Abstract:
An apparatus includes a streamer cable having one or more seismic devices disposed within a polymer body and about a core. The polymer body includes a channel defined therein for receiving one or more wires connecting the seismic devices. The wires include slack for withstanding the tensional forces experienced by the streamer cable during deployment and operation. Associated methods are also described.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 12/611667 filed Nov. 3, 2009, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    This disclosure generally relates to towed streamers for use in acquiring seismic data, and more specifically, to solid streamers and methods of manufacturing same. 
         [0003]    Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A seismic survey typically involves deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological formations creating pressure changes and vibrations along their way. Changes in elastic properties of the geological formation scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (hydrophones), others to particle motion (e.g., geophones), and industrial surveys may deploy only one type of sensors or both. In response to the detected seismic events, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits. 
         [0004]    Some surveys are known as “marine” surveys because they are conducted in marine environments. However, “marine” surveys may be conducted not only in saltwater environments, but also in fresh and brackish waters. In one type of marine survey, called a “towed-array” survey, an array of seismic sensor-containing streamers and sources is towed behind a survey vessel. 
         [0005]    Streamers are long cables that house various sensor networks and other devices useful in the acquisition of seismic data. Streamers may be manufactured as liquid-filled streamers or solid streamers. Prior art solid streamer cables are often constructed with a central core with transmission and power bundles that are continuous through the streamer section (a segmented portion of a streamer cable). The transmission and power bundles are typically connected to electronics modules between the streamer sections through end connectors. Also within a streamer section, there is a need to connect distributed sensors and (if present) sensor electronics by wires to transmit power and data to the electronics modules. 
         [0006]    In solid streamer cables, it is often a challenge to have wires run external to the stress member armoring because the bending forces experienced by the streamer cable impart local deformations that may introduce tensile or compressional stress in the wires. These stresses may eventually lead to deformations and/or breaks of the wires. The common way in the prior art to remove or reduce this effect is to twist the wires with a certain lay length around the stress member, which thus cancels the compressional and tensional forces experienced by the wires. However, the manufacturing and repair processes associated with utilizing twisted sensor wires and/or local electronics network wires are complicated. 
         [0007]      FIG. 1  illustrates a prior art arrangement in which a solid streamer cable  10  includes a central core  12  having a transmission bundle  14  surrounded by a strength member  16 . The central core  12  is typically pre-fabricated before adding sensors and/or sensor electronics. Local wiring  18 , which is used to connect the sensor and sensor electronics, is also disposed in the streamer cable  10  inside of a polymer body  20  and a skin  22 . The typical way to dispose the wiring  18  within the streamer cable  10  is to twist the wiring onto the central core  12  with a certain lay-length (or pitch) to allow for tensile cycling and bending of the streamer cable  10  without generating high stresses in the wires. Wiring layers in prior art solid cables are often pre-made with the central core  12 . 
         [0008]    One of the drawbacks associated with the prior art solid cable  10  of  FIG. 1  is that it complicates the manufacturing process by making it difficult to access and thereby connect the local wiring  18  to the sensors and/or sensor electronics. More particularly, it is difficult to open the local wiring  18  and cut the correct wires at the desired inline and rotational location. It is also challenging to obtain the desired slack in the wiring  18  to robustly establish connection between the wiring and the sensors and/or sensor electronics. In addition, connection of the wiring  18  to the sensors and/or sensor electronics has to be done late in the assembly process of the cable  10 . This makes the manufacturing process complex as many units have to come together at the same production step. 
       SUMMARY 
       [0009]    This disclosure is related to a solid streamer cable and a method of manufacturing same. In one embodiment, the streamer cable includes a local wiring scheme that imparts elastic elongation in a simple manner. The wiring scheme may be designed to run inline with the cable core and may be S-shaped or corrugated to thus incorporate the desired slack such that the wiring scheme can withstand both tension variations as well as bending forces. In some embodiments, a simpler manufacturing process can be employed as seismic sensors and the local wiring network can be pre-made prior to manufacturing the total seismic streamer section. 
         [0010]    Advantages and other features of the present disclosure will become apparent from the following drawing, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0011]      FIG. 1  is a cross-sectional view of a prior art solid streamer cable. 
           [0012]      FIG. 2  is a schematic diagram of a marine seismic data acquisition system according to an embodiment of the disclosure. 
           [0013]      FIG. 3  is a cut-away view of a streamer cable according to one embodiment of the present disclosure. 
           [0014]      FIG. 4  is a cross-sectional view of the streamer cable taken along the line  4 - 4  in  FIG. 3 . 
           [0015]      FIG. 5  is a cross-sectional view of the streamer cable taken along the line  5 - 5  in  FIG. 4 . 
