Abstract:
A method of measuring acoustic energy impinging upon a cable includes, interrogating at least one optical fiber of the cable with electromagnetic energy, the at least one optical fiber is nonconcentrically surrounded by and strain locked to a sheath of the cable, monitoring electromagnetic energy returned in the at least one optical fiber, and determining acoustic energy impinging on the cable.

Description:
BACKGROUND 
     Conventional distributed acoustic sensing (DAS) systems rely on the coupling of energy in propagating seismic waves into longitudinal vibrational modes of the fiber (i.e. vibrations of the fiber that are parallel to the axis of the fiber). Typical DAS interrogators send coherent laser pulses into a fiber and measure the Rayleigh backscattered light from those pulses as a function of time (which is then mapped to fiber position). Backscatter from distinct points within the region illuminated by the pulse as it propagates through the fiber interfere and therefore the phase and amplitude of backscatter power received from any given region (corresponding to a pulse width) is very sensitive to the distance between the points in the region where backscatter occurs. Acoustic signals that create longitudinal vibrations in the fiber are detected as variations in the backscattered power from any given region of the fiber as successive laser pulses are sent and the backscatter signals measured as a function of fiber position. Traditional DAS systems are therefore sensitive to any excitations that create vibrations which stretch/compress the fiber along its axis (i.e. longitudinally). 
     These systems are however insensitive to acoustic energy that is not parallel to the axis of the optical fiber. Methods that allow for determination of acoustic energy in nonlongitudinal orientations to the optical fiber are of interest to those who practice in the art. 
     BRIEF DESCRIPTION 
     Disclosed herein is a method of measuring acoustic energy impinging upon a cable. The method includes, interrogating at least one optical fiber of the cable with electromagnetic energy, the at least one optical fiber is nonconcentrically surrounded by and strain locked to a sheath of the cable, monitoring electromagnetic energy returned in the at least one optical fiber, and determining acoustic energy impinging on the cable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
         FIG. 1  depicts a cross sectional view of a fiber optic cable disclose herein; 
         FIG. 2  depicts a cross sectional view of an alternate embodiment of a fiber optic cable disclosed herein; 
         FIG. 3  depicts a cross sectional view of another alternate embodiment of a fiber optic cable disclosed herein; and 
         FIG. 4  depicts a partial perspective view of the fiber optic cable of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
     Embodiments of fiber optic cables disclosed herein employ nonconcentric (i.e. off center) mechanical coupling (strain locking) of optical fiber within cables. This nonconcentric coupling allows sensing of both longitudinal and transverse (bending) deformations of the cables, since all deformations in the cable are directly couple to longitudinal changes in fiber length. 
     Referring to  FIG. 1 , an embodiment of a fiber optic cable disclose herein is illustrated in cross section at  10 . The fiber optic cable  10  includes, at least one optical fiber  14 , with just one being shown in this embodiment, and a sheath  18  surrounding the optical fiber  14 . The optical fiber  14  is strain locked to the sheath  18  nonconcentrically such as by an adhesive  20 . In this embodiment the optical fiber  14  is sized such that its radial dimension  22  is about or less than one tenth of an inner radial dimension  26  defined by walls  30  of the sheath  18 , although any difference in the dimensions  22  and  26  assures that the optical fiber  14  will be strain locked nonconcentrically to the sheath  18  as long as an axis  34  of the optical fiber  14  is not coaxial with a longitudinal axis  38  of the sheath  18 . 
     In the fiber optic cable  10  the axis  34  is parallel to the axis  38  while being offset therefrom. The cable  10  is configured to determine strain exhibited both axially as well as strain created by bending of the cable  10  about an X axis. However, when the cable  10  is bent about a Y axis strain in the sheath  18  may not be readily determined by the optical fiber  14 . Additionally, while both axial strain in the cable  10  and strain due to bending of the cable  10  about the X axis are sensible by the optical fiber  14 , the sensed strain cannot be readily separated into what portion is due to axial loading and what portion is due to the bending. 
     Referring to  FIG. 2 , another embodiment of a fiber optic cable disclosed herein is illustrated in cross section at  110 . The fiber optic cable  110  has some similarities to the cable  10 , as such like elements are numbered with the same reference characters and only the differences are elaborated on hereunder. The cable  110  differs from the cable  10  in the use of two optical fibers  114 A,  114 B instead of just one. The optical fibers  114 A and  114 B are oriented about 90 degrees apart, and both are strain locked nonconcentrically to the walls  30  of the sheath  18  such as by the adhesive  20 . In one embodiment, axis  134 A of the fiber  114 A and axis  134 B of the fiber  114 B are both parallel to the axis  38  of the sheath  18 . The foregoing structure allows the cable  110  to sense axial strain as well as bending induced strain in all possible orientations. While all orientations of bending induced strain are sensible there are a few orientations of bending that will produce similar sensed values in the fibers  114 A,  114 B. These include bending the cable  110  about a 45 degree angle relative to the X and Y axis either toward or away from a plane  140  connecting the axis  134 A and  134 B. 
