Abstract:
A hydrophone streamer including a central member running substantially the length of the streamer with a strength member and a plurality of conductors has formed therein space adapted to receive a plurality of a spaced apart pairs of collars, a cylindrical chamber wall between each of the pairs of collars defining a chamber, and one or more hydrophones within the chamber. The chamber wall has one or more opening through it for the free passage of sea water into the chamber, thereby shielding the hydrophones from extraneous noise while exposing the hydrophones to a seismic signal conducted by the sea water surrounding the streamer.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of seismic exploration and more particularly, to a hydrophone carrier in a marine seismic streamer. 
     BACKGROUND OF THE INVENTION 
     Marine seismic exploration operations commonly include towing a seismic streamer behind a vessel. The seismic streamer includes data communications channels, power conductors, one or more strength members, and a number of sound-sensitive hydrophones. To maintain a very nearly neutral buoyancy, the streamer is commonly filled with a ballast fluid, such as kerosene or oil. 
     As the streamer is towed through the water during seismic operations, its primary function is to receive seismic signals at the plurality of hydrophones from subsurface geological structures, convert these signals to a voltage signal, and transmit these voltage signals to a central receiving station on board the vessel. The seismic signals are very often very weak, and can be masked by noise from a variety of sources. It is therefore imperative that these sources of noise be minimized so as not to interfere with the seismic signal of interest. This means that the signal to noise ratio of the sound receiving apparatus should be as high as possible. 
     Large diameter fluid-filled cables have achieved excellent signal to noise characteristics, but these cables are expensive, cumbersome, heavy, and not well suited to seismic operations in heavy weather at sea. Consequently, more recent fluid-filled cables have smaller diameters, at the cost of very fine signal quality. However, these smaller diameters cables are more robust, lighter, less expensive, easier to tow and operate, and have demonstrated adequate signal quality in most operating situations. 
     Even the smaller diameter streamer cables have their ballast fluid contained with a thin plastic jacket, typically 3-4 mm thick. This skin is susceptible to damage during normal streamer deployment and retrieval operations, and may also be easily damaged by objects in the water, by accidental contact with other streamers, and by a number of common hazards. Other internal components of the streamer cable are also susceptible to damage during normal streamer deployment and retrieval and from hazardous operating conditions. These factors, among others, have led to the developments today in solid-filled cables. Solid-filled cables are more robust and suffer less damage from normal operations and hazard conditions. 
     Solid-filled streamers include groups of hydrophones spaced apart along the length of the cable. Ideally, the hydrophones would be isolated from any noise in the cable, while positioned to receive the maximum amount of the seismic signal of interest. The hydrophones along the cable are commonly mounted within a hydrophone carrier, which is an integral portion of the cable. 
     Thus, there is a need for a hydrophone carrier in a solid-filled seismic cable which is robust, inexpensive, and easily accessible for repairs while the towing vessel is deployed at sea. The carrier should be as strong as the rest of the cable, during all phases of operation, including steady state steaming, heavy weather (which can induce longitudinal jerks in the cable) and deployment and retrieval operations in which the cable is reeled onto a winch. The carrier should also isolate the hydrophones from noise conducted along the cable, while exposing the hydrophones to the seismic signal without damping the signal. 
     SUMMARY OF THE INVENTION 
     The present invention addresses these and other challenges of the prior art in a solid seismic streamer. One or more hydrophones is mounted within a chamber of the streamer and the chamber includes openings which permit sea water to flow into the chamber. The openings also provide for free communication of seismic signals directly onto the hydrophone elements within the chamber. 
     The chamber is formed by a cylindrical chamber wall, preferably made of titanium, which is formed as a top half and a bottom half which may be bolted together or otherwise joined. The chamber wall mates with a complementary annular groove in each of a pair of collars. The collars are similarly formed as a top half and a bottom half which may be bolted together or otherwise joined. The chamber is also bounded on its interior by a central member, or a covering for the central member. The central member includes the power and data communications conductors, as well as at least one strength member. 
     The hydrophone element is preferably formed as a pair of opposed piezoelectric elements or fiber optic sensors mounted to a common support structure. The support structure is in turn enclosed within a sealed tube which is filled with a fluid, preferably a nonorganic oil. The piezoelectric elements are electrically coupled through a stuffing tube plugging one end of the tube for connection to the central member. Other structures for the hydrophone element are equally preferred, including a free-flooding hydrophone mounting, or an optical fiber sensor. 
