Abstract:
An apparatus includes a cable; and seismic sensors that are disposed in the cable. The apparatus also includes spacers that are distributed in the cable such that each seismic sensor is disposed in an interval of the cable separating a different adjacent pair of the spacers. The spacers of each pair are separated by at least twenty-five centimeters.

Description:
BACKGROUND 
       [0001]    The invention generally relates to a seismic sensor cable, such as a streamer, for example. 
         [0002]    Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A 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. 
         [0003]    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. 
       SUMMARY 
       [0004]    In an embodiment of the invention, an apparatus includes a cable; and seismic sensors that are disposed in the cable. The apparatus also includes spacers that are distributed in the cable such that each seismic sensor is disposed in an interval of the cable separating a different adjacent pair of the spacers. The spacers of each pair are separated by at least twenty-five centimeters. 
         [0005]    In another embodiment of the invention, a technique includes disposing seismic sensors in a cable and distributing spacers in the cable such that each seismic sensor is disposed in an interval of the cable separating a different adjacent pair of the spacers. The technique includes separating the spacers of each pair by at least twenty-five centimeters. 
         [0006]    In another embodiment of the invention, an apparatus includes a cable that includes a skin and a seismic sensor that is disposed in the cable. The apparatus also includes a spacer that is disposed in the cable. The spacer includes at least one extended portion to support the skin and at least one recessed portion to form a region between the spacer and the skin to receive a filler material. 
         [0007]    In yet another embodiment of the invention, a technique includes disposing seismic sensors and spacers inside a cable. For each spacer, at least one portion of the spacer is extended to support a skin of the cable and at least one portion of the spacer is recessed to receive a filler material between the spacer and the skin. 
         [0008]    Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]      FIG. 1  is a schematic diagram of a marine seismic data acquisition system according to an embodiment of the invention. 
           [0010]      FIG. 2  is a cross-sectional view of a streamer of the prior art. 
           [0011]      FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 1  according to an embodiment of the invention. 
           [0012]      FIG. 4  is a flow diagram depicting a technique to reduce flow noise in a gel-filled streamer according to an embodiment of the invention. 
           [0013]      FIG. 5  is a cross-sectional view of a spacer of a streamer of the prior art. 
           [0014]      FIG. 6  is a side view of the spacer of  FIG. 5 . 
           [0015]      FIG. 7  is a cross-sectional view of a spacer of a streamer according to an embodiment of the invention. 
           [0016]      FIG. 8  is a side view of the spacer of  FIG. 7  according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  depicts an embodiment  10  of a marine seismic data acquisition system in accordance with some embodiments of the invention. In the system  10 , a survey vessel  20  tows one or more seismic streamers  30  (one exemplary streamer  30  being depicted in  FIG. 1 ) behind the vessel  20 . The seismic streamers  30  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  30 . In general, each streamer  30  includes a primary cable into which is mounted seismic sensors  58  that record seismic signals. 
         [0018]    In accordance with embodiments of the invention, the seismic sensors  58  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  59 , for example)) of a particle velocity and one or more components of a particle acceleration. 
         [0019]    Depending on the particular embodiment of the invention, 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. 
         [0020]    For example, in accordance with some embodiments of the invention, 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 invention. 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. 
         [0021]    The marine seismic data acquisition system  10  includes a seismic source  104  that may be formed from one or more seismic source elements, such as air guns, for example, which are connected to the survey vessel  20 . Alternatively, in other embodiments of the invention, the seismic source  104  may operate independently of the survey vessel  20 , in that the seismic source  104  may be coupled to other vessels or buoys, as just a few examples. 
         [0022]    As the seismic streamers  30  are towed behind the survey vessel  20 , acoustic signals  42  (an exemplary acoustic signal  42  being depicted in  FIG. 1 ), often referred to as “shots,” are produced by the seismic source  104  and are directed down through a water column  44  into strata  62  and  68  beneath a water bottom surface  24 . The acoustic signals  42  are reflected from the various subterranean geological formations, such as an exemplary formation  65  that is depicted in  FIG. 1 . 
         [0023]    The incident acoustic signals  42  that are acquired by the sources  40  produce corresponding reflected acoustic signals, or pressure waves  60 , which are sensed by the seismic sensors  58 . It is noted that the pressure waves that are received and sensed by the seismic sensors  58  include “up going” pressure waves that propagate to the sensors  58  without reflection, as well as “down going” pressure waves that are produced by reflections of the pressure waves  60  from an air-water boundary  31 . 