     Referring to  FIG. 3 , another embodiment of a fiber optic cable disclosed herein is illustrated in cross section at  210 . The fiber optic cable  210  has some similarities to the cables  10  and  110 , as such like elements are numbered with the same reference characters and only the differences are elaborated on hereunder. The cable  210  includes three optical fibers  214 A,  214 B and  214 C. The optical fibers  214 A,  214 B and  214 C are oriented about 120 degrees from one another, and all three are strain locked nonconcentrically to the walls  30  of the sheath  18  such as by the adhesive  20 . In one embodiment axis  234 A of the fiber  214 A and axis  234 B of the fiber  214 B and axis  234 C of fiber  214 C are parallel to the axis  38  of the sheath  18 . The foregoing structure allows the cable  210  to sense axial strain as well as bending induced strain in all possible orientations. This sensing also allows separation of axial strain in the cable  210  from bending strain as well as discernment of direction of the bending strain relative to the X and Y axis of the cable  210 . This allows for determination of displacement in directions other than parallel to the axis  38  including directions orthogonal to the axis  38 . 
     It should be noted that other embodiments contemplated could have the optical fibers  14 ,  114 A,  114 B,  214 A,  214 B,  214 C of any of the cables  10 ,  110 ,  210  oriented in a helical or spiral pattern relative to the sheath  18 . One example is shown in  FIG. 4  as an embodiment of the cable  10 , wherein the optical fiber  14  is attached to the sheath  18  in a helical pattern. Such a configuration causes bending of the cable  10 ,  110 ,  210  to impart a longitudinal strain to the fibers  14 ,  114 A,  114 B,  214 A,  214 B,  214 C since the fibers  14 ,  114 A,  114 B,  214 A,  214 B,  214 C are not displaced from the axis  38  of the sheath  18  in a constant direction. Decreasing a pitch of the helical pattern can allow for increases in spatial resolution of measurements sensed along the cable  10 ,  110 ,  210 . 
     The cables  10 ,  110 ,  210  disclosed herein, with the mechanical coupling of the optical fibers  14 ,  114 A,  114 B,  214 A,  214 B,  214 C to the sheath  18  allow both longitudinal and orthogonal or transverse (bending) deformations of the cable to directly couple to longitudinal changes in fiber length. This contrasts with cable designs in which the fiber is not mechanically coupled to the cable or is coupled but in a concentric way. 
     The fibers  14 ,  114 A,  114 B,  214 A,  214 B,  214 C being mechanically coupled (strain locked) to the cable  10 ,  110 ,  210  experience the same strain profile as the cable  10 ,  110 ,  210  when it is under mechanical deformation. If properly placed in the cable  10 ,  110 ,  210  cross section, localized strain measurements derived from one or more of the mechanically coupled fibers  14 ,  114 A,  114 B,  214 A,  214 B,  214 C can therefore be used to reconstruct the cable  10 ,  110 ,  210  deformation. Longitudinal stretching/compression as well as magnitude and direction of orthogonal or transverse/bending deformations can be determined locally along the cable  10 ,  110 ,  210 . Local measurements of the full vibration profile of the cable can be calculated at regular intervals along the cable, yielding a distributed acoustic sensor that is sensitive to vibrations in all directions. 
     The fiber optic cables  10 ,  110 ,  210  disclosed herein are employable in distributed acoustic sensing systems used in earth formation boreholes in the hydrocarbon recovery and carbon dioxide sequestration industries. The cables  10 ,  110 ,  210  can be attached to a downhole tool  216  (shown in  FIG. 3  only) such as a drillstring, casing or liner, for example, to provide a well operator with static strain measurements of the tool  216  in addition to acoustic and vibration measurements available, whether the tool  216  is stationary or in motion. These measurements include longitudinal as well as nonlongitudinal directions and even directions orthogonal to the axis  38 . 
     Methods of distributed acoustic sensing (DAS) disclosed herein employ the optical cables  10 ,  110 ,  210  with the optical fibers  14 ,  114 A,  114 B,  214 A,  214 B,  214 C strain locked within the protective metal sheath  18  such that strain on the sheath is transferred effectively to the fiber  14 ,  114 A,  114 B,  214 A,  214 B,  214 C. There are several ways to interrogate the fibers  14 ,  114 A,  114 B,  214 A,  214 B,  214 C to extract the acoustic signal data as will be discussed hereunder. 
