Patent Publication Number: US-8976623-B2

Title: Towing methods and systems for geophysical surveys

Description:
BACKGROUND 
     The present invention relates generally to the field of marine geophysical surveying. More particularly, in one or more embodiments, this invention relates to towing methods and systems for controlling spread and/or depth in a geophysical survey. 
     Certain types of marine geophysical surveying, such as seismic or electromagnetic surveying, include towing an energy source at a selected depth in a body of water. One or more geophysical sensor streamers also may be towed in the water at selected depths. The streamers are essentially long cables having geophysical sensors disposed thereon at spaced apart locations. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with the rock formations below the water bottom. Energy that interacts with interfaces, generally at the boundaries between layers of rock formations, is returned toward the surface and is detected by sensors on the one or more streamers. The detected energy is used to infer certain properties of the subsurface rock, such a structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons. 
     Current electromagnetic survey techniques are generally based on a two-dimensional arrangement with a survey vessel towing a single streamer. As the streamer is pulled through the water, one or more hydrodynamic depressors can be used to pull the streamer down to a pre-selected depth. The length of the lead-in cable interconnecting the streamer with the survey vessel can be adjusted to regulate depth of the streamer. More line depth adjustments can be made with commercially available depth control devices cooperatively engaged with the streamer. 
     For electromagnetic surveying, it can be important that a streamer is maintained as close as possible to a selected depth profile in the water. For example, it may be important to increase the towing depth with an optimum depth being as close as possible to the seafloor while keeping the streamer as level as possible. This towing arrangement should reduce noise originating from towing the streamer through the water. Another important issue in electromagnetic surveying is cross-line sensitivity. In general, cross-line sensitivity is the distance in the horizontal plane perpendicular to the streamer direction of travel where the sensitivity drops below a detectable limit. In seismic surveying, cross-line sensitivity has been addressed by use of a three-dimensional survey arrangement in which multiple streamers are towed at selected lateral distances from one another. Spreading devices are used in seismic surveying to achieve the desired lateral spread between the streamers, thus improving the cross-line sensitivity of the seismic survey. However, the streamers in the seismic surveys are typically towed at shallow depths (e.g., &lt;20 m), which would result in low sensitivity due to streamer distance from the seafloor if used in an electromagnetic survey. 
     Accordingly, there is a need for improved methods and systems for controlling depth and spread in geophysical surveys to, for example, increase cross-line sensitivity while keeping the streamer as close to the seafloor as possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These drawings illustrate certain aspects of some of the embodiments of the present invention and should not be used to limit or define the invention. 
         FIG. 1  is a schematic diagram illustrating a marine electromagnetic survey system comprising submersible deflectors and three streamers in accordance with one embodiment of the present invention. 
         FIG. 2  a schematic diagram illustrating a marine electromagnetic survey system comprising submersible deflectors and a multi-tow lead-in cable branching into three streamers in accordance with one embodiment of the present invention. 
         FIG. 3  a schematic diagram illustrating a marine electromagnetic survey system comprising submersible deflectors and three streamers without deflector tow ropes in accordance with one embodiment of the present invention. 
         FIG. 4  a schematic diagram illustrating a marine electromagnetic survey system comprising submersible deflectors and two streamers without a spreader cable extending between the streamers in accordance with one embodiment of the present invention. 
         FIG. 5  a schematic diagram illustrating a marine electromagnetic survey system comprising submersible deflectors and hydrodynamic depressors in accordance with one embodiment of the present invention. 
         FIG. 6  is a perspective view of a hydrodynamic depressor in accordance with one embodiment of the present invention. 
         FIG. 7  is a rear perspective view of a submersible deflector in accordance with one embodiment of the present invention. 
         FIG. 8  is a front perspective view of a submersible deflector in accordance with one embodiment of the present invention. 
         FIG. 9  is a top end view of a submersible deflector in accordance with one embodiment of the present invention. 
         FIG. 10  is a bottom end view of a submersible deflector in accordance with one embodiment of the present invention. 
         FIG. 11  is a cross-sectional view of a submersible deflector in accordance with one embodiment of the present invention. 
         FIG. 12  is a perspective view of a submersible deflector in accordance with one embodiment of the present invention. 
         FIG. 13A  is a perspective view of a submersible deflector coupled to a streamer in accordance with one embodiment of the present invention. 
         FIG. 13B  is a perspective view of a submersible deflector coupled to a streamer in accordance with another embodiment of the present invention. 
         FIG. 14  is a perspective view of a submersible deflector comprising adjustable flaps in accordance with one embodiment of the present invention. 
         FIG. 15  is a cross-sectional view of a submersible deflector comprising adjustable flaps in accordance with one embodiment of the present invention. 
         FIG. 16  is a perspective view of a submersible deflector comprising adjustable flaps in accordance with one embodiment of the present invention. 
         FIG. 17  is a cross-sectional view of a submersible deflector comprising adjustable flaps in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to the field of marine geophysical surveying. More particularly, in one or more embodiments, this invention relates to towing methods and systems for controlling spread and/or depth in a geophysical survey. 
