depth, desired lateral position, actual lateral position and roll angle of the bird. The bird on-board controllers may communicate with local controllers mounted on or in elongate members of apparatus  100  of  FIG. 1 , such as described in  FIG. 4 , and/or communicate with other local controllers an/or remote controllers, such as a supervisory controller. Such a control system is discussed in reference to  FIG. 13 . Optionally, one or more birds controlled by a controlled scheme as illustrated in  FIG. 13  may work in tandem with the controller and control scheme on-board apparatus  100  of  FIG. 1 , described in  FIG. 4 . For example, the control schemes could be cascaded. Working independently of or with apparatus  100 , the bird control circuit may then adjust each of its wings independently by means of the stepper motors so as to start to achieve the calculated bird roll angle and wing angular positions. There may be instances where apparatus  100  is not operational and acting merely as a passive connector between streamers  2  and  2 ′, such as in embodiment  300  of  FIGS. 8-9 , in which case birds attached to each streamer may function as orientation members to control relative position between streamers and/or steamer pairs. The wings may include quick release mechanisms. Birds useful herein may include seismic receivers such as hydrophones, and in such instances may include an elongate, partly flexible body to house one or more receivers. 
   As mentioned herein, materials of construction of apparatus of the invention may vary. However, there may be a need to balance the apparatus with the remainder of the seismic equipment so that the system is balanced to be neutrally buoyant in the water, or nearly so, to perform its intended function. Polymeric composites, with appropriate fillers used to adjust buoyancy and mechanical properties as desired, may be employed. 
   In use the position of a pair of streamers may be actively controlled by GPS or other position detector sensing the position of the streamer pair, and tilt sensors, acoustic sensors, or other means may sense the orientation of one or more individual streamers and feed this data to navigation and control systems. Alternatively, data may be fed-forward to local controllers on apparatus of the invention. Gross positioning and local movement of the streamer pair may be controlled on board a tow vessel, on some other vessel, locally, or indeed a remote location. By using a communication system, either hardwire or wireless, information from the remote controller may be sent to one or more local controllers on apparatus of the invention, including connectors and, when present and when desired, one or more birds. The local controllers in turn are operatively connected to adjustment mechanisms comprising motors or other motive power means, and actuators and couplers connected to the orientation members (flaps), and, if present, birds, which function to move the apparatus as desired. This in turn adjusts the position of the streamer pair, causing it to move as desired. Feedback control may be achieved using local sensors positioned as appropriate depending on the specific embodiment used, which may inform the local and remote controllers of the position of one or more orientation members, the tilt angle of a pair of streamers, distance between streamer pairs, a position of an actuator, the status of a motor or hydraulic cylinder, the status of a bird, and the like. A computer or human operator can thus access information and control the entire positioning effort, and thus obtain much better control over the seismic data acquisition process. 
   Over/under towing may improve the seismic image considerably as one may be able to separate the downward propagating acoustic wave field from the upward propagating wave field. Among geophysicists this is called de-ghosting. By different means of configuring the towing system it is possible to place pairs of streamers at lateral spacings between the pairs to form an array so as to cover a rectangle.  FIG. 7  illustrates one arrangement employing apparatus or systems and methods of the invention. Many variations are possible. A seismic vessel  702  is shown towing an array  240  of seismic hydrophones (not shown) hidden within the streamers  2 ,  2 ′. The number of streamer pairs may exceed ten, but four to eight will probably be common. An example of a four-streamer pair configuration is shown in  FIG. 7 . In the embodiment illustrated, each streamer pair  2 ,  2 ′ comprises one streamer  2 ′ placed as accurate as possible on top of the other streamer  2 . A seismic source  260  towed by tow members  261  (only two source tow members are shown for clarity) provides a pressure pulse that is reflected in the sub surface layers of the sea bottom and recorded by the seismic hydrophones. This signal is used to map the geological structure beneath the sea floor. One set of streamers  2  is towed deep and one set of streamers  2 ′ are