Abstract:
A seismic streamer includes a jacket covering an exterior of the streamer. At least one strength member extends along the length of the jacket. The strength member is disposed inside the jacket. Seismic sensors are disposed at spaced apart locations along the interior of the jacket. A flexible, acoustically transparent material fills the space inside the jacket. The material is introduced into the inside of the jacket in liquid form and undergoes a state change thereafter. The strength member, prior to and during the state change, is maintained in substantially a same position with respect to the jacket as would occur during ordinary operation of the streamer. The maintaining position is performed at least at a location along the jacket where a device is to be coupled externally to the jacket.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       BACKGROUND OF INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The invention relates generally to the field of marine seismic data acquisition equipment. More specifically, the invention relates to structures for a marine seismic streamer, and methods for making such streamers.  
         [0005]     2. Background Art  
         [0006]     Marine seismic surveying is typically performed using “streamers” towed near the surface of a body of water. A streamer is in the most general sense a cable towed by a seismic vessel having a plurality of seismic sensors disposed thereon at spaced apart locations. The sensors are typically hydrophones, but can also be any type of sensor that is responsive to the pressure in the water, or in changes therein with respect to time. The sensors may also be any type of particle motion sensor or acceleration sensor known in the art. Irrespective of the type of such sensors, the sensors generate an electrical or optical signal that is related to the parameter being measured by the sensors. The electrical or optical signals are conducted along electrical conductors or optical fibers carried by the streamer to a recording system. The recording system is typically disposed on the seismic vessel, but may be disposed elsewhere.  
         [0007]     In a typical marine seismic survey, a seismic energy source is actuated at selected times, and a record, with respect to time, of the signals detected by the one or more sensors is made in the recording system. The recorded signals are later used for interpretation to infer structure of, fluid content of, and composition of rock formations in the Earth&#39;s subsurface.  
         [0008]     A typical marine seismic streamer can be up to several kilometers in length, and can include hundreds of individual seismic sensors. Because of the weight of all of the materials used in a typical marine seismic sensor, because of the friction (drag) caused by the streamer as it is moved through the water, and because of the need to protect the sensors, electrical and/or optical conductors and associated equipment from water intrusion, a typical seismic streamer includes certain features. First, the streamer includes one or more strength members to transmit axial force along the length of the streamer. The strength member is operatively coupled to the seismic vessel and thus bears all the loading caused by drag (friction) of the streamer in the water. The streamer also includes, as previously explained, electrical and/or optical conductors to carry electrical power and/or signals to the various sensors and (in certain streamers) signal conditioning equipment disposed in the streamer and to carry signals from the various sensors to a recording station. The streamer typically includes an exterior jacket that surrounds the other components in the streamer. The jacket is typically made from a strong, flexible plastic such as polyurethane, such that water is excluded from the interior thereof, and seismic energy can pass essentially unimpeded through the jacket to the sensors. A typical streamer also includes buoyancy devices at spaced apart locations therealong, so that the streamer so that the cable is substantially neutrally buoyant in the water. The interior of the jacket is typically filed with oil or similar electrically insulating fluid that is substantially transparent to seismic energy.  
         [0009]     Another device that is typically affixed to a streamer at spaced apart locations therealong is known as a “compass bird.” A compass bird includes a directional sensor, typically a magnetometer, to determine the orientation of the streamer at the position of the compass bird. The compass bird may include an electromagnetic transducer to communicate its measurements through the streamer jacket to a detector inside the jacket. Direction measurements are used to infer the position of the streamer along its length, because currents in the body of water can cause the streamer to move transversely with respect to the direction of motion of the seismic vessel.  
         [0010]     A seismic streamer including the various components described above is typically made by inserting the various components inside the jacket, and filling the interior space within the jacket with oil or other electrically insulating material. During manufacture, axial stress may be applied to the strength member, and during handling and storage, essentially no axial stress is applied. As a result, the various components within the jacket may move laterally and/or axially with respect to the jacket. Thus, the geometry of the typical streamer may change between handling, storage, deployment and actual operation, where substantial axial force is applied to the strength member. Compass bird orientation with respect to the streamer jacket and internal components is particularly susceptible to error due to changes in streamer component geometry.  
         [0011]     There is a need for a marine seismic streamer that has precisely controlled geometry during manufacture, and which geometry substantially does not change between manufacture, handling, storage and use.  
