Abstract:
A seismic streamer includes a jacket covering an exterior of the streamer, at least one strength member extending along the length of and disposed inside the jacket, at least one seismic sensor mounted in a sensor spacer affixed to the at least one strength member, and a void filler made from a material introduced into the jacket in liquid form and undergoing state change thereafter. The jacket includes an inner layer in contact with and having adhesiveness to the void filler, and an outer layer disposed over the outer layer and having substantially no adhesiveness.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE 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 geophysical surveying such as seismic surveying is typically performed using sensor “streamers” towed near the surface of a body of water. A streamer is in the most general sense a cable towed by a vessel. The cable has a plurality of sensors disposed thereon at spaced apart locations along the length of the cable. In the case of marine seismic surveying 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 or may be any type of particle motion sensor or acceleration sensor known in the art. Irrespective of the type of such sensors, the sensors typically 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 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. Structure, fluid content and mineral composition are typically inferred from characteristics of seismic energy that is reflected from subsurface acoustic impedance boundaries. One important aspect of interpretation is identifying those portions of the recorded signals that represent reflected seismic energy and those portions which represent noise. 
         [0008]    A typical sensor streamer is assembled by coupling a plurality of streamer segments together end to end. Each segment typically includes a jacket covering the exterior, one or more strength members extending along the segment from end to end, buoyancy spacers and sensors disposed in sensor spacers at selected positions along the strength member all disposed within the jacket. Void space within the jacket not occupied by the foregoing is typically filled with a material that is introduced into the jacket in liquid form and undergoes state change to a gel like material thereafter (called buoyancy void filler or “BVF”.) 
         [0009]    A desirable property of the material used to form the jacket is adhesiveness. Adhesiveness between the inner wall of the streamer jacket and the BVF is desirable because it can extend the lifetime of a streamer segment in case of jacket damage. Adhesion of the jacket material to the BVF can reduce salt water penetration into the interstices of the streamer segment. However, adhesiveness in the outer surface of the jacket can cause difficulties in handling a streamer cable. It is especially a disadvantage during deployment and/or in emergencies when tangled streamer(s) need to be untangled. 
         [0010]    There is a need for a streamer jacket for use in marine streamers that is adhesive on its inner wall and substantially non-adhesive on its outer wall. 
       SUMMARY OF THE INVENTION 
       [0011]    One aspect of the invention is a marine sensor streamer. A seismic streamer according to this aspect of the invention includes a jacket covering an exterior of the streamer, at least one strength member extending along the length of and disposed inside the jacket, at least one seismic sensor mounted in a sensor spacer affixed to the at least one strength member, and a void filler made from a material introduced into the jacket in liquid form and undergoing state change thereafter. The jacket includes an inner layer in contact with and having adhesiveness to the void filler, and an outer layer disposed over the outer layer and having substantially no adhesiveness. 
         [0012]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows typical marine data acquisition using a streamer according to one example of the invention. 
           [0014]      FIG. 2  shows a cut away view of one embodiment of a streamer segment according to the invention. 
           [0015]      FIG. 3  shows a two layer jacket that can be used in some examples. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  shows an example marine seismic data acquisition system as it is typically used on acquiring seismic data. 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 sources in the water  12 . The seismic vessel  14  or a different vessel tows at least one seismic streamer  10  near the surface of the water  12 . The streamer  10  is coupled to the vessel  14  by a lead in cable  26 . A plurality of sensor elements  24 , or arrays of such sensor elements, are disposed at spaced apart locations along the streamer  10 . The sensor elements  24 , are formed by mounting a seismic sensor inside a sensor spacer. 
         [0017]    During operation, certain 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 in each sensor element  24 . Structure of the formations  21 ,  23 , among other properties of the Earth&#39;s subsurface, can be inferred by travel time of the energy  19  and by characteristics of the detected energy such as its amplitude and phase. 
         [0018]    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 , which is a cut away view of a portion (segment)  10 A of a typical 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  10 A as shown in  FIG. 2  connected end to end behind the vessel ( 14  in  FIG. 1 ). 
         [0019]    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  thus 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 can be made from 3.5 mm thick polyurethane and has a nominal external diameter of about 62 millimeters. The jacket  30  will be explained in more detail below with reference to  FIG. 3 . In each segment  10 A, each axial end of the jacket  30  may be terminated by a coupling/termination plate  36 . 
