Abstract:
A technique includes distributing particle motion sensors along the length of a seismic streamer. Each particle motion sensor is eccentrically disposed at an associated angle about an axis of the seismic streamer with respect to a reference line that is common to the associated angles. The sensors are mounted to suppress torque noise in measurements that are acquired by the particle motion sensors. This mounting includes substantially varying the associated angles.

Description:
BACKGROUND 
     The invention generally relates to a system and technique to suppress the acquisition of torque noise on a multi-component streamer. 
     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. 
     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 
     In an embodiment of the invention, a technique includes distributing particle motion sensors along the length of a seismic streamer. Each particle motion sensor is eccentrically disposed at an associated angle about an axis of the seismic streamer with respect to a reference line that is common to the associated angles. The sensors are mounted to suppress torque noise in measurements that are acquired by the particle motion sensors. This mounting includes substantially varying the associated angles. 
     In another embodiment of the invention, a technique includes providing particle motion sensors to acquire a seismic signal and torque noise while in tow. The technique includes orienting the sensors to modulate a wavenumber of the acquired torque noise. 
     In yet another embodiment of the invention, a system includes a seismic streamer and particle motion sensors that are distributed along the length of a seismic streamer. Each particle motion sensor is eccentrically disposed at an associated angle about an axis of the seismic streamer with respect to a reference line that is common to the associated angles. The sensors are mounted such that the associated angles are substantially varied to suppress torque noise in measurements that are acquired by the sensors. 
     Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram of a marine seismic acquisition system according to an embodiment of the invention. 
         FIG. 2  is a cross-sectional view taken along line  2 - 2  of  FIG. 1  according to an embodiment of the invention. 
         FIG. 3  is a synthetically-generated noise record in frequency-wavenumber space illustrating traversal vibration noise and torque noise. 
         FIG. 4  is a transverse cross-sectional view of a streamer having eccentrically-disposed particle motion sensors, in accordance with an embodiment of the invention. 
         FIG. 5  is a lengthwise cross-sectional view of the streamer of  FIG. 4 . 
         FIG. 6  is a lengthwise cross-sectional view of a streamer having eccentrically-disposed particle motion sensors according to an embodiment of the invention. 
         FIGS. 7 and 9  are illustrations of crossline cross-sections of streamers at adjacent sensor locations along the streamers according to embodiments of the invention. 
         FIG. 8  is a synthetically-generated noise record in frequency-wavenumber space acquired by sensors of the streamer of  FIG. 6  according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts an embodiment  10  of a marine-based 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 . In one non-limiting example, the streamers  30  may be arranged in a spread in which multiple streamers  30  are towed in approximately the same plane at the same depth. As another non-limiting example, the streamers may be towed at multiple depths, such as in an over/under spread, for example. 
     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 that record seismic signals. The streamers  30  contain seismic sensor units  58 , which include, in accordance with embodiments of the invention, multi-component sensors. Each multi-component 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 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. 
     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. 
     For example, in accordance with some embodiments of the invention, a particular seismic sensor unit  58  may include at least one particle motion sensor  70  for purposes of measuring a component of particle motion along a particular sensitive axis  59  (the x, y or z axis, for example). As a more specific example, the seismic sensor unit  58  may include a particle velocity sensor that is oriented to acquire a measurement of a particle velocity along the depth, or z, axis; a particle velocity sensor to sense a particle velocity along the crossline, or y, axis; a particle velocity sensor to sense a velocity along the inline, or x, axis; multiple particle velocity sensors to sense particle velocities along all three (x, y and z) axes; etc. Alternatively, in other embodiments of the invention, the particle motion sensor(s) of each seismic sensor unit  58  may sense a particle motion other than velocity (an acceleration, for example). 
     In addition to the seismic sensor units  58 , the marine seismic data acquisition system  10  also includes one or more seismic sources  40  (two exemplary seismic sources  40  being depicted in  FIG. 1 ), such as air guns and the like. In some embodiments of the invention, the seismic source(s)  40  may be coupled to, or towed by, the survey vessel  20 . Alternatively, in other embodiments of the invention, the seismic source(s)  40  may operate independently of the survey vessel  20 , in that the source(s)  40  may be coupled to other vessels or buoys, as just a few examples. 
     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(s)  40  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 . 
     The incident acoustic signals  42  that are created by the source(s)  40  produce corresponding reflected acoustic signals, or pressure waves  60 , which are sensed by the seismic sensors of the seismic sensor unit  58 . It is noted that the pressure waves that are received and sensed by the seismic sensors include “up going” pressure waves that propagate to the sensors without reflection, as well as “down going” pressure waves that are produced by reflections of the pressure waves  60  from an air-water boundary, or free surface  31 . 
     The seismic sensors of the seismic sensor units  58  generate signals (digital signals, for example), called “traces,” which indicate the acquired measurements of the pressure wavefield and particle motion. 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 seismic sensor unit  58  may provide a trace, which corresponds to a measure of a pressure wavefield by its hydrophone; and the seismic sensor unit  58  may provide (depending on the particular embodiment of the invention) one or more traces that correspond to one or more components of particle motion. 
     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 . In accordance with other embodiments of the invention, the representation may be processed by a data processing system that may be, for example, located on land or on the vessel  20 . 
     For particularly the case in which the streamer  30  has a solid core, the particle motion sensors  70  may acquire a significant degree of noise in addition to the desired particle motion signal. The noise is attributable to several types of noise sources and may include vibration noise, flow noise, acoustic noise, incoherent noise, etc. The noise acquired by the particle motions sensors  70  may also be a function of torque noise, which is introduced due to the eccentric positions of the particle motion sensors  70  with respect to the inline axis of the streamer  30 . 
     More specifically, as depicted in an exemplary cross-section of the streamer  30  in  FIG. 2 , a particle motion sensor  70  may be mounted in the streamer  30  such that the sensor  70  is positioned eccentrically with respect to a central axis  80  of the streamer cable. In other words, the particle motion sensor  70  may be mounted a distance (called “e” in  FIG. 2 ) away from the cable&#39;s central inline axis  80 . Due to this eccentric positioning, the particle motion sensor  70  is subject to rotation about the axis  80  as the streamer  30  is being towed, and as a result, the measurement acquired by the particle motion sensor  70  contains torque noise that is attributable to this rotation. 
     In  FIG. 2 , the particle motion sensor  70  has three primary axes for purposes of sensing particle motion: a vertical, or z, axis  72 ; a crossline, or y, axis  74 ; and an inline, or x, axis  76 , which is directed out of the page and indicated by the “dot” in  FIG. 2 . As also depicted in  FIG. 2 , the streamer  30  may contain a solid core  82 . 
     Because the torque noise is related to the rotation of the streamer  30  about the inline, or x, axis  80 , the effect of the torque noise on the measured local crossline, or y, and vertical, or z, components are different, as described below:
 