           [0016]      FIG. 6  is a modification of  FIG. 5  to illustrate another embodiment of the present disclosure. 
           [0017]      FIG. 7  is a stress diagram illustrating exemplary stress forces undergone by the streamer cable of  FIGS. 3-5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 2  depicts an embodiment  30  of a marine seismic data acquisition system in accordance with some embodiments of the disclosure. In the system  30 , a survey vessel  32  tows one or more seismic streamers  34  (one exemplary streamer  34  being depicted in  FIG. 1 ) behind the vessel  20 . The seismic streamers  34  may be several thousand meters long and may contain various support cables (not shown), as well as wiring and/or circuitry (not shown) that may be used to support communication along the streamers  34 . In general, each streamer  30  includes a primary cable into which is mounted seismic sensors  36  that record seismic signals. It is to be appreciated that the sensors  36  are illustrated schematically for emphasis in  FIG. 2 , and that in practice, the sensors  36  are disposed within the streamer cable  34 . 
         [0019]    In accordance with embodiments of the disclosure, the seismic sensors  36  may be pressure sensors only or may be multi-component seismic sensors. For the case of multi-component seismic sensors, each sensor is capable of detecting a pressure wavefield and at least one component of a particle motion that is associated with acoustic signals that are proximate to the multi-component seismic sensor. Examples of particle motions include one or more components of a particle displacement, one or more components (inline (x), crossline (y) and vertical (z) components (see axes  38 , for example)) of a particle velocity and one or more components of a particle acceleration. 
         [0020]    Depending on the particular embodiment of the disclosure, the multi-component seismic sensor may include one or more hydrophones, geophones, particle displacement sensors, particle velocity sensors, accelerometers, pressure gradient sensors, or combinations thereof 
         [0021]    For example, in accordance with some embodiments of the disclosure, a particular multi-component seismic sensor may include a hydrophone for measuring pressure and three orthogonally-aligned accelerometers to measure three corresponding orthogonal components of particle velocity and/or acceleration near the seismic sensor. It is noted that the multi-component seismic sensor may be implemented as a single device or may be implemented as a plurality of devices, depending on the particular embodiment of the disclosure. A particular multi-component seismic sensor may also include pressure gradient sensors, which constitute another type of particle motion sensors. Each pressure gradient sensor measures the change in the pressure wavefield at a particular point with respect to a particular direction. For example, one of the pressure gradient sensors may acquire seismic data indicative of, at a particular point, the partial derivative of the pressure wavefield with respect to the crossline direction, and another one of the pressure gradient sensors may acquire, a particular point, seismic data indicative of the pressure data with respect to the inline direction. 
         [0022]    The marine seismic data acquisition system  10  includes a seismic source  40  that may be formed from one or more seismic source elements, such as air guns, for example, which are connected to the survey vessel  32 . Alternatively, in other embodiments of the disclosure, the seismic source  40  may operate independently of the survey vessel  32 , in that the seismic source  40  may be coupled to other vessels or buoys, as just a few examples. 
         [0023]    As the seismic streamers  34  are towed behind the survey vessel  32 , acoustic signals  42  (an exemplary acoustic signal  42  being depicted in  FIG. 2 ), often referred to as “shots,” are produced by the seismic source  40  and are directed down through a water column  44  into strata  46  and  48  beneath a water bottom surface  50 . The acoustic signals  42  are reflected from the various subterranean geological formations, such as an exemplary formation  52  that is depicted in  FIG. 2 . 
         [0024]    The incident acoustic signals  42  that are produced by the sources  40  produce corresponding reflected acoustic signals, or pressure waves  54 , which are sensed by the seismic sensors  36 . It is noted that the pressure waves that are received and sensed by the seismic sensors  36  include “up going” pressure waves that propagate to the sensors  36  without reflection, as well as “down going” pressure waves that are produced by reflections of the pressure waves  54  from an air-water boundary  56 . 
         [0025]    The seismic sensors  36  generate signals (digital signals, for example), called “traces,” which indicate the acquired measurements of the pressure wavefield and particle motion (if the sensors are particle motion sensors). The traces are recorded and may be at least partially processed by a signal processing unit  58  that is deployed on the survey vessel  32 , in accordance with some embodiments of the disclosure. For example, a particular multi-component seismic sensor may provide a trace, which corresponds to a measure of a pressure wavefield by its hydrophone; and the sensor may provide one or more traces that correspond to one or more components of particle motion, which are measured by its accelerometers. 
         [0026]    The goal of the seismic acquisition is to build up an image of a survey area for purposes of identifying subterranean geological formations, such as the exemplary geological formation  52 . Subsequent analysis of the representation may reveal probable locations of hydrocarbon deposits in subterranean geological formations. Depending on the particular embodiment of the disclosure, portions of the analysis of the representation may be performed on the seismic survey vessel  32 , such as by the signal processing unit  58 . 
