BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the field of marine seismic exploration. More particularly, the invention relates to an improved apparatus and method for improving marine cable tow efficiency and of extending the useful life of marine cables.  
           [0002]    Cables are towed through water in marine seismic operations to pull acoustic energy sources, flotation buoys, hydrophones, and other marine seismic equipment through the water. For large seismic vessels, multiple cables are simultaneously towed in a large swath through the water. The cables may extend thousands of meters behind the seismic tow vessel.  
           [0003]    Large drag forces are generated as the cables are towed through the water. The drag forces are caused by friction between the water and cable exterior surface and increase exponentially as the vessel speed increases. Uneven water flow around the cables produces alternating shedding forces which cause transverse vibrations, known as “strumming”, in the cables. Strumming increases cable drag and further generates acoustic noise interfering with seismic data collection operations. Cable movement generates stresses at equipment connection points and accelerates cable wear.  
           [0004]    Numerous techniques have been developed to reduce frictional drag forces acting on a cable towed through water. One technique uses multiple fibers or “hairs” to interrupt vortex shedding and other friction inducing processes. For example, U.S. Pat. No. 4,084,065 to Swenson (1978) disclosed polyester yarn or fiber hairs extending from a braided cable to reduce cable strumming in water. U.S. Pat. No. 4,756,270 to Boscov (1988) disclosed a cable fairing having hairs formed with a filamentary yarn woven into a cable.  
           [0005]    Another technique uses protrusions extending outwardly from the cable to interrupt drag inducing eddies. U.S. Pat. No. 4,190,012 to Rispin et al. (1980) disclosed a cable fairing having a plurality of stubs formed in a spiral pattern about the exterior surface of a cable. U.S. Pat. No. 5,214,244 to Cummings et al. (1993) disclosed a flexible jacket having a helically displaced phase shifter for decoupling a seismic cable from strumming induced by von Karmen vortex sheets. Each of the phase shifting sections had a length less than one quarter the coherence length of the flexible jacket. U.S. Pat. No. 5,275,120 to Ruffa et al. (1994) disclosed a tow cable fairing having a plurality of radially extending tab members indexed in a helical pattern longitudinally along the cable.  
           [0006]    A technique converse to outwardly extending protrusions was proposed in U.S. Pat. No. 5,228,005 to Bjelland (1993), which disclosed a seismic streamer constructed with an external, tubular cover having a plurality of continuous, longitudinally extending grooves in the tubular cover. The grooves ran continuous with the cable length to avoid the creation of surface features obstructive to water flow past the cable.  
           [0007]    Other friction reducing techniques attach fairings to cables to reduce cable strumming in the water. U.S. Pat. No. 5,335,620 to Small (1993) disclosed a continuous, longitudinally extending fairing comprised of a single plastic extrusion and fastening means for providing shock and impact protection to seismic array sensors and cables. U.S. Pat. No. 5,367,971 to Carpenter et al. (1994) disclosed an outer cable layer having a twisted surface to create ridges for minimizing vortex shedding and cable strumming. U.S. Pat. No. 4,075,967 to Silvey (1978) disclosed a fairing having a plurality of flat vanes extending parallel to the others and tapering toward the fairing trailing edge. U.S. Pat. No. 5,678,504 to Toplosky (1997) disclosed a towing cable fitted with fairings having negative lift vanes.  
           [0008]    Improvements to fairings and the connection mechanisms between fairings and cables were shown in U.S. Pat. No. 4,542,708 to Holcombe et al. (1985), which disclosed a fairing cable segment having a rotatable spring steel clip for minimizing interfacial friction between the fairing and cable. U.S. Pat. No. 4,700,651 to Hale (1987) disclosed a fairing having a relatively large clearance around the cable to permit frictionless fairing movement and to reduce cable wear.  
           [0009]    As the size of marine seismic arrays and the vessel tow speed increases, a need exists for improved techniques for reducing drag forces. The techniques should reduce strumming noise interfering with acoustic data collection and should increase tow efficiency through the water.  
         SUMMARY OF THE INVENTION  
         [0010]    The invention provides an apparatus and method for reducing friction acting against a marine cable towable through water. The apparatus comprises a cable sheath having an exterior surface in contact with the water and a plurality of indentations in the sheath exterior surface for reducing friction between the cable and the water as the cable is towed through the water. In different embodiments of the invention, the indentions can comprise dimples, can be symmetrically shaped, can be oriented in a selected pattern, and can have different sizes and configurations. The apparatus is particularly suited to a cable for reducing strumming of the cable as the cable is towed through water.  
           [0011]    The method of the invention comprises the steps of deploying a cable having a cable sheath in the water behind a tow vessel, and of towing the cable through the water. In other embodiments, at least two cables can be deployed in the water and such cables can be towed at a selected separation distance between the cables. Marine seismic equipment can be connected to the cable before the cable is deployed in the water. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 illustrates a seismic tow vessel in water.  
         [0013]    [0013]FIG. 2 illustrates indentations formed in the exterior surface of a cable sheath.  
         [0014]    [0014]FIG. 3 illustrates different shapes and forms of indentions.  
         [0015]    [0015]FIG. 4 illustrates one pattern of indentations.  