 N   y ( t,x )= V   y ( t,x )+ e{umlaut over (θ)} ( t,x )+ R   y ( t,x ), and  Eq. 1
 
 N   z ( t,x )= V   z ( t,x )+ e{dot over (θ)}   2 ( t,x )+ R   z ( t,x ),  Eq. 2
 
where “t” represents time; “×” represents the inline coordinate of the sensor position; “N y ” and “N z ” represent the y and z components, respectively, of the total noise present in the particle motion measurements; “V y ” and “V z ” represent the transversal vibration noise components along the y and z axes, respectively; “{dot over (θ)}” represents the angular velocity about the axis  80 ; “{umlaut over (θ)}” represents the angular acceleration about the axis  80 ; and “R y ” and “R z ” represent the remaining noise components (acoustic noise, ambient noise, etc.) along the y and z axes, respectively.
 
     In Eq. 1, the term “e{umlaut over (θ)}(t,x)” represents the y, or crossline, component of the torque noise, called “τ y (t,x),” as set forth below:
 
τ y ( t,x )= e{umlaut over (θ)} ( t,x ).  Eq. 3
 
     The τ y  (t,x) torque noise is usually significant for moderate values of eccentricity e and is referred to as the “torque noise” in the following discussion. It is noted that as set forth in Eq. 2, the cross component of the noise, N z (t,x), also contains a torque noise component, e{dot over (θ)} 2 (t,x). However, because the torque induced noise on the local z, or vertical, component is proportional to the square of the angular velocity, the amplitude of this term is relatively small and is considered to be negligible in the following discussion. 
     In general, the propagation of the torque noise on the y component is slower than the seismic signal and faster than the transversal vibration noise. More specifically,  FIG. 3  depicts a frequency-wavenumber (f-k) plot  100  of a synthetically-generated noise record acquired by particle motion sensors. This noise record includes transversal vibration noise  106  and torque noise  108 . Also depicted in  FIG. 3  is a signal cone  104 , which defines the boundaries in f-k space for the expected seismic signal. As can be seen in this particular example, the torque noise  108  contaminates mostly the lowest frequencies of the useful seismic frequency band, as the torque noise  108  intersects the lower frequency portion of the signal cone  104 . In general, it is difficult through signal processing to discriminate the torque noise inside the signal cone  104  from the desired particle motion signal. 
     Referring to  FIG. 4 , for purposes of suppressing, if not eliminating, the degree of torque noise that is present in the signal cone  104 , two multi-component sensors may be disposed at each inline sensor position. In this regard, referring to  FIG. 4  (which depicts a transverse cross-section) a streamer  150 , in accordance with embodiments of the invention, may contain two multi-component particle motion sensors (i.e., an upper sensor  70   a  and a lower sensor  70   b , where each sensor  70   a ,  70   b  has the same design  70 ) at each inline sensor location. In accordance with some embodiments of the invention, the upper  70   a  and lower  70   b  sensors are located at different positions in the same y-z plane, and more particularly, each sensor  70   a ,  70   b  has a different phasing, or angle, about the inline axis  80 . In this manner, the upper sensor  70   a  is disposed at an angle  162  of 90° (measuring in the clockwise direction) with respect to a crossline axis  81  of the streamer; and the lower sensor  70   b  is disposed at an angle  164  of 270° with respect to the crossline axis  81 . In other words, the upper  70   a  and lower  70   b  sensors are spaced apart by 180° about the inline axis  80 . 