       SUMMARY OF INVENTION  
       [0012]     One aspect of the invention is a seismic streamer, including a jacket covering an exterior of the streamer. At least one strength member extends along the length of the jacket. The strength member is disposed inside the jacket. Seismic sensors are disposed at spaced apart locations along the interior of the jacket. A flexible, acoustically transparent material fills the space inside the jacket. The material is introduced into the inside of the jacket in liquid form and undergoes a state change thereafter. The strength member is maintained at least near a position along the jacket to which a device is to be attached externally, during the state change in substantially axial alignment with the jacket.  
         [0013]     Another aspect of the invention is a method for making a seismic streamer. A method according to this aspect includes inserting at least one strength member and seismic sensors into a jacket. The jacket is then filled with a liquid having a composition adapted to undergo a change in state from liquid to substantially solid after the filling. The strength member is held, during the state change, in substantially axial alignment with the jacket. The holding is performed at least at a location along the jacket at which a device is to be externally affixed. In one embodiment, a selected tension is applied to the at least one strength member to effect the holding. In one embodiment, the tension is an amount selected to maintain the strength member and the sensors in essentially the desired position of the strength member with respect to the jacket when the streamer is towed by a seismic vessel in a body of water.  
         [0014]     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]      FIG. 1  shows typical marine seismic data acquisition using a streamer according to one embodiment of the invention.  
         [0016]      FIG. 2  shows a cut away view of one embodiment of a streamer segment according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]     An example marine seismic data acquisition system as it is typically used is shown in  FIG. 1 . A seismic vessel  14  moves along the surface of a body of water  12  such as a lake or the ocean. The marine seismic survey is intended to detect and record seismic signals related to structure and composition of various subsurface Earth formations  21 ,  23  below the water bottom  20 . The seismic vessel  14  includes source actuation, data recording and navigation equipment, shown generally at  16 , referred to for convenience as a “recording system.” The seismic vessel  14 , or a different vessel (not shown), can tow one or more seismic energy sources  18 , or arrays of such source(s) in the water  12 . The system includes at least one seismic streamer  10 , which includes a strength member  26  operatively coupled to the seismic vessel  14 , and a plurality of sensors  24  or arrays of such sensors, disposed at spaced apart locations along the streamer  10 . During operation, equipment (not shown separately) in the recording system  16  causes the source  18  to actuate at selected times. When actuated, the source  18  produces seismic energy  19  that emanates generally outwardly from the source  18 . The energy  19  travels downwardly, through the water  12 , and passes, at least in part, through the water bottom  20  into the formations  21 ,  23  below. Seismic energy  19  is at least partially reflected from one or more acoustic impedance boundaries  22  below the water bottom  20 , and travels upwardly whereupon it may be detected by the sensors  24 . Structure of the formations  21 ,  23  can be inferred by travel time of the energy  19  and by characteristics of the detected energy such as its amplitude and phase.  
         [0018]     An important aspect of inferring the structure of the formations  21 ,  23  is precise knowledge of the geographic position of the sensors  24  during the survey, so that the geographic position of the boundaries  22  may be correctly inferred and so that the geographic position of various compositions of the formations  21 ,  23  may be estimated accurately.  
         [0019]     Having explained the general method of operation of a marine seismic streamer, an example embodiment of a streamer according to the invention will be explained with reference to  FIG. 2 .  FIG. 2  is a cut away view of a portion (segment)  10 A of a marine seismic streamer ( 10  in  FIG. 1 ). A streamer as shown in  FIG. 1  may extend behind the seismic vessel ( 14  in  FIG. 1 ) for several kilometers, and is typically made from a plurality of streamer segments as shown in  FIG. 2  connected end to end behind the vessel ( 14  in  FIG. 1 ).  
         [0020]     The streamer segment  10 A in the present embodiment may be about 75 meters overall length. A streamer such as shown at  10  in  FIG. 1  may be formed by connecting a selected number of such segments  10 A end to end. The segment  10 A includes a jacket  30 , which in the present embodiment is made from 3.5 mm thick transparent polyurethane, having a nominal external diameter of about 62 millimeters. In some embodiments, the jacket  30  may be externally banded in selected places with an alloy number 304 stainless steel, copper flashed band (not shown).  
