Abstract:
A method for estimating a parameter includes: generating an optical signal, the optical signal modulated via a modulation signal; transmitting the modulated optical signal from a light source into an optical fiber, the optical fiber including a plurality of sensing locations disposed along the optical fiber and configured to reflect light; receiving a reflected signal including light reflected from the plurality of sensing locations; and combining, in parallel, each of a plurality of reference signals with the reflected signal to estimate a value of the parameter.

Description:
BACKGROUND 
       [0001]    Parameter monitoring systems can be incorporated with downhole components as fiber-optic distributed sensing systems (DSS). Examples of DSS techniques include Optical Frequency Domain Reflectometry (OFDR), which includes interrogating an optical fiber sensor with an optical signal to generate reflected signals in the optical fiber sensor. 
         [0002]    Many downhole applications typically require measuring parameters at extremely long depths, which are further extended in marine applications. Lead-in lengths (i.e., the length of the optical fiber from an optical interrogator to the region of interest) can thus be quite long, which can reduce the effective measurement range of DSS systems. 
       SUMMARY 
       [0003]    A method for estimating a parameter includes: generating an optical signal, the optical signal modulated via a modulation signal; transmitting the modulated optical signal from a light source into an optical fiber, the optical fiber including a plurality of sensing locations disposed along the optical fiber and configured to reflect light; receiving a reflected signal including light reflected from the plurality of sensing locations; and combining, in parallel, each of a plurality of reference signals with the reflected signal to estimate a value of the parameter. 
         [0004]    A system for estimating a parameter includes: a light source in optical communication with an optical fiber, the light source configured to generate an optical signal, the optical fiber configured to receive the optical signal and including a plurality of sensing locations disposed along the optical fiber and configured to reflect light; a modulator configured to modulate the optical signal via a modulation signal; a detector configured to receive a reflected signal including light reflected from the plurality of sensing locations; and a processor configured to combine, in parallel, each of a plurality of reference signals with the reflected signal to estimate a value of the parameter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings, wherein like elements are numbered alike, in which: 
           [0006]      FIG. 1  illustrates an exemplary embodiment of a downhole drilling, monitoring, evaluation, exploration and/or production system; 
           [0007]      FIG. 2  illustrates an exemplary embodiment of a measurement unit of the system of  FIG. 1 ; 
           [0008]      FIG. 3  illustrates another exemplary embodiment of the measurement unit of the system of  FIG. 1 ; 
           [0009]      FIG. 4  is a flow chart illustrating an exemplary embodiment of a method of estimating a parameter; 
           [0010]      FIG. 5  is an illustration of a modulation frequency of a modulated optical signal; and 
           [0011]      FIG. 6  is an illustration of a modulation frequency of a plurality of demodulation signals. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    There are provided systems and methods for interrogating one or more optical fibers. An exemplary method includes generating an optical signal and modulating the optical signal by a modulation signal having a modulation frequency. This modulated optical signal is launched by an interrogator into an optical fiber that includes one or more measurement locations, and a return signal including light reflected and/or backscattered from the measurement locations is received. A plurality of oscillating reference signals are generated (e.g., by a plurality of respective mixers) and mixed or otherwise combined in parallel with the return signal. In one embodiment, each reference signal includes a delay to correspond to a region or length of the optical fiber. In one embodiment, a system includes one or more mixers or other processors configured to receive the return signal from the optical fiber and process the return signal using each reference signal. For example, the system includes a plurality of mixers, each mixer outputting a signal indicative of the difference in frequency between the modulation signal and the reflected signal. This frequency difference may be analyzed to estimate parameters of each optical fiber sensing region. 
         [0013]    Referring to  FIG. 1 , an exemplary embodiment of a downhole drilling, monitoring, evaluation, exploration and/or production system  10  disposed in a wellbore  12  in an earth formation  16  is shown. A borehole string  14  is disposed in the wellbore  12  and performs or facilitates functions such as drilling, production and formation evaluation. The borehole string  14  is made from, for example, a pipe, multiple pipe sections or flexible tubing. The borehole string  14  includes for example, a drilling system and/or a bottomhole assembly (BHA). The system  10  and/or the borehole string  14  include any number of downhole tools  18  for various processes including drilling, hydrocarbon production, and formation evaluation (FE) for measuring one or more physical quantities in or around a borehole. Various measurement tools  18  may be incorporated into the system  10  to affect measurement regimes such as wireline measurement applications or logging-while-drilling (LWD) applications. 