     Both sensors  70   a  and  70   b  are disposed by the distance e from the inline streamer axis  80 ; and inline (x)  76  (denoted by a “dot” to show the axis  76  pointing out of the page), crossline (y)  74  and depth (z)  72  axes of the sensors  70   a  and  70   b  are oriented in the same directions such that the inline axes  76  of the sensors  70   a  and  70   b  are parallel, the crossline axes  74  of the sensors  70   a  and  70   b  are parallel and the depth axes  72  of the sensors  70   a  and  70   b  are parallel. The corresponding lengthwise cross-section of the streamer  150  is depicted in  FIG. 5 , in which each “dot” in the sensor  70  represents the crossline axis  74  pointing out of the page. 
     The different phasings of the particle motion sensors  70   a  and  70   b  induce torque noise with opposite polarities on the measurements that are acquired by the sensors  70   a  and  70   b , as described below:
 
 N   y     1   ( t,x )= V   y ( t,x )+ e{umlaut over (θ)} ( t,x )+ R   y     1   ( t,x ), and  Eq. 4
 
 N   y     2   ( t,x )= V   y ( t,x )− e{umlaut over (θ)} ( t,x )+ R   y     2   ( t,x ),  Eq. 5
 
where “N y     1   (t,x)” represents the crossline, or y, component of the total noise acquired by the particle motion sensors  70   a ; and “N y     2   (t,x)” represents the crossline, or y, component of the total noise acquired by the particle motion sensors  70   b . As can be seen from Eqs. 4 and 5, the measurements acquired by the particle motion sensors  70   a  and  70   b  may be added together to significantly suppress, or even eliminate, the sensed torque noise, as the sum of the measurements from particle motion sensors does not have a torque noise component because the opposite polarity components cancel each other out. However, the number of particle motion sensors for the streamer  150  is doubled, as compared to conventional arrangements. Additionally, the extra x and z components of the particle motion sensors are subject to similar noise modes and allow for only negligible additional noise attenuation.
 
     Referring to  FIG. 6 , in accordance with other embodiments of the invention, a seismic streamer  200  (a lengthwise cross-section of which is depicted in  FIG. 6 ) may be used. For the streamer  200 , a single multi-component particle motion sensor  70  is disposed at each inline sensor position along the length of the streamer  200 , and each particle motion sensor  200  is disposed the same distance e away from the inline axis  80  of the streamer  200 . Unlike conventional arrangements, however, the phasing of the particle motion sensors  70  about the inline streamer axis  80  vary along the length of the streamer  200 . The variation in phasing can be seen more clearly in  FIG. 7 , which is an illustration  220  of crossline cross-sections of the streamer  200  (i.e., y-z plane cross-sections) at adjacent sensor locations. As can be seen, the multi-component axes of the sensors  70  are aligned, and the sensor locations in the y-z plane vary by 180° from one adjacent sensor  70  to the next. 
     More specifically, beginning with the leftmost cross-section that is depicted in  FIG. 7 , the sensor  70   a  is located at the 90° angle  162  about the inline axis  80 ; the next adjacent sensor  70   b  (to the left) is located at the 270° angle  164  about the inline axis  80 ; the next adjacent sensor  70   a  (to the left) is located at the 90° angle  162  about the inline axis  80 ; etc. In other words, a 180° phasing scheme is used such that (excluding the first and last sensors  70  on the streamer  200 ), each sensor  70  is disposed at an angle about the inline axis  80  that is 180° apart from the angles at which the immediately adjacent sensors (one on each side) are disposed. 
     Due to the above-described phasing, the measured torque noise is wavenumber modulated, as described below for the crossline component of the total noise:
 