     One of the many potential advantages of the systems and methods of the present invention, only some of which are disclosed herein, is that a marine electromagnetic survey system may be used in a three-dimensional survey arrangement. For example, it is believed that submersible deflectors may be used to achieve the desired spread between streamers in an electromagnetic survey while the streamers are maintained at a greater depth than has been obtainable heretofore. In certain embodiments, hydrodynamic depressors may also be deployed to further increase the towing depth of the streamers. In one embodiment, the methods and systems may be used to tow streamers at a depth of at least about 25 meters and at a depth of at least about 100 meters, in another embodiment. In one particular embodiment, the streamers may be towed at a depth up to about 500 meters or more. In one embodiment, the methods and systems may be used to achieve a spread between outer streamers of at least about 150 meters, at least about 500 meters in another embodiment, and at least about 1,000 meters in yet another embodiment. In one particular embodiment, the methods and systems may be used to achieve a spread between outer streamers up to about 1,500 meters. Accordingly, embodiments of the methods and systems may provide improved operating efficiencies for a marine electromagnetic survey system by, for example, increasing cross-line sensitivity due to the lateral spread and reducing signal noise by increasing the depth at which the streamers can be towed. In addition, embodiments of the methods and systems may enable measurement of cross-line field components as multiple streamers may be employed. 
       FIG. 1  illustrates a marine electromagnetic survey system  10  in accordance with one embodiment of the present invention. In the illustrated embodiment, the system  10  may include a survey vessel  12  that moves along the surface of a body of water  14 , such as a lake or ocean. The vessel  12  includes thereon equipment, shown generally at  16  and collectively referred to herein as a “recording system.” The recording system  16  may include devices (none shown separately) for determining geodetic position of the vessel (e.g., a global positioning system satellite receiver signal), detecting and making a time indexed record of signals generated by each of electromagnetic sensors  18  (explained further below), and actuating one or more energy sources (not shown) at selected times. The energy sources may be any selectively actuable sources suitable for subsurface electromagnetic surveying, such as one or more electromagnetic field transmitters. The energy sources may be towed in any suitable pattern for electromagnetic surveying, including in a parallel or orthogonal pattern. 
     The electromagnetic sensors  18  may be any sensor suitable for subsurface electromagnetic surveying. By way of example, the electromagnetic sensors  18  may include, without limitation, any of a variety of electromagnetic field sensors, such as electrodes, magnetic field sensors, or magnetometers. The electromagnetic sensors  18  may generate response signals, such as electrical or optical signals, in response to detecting energy emitted from the source after it has interacted with rock formations (not shown) below the water bottom (not shown). 
     As illustrated by  FIG. 1 , the system  10  may further include laterally spaced apart streamers, such as outer streamers  20  and inner streamer  22 , on which the electromagnetic sensors  18  may be disposed at spaced apart locations. “Lateral” or “laterally,” in the present context, means transverse to the direction of the motion of the survey vessel  12 . In the illustrated embodiment, the system  10  includes two outer streamers  20  and a single inner streamer  22 . The outer streamers  20  and inner streamer  22  may each be formed, for example, by coupling a plurality of streamer segments end-to-end as explained in U.S. Pat. No. 7,142,481, the disclosure of which is incorporated herein by reference. In one embodiment, the outer streamers  20  and the inner streamer  22  may each include a lateral force and depth (“LFD”) control device (not shown). The LFD control devices may be deployed, for example, to regulate streamer depth so that the outer streamers  20  and the inner streamer  22  may be kept as level as possible while towed through the water  14 . The LFD control device may be any of a variety of different devices suitable for regulating streamer depth, including “birds” having variable-incidence wings. One example of an LFD control device is described in U.S. Patent Application No. 2008/0192570, the disclosure of which is incorporated herein by reference. It should be noted that, while the present example, shows only three streamers, the invention is applicable to any number of laterally spaced apart streamers towed by survey vessel  12  or any other vessel. For example, in some embodiments, 8 or more laterally spaced apart streamers may be towed by survey vessel  12 , while in other embodiments, up to 26 laterally spaced apart streamers may be towed by survey vessel  12 . 
     In an embodiment, the outer streamers  20  and the inner streamer  22  may be coupled directly to the survey vessel  12  using a corresponding lead-in line, such as outer lead-in lines  24  and inner lead-in line  26 . In the illustrated embodiment, the outer lead-in lines  24  and the inner lead-in line  26  are used, for example, to deploy the outer streamers  20  and the inner streamer  22  from the survey vessel  12  and to maintain the outer streamers  20  and the inner streamer  22  at a selected distance behind the vessel  12 . As illustrated, each of the outer lead-in lines  24  may be coupled at one end to the survey vessel  12  and at the other end to the corresponding outer streamer  20 . In a similar manner, the inner lead-in line  26  may be coupled at one end to the survey vessel  12  and at the other end to the inner streamer  22 . Each of the outer lead-in lines  24  and the inner lead-in line  26  may be deployed by a respective winch  28 , or similar spooling device, disposed on the vessel  12 , such that the length of each of the outer lead-in lines  24  and inner lead-in line  26  may be changed, for example. The outer lead-in lines  24  and the inner lead-in line  26  may be, for example, any of a variety of spoolable lines suitable for use in electromagnetic survey systems, including, without limitation, fiber ropes, armored cables, or any similar device or combination thereof. In some embodiments, the outer lead-in lines  24  and the inner lead-in line  26  may transmit towing force from the vessel  12  to the outer streamers  20  and the inner streamer  22 . In some embodiments, the outer lead-in lines  24  and inner lead-in lines  26  may communicate power and/or signals between the recording system  16  and the various electronic components (e.g., electromagnetic sensors  18 ) on the outer streamers  20  and the inner streamer  22 . For example, lead-in terminations  30  may be disposed at an axial end furthest away from the vessel  12  (“distal end”) of each of the outer lead-in lines  24  and the inner lead-in lines  26 . Electrical and/or optical connection between the recording system  16  and electrical components on the outer streamers  20  and the inner streamer  22  may be made through the lead-in terminations  30  using the outer lead-in lines  24  and the inner lead-in lines  26 . 