towed shallower. Streamers  2  and  2 ′ are deflected laterally with seismic deflectors  250 ,  251 ,  252 , and  253 , which may be passive or remotely controllable. Eight streamers  2  and  2 ′ are illustrated towed by respective eight tow members  3   a - 3   h  as indicated, with separation members  4 ,  5 ,  6 , and  7  provided between adjacent deep streamers  2  and adjacent shallow streamers  2 ′. Passive or active tow members (not shown) may connect source  260  with one or more streamer tow members. The vertical distance between streamers  2 ,  2 ′ in a streamer pair may range from 1 meter to 50 meters, and may be about 5 meters. A selected number of hydrophones, either mounted within the streamer or in/on equipment mounted onto the streamer, may be used as receivers in an acoustic ranging system and thereby provide knowledge of the horizontal and vertical position of streamers  2  and  2 ′. Horizontal streamer separations may range from about 25 to about 180 meters. Depth control of streamers  2  and  2 ′ in this embodiment may be optionally provided by so-called birds  116  which may be of any type, such as small hydrofoils that can provide forces in the vertical plane. One suitable depth control device is the previously described device known under the trade designation “Q-FIN”; another suitable device is that known under the trade designation “DigiBIRD”, available from Input/Output, Inc., Stafford, Tex. Illustrated in  FIG. 7  is a plurality of connection apparatus  100 , which may be configured as more fully illustrated in  FIG. 1 , embodiment  200  of  FIG. 6 , embodiment  300  of  FIG. 8 , or some other configuration. There are many possibilities for the type, number and position of connection apparatus  100 , and this will also depend on whether birds  116  are employed. Apparatus  100  may be equally spaced along the length of the streamers, with optional birds  116  in close proximity to connection apparatus  100 . Birds  116  may be moved in close proximity to connection apparatus  100  and clamped to streamers  2 ,  2 ′, hung from streamers  2 ,  2 ′, or inserted inline in streamers  2 ,  2 ′ to provide optional supplementary position control, while birds  117 , or other streamer positioning device, such as the devices described in U.S. Pat. Nos. 3,774,570; 3,560,912; 5,443,027; 3,605,674; 4,404,664; 6,525,992 and EP patent publication no. EP 0613025, may be placed at intervals between connection apparatus  100  for supplemental position control, for example to reduce streamer “sagging.” 
     FIG. 8  illustrates a perspective view of another embodiment  300  of the invention. Streamers  2  and  2 ′ are illustrated connected together by an elongate rigid or semi-rigid member  110 ′ having first and second ends  112 ′ and  114 ′, end  112 ′ being connected with a first streamer coupler  140 , and second end  114 ′ connected via a second streamer coupler  145 . An orientation member  116 , such as a bird having a body  12  and two independently moveable control surfaces  24 , sometimes referred to herein as wings, is illustrated mounted to or attached inline in streamer  2  and aft of elongate member  110  (referenced to flow direction, indicated by arrow “F” in  FIG. 8 ). A second orientation member  116 ′ is mounted to or attached inline in streamer  2 ′. While orientation members  116  and  116 ′ are depicted as substantially identical, they may be different, as long as they are able to function together to control orientation of the pair of streamers. Alternatively, or in addition thereto, orientation members  116  and  116 ′ may be mounted on streamer couplers  140  and  145 , as discussed in more detailed herein. Orientation members  116  and  116 ′ may number more than two. An even or odd number may be employed. 
     FIG. 9  illustrates a cross-sectional view along the section C-C of  FIG. 8 , illustrating one possible construction of elongate member  110 ′, here illustrated as a hollow, cylindrical conduit or pipe. Elongate member  110 ′ may comprise more than one part or component, and may include communications components, sensors, and power components, all of which are not shown. In as much as the functions of elongate member  110 ′ are primarily to connect streamers  2  and  2 ′, and serve in controlling distance between streamers  2  and  2 ′, member  110 ′ may be any shape, cross-section, or material of construction as desired. For example, the cross-section of elongate member  110  could be oval or rectangular; its material of construction may be metal, plastic, composite, and the like. One or more parallel, closely spaced elongate members are possible. More than one elongate member may be employed, for example attached together end-to-end. Elongate member  110 ′ could comprise any number of alternative arrangements, including pipe-in-pipe, solid rod-in-pipe, solid rod-in-box arrangements, and the like, allowing sensors, transmitters, receivers, and the like to be carried by elongate member  110 ′. 