         [0024]    The seismic sensors  58  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  23  that is deployed on the survey vessel  20 , in accordance with some embodiments of the invention. 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. 
         [0025]    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  65 . Subsequent analysis of the representation may reveal probable locations of hydrocarbon deposits in subterranean geological formations. Depending on the particular embodiment of the invention, portions of the analysis of the representation may be performed on the seismic survey vessel  20 , such as by the signal processing unit  23 . 
         [0026]    The main mechanical parts of a conventional streamer typically include skin (the outer covering); one or more stress members; seismic sensors; spacers to support the skin and protect the seismic sensors; and a filler material. In general, the filler material typically has a density to make the overall streamer neutrally buoyant; and the filler material typically has properties that make the material acoustically transparent and electrically non-conductive. 
         [0027]    Certain fluids (kerosene, for example) possess these properties and thus, may be used as streamer filler materials. However, a fluid does not possess the ability to dampen vibration, i.e., waves that propagate in the inline direction along the streamer. Therefore, measures typically are undertaken to compensate for the fluid&#39;s inability to dampen vibration. For example, the spacers may be placed either symmetrically around each seismic sensor (i.e., one spacer on each side of the sensor); or two sensors may be placed symmetrically about each spacer. The vibration is cancelled by using two spacers symmetrically disposed about the seismic sensor because each spacer sets up a pressure wave (as a result of inline vibration), and the two waves have opposite polarities, which cancel each other. Two seismic sensors may be disposed symmetrically around one spacer to achieve a similar cancellation effect, but this approach uses twice as many sensors. Furthermore, the latter approach may degrade performance due to nonsymmetrical positioning of the other seismic sensors. 
         [0028]    When gel is used as the filler material, the noise picture changes, as flow noise (instead of vibration) becomes the dominant noise source. More specifically, the main mechanical difference between fluid and gel as a filler material is the shear stiffness. A fluid has zero shear stiffness, and shear stresses from viscous effects typically are negligible. The shear stiffness is what makes a gel possess solid-like properties. It has been discovered through modeling that the shear stiffness of the gel degrades the averaging of flow noise. The degradation in the flow noise cancellation may be attributable to relatively little amount of gel being effectively available to communicate the pressure between each side of the spacer. 
         [0029]    In accordance with embodiments of the invention described herein, the streamer  30  is filled with a gel-based filler material. Techniques and structures are described herein for purposes of increasing the volume, area and/or length of continuous gel, which is available for effectively canceling flow noise that is introduced by turbulent pressure fluctuations. 
         [0030]    One way to increase the gel available for noise cancellation involves increasing the separation between spacers, as compared to the spacer separation used in streamers of the prior art. More specifically,  FIG. 2  depicts a conventional streamer  100 , which includes spacers  108  (spacers  108   a ,  108   b ,  108   c  and  108   d , being depicted as examples) that are located inside a cable  101  of the streamer  100 . The spacers  108  are primarily used to support an outer skin  104  of the cable  101  and protect the seismic sensors (such as an exemplary seismic sensor  110 ) of the streamer  100 . As depicted in  FIG. 2 , conventionally, the spacers  108  may have two different spacings: a first smaller center-to-center spacing distance (called “d 1 ” in  FIG. 2 ) when the spacers  108  straddle a seismic sensor (such as the exemplary sensor  110 ) and a larger center-to-center spacing distance (called “d 2 ” in  FIG. 2 ) when no seismic sensor is located in between. As depicted in  FIG. 2 , the spacers  108   a  and  108   b  are located on either side of a seismic sensor  110  and are separated by the distance d 1 , which is less than the distance d 2  between the spacers  108   c  and  108   a  or the distance d 2  between the spacers  108   b  and  108   d.    
         [0031]    As a specific example, conventionally, the distance d 2  may be approximately 240 to 260 millimeters (mm), and the distance d 1  may be approximately 140 to 160 mm. 
         [0032]    It has been discovered through simulations that for a gel-filled streamer, acceptable noise cancellation may be achieved by moving the spacers farther away from the seismic sensors. More specifically, the distances between the spacers and the seismic sensors may be increased, which provides more volume of the gel-based filler material for noise cancellation, while still providing sufficiently close sensor-to-spacer separation to protect the seismic sensors. 
         [0033]      FIG. 3  depicts an exemplary embodiment of the streamer  30  in accordance with some embodiments of the invention. The cross-sectional view depicted in  FIG. 3  is simplified, in that communication and support lines of the streamer  30  are not depicted, for purposes of clarifying the relationship of the spacers and seismic sensors. 