     In the illustrated embodiment, the outer streamers  20  and inner streamer  22  are coupled at their forward ends to one or more spreader lines  32 , which extend between outer streamers  20 . As illustrated, the spreader lines  32  may interconnect the outer streamers  20  and the inner streamer  22 . In general, the spreader lines  32  may extend in the water  14  transversely to the direction of motion of the survey vessel  12  and, for example, when maintained in correct tension, should help to maintain the lateral positions of the forward ends of the outer streamers  20  and inner streamer  22 . The spreader lines  32  may be, for example, any of a variety of lines suitable for use in electromagnetic survey systems, including, without limitation, fiber ropes, armored cables, or any similar device or combination thereof. In one embodiment, the spreader lines  32  may include hydrodynamic depressors (e.g., hydrodynamic depressors  52  shown on  FIG. 5 ) disposed thereon. The hydrodynamic depressors may be deployed on the spreader lines  32 , for example, to provide downward thrust on the spreader lines  32 , thereby forcing down the forward ends of the outer streamers  20  and inner streamer  22 . The hydrodynamic depressors may be any of a variety of different devices for forcing down the spreader lines  32 , including depth control foils. A non-limiting example of a hydrodynamic depressor is described below with reference to  FIG. 6 . 
     The system  10  may further include submersible deflectors  34  in accordance with embodiments of the present invention. As illustrated, the outer streamers  20  are each coupled to a corresponding one of the submersible deflectors  34 . In one embodiment, spur lines  38  couple the outer streamers  20  to the submersible deflectors  34 . The spur lines  38  may be any of a variety of lines suitable for use in electromagnetic survey systems, including, without limitation, fiber ropes, armored cables, or any similar device or combination thereof. 
     In accordance with present embodiments, the submersible deflectors  34  do not have a surface reference (e.g., attached buoy or other flotation device) and are free to move on a vertical plane. In some embodiments, the submersible deflectors  34  may be configured to have a negative buoyancy. For example, the submersible deflectors  34  may have a weight that is at least ⅓ the lift force generated as the submersible deflectors  34  are towed through the water at a speed of about 2 to about 6 knots. It should be noted that, while the present example shows only two submersible deflectors  34 , the invention is applicable to any number of submersible deflectors  34  that may be used as desired for a particular application. For example, while not illustrated, more than two submersible deflectors  34  may be used in embodiments where more than three streamers are used. A non-limiting example of a structure suitable for a submersible deflector  34  is described below with respect to  FIGS. 7-10 . In present embodiments, the submersible deflectors  34  are each shaped to provide a lateral component of force to the corresponding outer streamers  20  as the submersible deflectors  34  are moved through the water  14 . By way of example, the submersible deflectors  34  may comprise one or more foils that create lateral thrust as the submersible deflectors  34  are moved through the water  14 . In one embodiment, the foils also are configured to create vertical thrust as they are moved through the water. Submersible deflectors that may be used, in certain embodiments, include two-foil or three-foil deflectors. 
     The lateral component of motion of each of the submersible deflectors  34  is opposed to that of the other of the submersible deflectors  34 , and is generally, for example, in a direction transverse to the direction of the motion of the vessel  12 . The combined lateral motion of the submersible deflectors  34  separates the submersible deflectors  34  from each other until they place the outer streamers  20  in selected lateral positions. In one example, the separation is selected to place tension in the spreader lines  32 . In one embodiment, the submersible deflectors  34  also have a downward component of motion to force the outer streamers  20  downward in the water  14  to a selected depth. Due to tension in the spreader lines  32 , the inner streamer  22  should also be placed at the selected depth. It should be understood that the spreader lines  32  may be interconnected across the entire span between the submersible deflectors  34 , or in another embodiment may be separated. As will be discussed in more detail below, the yaw and roll angles of the submersible deflectors  34  may be controlled to obtain a selected depth and spread in accordance with embodiments of the present invention. The “yaw angle,” which is sometimes referred to as the “angle of attack,” refers to the rotation angle about the vertical axis in relation to the heading of a particular submersible deflector  34  as it is towed through the water  14 . The yaw angle can be adjusted to modify the lateral thrust generated by the particular submersible deflector  34 , thus increasing or decreasing the spread as desired for a particular application. In addition, as the submersible deflector  34  is not connected to a surface reference, adjusting the yaw angle may also result in a new equilibrium, which may be at a different depth. The “roll angle,” sometimes referred to as the “heel angle,” refers to the rotation angle along the longitudinal axis in the relation to the vertical axis. The roll angle can be adjusted to modify the vertical thrust generated by the particular submersible deflector  34 , thus increasing or decreasing the depth as desired for a particular application. In one embodiment, signals may be sent from the recording system  16  to control the yaw and roll angles of the submersible deflectors  34 . 