         [0021]     In each segment  10 A, each axial end of the jacket  30  may be terminated by a coupling/termination plate  36 . The termination plate  36  may include elements  36 A on a surface inserted into the end of the jacket  30  to seal against the inner surface of the jacket  30 , and to grip the termination plate  36  to the jacket  30  when clamped externally (not shown). In the present embodiment, two strength members  42  are coupled to the interior of each termination plate  36  and extend the length of the segment  10 A. In a particular implementation of the invention, the strength members  42  may be made from a fiber rope, using a fiber sold under the mark VECTRAN, which is a registered trademark of Hoechst Celanese Corp., New York, N.Y. The strength members  42  transmit axial force along the length of the segment  10 A. When one segment  10 A is coupled end to end to another segment (not shown in  FIG. 2 ), mating termination plates  36  are coupled together using any suitable connector, so that the axial force is transmitted through the termination plates  36  from the strength members  42  in one segment  10 A to the strength member in the adjoining segment.  
         [0022]     The segment  10 A includes buoyancy spacers  32  disposed in the jacket  30  at spaced apart locations along its length. The buoyancy spacers  32  may be made from foamed polypropylene. The buoyancy spacers  32  have a density selected to provide the segment  10 A with approximately the same overall density as water ( 12  in  FIG. 1 ), so that the streamer ( 10  in  FIG. 1 ) will be substantially neutrally buoyant in the water. As a practical matter, the buoyancy spacers  32  provide the segment  10 A with an overall density very slightly less than that of fresh water. Appropriate overall density may then be adjusted in actual use by adding selected amounts of dense ballast (not shown) to the exterior of the jacket, thus providing adjustment in the buoyancy for changes in water temperature and salinity.  
         [0023]     The segment  10 A includes a generally centrally located conductor cable  40  which includes a plurality of insulated electrical conductors (not shown separately), and may include one or more optical fibers (not shown). The cable conducts electrical and/or optical signals from the sensors (which will be further explained below) to the recording system ( 16  in  FIG. 1 ). The cable may also carry electrical power to various signal processing circuits (not shown separately) disposed in one or more segments  10 A or disposed elsewhere along the streamer ( 10  in  FIG. 1 ). The length of conductor cable  40  within a cable segment  10 A is generally longer than the axial length of the segment  10 A under the largest expected axial stress, so that the electrical conductors and optical fibers will not experience any substantial axial stress when cable  10  is towed through the water by a vessel. The conductors and optical fibers may be terminated in a connector  38  disposed in each termination plate  36  so that when the segments  10 A are connected end to end, corresponding electrical and/or optical connections may be made between the electrical conductors and optical fibers in the conductor cable  40  in adjoining segments  10 A.  
         [0024]     Sensors, which in the present embodiment may be hydrophones, can be disposed in selected ones of the buoyancy spacers, shown in  FIG. 2  generally at  34 . The hydrophones in the present embodiment can be or a type known to those of ordinary skill in the art, including but not limited to those sold under model number T-2BX by Teledyne Geophysical Instruments, Houston, Tex. In the present embodiment, each segment  10 A may include  96  such hydrophones, disposed in arrays of sixteen individual hydrophones connected in electrical series. In a particular implementation of the invention, there are thus six such arrays, spaced apart from each other at about 12.5 meters. The spacing between individual hydrophones in each array should be selected so that the axial span of the array is at most equal to about one half the wavelength of the highest frequency seismic energy intended to be detected by the streamer ( 10  in  FIG. 1 ). It should be clearly understood that the types of sensors used, the electrical and/or optical connections used, the number of such sensors, and the spacing between such sensors are only used to illustrate one particular embodiment of the invention, and are not intended to limit the scope of this invention. In other embodiments, the sensors may be particle motion sensors such as geophones or accelerometers. A marine seismic streamer having particle motion sensors is described in U.S. patent application Ser. No. 10/233,266, filed on Aug. 30, 2002, entitled, “Apparatus and Method for Multicomponent Marine Geophysical Data Gathering”, assigned to an affiliated company of the assignee of the present invention and incorporated herein by reference.  
