Abstract:
A system and method to obtain acoustic information from a borehole penetrating the earth are described. The system includes a light source to provide a continuous output beam and a modulator to modulate the continuous output beam with a modulation signal to provide a frequency modulated continuous wave (FMCW) to be sent out on an optical fiber disposed along the borehole, the optical fiber including a plurality of reflectors at known locations along the optical fiber. The system also includes a processor to process a light reflection signal from the optical fiber to determine the acoustic information.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a non-provisional of U.S. Provisional Application Ser. No. 61/882,287 filed Sep. 25, 2013, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    In downhole exploration and production, sensors and monitoring systems provide information about the downhole environment and the formation. One of the parameters of interest is acoustic signals, which may indicate the status of and changes in drilling and formation conditions, for example. 
       SUMMARY 
       [0003]    According to an aspect of the invention, a system to obtain acoustic information from a borehole penetrating the earth includes a light source configured to provide a continuous output beam; a modulator configured to modulate the continuous output beam with a modulation signal to provide a frequency modulated continuous wave (FMCW) to be sent out on an optical fiber disposed along the borehole, the optical fiber including a plurality of reflectors at known locations along the optical fiber; and a processor configured to process a light reflection signal from the optical fiber to determine the acoustic information. 
         [0004]    According to another aspect of the invention, a method of obtaining acoustic information from a borehole penetrating the earth includes disposing an optical fiber along the borehole, the optical fiber including a plurality of reflectors at known locations along the optical fiber; modulating a continuous output beam with a modulation signal to provide a frequency modulated continuous wave (FMCW) to be sent out on the optical fiber; and processing a light reflection signal from the optical fiber to determine the acoustic information. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
           [0006]      FIG. 1  is a cross-sectional illustration of a borehole and an acoustic sensory system according to an embodiment of the invention; 
           [0007]      FIG. 2  is a block diagram of components of the acoustic sensor system according to an embodiment of the invention; 
           [0008]      FIG. 3  depicts an exemplary modulation signal in the time domain; 
           [0009]      FIG. 4  illustrates an exemplary electronic signal resulting from three reflections; 
           [0010]      FIG. 5  illustrates an exemplary output signal and corresponding detected signal; 
           [0011]      FIG. 6  illustrates a detected signal that includes an acoustic component; and 
           [0012]      FIG. 7  is a process flow of a method of obtaining acoustic measurements along a fiber according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    As noted above, downhole acoustic signals are among the parameters that are used to characterize the downhole environment. Embodiments of the system and method described herein relate to determining the movement of reflections from an optical fiber and correlating that movement to an acoustic event. 
         [0014]      FIG. 1  is a cross-sectional illustration of a borehole  1  and an acoustic sensory system  100  according to an embodiment of the invention. The borehole  1  penetrates the earth  3  including a formation  4 . A set of tools  10  may be lowered into the borehole  1  by a string  2 . In embodiment of the invention, the string  2  may be a casing string, production string, an armored wireline, a slickline, coiled tubing, or a work string. In measure-while drilling (MWD) embodiments, the string  2  may be a drill string, and a drill would be included below the tools  10 . Information from the sensors and measurement devices included in the set of tools  10  may be sent to the surface for processing by the surface processing system  130  via a fiber link or telemetry. The surface processing system  130  (e.g., computing device) includes one or more processors and one or more memory devices in addition to an input interface and an output interface. The acoustic sensor system  100  includes an optical fiber  110  with two or more reflectors  115  (e.g., fiber Bragg gratings (FBGs)). The reflectors  115  may be positioned at known distances apart from each other. The acoustic sensor system  100  also includes components  120  shown at the surface of the earth  3  in  FIG. 1  and further detailed below with reference to  FIG. 2 . 