     These and other features of this invention will be apparent to those skilled in the art from a review of the following description along with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal cross section of a seismic streamer including the hydrophone carrier of this invention. 
     FIG. 2 is an axial cross section of a portion of the hydrophone carrier taken along section  2 — 2  of FIG.  1 . 
     FIG. 3 is an axial cross section of another portion of the hydrophone carrier taken along section  3 — 3  of FIG.  1 . 
     FIG. 4 is a section view of a detail of the carrier of this invention taken along section  4 — 4  of FIG.  1 . 
     FIG. 5 is a partial side view of a fluid port into the hydrophone carrier. 
     FIG. 6 is a longitudinal section view of a hydrophone and its surrounding capsule. 
     FIG. 7 is an axial section view of the hydrophone and its capsule as taken along section  7 — 7  of FIG.  6 . 
     FIG. 8 is a longitudinal cross section of a seismic streamer including another preferred embodiment of the hydrophone carrier of this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a side view of the seismic streamer  10  and its component hydrophone carrier  12  in section. The streamer  10  includes a plurality of hydrophones  14 , of which only two are shown. A seismic streamer is commonly kilometers long, and thus may include hundreds of such hydrophones. The hydrophones are depicted as positioned on either side of the streamer, but the streamer may include only one of the pair of hydrophones disposed within the carrier  12 , or there may be more than two hydrophones within a carrier, depending on the choice of design and the application. 
     The hydrophone  14  develops an electrical signal in response to an acoustic stress imparted to the hydrophone and this electrical signal is conducted over a set of wires  16  to a central member  18 . The central member, shown in partial section in FIG. 1, includes conductors for data and power, as well as one or more strength members to carry the stress of the length of the streamer. The central member may further include fiber optic data communication channels for carrying the seismic data acquired by the streamer to increase the bandwidth for the data. 
     A set of opposed collars  20  are clamped around the central member  18 , as shown in axial cross section in FIG.  2 . The collars  20  are made up of a top half  22  and a bottom half  24 , held together as by bolts  26 , by way of example and not by way of limitation. The smooth contour of the streamer is maintained at the collars  20  by potting the bolt holes with a potting material  28 , or by other appropriate means. The collars are also secured to the central member  18  with a potting material  30 . The primary function of the potting material  30 , however, is to seal a chamber  32 , which in operation is filled with sea water, from the remainder of the streamer up and down the central member  18 . The central member  18  is enclosed by a polymeric material filler  34  along the streamer between the carriers  12 , and the potting material  30  seals against sea water migrating along the central member  18  beneath the polymeric material filler  34 . 
     The filler  34  is preferably of a synthetic plastic that provides approximately neutral buoyancy to the streamer. The filler is surrounded and enclosed by a jacket  36  which provides the smooth, cylindrical surface to the exterior of the streamer. Each of the filler  34  and the jacket  36  is preferably extruded onto the central member  18  in the manufacturing process of the streamer. 
     The collar  20  defines an annular groove  38  and a circular flange  40 , which provide a mating surface for a cylindrical chamber wall  42 . The chamber wall  42  encloses the chamber  32  and is shown in section in FIG.  3 . The chamber wall  42  is made up of a top half  44  and a bottom half  46 , held together as by bolts  48 , by way of example and not by way of limitation. The smooth contour of the streamer is maintained at the chamber wall  42  by potting the bolt holes with a potting material  50 . 
     At each end of the chamber wall  42  is a circular flange  52 , which mates with the annular groove  38  and the circular flange  40  of the collar  20 . This structure provides strength to the streamer as a whole and defines the chamber  32 , which is open to sea water via openings  54 , shown in greater detail in FIGS. 4 and 5. The chamber wall  42  present a gently curving contour on its outside surface and the openings  54  are located within an elongate groove  56 . The groove  56  presents a gently sloping shoulder  58  down to the opening  54 , which offers no sharp corners to the sea water as the streamer is drawn through the water, although other appropriate shapes and contours may be used. This helps to further reduce turbulence and therefore self noise of the hydrophone carrier. Note also that the opening  54  is preferably an elongated circle oriented lengthwise in the groove  56 , although other shapes and orientations are also possible. 