         [0014]    In one embodiment, a parameter measurement system is included as part of the system  10  and is configured to measure or estimate various downhole parameters of the formation  16 , the borehole  14 , the tool  18  and/or other downhole components. The measurement system includes an optical interrogator or measurement unit  20  connected in operable communication with at least one optical fiber  22 . The measurement unit  20  may be located, for example, at a surface location, or may be incorporated with the borehole string  12  or tool  18  or otherwise disposed downhole as desired. The measurement unit  20  includes, for example, an electromagnetic signal source  24  such as a tunable light source, a LED and/or a laser, and a signal detector  26 . In one embodiment, a processing unit  28  is in operable communication with the signal source  24  and the detector  26  and is configured to control the source  24 , receive reflected signal data from the detector  26  and/or process reflected signal data. Although the measurement unit  20  is shown as a single unit, it can also be configured as multiple units. Furthermore, the measurement system described herein is not limited to downhole applications. The measurement system may be used in conjunction with any surface or downhole environment, particularly those that would benefit from distributed parameter (e.g., temperature or pressure) measurements. 
         [0015]    The optical fiber  22  is operably connected to the measurement unit  20  and is configured to be disposed downhole. The optical fiber  22  includes one or more sensing locations  30  disposed along a length of the optical fiber. The sensing locations  30  are configured to reflect and/or scatter optical interrogation signals transmitted by the measurement unit  20 . Examples of sensing locations include fiber Bragg gratings (FBG), mirrors, Fabry-Perot cavities and locations of intrinsic scattering, as well as any induced reflections. Locations of intrinsic scattering include points in or lengths of the fiber that reflect interrogation signals, such as Rayleigh scattering, Brillouin scattering and Raman scattering locations. The sensing locations  30  are configured to return reflected and/or backscattered signals (referred to herein collectively as “reflected signals” or “return signals”) from the sensing locations  30  in response to optical measurement signals (i.e., interrogation signals) launched into the optical fiber  22 . 
         [0016]    In one embodiment, the measurement system is configured as an optical frequency-domain reflectometry (OFDR) system. In this embodiment, the source  24  includes a continuously tunable laser that is used to spectrally interrogate the optical fiber sensor  22 . Scattered signals reflected from intrinsic scattering locations, sensing locations  30  and other reflecting surfaces in the optical fiber  22  are detected as a function of frequency and analyzed. Each scattered signal can be correlated with a location by, for example, a mathematical transform or interferometrically analyzing the scattered signals in comparison with a selected common reflection location. Each scattered signal can be integrated to reconstruct the total length and/or shape of the cable. 
         [0017]    One type of OFDR is referred to as incoherent OFDR (IOFDR) (also referred to as Frequency Modulated Continuous-wave (FMCW) techniques). In IOFDR, the interrogation signal is frequency modulated over time (e.g., periodically) via a modulation signal. The frequency may be modulated in a step-wise manner or continuously (swept frequency). The interrogation signal is transmitted into the optical fiber and reflected signals are returned from the sensing locations and detected as a function of modulation frequency. In one embodiment, the reflected signals are mixed with the original modulation signal or another modulation signal in the electrical domain, to generate an interference signal. A Fourier transformation (e.g., Fast Fourier Transform) of the interference signal as a function of frequency provides the time-domain signal, which can be used to correlate the interference signal with locations along the fiber. 
         [0018]    An example of the measurement unit  20  is shown in  FIG. 2 . In this example, the measurement unit is an incoherent OFDR device. The measurement unit  20  includes the optical source  24 , such as a continuous wave (cw) frequency (or wavelength) tunable diode laser optically connected to the optical fiber  22 . A modulator (e.g., function generator)  32  in optical communication with the tunable optical source  24  modulates the optical source  24 , such as by power, intensity or amplitude, using a modulation signal. The modulation signal is generally an oscillating waveform, such as a sine wave, having a modulation frequency. In one embodiment, the modulator  32  may be incorporated as part of the optical source  24 . A detector  26 , such as a photodiode, is included to detect reflected signals from the optical fiber  22  in response to modulated optical signal launched from the optical source  24 . 