   Although orientation members  116  and  116 ′ are illustrated in  FIG. 8  positioned aft of elongate member  110 ′, it will be understood by those of ordinary skill in the art that the orientation member or members may be positioned forward of elongate member  110 . Moreover, the use of both fore and aft orientation members are deemed variants within the present invention. 
   Very often, as mentioned earlier in reference to  FIGS. 1-3 , water currents often vary significantly with depth and the two streamers in a pair are easily brought out of the ideal position, which may be directly on top of each other in an “over and under” configuration, or the streamers may “snake” or “feather.” To correct for these movements, the apparatus and system of  FIG. 8  may enforce a moment on the streamer pair. A moment may be accomplished by moving wings  24  of orientation members  116  and  116 ′ in opposing directions, and translation force may be imposed by positioning wings  24  in identical directions. 
   The orientation members, or “birds”, illustrated in  FIG. 8  generally at  116  and  116 ′, may comprise an elongate streamlined body  12 ,  12 ′ adapted to be mechanically and electrically connected in series in a multi-section marine seismic streamer  2  or  2 ′ of the kind which is towed by a seismic survey vessel and which is used, in conjunction with a seismic source also towed by the vessel, to conduct seismic surveys, as briefly described hereinbefore. To permit such connection, each end of body  12  and body  12 ′ is provided with a respective mechanical and electrical connector, these connectors being complementary to, and designed to interconnect with, streamer end connectors that are normally used to join together adjacent sections of a streamer. Birds  116  and  116 ′ may be provided with two opposed control surfaces, or wings,  24 ,  24 ′, which may be molded from a fiber-reinforced plastics material, which project outwardly from body  12 ,  12 ′ and which are independently rotatable about a common axis extending substantially perpendicularly through the longitudinal axis of the body. Rotation of wings  24 ,  24 ′ may be effected under the control of a control system sealingly housed within body  12 ,  12 ′. Wings  24 ,  24 ′ may be generally rounded and swept back with respect to the direction of tow of streamers  2  and  2 ′ (which direction is opposite of that indicated by the arrow F), in order to reduce the possibility of debris becoming hooked on them. To facilitate their rapid removal and reattachment, wings  24 ,  24 ′ may be secured to body  12 ,  12 ′ by a quick-release attachment. 
   As mentioned hereinbefore, streamers  2  and  2 ′ include hydrophones distributed along their length; they also may include 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. If birds or other like devices are employed, all these lines may be coupled together from one streamer section to another streamer section via respective corresponding lines which may extend through body  12  of bird  116  between coupler  140  and its nearest neighboring coupler  140 , and so on down the length of the streamer. Alternatively or additionally, wireless and optical transmission signals may be generated and received by functional components in or on streamers  2  and  2 ′ and bird body  12 . 
     FIGS. 10 to 12  illustrate the operation of bird  116  in the case where streamer  2  or  2 ′ is slightly heavy (slightly negative buoyancy), and bird  116  thus needs to produce lift to maintain the streamer at the desired depth. As streamers  2  and  2 ′ are connected by elongate member  110  ( FIG. 8 ), another bird or other streamer positioning device may be required on or inline with streamer  2 ′ to help move streamer  2 ′, since bird  116  will not only have to overcome cross flow drag and gravity forces on streamer  2 , but cross flow drag produced by streamer  2 ′ and elongate member  110 . This lift is produced by the flow of the water over the wings  24  of the bird  116 , resulting from the desired towing speed of streamers  2 ,  2 ′ 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 moving streamer  2  when by itself (disconnected from a streamer pair) is indicated by the length of the arrows  60 . These arrows may be incrementally higher or lower when streamers  2  and  2 ′ are connected with an elongate member  110 . If streamer  2  now needs to be moved laterally to the right (as viewed in  FIGS. 10 to 12 ), the angular position of left wing  24  of bird  116  may be first adjusted to increase its lift, while the angular position of right wing  24  is adjusted to decrease its lift, as represented by the length of the arrows  64  in  FIG. 11 , thus causing bird  116  to roll clockwise from the position shown in  FIG. 10  to the position shown in  FIG. 11 . This clockwise roll may continue until bird  116  reaches a steady state condition shown in  FIG. 12 , where it can be seen that the vertical component of the lift produced by wings  24 , indicated by arrows  66 , is equal to the lift represented by arrows  60  of  FIG. 10  required to maintain streamer  2  at the desired depth, while the much larger horizontal component, represented by the arrows  68 , moves streamer  2  to the right. 