         [0034]    In general, the streamer  30  includes a cable  114 , which has an outer skin  124 , and in general, the outer skin  124  defines an interior space that is filled with a gel-based filler material  126 . The streamer  30  contains seismic sensors  58  (one seismic sensor  58  being depicted in  FIG. 3  as an example), which may be multicomponent and/or pressure sensors, depending on the particular embodiment of the invention. To support the outer skin  124  and protect the seismic sensors  58 , the streamer  30  contains spacers  120 , which are distributed along the length of the cable  114 . Two exemplary spacers  120   a  and  120   b  of the streamer  30  are depicted in  FIG. 3 . 
         [0035]    As depicted in  FIG. 3 , the spacers  120   a  and  120   b  are adjacent spacers  120 , are located on either side of the seismic sensor  58  and are separated from each other by a center-to-center spacing distance (called “d 3 ” in  FIG. 3 ). The distance d 3  is significantly greater than the center-to-center distance d 1  (see  FIG. 2 ) between the spacers  108   a  and  108   b  of a conventional streamer. Due to the increased spacing distance d 3 , more of the gel-based filler material  126  is available to attenuate flow noise. 
         [0036]    As a more specific example, in accordance with some embodiments of the invention, the distance d 3  may be greater than 25 centimeters (cm), and as a more specific example, the distance d 3  may be approximately 60 cm, in accordance with some embodiments of the invention. 
         [0037]    Depending on the particular embodiment of the invention, the spacer spacing may vary, depending on whether the spacers  120  are in proximity to a seismic sensor  58 . More specifically, in accordance with some embodiments of the invention, the spacers  120  may be more closely spaced together (at a spacing of 25 cm or greater) when disposed on either side of the seismic sensor  58 . However, when no seismic sensor  58  is disposed in between, the spacers  120  may be moved even farther apart. In other embodiments of the invention, the spacers  120  may be uniformly spaced apart (i.e., a spacing of at least 25 cm between adjacent spacers  120 ) along the entire length of the streamer  30 . Thus, many variations are contemplated and are within the scope of the appended claims. 
         [0038]    To summarize,  FIG. 4  depicts a technique  150  in accordance with embodiments of the invention. Pursuant to the technique  150 , spacers are distributed (block  154 ) along the length of a streamer cable. The spacers are positioned (block  158 ) so that a minimum distance between adjacent spacers located on either side of a seismic sensor is at least 25 cm. 
         [0039]    Another technique to increase the amount of gel available to attenuate flow noise involves specifically constructing the spacer to receive and communicate gel between either side of the spacer. Such a spacer construction is to be contrasted to a spacer  200  of the prior art, which is depicted in a cross-sectional view in  FIG. 5  and in a side view of  FIG. 6 . More specifically, the conventional spacer  200  generally has a body  201  with a cylindrical cross-section and openings  205  and  203  to route communication and structural lines, respectively, through the spacer  200 . The spacer  200  is designed so that an outer surface  202  of the body  201  contacts and generally supports the outer skin of the streamer along the entire outer periphery of the body  201 . However, such a design limits the amount of gel, which is available to attenuate flow noise. 
         [0040]    Referring to  FIGS. 7  (depicting a cross-sectional view) and  8  (depicting a side view), contrary to conventional arrangements, a spacer  220  in accordance with embodiments of the invention may be formed from a spacer body  224 , which has radially recessed regions  240  for purposes of communicating gel between either side of the spacer  220 . In this regard, in accordance with some embodiments of the invention, the spacer body  224  includes vertically extending arms  230  and horizontally extending arms  234 , which generally form a T-shaped structure for supporting the outer skin of the streamer  30 . Thus, at their upper and lower ends, the arms  230  have surfaces  231  that contact the interior surface of the outer skin of the streamer  30 ; and similarly, the horizontally extending arms  234  have contact surfaces  235  for purpose of contacting the inner surface of the outer skin of the streamer  30 . However, the contact between the outer surface of the spacer body  224  and the inner surface of the outer skin is not continuous, thereby creating the recessed regions  240 , which receive and communicate the gel-based filler material between either side of the spacer  220 . 
         [0041]    The spacers may have other shapes other than the shape depicted in  FIGS. 7 and 8  for purposes of creating a sufficient volume of gel-based filler material to attenuate flow noise, in accordance with other embodiments of the invention. 
         [0042]    While the present invention 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. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.