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CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a U.S. National Stage Application of International Application No. PCT/US2013/057702 filed Aug. 30, 2013, which is incorporated herein by reference in its entirety for all purposes. 
     BACKGROUND 
     This disclosure generally relates to monitoring of hydrocarbon wellbores. In particular, this disclosure relates to systems and methods for monitoring a wellbore using Distributed Acoustic Sensing (DAS). 
     When performing subterranean operations, acoustic sensing may be used to measure many important properties and conditions of a wellbore, pipeline, other conduit/tube, or fluids used. For example, when performing subterranean operations, it may be desirable to monitor a number of properties related to the subterranean formation and/or conduits used downhole, including, but not limited to, pressure, temperature, porosity, permeability, density, mineral content, electrical conductivity, and bed thickness. Further, certain properties of fluids used in conjunction with performance of subterranean operations, such as pressure, temperature, density, viscosity, chemical elements, and the content of oil, water, and/or gas, may also be important measurements. In addition, downhole-logging tools based on sonic well logging systems may be used to measure downhole properties such as formation porosity, location of bed boundaries and fluid interfaces, well casing condition, and behind casing cement location and bonding quality. Monitoring properties and conditions over time may have significant value during exploration and production activities. 
     A DAS system may be capable of producing the functional equivalent of 10s, 100s, or even 1000s of acoustic sensors. Properties of downhole formations surrounding or otherwise adjacent to a wellbore may be monitored over time based on the acoustic sensing. Further, hydrocarbon production may be controlled, or reservoirs may be managed based on the downhole formation properties sensed by in-well acoustic measurement methods using a DAS system. 
     Acoustic sensing based on DAS may use the Rayleigh backscatter property of a fiber&#39;s optical core and may spatially detect disturbances that are distributed along the fiber length. Such systems may rely on detecting phase changes brought about by changes in strain along the fiber&#39;s core. Externally-generated acoustic disturbances may create very small strain changes to optical fibers. The acoustic disturbance may also be reduced or masked by a cable in which the fiber is deployed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These drawings illustrate certain aspects of certain embodiments of the present disclosure. They should not be used to limit or define the disclosure. 
         FIG. 1  depicts a hydrocarbon drilling site in accordance with one embodiment of the present disclosure. 
         FIG. 2  depicts a distributed acoustic sensing system. 
         FIG. 3  depicts a distributed acoustic sensing system in accordance with one embodiment of the present disclosure. 
         FIG. 4  depicts a distributed acoustic sensing system in accordance with an alternative embodiment of the present disclosure. 
         FIG. 5  depicts a distributed acoustic sensing system in accordance with an alternative embodiment of the present disclosure. 
         FIG. 6  depicts a distributed acoustic sensing system in accordance with an alternative embodiment of the present disclosure. 
         FIG. 7  depicts a distributed acoustic sensing system in accordance with an alternative embodiment of the present disclosure. 
     
    
    
     While embodiments of this disclosure have been depicted and described and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure. 
     DETAILED DESCRIPTION 
     Illustrative embodiments of the present disclosure are described in detail herein. In the interest of clarity, not all features of an actual implementation may be described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions may be made to achieve the specific implementation goals, which may vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of the present disclosure. 
     The terms “couple” or “couples” as used herein are intended to mean either an indirect or a direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect electrical or mechanical connection via other devices and connections. The term “upstream” as used herein means along a flow path towards the source of the flow, and the term “downstream” as used herein means along a flow path away from the source of the flow. The term “uphole” as used herein means along the drillstring or the hole from the distal end towards the surface, and “downhole” as used herein means along the drillstring or the hole from the surface towards the distal end. 
     It will be understood that the term “oil well drilling equipment” or “oil well drilling system” is not intended to limit the use of the equipment and processes described with those terms to drilling an oil well. The terms also encompass drilling natural gas wells or hydrocarbon wells in general. Further, such wells can be used for production, monitoring, or injection in relation to the recovery of hydrocarbons or other materials from the subsurface. This could also include geothermal wells intended to provide a source of heat energy instead of hydrocarbons. 
     For purposes of this disclosure, an information handling system may include any instrumentality or aggregate of instrumentalities operable to compute, classify, process, transmit, receive, retrieve, originate, switch, store, display, manifest, detect, record, reproduce, handle, or utilize any form of information, intelligence, or data for business, scientific, control, or other purposes. For example, an information handling system may be a personal computer, a network storage device, or any other suitable device and may vary in size, shape, performance, functionality, and price. The information handling system may include random access memory (“RAM”), one or more processing resources such as a central processing unit (“CPU”) or hardware or software control logic, ROM, and/or other types of nonvolatile memory. Additional components of the information handling system may include one or more disk drives, one or more network ports for communication with external devices as well as various input and output (“I/O”) devices, such as a keyboard, a mouse, and a video display. The information handling system may also include one or more buses operable to transmit communications between the various hardware components. 