     In an embodiment, the submersible deflectors  34  may be coupled directly to the survey vessel  12  using deflector tow lines  36 . In the illustrated embodiment the deflector tow lines  36  are used, for example, to deploy the submersible deflectors  34  from the survey vessel  12  and to maintain the submersible deflectors  34  at a selected distance behind the vessel  12 . In one embodiment, the length of the deflector tow lines  36  may be controlled to obtain a desired depth as the submersible deflectors  34  are towed through the water  14 . As illustrated, each of the deflector tow lines  36  may be coupled at one end to the survey vessel  12  and at the other end to the corresponding one of the submersible deflectors  34 . Each of the deflector tow lines  36  may be deployed by a respective winch  28 , or similar spooling device, disposed on the vessel  12 , such that the length of each of the deflector tow lines  36  may be changed, for example. The outer deflector tow lines  36  may be, for example, any of a variety of spoolable lines suitable for use in electromagnetic survey systems, including, without limitation, fiber ropes, armored cables, or any similar device or combination thereof. In some embodiments, the deflector tow lines  36  may transmit towing force from the vessel  12  to the submersible deflectors  34 . In some embodiments, the deflector tow lines  36  may communicate power and/or signals between the recording system  16  and the various electronic components of the system  10 . 
       FIG. 2  illustrates a marine electromagnetic survey system  10  that utilizes a multi-tow lead-in line  40  to couple the outer streamers  20  and the inner streamer  22  to the survey vessel  12  in accordance with embodiments of the present invention. As illustrated, the system  10  may include a survey vessel  12  that moves along the surface of a body of water  14 , wherein the vessel  12  includes recording system  16  and winches  28 . The system  10  may further include laterally spaced apart streamers, such as outer streamers  20  and an inner streamer  22 , on which electromagnetic sensors  18  may be disposed at spaced apart locations. The system  10  may further include submersible deflectors  34 , which are each coupled to a corresponding one of the outer streamers  20 . In accordance with present embodiments, the submersible deflectors  34  may create lateral and vertical thrust as they are moved through the water  14  to obtain a selected depth and spread. Deflector tow lines  36  coupled to the winches  28  on the vessel  12  may be used, for example, to deploy the submersible deflectors  34  from the survey vessel  12  and to maintain the submersible deflectors  34  at a selected distance behind the vessel  12 . 
     Rather than using separate lead-in lines that are each directly coupled to the survey vessel  12 , a multi-tow lead-in line  40  is used to couple the outer streamers  20  and the inner streamer  22  to the vessel  12  in the embodiment illustrated by  FIG. 2 . For example, the multi-tow lead-in  40  may be used to deploy the outer streamers  20  and the inner streamer  22  from the survey vessel  12  and to maintain the outer streamers  20  and the inner streamer  22  at a selected distance behind the vessel  12 . 
     As illustrated, the multi-tow lead-in line  40  includes a primary line  42  and branches  44 ,  46  that extend from the distal end of the primary line  42  at the primary line termination  48 . The primary line  42  may be coupled at one end to the survey vessel  12  and at the other end to the inner streamer  22 . The branches  44 ,  46  may each be coupled at one end to one of the outer streamers  20  and at the other end to the primary line  42 . In the illustrated embodiment, spreader lines (e.g., spreader lines  32  shown on  FIG. 1 ) are not used to interconnect the inner streamer  22  and the outer streamers  20 . While not illustrated, hydrodynamic depressors (e.g., hydrodynamic depressor  52  shown on  FIG. 6 ) may be deployed on the multi-tow lead-in, in certain embodiments. Where used, the hydrodynamic depressors may be coupled, for example, to the branches  44 ,  46  proximate to the primary line termination  48 . The multi-tow lead-in line  40  may be deployed by a respective winch  28 , or similar spooling device, disposed on the vessel  12 , such that the length of the multi-tow lead-in line  40  may be changed, for example. The multi-tow lead-in line  40  may be, for example, any of a variety of spoolable lines suitable for use in electromagnetic survey systems, including, without limitation, fiber ropes, armored cables, or any similar device or combination thereof. In some embodiments, the multi-tow lead-in line  40  may transmit towing force from the vessel  12  to the outer streamers  20  and the inner streamer  22 . In some embodiments, the multi-tow lead-in line  40  may communicate power and/or signals between the recording system  16  and the various electronic components (e.g., electromagnetic sensors  18 ) on the outer streamers  20  and the inner streamer  22 . For example, branch terminations  50  and primary line termination  48  may be disposed at the distal end of the primary line  42 , and the branch terminations  50  may be at the end of corresponding branches  44 ,  46  that is opposite the primary line termination  48 . Electrical and/or optical connection between the recording system  16  and electrical components on the outer streamers  20  and the inner streamer  22  may be made through the branch terminations  50  and primary line termination  48  using the primary line  42  and branches  44 ,  46 . 