     The purpose of the openings  54  is to permit the free flow of sea water into the chamber  32  and therefore to conduct the seismic signal directly from the water surrounding the streamer into contact with the hydrophones  14 . The hydrophones  14 , located within the chamber  32 , are therefore insulated from exterior disturbances while in direct fluid communication with the seismic signal of interest. 
     The hydrophone component  14  is shown in greater detail in FIGS. 6 and 7. This component comprises an elongated tube  60 , sealed at one end  62  with an open end  64 . The open end  64  of the tube  60  is partly closed with an electrical stuffing tube  66  through which the wires  16  pass. The tube  60  is preferably filled with a fluid  68  of an inorganic oil. The tube  60 , once filled with the fluid, is then plugged with a plug  68 . 
     The tube  60  encloses a hydrophone support element  70  on which is mounted a top piezoelectric element  72  and a bottom piezoelectric element  74 . This structure may incorporate the low distortion features described in one or more U.S. Pat. Nos. 5,541,894; 5,663,931; 5,675,556; and 5,677,894; all of which are incorporated herein by reference, in order to reduce the effects of the second harmonics of the piezoelectric elements  72  and  74 . The support element is shown as a closed parallelepiped, closed on all six sides, with a rectangular cross section. The support element thus is hollow inside and filled with air, so that the top and bottom surface of the support element are free to flex under the influence of a seismic signal and thereby flex the elements  72  and  74  supported thereon. 
     FIG. 8 shows a side view of another preferred embodiment of the seismic streamer  10  and its component hydrophone carrier  80  in section. The streamer  10  includes one or more hydrophones  14 , of which only one is shown. As with the embodiment of FIG. 1, the hydrophone  14  develops an electrical signal in response to an acoustic stress imparted to the hydrophone and this electrical signal is conducted over a set of wires  16  to a central member  18 . The central member is substantially the same as that of FIG.  1  and the hydrophones are constructed substantially as shown in FIGS. 6 and 7. 
     A set of opposed collars  82  are clamped around the central member  18 . The collars  82  are made up of top and bottom halves, and the halves are held together much the same as shown in FIG.  2 . The collars  82  are held apart by a pair of rods  84  and one each of the rods  84  may be formed as an integral part of the respective half of the collar  82 . In the embodiment of FIG. 8, the structure retains the advantage of distancing the stress bearing rods  84  away from the central member  18  and reducing the amount of noise conducted directly to the hydrophones. 
     The collars are also secured to the central member  18  with a potting material  30 . The central member  18  is enclosed by a polymeric material filler  34  along the streamer between the carriers  80 , and the potting material  30  seals against sea water migrating along the central member  18  beneath the polymeric material filler  34 . The filler is surrounded and enclosed by a jacket  36  which provides the smooth, cylindrical surface to the exterior of the streamer. Each of the filler  34  and the jacket  36  is preferably extruded onto the central member  18  in the manufacturing process of the streamer. 
     The collars  82  and the jacket  36  are sealed off by an outer sleeve  86 . The outer sleeve  86  is not a stress bearing member, and is made of a pliant material that closely matches the sound transmissive qualities of water. In this way, the hydrophone  14  is more directly subjected to the seismic signal without the interference of intervening materials as in previous designs. 
     The sleeve  86  forms a chamber  88 , which is entirely filled with a fluid, such as water or oil. The chamber is filled through a fill port  90 , and more than one port may be provided to assist in venting all the air from the chamber  88  as it is being filled with fluid. The sleeve is preferably bonded against the collars  82  and the jacket  36  to seal the streamer from end to end. The sleeve  86  may also include longitudinal expansion slits which are “welded” once the sleeve is in place to from a tight seal. 
     Both the carrier of FIG.  1  and the carrier of FIG. 8 provide a solid streamer with a section of the streamer provided for discrete, distributed hydrophones. Thus, both embodiments eliminate the drawbacks of the fluid filled streamers known in the art, while providing a smoothly contoured streamer to reduce self-noise. 
     The principles, preferred embodiment, and mode of operation of the present invention have been described in the foregoing specification. This invention is not to be construed as limited to the particular forms disclosed, since these are regarded as illustrative rather than restrictive. Moreover, variations and changes may be made by those skilled in the art without departing from the spirit of the invention.