         [0020]    The coupling/termination block  36  may include ribs or similar elements  36 A on an external surface of the coupling/termination plate  36  that is inserted into the end of the jacket  30 , so as to seal against the inner surface of the jacket  30  and to grip the coupling/termination plate  36  to the jacket  30  when the jacket  30  is secured by and external clamp (not shown). In the present embodiment, two strength members  42  are coupled to the interior of each coupling/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 made from a fiber sold under the trademark VECTRAN, which is a registered trademark of Hoechst Celanese Corp., New York, N.Y. The strength members  42  transmit axial load along the length of the segment  10 A. When one segment  10 A is coupled end to end to another such segment (not shown), the mating coupling/termination plates  36  are coupled together using any suitable connector, so that the axial force is transmitted through the coupling/termination blocks  36  from the strength members  42  in one segment  10 A to the strength member in the adjoining segment. 
         [0021]    The segment  10 A can include a selected number of buoyancy spacers  32  disposed in the jacket  30  and coupled to the strength members  42  at spaced apart locations along their length. The buoyancy spacers  32  may be made, for example, from foamed polyurethane or other suitable material. The buoyancy spacers  32  have a density selected to provide the segment  10 A with a selected overall density, preferably approximately the same overall density as the water ( 12  in  FIG. 1 ), so that the streamer ( 10  in  FIG. 1 ) will be substantially neutrally buoyant in the water ( 12  in  FIG. 1 ). 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 buoyancy spacers  32  and fill media having suitable specific gravity. 
         [0022]    The segment  10 A includes a generally centrally located conductor cable  40  which can include a plurality of insulated electrical conductors (not shown separately), and may include one or more optical fibers (not shown). The cable  40  conducts electrical and/or optical signals from the sensors (not shown) to the recording system ( 16  in  FIG. 1 ). The cable  40  may in some implementations 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 the 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 on the segment  10 A, so that the electrical conductors and optical fibers in the cable  40  will not experience any substantial axial stress when the streamer  10  is towed through the water by a vessel. The conductors and optical fibers may be terminated in a connector  38  disposed in each coupling/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. 
         [0023]    Sensors, which in the present example may be hydrophones, can be disposed inside sensor spacers, shown in  FIG. 2  generally at  34 . The hydrophones in the present embodiment can be of 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. 
         [0024]    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 is known in the art, to infer the position of the various sensors 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). 
         [0025]    In the present embodiment, the interior space of the jacket  30  may be filled with a material  46  such as BVF (described in the Background section herein) which may be a curable, synthetic urethane-based polymer. The BVF  46  serves to exclude fluid (water) from the interior of the jacket  30 , to electrically insulate the various components inside the jacket  30 , to add buoyancy to a streamer section and to transmit seismic energy freely through the jacket  30  to the sensors  34 . The BVF  46  in its uncured state is essentially in liquid form. Upon cure, the BVF  46  no longer flows as a liquid, but instead becomes substantially solid. However, the BVF  46  upon cure retains some flexibility to bending stress, substantial elasticity, and freely transmits seismic energy to the sensors  34 . It should be understood that the BVF used in the present example only is one example of a gel-like substance that can be used to fill the interior of the streamer. Other materials could be also used. 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. 
         [0026]    An example streamer jacket made according to the invention is shown in cross section in  FIG. 3 . The jacket  30  may include an inner layer  50  and an outer layer  52 . One example of a method for producing such a two layer jacket is double extrusion. The inner layer may be made from polyurethane that has the property of adhesiveness. Examples of such materials include one sold under product designation BFG-58887 by The Spiratex Company. Another suitable material for the inner layer  50  is sold under product designation 1185 A IOU by BASF Group. 
         [0027]    The outer layer  52  may also be polyurethane, preferably formulated to substantially lack adhesion. The foregoing example materials for polyurethane may be used, and in addition, the outer layer material may include an anti-adhesion additive, such as one sold under product designation NMP 959 by Nu-Methods Plastics Incorporated, 4321 Northampton Rd., Cuyahoga Falls, Ohio 44223 or one sold by Americhem Inc., 2000 Americhem Way, Cuyahoga Falls, Ohio 44221. In some examples, the additive may be three percent (3%) by weight of the material used to make the outer layer  52 . The additive causes the outer layer to substantially lack adhesiveness. The outer layer  52  material, however, will bond tightly to the inner layer  50  because it is also polyurethane. 
         [0028]    In the example shown in  FIG. 3 , the overall thickness of each layer  50 ,  52  may be about 1.75 mm so that the overall thickness of the jacket  30  us about 3.5 mm as explained above with reference to  FIG. 2 . 
         [0029]    Streamers and streamer segments made according to the various aspects of the invention may have improved durability in the event of jacket damage as well as improved handling ability due to lack of adhesiveness of the outer surface of the jacket. 
         [0030]    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.