       FIG. 3  illustrates a marine electromagnetic survey system  10  that utilizes outer lead-in lines  24  to interconnect the submersible deflectors  34  to the survey vessel  12  in accordance with embodiments of the present invention. As illustrated, the system  10  may include a survey vessel  12  that moves along the surface of a body of water  14 , wherein the vessel  12  includes recording system  16  and winches  28 . The system  10  may further include laterally spaced apart streamers, such as outer streamers  20  and an inner streamer  22 , on which electromagnetic sensors  18  may be disposed at spaced apart locations. The outer streamers  20  and the inner streamer  22  may be coupled, for example, directly to the survey vessel  12  using a corresponding lead-in line, such as outer lead-in lines  24  and inner lead-in line  26 . In embodiments, the outer lead-in lines  24  and the inner lead-in line  26  are used, for example, to deploy the outer streamers  20  and the inner streamer  22  from the survey vessel  12  and to maintain the outer streamers  20  and the inner streamer  22  at a selected distance behind the vessel  12 . As illustrated, the outer streamers  20  and inner streamer  22  may be coupled at their forward ends to one or more spreader lines  32 , which extend between outer inner streamers  20 . In certain embodiments, hydrodynamic depressors (e.g., hydrodynamic depressors  32  shown on  FIG. 5 ) may be deployed on the spreader lines  32 . The system  10  may further include submersible deflectors  34 , which are each coupled to a corresponding one of the outer streamers  20 . In accordance with present embodiments, the submersible deflectors  34  may create lateral and vertical thrust as they are moved through the water  14  to obtain a selected depth and spread. Rather than using separate deflector tow lines  36  (e.g., shown on  FIGS. 1 and 2 ), the outer lead-in lines  24  couple the submersible deflectors  34  to the survey vessel  12 . 
       FIG. 4  illustrates a marine electromagnetic survey system  10  that includes only outer streamers  20  in accordance with embodiments of the present invention. As illustrated, the system  10  may include a survey vessel  12  that moves along the surface of a body of water  14 , wherein the vessel  12  includes recording system  16  and winches  28 . The system  10  may further include laterally spaced apart outer streamers  20  on which electromagnetic sensors  18  may be disposed at spaced apart locations. In contrast to the previously described embodiments, the system  10  in this example does not include a central streamer  22  (e.g., shown on  FIGS. 1-3 ). In addition, the system  10  also does not include spreader lines  32  (e.g., shown on  FIGS. 1 and 3 ) or other similar lines for maintaining the spread between the outer streamers  20 . The outer streamers  20  may be coupled, for example, directly to the survey vessel  12  using outer lead-in lines  24 . In embodiments, the outer lead-in lines  24  are used, for example, to deploy the outer streamers  20  from the survey vessel  12  and to maintain the outer streamers  20  at a selected distance behind the vessel  12 . The system  10  may further include submersible deflectors  34 , which are each coupled to a corresponding one of the outer streamers  20 . In accordance with present embodiments, the submersible deflectors  34  may create lateral and vertical thrust as they are moved through the water  14  to obtain a selected depth and spread. Rather than using separate deflector tow lines  36  (e.g., shown on  FIG. 1 ), the outer lead-in lines  24  interconnect the submersible deflectors  34  to the survey vessel  12 . Alternatively, separate deflector tow lines  36  may be employed to deploy the submersible deflectors  34  from the survey vessel  12 . 
       FIG. 5  illustrates a marine electromagnetic survey system  10  in which hydrodynamic depressors  52  have been employed in accordance with embodiments of the present invention. As previously described, hydrodynamic depressors  52  may be used, for example, to provide downward thrust to force down the forward ends of the outer streamers  20  and inner streamer  22 . In the illustrated embodiment, the hydrodynamic depressors  52  have been installed on the one or more spreader lines  32 . It should be understood that hydrodynamic depressors  52  may also be used in alternative embodiments of the system  10  (e.g., the systems  10  shown on  FIGS. 2 and 3 ). While not illustrated, the hydrodynamic depressors  52 , in one embodiment, may be placed on the branches of a multi-tow lead-in line  40  (e.g., branches  44  and  46  of the multi-tow lead-in  40  shown on  FIG. 2 ). 
       FIG. 6  illustrates a hydrodynamic depressor  52  that may be employed in accordance with embodiments of the present invention. As illustrated, the hydrodynamic depressor  52  is a depth control foil that includes an opening  54  proximate the forward (with respect to the direction of motion through the water) end  56  for coupling the depressor  52  on the spreader line  32  (e.g., shown  FIG. 5 ). The forward end  56  of the depressor  52  may be shaped to reduce hydrodynamic drag as the survey system  10  (see, e.g.,  FIG. 5 ) is towed through the water. The depressor  52  may include a curved upper surface  58  and a tail  60  that extends from the upper surface  58  of the depressor  52 . The respective lengths of the upper surface  58 , the tail  60 , and the lower surface  62  of the depressor are configured to generate the desired hydrodynamic force. Those of ordinary skill in the art with the benefit of this disclosure will recognize that the present invention is not limited to the hydrodynamic depressors illustrated by  FIG. 6 , but is broad enough to include other devices suitable for forcing down a spreader line  32 , such as a weighted rope, for example. 
       FIGS. 7-8  illustrate a submersible deflector  34  that may be used in accordance with embodiments of the present invention. In the illustrated embodiment, the submersible deflector  34  may have a front side  65  ( FIG. 8 ) and a rear side  67  ( FIG. 7 ). As illustrated, the submersible deflector  34  may comprise an upper submersible deflector portion  66  and a lower submersible deflector portion  68  joined together by a center plate  70 . The center plate  70  may comprise a fin  71  that projects from the rear side  67  of the submersible deflector  34 , as illustrated in  FIG. 7 . In one embodiment (not illustrated), the upper submersible deflector portion  66  and the lower submersible deflector portion  68  are coupled without a center plate  70 . Each of the submersible deflector portions  66 ,  68  comprises a first foil  72 , a second foil  74 , and a third foil  76 . In an embodiment, the foils  72 ,  74 ,  76  may each be constructed from a material comprising stainless steel or other suitable material. In the illustrated embodiment of  FIG. 8 , a first slot  78  is defined between first foil  72  and second foil  74  of each of the submersible deflector portions  66 ,  68  with the first slot  78  extending substantially the entire length of the first foil  72  and the second foil  74 . A second slot  80  may be defined between second foil  74  and third foil  76  of each of the submersible deflector portions  66 ,  68 , with the second slot  80  extending substantially the entire length of the second foil  74  and the third foil  76 . As the submersible deflector  34  is towed, water may pass through the first slot  78  and second slot  80 , exerting hydrodynamic force on the foils  72 ,  74 ,  76 . 