         [0025]     At selected positions along the streamer ( 10  in  FIG. 1 ) a compass bird  44  may be affixed to the outer surface of the jacket  30 . The compass bird  44  includes a directional sensor (not shown separately) for determining the geographic orientation of the segment  10 A at the location of the compass bird  44 . The compass bird  44  may include an electromagnetic signal transducer  44 A for communicating signals to a corresponding transducer  44 B inside the jacket  30  for communication along the conductor cable  40  to the recording system ( 16  in  FIG. 1 ). Measurements of direction are used, as known in the art, to infer the position of the various sensors  34  in the segment  10 A, and thus along the entire length of the streamer ( 10  in  FIG. 1 ). Typically, a compass bird will be affixed to the streamer ( 10  in  FIG. 1 ) about every 300 meters (every four segments  10 A). One type of compass bird is described in U.S. Pat. No. 4,481,611 issued to Burrage and incorporated herein by reference.  
         [0026]     In the present embodiment, the interior space of the jacket  30  may be filled with a material  46  such as a gel, which may be a curable, synthetic urethane-based polymer. The gel  46  serves to exclude fluid (water) from the interior of the jacket  30 , to electrically insulate the various components inside the jacket  30 , and to transmit seismic energy freely through the jacket  30  to the sensors  34 . The gel  46  in its uncured state is essentially in liquid form. Upon cure, the gel  46  no longer flows as a liquid, but instead becomes substantially solid. However, the gel upon cure retains some flexibility to bending stress, some elasticity, and freely transmits seismic energy to the sensors  34 . For purposes of defining the scope of the invention, it should be understood that the gel used in the present embodiment only is one example of a substance which would perform according to the invention. Chemical and/or evaporative curing of a urethane compound is a convenient method for forming a streamer segment according to the invention, however other methods could be used with other materials. For example, heating a selected substance, such as a thermoplastic, above its melting point, and introducing the melted plastic into the interior of the jacket  30 , and subsequent cooling, may also be used in a streamer according to the invention. It is preferable that the material used has similar acoustic properties, density and electrical properties as the disclosed BVF-25 urethane so that the streamer will have similar mechanical and acoustic response characteristics to the disclosed streamer. All that is required for the invention to work is that the material undergo a state change from liquid at the time of filling the interior of the jacket to substantially solid thereafter.  
         [0027]     In making a streamer according to the invention, first, the components described above including the sensors  34 , buoyancy spacers  32 , strength members  42  and conductor cable  40  are inserted into the jacket  30 . In the present embodiment, the strength members  42  are then stretched to approximately the same degree as would be the case when the streamer is in use towed by the seismic vessel ( 10  in  FIG. 1 ). By applying the appropriate amount of axial tension to the strength members  42 , the spacers  32  and the strength members  42  may be maintained in essentially the same geometry with respect to the jacket  30  that they will assume during operation of the streamer as towed by the seismic vessel. Then, the uncured urethane compound (gel  46 ) is inserted into the interior of the jacket  30  to fill the space therein. During the time needed for the urethane compound to cure, which may be on the order of two weeks for the present embodiment, the axial tension is maintained on the strength members  42 . When the urethane compound is cured, the streamer may be made ready for storage and transportation, such as on a reel (not shown). For the segment embodiment shown in  FIG. 2 , during assembly of the segment  10 A, the termination plates  36  are coupled to the strength member  42 , and inserted into the jacket  30 . Tension may be applied to the strength members  42  during cure by way of the termination plates  36 , thus making a completed segment  10 A. Made according to this embodiment, the streamer will maintain essentially the same geometry of the various internal components, including the spacers  32 , the sensors  34  and the strength members  42  irrespective of the amount the tension applied to the strength member  42 .  
         [0028]     In other embodiments, the stretching of the strength members may be made only at the position along the jacket  30  at which the compass bird  44  is to be affixed to the exterior of the jacket.  
         [0029]     It should be understood that stretching the strength members is only one convenient way to cause the strength members to remain in their ordinary operating position during cure of the gel  46 . For purposes of defining the scope of the invention, it is only necessary to maintain the strength members  42  in their desired position during operation of the streamer, during cure of the gel  46 .  
         [0030]     Having a curable gel or similar filling the jacket  30 , rather than liquid as in prior art streamers, can also reduce the possibility of streamer failure in the event of breach of the jacket  30 . In the event of such breach, the substantially solid nature of the cured gel  46  will provide some mechanism to continue to exclude water from the active components of the streamer, including the sensors  34  and the cable conductor  40 , similar to the action of a potting compound.  
         [0031]     Streamers and streamer segments made according to the various aspects of the invention may have improved control over relative geometry of the internal components as compared with prior art streamers, and may provide more accurate placement of navigational devices thereon for increased accuracy in seismic surveying.  
         [0032]     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.