         [0015]      FIG. 2  is a block diagram of components  120  of the acoustic sensor system  100  according to an embodiment of the invention. A laser source  210  produces a continuous output beam  212  that is modulated by a modulation signal  214  output by a signal generator  220 .  FIG. 3  depicts an exemplary modulation signal  214  in the time domain (time on the x-axis). The exemplary modulation signal  214  has a sinusoidal envelope whose frequency is swept linearly in time over a given range. The modulated signal  216  resulting from modulating the continuous output beam  212  with the modulation signal  214  is a frequency modulated continuous wave (FMCW) and is sent out on the fiber  110 . The reflected light  217  (resulting from the FMCW interrogation of the fiber  110 ) is composed of a superposition of copies of the original signal ( 216 ) with varying delays corresponding with each area of reflection (reflectors  115 ) on the fiber  110 . The reflected light  217  is converted to an electronic signal  218  by a photodetector  219 , for example.  FIG. 4  illustrates an exemplary electronic signal  218  resulting from three reflections  410 ,  420 ,  430 . The exemplary signals shown in  FIG. 4  do not include an acoustic event. As such, the reflections  410 ,  420 ,  430  are not modulated by any acoustic noise. The electronic signal  218  representing the reflected signal is mixed with the modulation signal  214  (the reference signal) to produce an output signal  230  for further processing. The output signal  230  is a superposition of interference signals at fixed frequencies. The frequencies of the interference signals making up the output signal  230  match the frequency difference between the reflected signal (electronic signal  218 ) and the reference signal (modulation signal  214 ) and are proportional to time delays associated with the reflections that originated the reflected light  217  returned by the fiber  110 . 
         [0016]    The output signal  230  may be further processed by a processor  240  (e.g., the surface processing system  130 ). The processor  240  may be part of the components  120 , for example. During the processing, when a Fourier transform is taken of the output signal  230 , the resulting detected signal  242  in the frequency domain includes peaks corresponding to reflectors  115  in the fiber  110 . That is, just as the different time delays in the reflected electronic signal  218  correspond to the different reflectors  115 , the different frequencies in the detected signal  242  correspond with the different reflectors  115 .  FIG. 5  illustrates an exemplary output signal  230  and corresponding detected signal  242 . The exemplary signals in  FIG. 5  do not include acoustic noise.  FIG. 6  illustrates a detected signal  242  that includes an acoustic component. The detected signal  242  with ( 242   a ) and without ( 242   b ) the acoustic component are shown in  FIG. 6 . When acoustic excitation causes motion of a reflection event, the movement of a reflector  115  will show up in the detected signal  242  in the form of sidelobes (see e.g.,  610  in  FIG. 6 ) at the frequency corresponding with the effected reflector  115 . The detected signal  242  is input to a bandpass filter and demodulator to obtain the displacement signal  244  that indicates the displacement of reflectors  115  with respect to the start of the fiber  110 . By computing the difference between the obtained displacements associated with each of the reflectors  115 , local measurements of the acoustic excitation between two reflection events on the fiber  110  may be obtained. 
         [0017]      FIG. 7  is a process flow of a method of obtaining acoustic measurements along a fiber  110  according to an embodiment of the invention. At block  710 , modulating the light source includes modulating the laser source  210  output beam  212  with the modulation signal  214  before sending the resultant modulated signal  216  on the fiber  110 . Receiving the reflection from reflectors  115  on the fiber  110  at block  720  includes converting the received reflected light  217  to an electronic signal  218 . At block  730 , mixing with the reference signal (modulation signal  214 ) includes mixing the electronic signal  218  to generate the output signal  230 . As noted above, the output signal  230  is further processed by a processor  240  (e.g., the surface processing system  130 ). At block  740 , processing in the frequency domain to obtain displacements includes obtaining a Fourier transform of the output signal  230  to obtain the detected signal  242  and using demodulation techniques to find the displacements associated with the respective reflectors  115 . Obtaining acoustic information from the displacements at block  750  includes computing the difference between the obtained displacements to isolate the acoustic contribution to the resulting signal. 
         [0018]    While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.