     As illustrated, the upper submersible deflector portion  66  comprises a top wing section  82  at the top end of the submersible deflector  34 , and the lower submersible deflector portion  68  comprises a lower wing section  84  at the lower end of the submersible deflector  34 . The foils  72 ,  74 ,  76  of each of the submersible deflector portions  66 ,  68  may extend longitudinally between the top wing section  82  and the lower wing section  84 . In the illustrated embodiment, plates  86  separate the wing sections  82 ,  84  and the foils  72 ,  74 ,  76  in each of the submersible deflector portions  66 ,  68 . In an embodiment, the foils  72 ,  74 ,  76  of the upper submersible deflector portion  66  are fixed to the center plate  70  on one end and to one of the plates  86  on the other end, and the foils  72 ,  74 ,  76  of the lower submersible deflector portion  68  are fixed to the center plate  70  on one end and to one of the plates  86  on the other end. Each of the plates  86  may have a fin  88  that projects from the rear side  67  of the submersible deflector  34 , as illustrated in  FIG. 7 . 
     A number of different techniques may be used to couple the submersible deflector  34  to the survey vessel  12  (e.g., shown on  FIGS. 1-4 ). As illustrated by  FIG. 8 , a bridle comprising bridle lines  89  may be used to couple the submersible deflector  34  to the deflector tow line  36 . As illustrated, the bridle lines  89  may each be coupled to a corresponding point on one of the plates  86 . Those of ordinary skill in the art with the benefit of this disclosure should be able to select an appropriate technique for coupling the submersible deflector  34  to a deflector tow line  36 . 
     The submersible deflector  34  may have an aspect ratio (i.e., submersible deflector length L relative to submersible deflector width W) that is suitable for a particular application. In an embodiment, the submersible deflector  34  may have an aspect ratio of at least about 1.5:1. In another embodiment, the submersible deflector  34  may have an aspect ratio of at least about 2:1 and at least about 3:1, in yet another embodiment. Those of ordinary skill in the art with the benefit of this disclosure should be able to select an appropriate aspect ratio for a particular application. 
       FIG. 9  is a top end view of the submersible deflector  34  of  FIGS. 7-8  in accordance with one embodiment of the present invention. As illustrated, the upper fin section  82  may have a flat inner (towards the towing vessel) side surface  92  and a convex outer (away from the towing vessel) side surface  94 . In one embodiment, the upper fin section  82  has an interior chamber that may contain a selected material. For example, the interior chamber of the upper fin section  82  may contain a low density material that is buoyant to give buoyancy to the upper fin section  82 . Non-limiting examples of suitable low-density materials that may be used include foam materials, such as Syntac® syntactic foam, available from Trellborg Offshore Boston, Inc., Mansfield Mass., and Divinylcell® foams, available from the DIAB Group. Those of ordinary skill in the art will appreciate that the volume of the interior chamber may vary, depending on a number of factors including, for example, the size of the submersible deflector  34 , the desired buoyancy, and the like. The interior chamber may have a volume of about 0.1 m 3  to about 3 m 3 , in one embodiment, about 1 to about 2 m 3 , in another embodiment, and about 1.5 to about 2 m 3 , in yet another embodiment. In one particular embodiment, 1.66 m 3  of a foam (e.g., Divinylcell® foam) may be selected for placement in the interior chamber to provide a IT lift. In one embodiment, the upper fin section  82  may be constructed from a material comprising stainless steel. Those of ordinary skill in the art, with the benefit of this disclosure, will appreciate other suitable materials that may be used for the upper fin section  82 . 
     As further illustrated by  FIG. 9 , the fin  88  of the plate  86  separating the upper fin section  82  and the foils  72 ,  74 ,  76  (e.g., shown on  FIGS. 7-8 ) projects from the rear side  67  of the submersible deflector  34 . The center plate  70  (e.g., shown on  FIGS. 7-8 ) further may comprise a central tow point  96  projecting from the front side  65  of the submersible deflector  34 . 
       FIG. 10  is a bottom end view of the submersible deflector  34  of  FIGS. 7-8  in accordance with one embodiment of the present invention. As illustrated, the lower fin section  84  may have a flat inner side surface  98  and a convex outer side surface  100 . In one embodiment, the lower fin section  84  has an interior chamber that can be filled with a selected material. For example, the interior chamber of the lower fin section  84  may be filled with a ballast material. Non-limiting examples of suitable ballast materials include stainless steel plates. Those of ordinary skill in the art will appreciate that the volume of the interior chamber may vary, depending on a number of factors including, for example, the size of the submersible deflector  34 , the desired buoyancy, and the like. The interior chamber may have a volume of about 0.05 m 3  to about 3 m 3 , in one embodiment, about 0.1 to about 2 m 3 , in another embodiment, and about 0.1 to about 1 m 3 , in yet another embodiment. In one embodiment, the lower fin section  84  may be constructed from a material comprising stainless steel. Those of ordinary skill in the art, with the benefit of this disclosure, will appreciate other suitable materials that may be used for the lower fin section  84 . 