     FIG. 13  is a schematic diagram of a control scheme useful with apparatus, systems and methods described in reference to  FIGS. 8-12 . In  FIG. 13 , “N” refers to the Nth orientation member, while N 1 , N 2 , on so on refer to an N 1  orientation member, an N 2  orientation member, and so forth. Control system  26 N comprises a microprocessor-based control circuit  34 N having respective inputs  35 N to  39 N to receive control signals representative of desired depth, actual depth, desired lateral position, actual lateral position and roll angle of orientation member N (i.e. the angular position of body  12 N in a plane perpendicular to the longitudinal axis of streamer  2  or  2 ′). Control circuit  34 N may also receive information through input  33 N regarding the status or position of orientation members N 1 , N 2 , and the like. The desired depth signal can be either a fixed signal or an adjustable signal, while the actual depth signal is typically produced by a depth sensor  40 N mounted in or on orientation member N. The lateral position signals may be 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 WO9621163. The roll angle signal may be produced by an inclinometer  42 N mounted on or within orientation member N. Control circuit  34 N may have control outputs  44 N,  46 N, connected to control respective electrical stepper motors  48 N,  50 N, each of which is drivingly connected to a respective one of wings  24 N. Stepper motors  48 N,  50 N have respective outputs at which they produce signals representative of their respective current angular positions (and therefore of the current angular positions of wings  24 N), which outputs are connected to respective control inputs  52 N,  54 N of control circuit  34 N. 
   In operation, control circuit  34 N may receive between its inputs  35 N and  36 N a signal indicative of the difference between the actual and desired depths of orientation member N, and may receive between its inputs  37 N and  38 N a signal indicative of the difference between the actual and desired lateral positions of orientation member N. These two difference signals, along with any feed-forward information received through input  32 N, any information about other orientation members N 1 , N 2 , etc., through input  33 N, and any supervisory control signals received from a supervisory controller through input  45 N may be used by control circuit  34 N to calculate the roll angle of orientation member N and the respective angular positions of wings  24 N which together will produce the necessary combination of vertical force (upwardly or downwardly) and lateral force (left or right) required to move orientation member N to a desired depth and lateral position. Control circuit  34 N then adjusts each of wings  24 N independently by means of the stepper motors  48 N,  50 N, so as to start to achieve the calculated roll angle and wing angular positions. Information may also be sent to other orientation members N 1 , N 2 , etc., through output  43 N, and information may be sent to the supervisory controller (not shown), if any, through an output  41 N. Numerous variations in the control scheme are possible. Supervisory controllers, feed-forward controllers, and the like may be cascaded with control system  26 . A feed-forward controller, as indicated by input  32 N in  FIG. 13 , may utilize information about currents, wind, and other environmental conditions, in order to counteract for any deviations relative to the nominal that may be predicted to take place, and do so before the deviation actually takes place or to do so in an early stage of the deviation. An adaptive control scheme may also be used. 