         [0019]    Still referring to  FIG. 2 , a computer processing system  28  is coupled to at least the detector  26 , and is configured to process the reflected light signals. For example, the computer processing system  28  can demodulate the reflected signal using a de-modulation signal, such as the modulation signal used in launching the optical interrogation signal, or another local oscillator (referred to as a “reference signal”). The computer processing system  28  can be configured as a signal mixer, which measures the amplitude and phase of the modulation signal or reference signal with respect to the received reflected signal. The processing system  28  may also be configured to further process the demodulated signal. For example, the processing system  28  is configured to transform (e.g., via a FFT) the reflected signal to allow spatial correlation of the signal with the sensing locations  30  or selected locations or regions of the optical fiber  22 . 
         [0020]    Another embodiment of the measurement unit  20  is shown in  FIG. 3 . In this embodiment, the measurement unit  20  includes the optical source  24  optically connected to the optical fiber  22 , and the modulator  32  in optical communication with the tunable optical source  24 . The modulator  32  generates a modulation signal  34  such as an oscillating waveform. The modulator  32 , in addition to generating the modulation signal for use by the optical source, sends the modulation signal to the computer processing system  28 . 
         [0021]    In this embodiment, the measurement unit  20  includes a plurality of signal processors  36 ,  38  and  40  that are each configured to generate a respective reference signal  42 ,  44  and  46 , each of which can be applied to the reflected light signal, i.e., via mixing or demodulation. The reference signals  42 ,  44  and  46  are configured to be applied to the reflected signal in parallel, that is, each reference signal is applied to the same reflected signal. For example, each reference signal  42 ,  44  and  46  is applied to the same return signal over a substantially identical time window, and/or each reference signal is at least substantially simultaneously applied to the return signal. 
         [0022]    In one embodiment, each of the modulation signal  34  and the reference signals  42 ,  44  and  46  are oscillating signals having a time-varying oscillation frequency, also referred to as the “modulation frequency”. Each signal includes a respective oscillation frequency or modulation frequency that varies over time according to some function, such as a step function or a linear function. 
         [0023]      FIG. 3  illustrates an exemplary configuration for applying the reference signals. The measurement unit  20  includes a beam splitter  48  for splitting the return signal into multiple constituent return signals. Each constituent return signal beam may have the same or different power, but has the same wavelength and modulation. A detector (e.g., an opto-electrical converter)  50  converts each respective constituent return signal to an electrical signal. Each detector signal is mixed or otherwise combined with a respective reference signal  42 ,  44 ,  46  via the signal processors  36 ,  38  and  40 , which may be configured as, for example, radiofrequency (RF) mixers. 
         [0024]    The configuration shown in  FIG. 3  is not intended to be limiting, as any suitable configuration can be used for introducing multiple local oscillator signals and performing multiple combination or mixing operations in parallel on the same reflected or backscattered signal. Such configurations include employing multiple signal generation circuits, or utilizing other digital delay methodologies. For example, the reflected signal can be converted by a single detector into an electric signal, and the converted signal can be input into multiple signal generation circuits, each providing a different reference signal to combine with the reflected signal. Other embodiments may include introducing the reference signals optically to each constituent reflected signal, such as by optically delaying each constituent reflected signal and mixing each delayed reflected signal with the modulation signal. 
         [0025]    The computer processing system  28  is coupled to at least the detector  26 , and is configured to process the reflected light signals. For example, the computer processing system  28  can demodulate or mix the return signal with a reference signal such as the delayed reference signals  42 ,  44  and  46 . The computer processing system can be configured as a signal mixer, which measures the amplitude and phase of each modulation signal with respect to the received reflected signal. Various additional components may also be included as part of the measurement units  20  and  30 , such as a spectrum analyzer, beam splitter, light circulator, gain meter, phase meter, lens, filter and fiber optic coupler for example. 