 N   y ( t,x )= V   y ( t,x )+ e{umlaut over (θ)} ( t,x )(−1) n(x)   +R   y ( t,x ),  Eq. 6
 
where “n” refers to the index of the corresponding sensor. In other words, the odd-indexed sensors  70  perceive the torque noise with an opposite phase than the even-indexed sensors  70 . It is noted that the frequency-wavenumber spectrum of the particle motion signal and the transversal noise are not affected by these alternating orientation(s), because the sensors  70  at opposite sides of the central axis  80  have the same sensitivity (in amplitude and phase) to the signal and transversal vibration.
 
     The corresponding crossline component of the total noise measurement in the frequency-wavenumber domain may be described as follows: 
                         N   y     ⁡     (     f   ,   k     )       =         V   y     ⁡     (     f   ,   k     )       +                 τ   y     ⁡     (     f   ,     k   +       K   x     /   2         )       +                 τ   y     ⁡     (     f   ,     k   +       K   x     /   2         )             2     +       R   y     ⁡     (     f   ,   k     )           ,           Eq   .           ⁢   7               
where “f” represents the frequency; “k” represents the wavenumber; “τ y (f,k)” represents the frequency-wavenumber transform of the torque noise, “e{umlaut over (θ)}(t,x);” and “K x /2” represents the Nyquist wavenumber, which is one half of the inverse of the inline sensor spacing.
 
     Referring to  FIG. 8 , a frequency-wavenumber plot  250  of a synthetically-generated noise record illustrates the wavenumber modulation that may be achieved using the sensor orientation that is depicted in  FIG. 6 . The wavenumber modulation effectively moves the torque noise wavenumbers (“K x /2”) away from their original location (see  FIG. 3 , for example) and out of the signal cone  104 . 
     In accordance with some embodiments of the invention, the particle motion sensors  70  may be arranged in two groups (a first group of sensors  70   a  and a separate second group of sensors  70   b , for example) which form two separate sensor networks (one for each group) that may each independently transfer the acquired data to an onboard acquisition system. In other words, a dual sensor network may be used instead of a single sensor network. This type of implementation may increase the reliability of the data acquisition system. In this regard, if a failure occurs at one of the sensor networks, the other network is available to transfer the acquired data, although the sensor spacing is increased by a factor of two due to the failure. Other variations are contemplated and are within the scope of the appended claims. 
     Other embodiments are contemplated and are within the scope of the appended claims For example, a phasing scheme other than the above-described alternating 180° phasing scheme may be employed in accordance with other embodiments of the invention. As a specific example,  FIG. 9  is an illustration  300  of crossline cross-sections of a streamer  320  at inline sensor locations in accordance with another embodiment of the invention. For this embodiment, each inline sensor location has a single multi-component sensor  70 , similar to the streamer  200 . However, unlike the streamer  200 , the streamer  320  employs a 90° phasing scheme in that the sensor locations are rotated by 90° about the inline streamer axis  80  from one adjacent sensor  70  to the next. Thus, each sensor  70  has either a 90° angle (as shown by an exemplary sensor  70   a  being disposed at a 90° angle  302  about the inline axis  80  with respect to the crossline axis  81 ); a 0° angle (as shown by an exemplary sensor  70   c  being disposed at a 0° angle about the inline axis  80  with respect to the crossline axis  81 ); a 270° angle (as shown by an exemplary sensor  70   b  being disposed at a 270° angle  306  about the inline axis  80  with respect to the crossline axis  81 ); or an 180° angle (as shown by an exemplary sensor  70   d  being disposed at an 180° angle  308  about the inline axis  80  with respect to the crossline axis  81 ). As with the other embodiments, the respective crossline, inline and depth axes of the sensors  70  remain aligned, regardless of their angles about the inline streamer axis. 
     Comparing the streamers  200  and  320 , the streamer  200  pushes the torque noise further away from the desired seismic signal in the frequency-wavenumber domain than the streamer  320 . 
     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.