   Systems of the invention may become unstable due to geometry of the streamer pair, the point of application of, and direction of the applied forces. This may cause orientation members to generate undesirable torque on one or both streamers. To remove this undesirable effect, control system  26 N in  FIG. 13  may be programmed appropriately. While adjusting the angular positions of wings  24 N of orientation member N, control circuit  34 N may continuously receive signals representative of the actual angular positions of wings  24 N from the stepper motors  48 N,  50 N, as well as signals representative of the actual roll angles of orientation members N, N 1 , N 2 , etc., from an inclinometer  42 N and input  33 N, to enable control circuit  34 N to determine and/or predict when the calculated wing angular positions and bird roll angle have been or should be reached. And as the aforementioned difference signals at the inputs  35 N to  38 N of the control circuit  34  reduce, control circuit  34 N may repeatedly recalculate the progressively changing values of the roll angle of orientation member N and the angular positions of the wings  24 N required for orientation member N and streamer to reach the desired depth and lateral position, until orientation member N and the streamer to which it is attached actually reach the desired depth and lateral position. Body  12  of any particular orientation member may or may not rotate with respect to streamer  2  or  2 ′; if body  12  does not rotate it may then twist streamer  2 , and perhaps streamer  2 ′, as it rolls. Streamers  2  and  2 ′ then resist this twisting motion, acting together as a kind of torsion spring that tends to return the orientation members to their normal position. However, this torsional action may or may not be beneficial and is not essential, and the orientation members may if desired be designed to rotate to a certain extent with respect to the axis of the streamer to which they are attached or a part of inline. 
     FIGS. 14A and 14B  illustrate schematically, with some parts in phantom, two apparatus embodiments of the invention. In  FIG. 14A , elongate member  110  comprises an elongate, hydrofoil-shaped body having ends  112  and  114 , and a central portion  113 . Shown in phantom are two shafts  131  and  133  supported by bearings  132 ,  134 , and  136 , also in phantom. Shafts  131  and  133 , which may be any cross-sectional shape and may be hollow or solid, may rotate independently of one another in their respective bearings. Bearings  132 ,  134 , and  136  may be mounted inside elongate member  110  in any functional manner, such as welding, bolts, screws, or even molded as part of the structure of the elongate member, such as cast metal depressions made during the manufacture of elongate member  110 . In  FIG. 14A , shaft  131  supports flap  130 , while shaft  133  supports flap  135 . As shafts  131  and  133  rotate, their respective flaps  130  and  135  also rotate. Mechanisms responsible for this movement may vary, with three embodiments discussed in reference to  FIGS. 15A-C .  FIG. 14B  illustrates a slightly different arrangement. Flaps  130 ′ and  135 ′ are again mounted on respective shafts  131 ′ and  133 ′. In the embodiment depicted in  FIG. 14B , however, elongate member  110 ′ includes support brackets  132 ′,  134 ′, and  136 ′, which serve the function of bearings for shafts  131 ′ and  133 ′. Flaps  130 ′ and  135 ′ are also somewhat more extended rearward relative to elongate body  110 ′. 
     FIGS. 15A-C  illustrate schematically three modes of how to make orientation members move in accordance with the present invention.  FIG. 15A  shows a motor  150  connected to a drive shaft  151  and gear  152 . Gear  152  meshes with another gear  153  connected to shaft  131 , which in turn is connected through suitable fasteners, not illustrated, to flap  130 , and which may rotate within bearing  134 . Shaft  131  may be welded or brazed to flap  130 , for example.  FIG. 15B  illustrates another embodiment, wherein motor  150 , shaft  151 , and gear  152  are provided as in  FIG. 15A , however, in this embodiment an endless chain  154  extends around gear  152  and a second gear  153 .  FIG. 15C  illustrates a cross-sectional view of the embodiment of  FIG. 15A , similar to the view of  FIG. 2 , and shows how a linear actuator  155  might be employed with a bracket  156  attached to shaft  131 . Linear actuator  155  could be pneumatic, electric, or hydraulic in nature. 
   Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, no clauses are intended to be in the means-plus-function format allowed by 35 U.S.C. § 112, paragraph 6 unless “means for” is explicitly recited together with an associated function. “Means for” clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a clamp-on bird and an inline bird may not be structural equivalents in that a clamp-on bird employs one type of fastener, whereas an inline bird employs a different fastener, in the environment of using birds to position streamers, a clamp-on bird and an inline bird may be equivalent structures.