         [0026]      FIG. 4  illustrates a method  60  of measuring downhole parameters. The method  60  includes one or more stages  61 - 65 . Although the method  60  is described in conjunction with the system  10  and the measurement systems described above, the method  60  is not limited to use with these embodiments, and may be performed by the measurement unit  20  or other processing and/or signal detection device. In one embodiment, the method  60  includes the execution of all of stages  61 - 65  in the order described. However, certain stages may be omitted, stages may be added, or the order of the stages changed. 
         [0027]    In the first stage  61 , the optical fiber  22  along with the borehole string  12 , tools  18  and/or other components are lowered downhole. The components may be lowered via, for example, a wireline or a drillstring. 
         [0028]    In the second stage  62 , a modulated optical signal having a wavelength “λ” is generated and launched into the optical fiber  22 . The modulator  32  modulates the power, intensity and/or amplitude of the optical signal (e.g., using a modulation signal such as modulation signal  34 ) according to a sinusoidal or other oscillating function having a time-varying oscillation frequency or modulation frequency. In general, the modulation frequencies are in the radio frequency range, although other frequencies can be used down to zero Hertz. 
         [0029]    For example, as shown in  FIG. 5 , the oscillation frequency varies over time according to a selected function. For example, the optical signal modulation frequency is swept, i.e., changed, by the modulator  32  over a period of time, such as in a continuous or nearly continuous change (e.g., linear change, exponential). For example, the modulator  32  modulates the optical signal with a modulation signal  34  having a modulation frequency represented by a linear function  70 . This function may be produced by, e.g., a function generator, which regulates the current applied to the laser. In this example, the function begins at an initial time “t 0 ”, at which the modulation frequency is at a selected minimum “f 1 ” (e.g., at or near zero), and ends at a time “t f ”, at which the modulation frequency is a selected maximum “f 2 ”. The function may repeat as desired. Multiple modulated signals may be iteratively launched for multiple laser wavelengths. 
         [0030]    In the third stage  63 , a reflected signal is detected by the detector  26 ,  50  and corresponding return or reflected signal data is generated by the processor  28 . The reflected signals may include light reflected and/or backscattered from sensing locations  30 . For example, the reflected signal is a result of reflections and/or backscattering from FBGs, Rayleigh scattering, Raman scattering, and/or Brillouin scattering. 
         [0031]    In the fourth stage  64 , the reflected signal is mixed or demodulated with respect to multiple reference signals. The reflected signal is mixed with reference signals such as reference signals  42  and  44 , each of which has an oscillation frequency that varies differently, e.g., that varies according to a different function. In one embodiment, each reference signal modulation frequency varies according to a function having at least substantially the same form as the modulation signal, the function being temporally delayed relative to the modulation signal according to a selected time delay. For example, as shown in  FIG. 6 , each reference signal has at least substantially the same form as the modulation signal  34 , i.e., is a sinusoidal waveform having a modulation frequency that is varied over time. In this example, both the modulation signal  34  and the reference signals  42  and  44  have at least substantially the same sweep rate (i.e., rate of change of the modulation frequency from t 0  to “t f+d ” over some time period), but the reference signals  42  and  44  are time delayed. The time delays “d 1 ” and “d 2 ” in this example are represented by the time period from t 0  to “t d1 ” and from t 0  to “t d2 ”, respectively. The reference signal can be delayed by any suitable method or mechanism, such as by generating the delayed reference signal by the modulator  34  or a separate signal generation circuit. Other methods of introducing the delay include using digital delay devices such as first-in first-out (FIFO) buffers. As discussed above, the return optical signal can alternatively be split into multiple constituent signals having the same phase, and optically delayed (e.g., by d 1  and d 2 , respectively). The optically delayed signals can then each be mixed with the original modulation signal  34 . 
         [0032]    The amount of each delay corresponds to, for example, the time-of-flight of an optical signal between a launching location (e.g., input location of the optical source  24 ) and a selected location or region in the optical fiber  22 . The time of flight may be acquired or calculated by any suitable means, such as by using the measurement unit  20  or other optical source to send a pulsed signal and record the time of receipt of resulting reflected signals. Each delay may be at least substantially equal to the difference in the time of flight between two locations bounding a selected region of the fiber. 