         [0016]    [0016]FIG. 5 illustrates test data of a dimpled pipe under different test conditions.  
         [0017]    [0017]FIG. 6 illustrates test data comparing dimpled pipe to hairy fairings.  
         [0018]    [0018]FIG. 7 illustrates an embodiment of the invention having a splitter plate. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    The invention uniquely reduces frictional drag of a towed cable, strumming, and cable wear. Referring to FIG. 1, tow vessel  10  is positioned in water  12  and deploys cable  14  from drum or spool  16 . Marine seismic equipment such as acoustic energy source gun  18 , buoy  20 , and hydrophone  22  is attached to cable  14 .  
         [0020]    As shown in FIG. 2, cable  14  comprises core  24  having cable sheath  26  and exterior surface  28 . Cable sheath  26  can be formed with a metallic or extruded or wrapped plastic or composite sheath material resistant to damage. A plurality of indentations  30  are formed in exterior surface  28  for the purposes of reducing drag forces between cable  14  as cable  14  is towed through water  12 , for reducing the amount of tow energy expended by vessel  10 , for reducing strumming of cable  14  and the undesirable noise generated by such strumming, and for reducing the wear on cable  14  at connections between cable  14  and marine seismic equipment such as that identified above.  
         [0021]    Indentations  30  are formed in the exterior surface  28  of cable sheath  26  in contact with water  12 . The term “dimple” as used herein means a slight depression in exterior surface  28 , and can be dish-shaped, polygonal, or of another configuration. Each indentation  30  or dimple can be formed with a symmetric or asymmetric configuration as shown in FIG. 3 by symmetric indentation  32  and by asymmetric indentation  34 . FIG. 3 also shows geometric indentations  36  and  38  having selected configurations of different shapes, and indentation  40  is formed in a different size. Many different indentation  30  shapes, sizes, and configurations are possible within the scope of the invention, and different combinations of different indentations  30  can be provided to provide different flow dynamics. The size and depth of indentations  30  can be selected to accomplish different flow dynamics. For example, different indentations  30  having different depths from exterior surface  28  can be used to vary the fluid dynamics and resulting drag of cable exterior surface  28  through water  12 . In another embodiment of the invention, protrusions  42  can be intermingled within such indentations to vary the flow dynamics of cable  14  through water  12 .  
         [0022]    [0022]FIG. 4 illustrates a plurality of indentations  30  in a selected pattern. Although one pattern is illustrated in FIG. 4, many different patterns and configurations of indentations  30  are possible to accomplish different flow dynamics between cable  14  and water  12 . A plurality of indentations  30  resist formation of turbulent flow between cable  14  and  12  and the correspondent shedding vortexes leading to strumming and other undesirable cable  14  movement. As cable  14  is pulled through water  12 , indentations  30  break up the water flow and reduce strumming of cable  14 . Drag forces acting on cable  14  are accordingly reduced, resulting in less tow energy required to pull cable  14  through water  12 .  
         [0023]    Indentations  30  can be added to sheath exterior surface  28  before or after sheath  26  has been formed around cable core  24 . Rollers or other mechanisms can form indentations  30  such as dimples in exterior surface  28  while cable sheath  26  is formed around core  24 . Indentations  30  also provide traction against spool  16  or cable pullers for deploying and retrieving cable  14  from water  12  which lessens the radial gripping force necessary to grasp cable  14 .  
         [0024]    Multiple cables  14  are often deployed from a single seismic vessel  10 , and the invention facilitates such deployment and operation. By reducing the drag acting on cables  14 , each cable  14  can be pulled further from the center of vessel  10  travel while reducing noise induced by strumming. Reduced strumming also increases usable life of cables  14  as previously described.  
         [0025]    [0025]FIG. 5 illustrates drag tests of the invention under different test conditions. Typical tow ranges for seismic cables range between a Reynolds Number of 50,000 with a 1.45 inch diameter cable at 3.5 knots, and a Reynolds Number of 100,000 with a faired 2.0 inch vane tag line at 4.5 knots. Within this selected Reynolds Number range, which is not limiting to the scope of the invention, different cavitation tunnel and liquid tow tank tests provided substantially similar results. Larger sized dimples reduce the drag coefficient. FIG. 6 illustrates test results for the invention when compared to a conventional hairy fairing lead. As shown, the invention provided superior results in drag reduction.  
         [0026]    In another embodiment of the invention disclosed in FIG. 7, splitter plate  44  is attached to cable  14  to further reduce the drag coefficient. The combination of splitter plate  44  and dimples  30  on cable  14  reduced the drag coefficient to a range between 0.5 and 0.7.  
         [0027]    Although the invention has been described in terms of certain preferred embodiments, it will become apparent to those of ordinary skill in the art that modifications and improvements can be made to the inventive concepts herein without departing from the scope of the invention. The embodiments shown herein are merely illustrative of the inventive concepts and should not be interpreted as limiting the scope of the invention.
An apparatus and method for improving the efficiency of marine cable tow operations. Indentations such as dimples in the cable exterior surface reduce frictional drag forces and reduce strumming of the cables as the cables are towed through the water. The size, configuration and orientation of the indentations can be selected to control the desired water flow parameters.