     For fine spatial resolution, an optical frequency domain reflectometer (OFDR) might be used. In this case, a laser wavelength is swept between 2 wavelengths λs and λf such that Δλ=λf−λs. Then, spacing of points (the smallest possible spatial resolution) is Δz=λsλf/2 nΔλ. So, for example an OFDR sweeping between 1520 and 1560 nm to interrogate a fiber of index n=1.46 would have a spatial resolution of 20.3 um. Such a system could easily interrogate FBG (fiber Bragg grating) sensors spaced millimeters to meters apart, but the maximum length one could interrogate in a sweep is given by sampling theory to be L max (Rsλs^2)/(4 n dλ/dt) and the number of data points taken is N max =RsΔλ(dλ/dt) where Rs is the sampling rate and dλ/dt is the laser sweep rate. The sweep time is given by Δt=Δλ/(dλ/dt). So, for example to interrogate a 1 km fiber using OFDR swept from 1520 to 1560 nm one could choose a sweep rate of 100 nm/s which would require a fast sampling rate Rs of 253 MHz. Then the number of data points would be 101 million and the time to sweep would be 0.4 s. One concern with this interrogation approach is the huge data set which must be processed to strain data and acoustic data and the slow speed. Taking data for 0.4 s would limit the time response to a little more than 1 Hz, which is a bit slow for a meaningful DAS system. So, the sweep range should be decreased or the sweep rate increased for DAS, thus limiting either the distance or requiring a very fast sampling rate. For example, to achieve a 500 Hz DAS system, interrogation time might be limited to 0.1 ms. A laser might be swept at 10,000 nm/s, a very fast rate for a laser. The scan range would then only be 10 nm, and achieving a 1 km interrogated length would require a 25 GHz sampling rate. So, balancing is needed between length and speed of a DAS system using OFDR due to the speed of electronics employed to process the data. 
     The design of an OFDR system and the configuration of the cable  10 ,  110 ,  210  as described above are intimately related. So, the cable  10  as described above with the single optical fiber  14  strain locked in a helical pattern with several FBGs per helical period might be interrogated using OFDR for this purpose. For example, the fiber  10  might be helixed at a period of 16 cm (about 6 rotations per m) with FBGs spaced 2 cm apart (8 per helix), providing a spatial resolution of sensing of about 16 cm, being able to distinguish between longitudinal and transverse (orthogonal to the axis  38 ) acoustic waves on this scale, and being able to detect any type of acoustic wave on a 2 cm spatial scale. It would also be possible to use the cable  210  with the three fibers  214 A,  214 B,  214 C with FBGs 2 cm apart at 120 degree relative orientation, not helixed. All three of the fibers  214 A,  214 B,  214 C could be interrogated and provide 2 cm three-dimensional spatial resolution of a DAS signal based on this FBG spacing. The same system could utilize Rayleigh scattering rather than FBGs, providing a weaker signal but a much finer spatial resolution, determined as above by the sweep range of the laser. So, several configurations of sensors according to the embodiments described above could be interrogated by different OFDR systems to provide three-dimensional spatial resolution from a few microns to a few meters and interrogation lengths from a few meters to hundreds of meters, according to the equations above. All could take advantage of the unique characteristics of the cables  10 ,  110 ,  210  described. 
     A courser spatial resolution but much longer system length might be provided by a Coherent Optical Time Domain Reflectometer (CoOTDR) which is another embodiment for interrogating a DAS system disclosed herein. A narrow linewidth (coherent) source is pulsed and sent through the fiber  10 ,  110 ,  210 , causing interference that is a function of time. Time of flight determines what section of the fiber is interrogated at each time. A 10 ns pulse can produce a 1 m spatial resolution in a typical fiber and the pattern that returns is a complex function of the acoustic waves impinging on the fiber  10 ,  110 ,  210 . It is possible to extract the location (to within typically 1 m) and frequency content (to a few hundred Hz) of acoustic waves impinging on a many km fiber as long as the signal caused by Rayleigh scatter is strong enough to overcome signal to noise limitations. Hence, a CoOTDR based system could operate to interrogate a cable as described. A helical cable would not be useful unless the spatial period of the helix was greater than the spatial resolution of the CoOTDR system. However, the multifiber cables  110 ,  210  could be used to allow spatial resolution to match the spatial resolution of the CoOTDR system, typically about 1 m. If the cable  110 ,  210  was strain locked, and the multiple fibers  114 A,  114 B,  214 A,  214 B,  214 C were interrogated, the cables  110 ,  210  as described above would be advantageous for the reasons described above. 
     Another embodiment of a method of interrogation is to use wavelength division multiplexed (WDM) sensors. As long as only a countable finite number of sensors is needed, it would be possible to make each sensor an FBG at a different wavelength and interrogate each in its wavelength range. 
     Any number of hybrid systems, combining features of those described above or other similar techniques for discerning time varying strain signals can benefit from the use of the cables  10 ,  110 ,  210  disclosed herein. The signals can be used to determine magnitudes of acoustic wave, acoustic spectrum (to identify what type of thing created the wave), and phase (to identify direction the wave is traveling), for example. In some cases, acoustic energy is generated at a known location and then measurements are made downhole with the cables  10 ,  110 ,  210  to image the space between the source and the cables  10 ,  110 ,  210 . The measurements includes determining a disturbance in a reflected signal as a function of time (either phase, amplitude or both), which is processed to determine some characteristic of the acoustic wave (location, strength, direction of travel, image of what it passed through, etc). The presence of the fibers  14 ,  114 A,  114 B,  214 A,  214 B,  214 C at a distance from the longitudinal axis  38  makes the measurement more sensitive to transverse traveling acoustic waves. 
     While the invention has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. Also, in the drawings and the description, there have been disclosed exemplary embodiments of the invention and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention therefore not being so limited. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.