         [0033]    In one embodiment, the return optical signal is split into multiple constituent signals, and each constituent signal is converted into an electrical signal (e.g., via detector  50  including an O/E converter). Each electrical signal is individually mixed with a respective reference signal  42 ,  44  and  46  to generate a data set for a region of the optical fiber corresponding to the delay. In another embodiment, the return optical signal is converted into an electrical signal without splitting the optical signal, and the electrical signal is mixed in parallel with each reference signal. 
         [0034]    The mixing operations are each performed on the return signal over the same time window, so as to generate data corresponding to different lengths of the optical fiber. In one embodiment, the mixing operations performed on the return signal are each performed simultaneously. These operations in effect produce multiple data sets, each corresponding to some portion of the length of the optical fiber and/or some location or location range of different sensors along the optical fiber. Because longer fiber lengths produce signals with greater amounts of chirp noise, these reduced length data sets reduce the chirp noise, allowing for increased sweep rates for modulation signals relative to techniques that utilize a single reference signal. 
         [0035]    The demodulated reflected signals may then be inversely transformed using a mathematical algorithm such as a Fast Fourier Transform (FFT) into a spatial frequency domain. 
         [0036]    Stages  61 - 64  may be repeated for optical signals having multiple optical wavelengths. For example, stages  61 - 64  are performed using a modulated signal having a first substantially constant first wavelength λ 1 , and repeated for N subsequent signals having wavelengths λ 2 -λ N . Multiple sets of readings may be assembled into one composite set of readings, which provides a complex data set containing, among other parameters, amplitude of reflection (or transmission) and spatial location data for each of the components in optical communication with the optical fiber  22 . The modulated signal wavelength can be varied at any desired rate, e.g., swept in a step-wise manner or a continuous manner. 
         [0037]    In the fifth stage  65 , the mixed signal data is utilized to estimate various parameters along the optical fiber  22 . The reflected signal data is correlated to locations on or lengths of the optical fiber  22 , and parameters are estimated for one or more sensing locations  30 . Examples of such parameters include temperature, pressure, vibration, force, strain and deformation of downhole components, chemical composition of downhole fluids or the formation, acoustic events, and others. 
         [0038]    The systems and methods described herein provide various advantages over prior art techniques. The systems and methods provide a mechanism for compensating for or reducing/nullifying effects such as reduced signal-to-noise ratios (SNRs) due to increases in sensing lengths. For example, the systems and methods increase the measurement utility of OFDR systems by introducing parallel mixing operations on the same reflected and/or backscattered optical signal, which enables an increase in the effective number of sensors that can be monitored with an acceptable SNR and/or an increase in the SNR of each of the measurements, improving measurement fidelity. This can permit an effectively higher sweep rate for a given signal generation circuit, while enabling the analysis of several segments of the DUT in parallel. 
         [0039]    In support of the teachings herein, various analysis components may be used, including a digital and/or an analog system. Components of the system, such as the measurement unit  20  or  30 , the processor  28  and other components of the system  10 , may have components such as a processor, storage media, memory, input, output, communications link, user interfaces, software programs, signal processors (digital or analog) and other such components (such as resistors, capacitors, inductors and others) to provide for operation and analyses of the apparatus and methods disclosed herein in any of several manners well appreciated in the art. It is considered that these teachings may be, but need not be, implemented in conjunction with a set of computer executable instructions stored on a computer readable medium, including memory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks, hard drives), or any other type that when executed causes a computer to implement the method of the present invention. These instructions may provide for equipment operation, control, data collection and analysis and other functions deemed relevant by a system designer, owner, user or other such personnel, in addition to the functions described in this disclosure. 
         [0040]    It will be recognized that the various components or technologies may provide certain necessary or beneficial functionality or features. Accordingly, these functions and features as may be needed in support of the appended claims and variations thereof are recognized as being inherently included as a part of the teachings herein and a part of the invention disclosed. 
         [0041]    While the invention has been described with reference to exemplary embodiments, it will be understood 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 will be appreciated to adapt a particular instrument, 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 appended claims.