     As further illustrated by  FIG. 10 , the fin  88  of the plate  86  separating the lower fin section  84  and the foils  72 ,  74 ,  76  (e.g., shown on  FIGS. 7-8 ) projects from the rear side  67  of the submersible deflector  34 . The center plate  70  (e.g., shown on  FIGS. 7-8 ) further may comprise a central tow point  96  projecting from the front side  65  of the submersible deflector  34 . 
     Turning now to  FIGS. 11 and 12 , the profile of the foils  72 ,  74 ,  76  will be discussed in more detail in accordance with one embodiment of the present invention.  FIG. 11  is a cross-sectional view of the submersible deflector  34  of  FIGS. 7-8  as taken along a horizontal line passing through the upper submersible deflector portion  66  (e.g., shown on  FIG. 7 ) in accordance with one embodiment of the present invention.  FIG. 12  is a perspective view of the submersible deflector  34  with the upper fin section  82  (e.g., shown on  FIG. 7 ) removed in accordance with one embodiment of the present invention. With the upper fin section  82  removed, a clean view of the profile of the foils  72 ,  74 ,  76  of the upper submersible deflector portion  66  is illustrated in this example. As illustrated, the foils  72 ,  74 ,  76  may be attached to the center plate  70 . While the foils  72 ,  74 ,  76  illustrated on  FIGS. 11 and 12  have a specific profile, it should be understood that the present invention encompasses foils  72 ,  74 ,  76  having profiles that differ from those shown on  FIGS. 11 and 12 . 
     The first foil  72  may comprise a leading first foil edge  102 , a trailing first foil edge  104 , a first foil inner surface  106 , and a first foil outer surface  108 . In an embodiment, first foil inner surface  106  may be generally concave, and the first foil outer surface  108  may be generally convex, for example, so that the profile of the first foil  72  may be in the shape of an arc. In one embodiment, the widest point of the first foil  72  between the first foil inner surface  106  and the first foil outer surface  108  is less than about least 10% of the direct distance between the leading first foil edge  102  and the trailing first foil edge  104 . In another embodiment, the widest point of the first foil  72  is about 0.1% to about 5% of the direct distance between the leading first foil edge  102  and the trailing first foil edge  104 . In an embodiment, the first foil is formed from sheet metal. 
     In a similar manner to the first foil  72 , the second foil  74  may comprise a leading second foil edge  110 , a trailing second foil edge  112 , a second foil inner surface  114 , and a second foil outer surface  116 . In an embodiment, the second foil inner surface  114  may be generally concave. In an embodiment, the second foil outer surface  116  may be generally convex. In one embodiment, the widest point of the second foil  74  between the second foil inner surface  114  and the second foil outer surface  116  is at about least 25% the direct distance between the leading second foil edge  110  and the trailing second foil edge  112 . In another embodiment, the widest point of the second foil  74  is about 50% to about 100% of the direct distance between the leading second foil edge  110  and the trailing second foil edge  112 , and about 60% to about 90% of the distance, in yet another embodiment. 
     In a similar manner to the first foil  72  and the second foil  74 , the third foil  76  may comprise a leading third foil edge  118 , a trailing third foil edge  120 , a third foil inner surface  122 , and third foil outer surface  124 . In one embodiment, the leading third foil edge  118  is generally aligned with the leading second foil edge  110 . In one embodiment, the widest point of the third foil  76  between the third foil inner surface  122  and the third foil outer surface  124  is at about least 25% the direct distance between the leading third foil edge  118  and the trailing third foil edge  120 . In another embodiment, the widest point of the third foil  76  is about 25% to about 50% of the direct distance between the leading third foil edge  118  and the trailing third foil edge  120 . 
     As illustrated by  FIGS. 11 and 12 , the first foil  72  may be located on one side of the second foil  74  with the third foil  76  located on the other side. The first foil  72  may have a leading edge  102  that is spaced from the leading edge  110  of the second foil  74 . Because of the difference in profile between the inner surface  106  of the first foil and the outer surface  116  of the second foil  74 , first slot  78  is formed. Accordingly, first slot  78  may be defined by the first foil  72  and the second foil  74  and extend along the length of the first foil  72  and the second foil  74 . The third foil  76  has a leading edge  118  that is spaced from the second foil  74 . Because of this spread and the difference in profile between the inner surface  114  of the second foil  74  and the outer surface  124  of the third foil  76 , second slot  80  is formed. Accordingly second slot  80  may be defined by the second foil  74  and the third foil  76  and extend along the length of the second foil  74  and third foil  76 . As can be seen in  FIG. 11 , the second slot  80  may decrease in area as it moves from the leading third foil edge  118  along the profile of the second foil  74 . 