         [0027]    Referring to  FIG. 3 , a solid streamer cable  100  according to one embodiment of the present disclosure includes a skin  102  for enclosing a polymer body  104  and one or more seismic devices  108  for use in seismic data acquisition. The seismic devices  108  may include seismic sensors (e.g., geophone, hydrophone and/or accelerometer) and/or sensor electronics that generally manipulate data acquired by the seismic sensors, such as an analog to digital converter that digitizes the analog data acquired by the sensors. In practice, the seismic devices  108  may be disposed within a housing. A core  110  is also disposed within the streamer cable  100  and may comprise a strength member and often also a transmission bundle (not shown). In some embodiments, the core  110  is substantially solid. A channel  112  is formed in the polymer body  104  in an area generally adjacent to the core  110 . In some embodiments, the channel  112  is formed in the polymer body  104  away from the core  110 . Referring to  FIG. 4 , the channel  112  provides a pathway for a wire bundle  114  to connect the various seismic devices  108  disposed within the streamer cable  100 . In this embodiment, the wire bundle  114  extends through the channel inline with the central core, thus providing easy access to the wire bundle for technicians to connect and/or disconnect the wires to the associated seismic devices  108 . 
         [0028]    Referring to  FIG. 5 , the wires  114  are formed such that they have slack when extending through the streamer cable  100 . Slack may be imparted to the wires  114  by ensuring that the wires are longer when straight than the streamer cable  100 . The additional length of the wires  114  relative to the streamer cable may be referred to as “over-length.” To accommodate the over-length, the wires  114  may be formed to have a corrugated or S-shape when extending through the cable. In corrugated embodiments, the wires  114  may be run through teethed wheels or pre-formed plates to thus impart corrugation to the wires prior to insertion in the streamer cable  100 . By having slack, the wires  114  are able to withstand the various compressional or tensional loads experienced by the streamer cable  100  during deployment and operation. 
         [0029]    It is to be appreciated that additional manners for imparting slack to the wires  114  are contemplated. For example, with reference to  FIG. 6 , slack may be imparted to the wires  114  only at certain points along the channel  112 . To accommodate such slack, enlarged cavities, such as cavity  120 , may be defined in the polymer body  104  along certain portions of the channel  112 . Accordingly, in this embodiment, the wires  114  are substantially taut along some segments of the channel  112  but do incorporate slack at the enlarged cavities  120 . 
         [0030]    By imparting slack to the wires  114 , elongation or bending of the streamer cable will only impose a portion of the tensional forces experienced by the streamer cable  100  onto the wires compared to the greater amount of tensional forces that would be experienced by taut wires. In practice, streamer cables are typically rolled on a spool and placed on a vessel for deployment at sea. As can be appreciated, rolling a streamer cable on a spool introduces undesirable bending strains, particularly with respect to solid streamer cables. Referring to  FIG. 7 , the maximum bending strain over the cross section for the cable  100  will be influenced by the cable and spool diameter. In one example, if the cable diameter is 50 mm and the spool diameter is 1400 mm, the maximum bending strain would be calculated as 3.44% at the outermost portion of the cable (25 mm out of center). Such strain will be realized as compression and tensile strain over the cross section of the cable  100 . Compression and tensile strain experienced by the wires  114  can lead to undesirable wire breaks. Prior art streamer cables sought to address this problem by twisting or coiling the wire around the streamer core, thus canceling out the compression and tensile strains. The present disclosure, however, accounts for such strain by incorporating slack into the wires  114 , thus imposing only a portion of the tensional forces experienced by the streamer cable  100  onto the wires. This permits the wires  114  to be placed eccentrically within the streamer cable, which, in turn, allows for easy access to the wires for connection and/or repair. 
         [0031]    The manufacturing process associated with assembling the streamer cable  100  according to the present disclosure can thus be simplified. In particular, by placing the wires  114  through the inline channel  112 , the sensors  106  and wires can be connected, tested and pre-made before the step of assembling the sensors and core  110  together. In one embodiment, this can be realized if the polymer body  104  was manufactured in two halves (or other multiple) that are then secured together during manufacturing. In another embodiment, the sensor network (sensor  106 , wires  114  and electronics  108 ) may be pre-assembled inside a portion of the polymer body  104  and then later assembled together with the core  110 . 
         [0032]    While the present disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. For example, the wire bundle  114  may contain one or more wires and thus this disclosure is not limited to only those embodiments having a plurality of wires in the wire bundle. Also, the channel  112  and cavity  120  may be filled with air or a compliant material. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present disclosure.