       FIGS. 13A and 13B  illustrate alternative embodiments of the present invention in which a streamer (e.g., one of the outer streamers  22  on  FIGS. 1-5 ) may be towed from the rear side  67  of the submersible deflector  34 . As illustrated, the outer streamer  22  may be coupled to the submersible deflector  34 . In the illustrated embodiment of  FIG. 13A , the streamer  22  may be coupled to a central tow point  96  on the fin  71  of the center plate  70  of the submersible deflector. In the illustrated embodiment of  FIG. 138 , a bridle system comprising one or more bridle lines  73  may be used to interconnect the outer streamer  22  and the submersible deflector  34 . As illustrated by  FIG. 13B , each of the bridle lines  73  may be coupled to a central tow point  96  on a corresponding one of the plates  86 . As illustrated by  FIGS. 13A and 13B , the submersible deflector  34  may be coupled to a deflector tow line  36 . In the illustrated embodiment, the deflector tow line  36  is attached to a central tow point  96  on the center plate  70  of the submersible deflector  34 . Alternatively, a bridal system may be used to couple the deflector tow line  36  and the submersible deflector  34  as shown on  FIG. 8 . While not illustrated on  FIGS. 13A and 13B , the deflector tow line  36  may be coupled to a survey vessel  12  (e.g., shown on  FIGS. 1-4 ). Accordingly, the outer streamer  22  may be towed behind the submersible deflector  34  as the submersible deflector  34  is moved through the water  14  (see  FIGS. 1-4 ). 
     As previously mentioned, the yaw and roll angles of the submersible deflectors may be adjusted in accordance with embodiments of the present invention. The yaw and roll angles of the submersible deflectors  34  may be adjusted using any of a variety of different techniques suitable for use in electromagnetic surveying. In one embodiment, the length of the bridal cables  89  (see  FIG. 8 ) can be independently adjusted. For example, a controller (not illustrated) may be included for adjusting the length of the bridal cables  89 . By changing the length of the bridal cables  89  with respect to one another, the yaw and/or roll angles of the submersible deflector  34  can be adjusted, for example, as explained in U.S. Pat. Nos. 7,404,370 and 7,881,153, the disclosures of which are incorporated herein by reference. 
       FIGS. 14-17  illustrate another technique for adjusting the yaw and/or roll angles of the submersible deflectors  34  in accordance with embodiments of the present invention. While  FIGS. 14 and 16  illustrate the submersible deflector  34  without the lower fin section  84 , this is for illustration only, and it should be understood that the submersible deflector  34  could further include a lower fin section  84  in accordance with one embodiment of the present invention.  FIGS. 15 and 17  are cross-sectional views of the submersible deflector  34  of FIGS.  14  and  16  as taken along line  130 . As illustrated, the submersible deflector  34  may include an upper submersible deflector portion  66  and a lower submersible deflector portion  68 . In a similar manner to the embodiments shown on  FIGS. 7-12 , the upper submersible deflector portion  66  and the lower submersible deflector portion  68  each may comprise a first foil  72  and a second foil  74 . However, the third foil  76  in each of the upper and lower submersible deflector portions  66 ,  68  has been modified, in this example, to include one or more fixed foil portions  132  and one or more adjustable flaps  134 . In the illustrated embodiment, the upper submersible deflector portion  66  includes two fixed foil portions  132  and one adjustable flap  134  while the lower submersible deflector portion  68  includes one fixed foil portion  132  and one adjustable flap  134 . As illustrated, the adjustable flap  134  in the upper submersible deflector portion  66  is located between the fixed foil portions  132 . As further illustrated, the fixed foil portion  132  in the lower submersible deflector portion  68  is proximate the central plate  70  with the adjustable flap  134  proximate plate  84  (e.g., shown on  FIG. 7 ). 
     In one embodiment, the adjustable flaps  134  may be moved to adjust the roll/yaw angles of the submersible deflector. In the illustrated embodiment, the adjustable flaps  134  include a leading flap edge  136  and a trailing flap edge  138 . In one embodiment, moving the adjustable flaps  134  may include raising the trailing flap edge  138  of each of the adjustable flaps  134 , as shown in  FIGS. 14 and 15 . At least a portion of the second slot  80  may decrease in area as the trailing flap edge  138  is raised. In particular, the outer surface  140  of the adjustable flaps  134  may move closer to the trailing second foil edge  112  as the trailing flap edge  138  is raised, thus decreasing the area of the second slot  80 . In another embodiment, moving the adjustable flaps  134  may include lowering the trailing flap edge  138 , as shown in  FIGS. 16 and 17 . At least a portion of the second slot  80  may increase in area as the trailing flap edge  138  is lowered. In particular, the outer surface  140  of the adjustable flaps  134  may move away from the trailing second flow edge  112  as the trailing flap edge  138  is lowered, thus increasing the area of the second slot  80 . In one particular embodiment, the adjustable flaps  134  may be moved such that the adjustable flaps  134  have opposite angles, for example, with one of the adjustable flaps  134  raised (e.g., raising trailing flap edge  138 ) and the other one of the adjustable flaps  134  lowered (e.g., lowering trailing flap edge  138 ). By moving the adjustable flaps  134  opposite to one another, the submersible deflector  34  can be caused to pivot about the center plate  70 , thus changing the roll angle. 
     While the preceding description is directed to electromagnetic survey systems, those of ordinary skill in the art will appreciate that it may be desirable to use embodiments of the methods and systems of the present invention to control spread and/or depth in other geophysical surveys. For example, any of a variety of different energy sources may be used, including, for example, seismic air guns, water guns, vibrators, or arrays of such devices. In addition, any of a variety of different geophysical sensors may be used, including, for example, seismic sensors, such as geophones, hydrophones, or accelerometers. 
     Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Although individual embodiments are discussed, the invention covers all combinations of all those embodiments. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range are specifically disclosed. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted for the purposes of understanding this invention.