     For the purposes of this disclosure, computer-readable media may include any instrumentality or aggregation of instrumentalities that may retain data and/or instructions for a period of time. Computer-readable media may include, for example, without limitation, storage media such as a direct access storage device (e.g., a hard disk drive or floppy disk drive), a sequential access storage device (e.g., a tape disk drive), compact disk, CD-ROM, DVD, RAM, ROM, electrically erasable programmable read-only memory (“EEPROM”), and/or flash memory; as well as communications media such as wires. 
     To facilitate a better understanding of the present disclosure, the following examples of certain embodiments are given. In no way should the following examples be read to limit, or define, the scope of the disclosure. Embodiments of the present disclosure may be applicable to horizontal, vertical, deviated, multilateral, u-tube connection, intersection, bypass (drill around a mid-depth stuck fish and back into the wellbore below), or otherwise nonlinear wellbores in any type of subterranean formation. Certain embodiments may be applicable, for example, to logging data acquired with wireline, slickline, and logging-while-drilling/measurement-while-drilling (LWD/MWD). Certain embodiments may be applicable to subsea and/or deep sea wellbores. Embodiments described below with respect to one implementation are not intended to be limiting. 
       FIG. 1  illustrates an example drilling system  100  according to aspects of the present disclosure. The drilling system  100  includes a rig  101  located at a surface  111  and positioned above a wellbore  103  within a subterranean formation  102 . In certain embodiments, a drilling assembly  104  may be coupled to the rig  101  using a drill string  105 . In other embodiments, the drilling assembly  104  may be coupled to the rig  101  using a wireline or a slickline, for example. The drilling assembly  104  may include a bottom hole assembly (BHA)  106 . The BHA  106  may include a drill bit  109 , a steering assembly  108 , and a LWD/MWD apparatus  107 . A control unit  110  located at the surface  111  may include a processor and memory device, and may communicate with elements of the BHA  106 , in the LWD/MWD apparatus  107  and the steering assembly  108 . In certain implementations, the control unit  110  may be an information handling system. The control unit  110  may receive data from and send control signals to the BHA  106 . Additionally, at least one processor and memory device may be located downhole within the BHA  106  for the same purposes. The LWD/MWD apparatus  107  may log the formation  102  both while the wellbore  103  is being drilled, and after the wellbore is drilled to provide information regarding ongoing subterranean operations. The steering assembly  108  may include a mud motor that provides power to the drill bit  109 , and that is rotated along with the drill bit  109  during drilling operations. The mud motor may be a positive displacement drilling motor that uses the hydraulic power of the drilling fluid to drive the drill bit  109 . In accordance with an exemplary embodiment of the present disclosure, the BHA  106  may include an optionally non-rotatable portion. The optionally non-rotatable portion of the BHA  106  may include any of the components of the BHA  106  excluding the mud motor and the drill bit  109 . For instance, the optionally non-rotatable portion may include a drill collar, the LWD/MWD apparatus  107 , bit sub, stabilizers, jarring devices and crossovers. In certain embodiments, the steering assembly  108  may angle the drill bit  109  to drill at an angle from the wellbore  103 . Maintaining the axial position of the drill bit  109  relative to the wellbore  103  may require knowledge of the rotational position of the drill bit  109  relative to the wellbore  103 . 
     Referring now to  FIG. 2 , a system for performing Distributed Acoustic Sensing (DAS) is referenced generally by reference numeral  200 . The system  200  may be incorporated into the drilling assembly  104  and lowered downhole using a drill string, by wireline, slickline, coiled tubing, or by any other means known to those in the art having the benefit of this disclosure. Alternatively, the system  200  or a portion of the system  200  may be positioned downhole for permanent monitoring and coupled to the casing or tubing. The system  200  may be a single pulse coherent Rayleigh scattering system with a compensating inferometer but is not intended to be limited to such. 
     Still referring to  FIG. 2 , a pulse generator  214  may be coupled to a first coupler  210  using the optical fiber  212 . The pulse generator  214  may be located at any suitable location when performing subterranean operations. For instance, in some embodiments, the pulse generator  214  may be located at the surface of the wellbore  103 . The pulse generator  214  may include associated opto-electronics and laser. The first coupler  210  may be a traditional fused-type fiber optic splitter, a circulator, a PLC fiber optic splitter, or any other type of splitter known to those with ordinary skill in the art having the benefit of this disclosure. In other embodiments, the first coupler  210  may be a circulator. Optical pulses from the pulse generator  214  may be amplified using optical gain elements, such as any suitable amplification mechanisms including, but not limited to, Erbium Doped Fiber Amplifiers (EDFAs) or Semiconductor Optical Amplifiers (SOAs). 
     Still referring to  FIG. 2 , a second coupler  208  may be coupled to an interferometer  202 . The second coupler  208  may split light from the optical fiber  232  into two paths along a top interferometer arm  224  and a bottom interferometer arm  222 . In other words, the second coupler  208  may split the backscattered light (e.g., backscattered light  228 ) from the optical fiber  232  into a first backscattered pulse and a second backscattered pulse. The first backscattered pulse may be sent into the top interferometer arm  222 . The second backscattered pulse may be sent into the bottom interferometer arm  224 . The first and second backscattered pulses from the top and bottom interferometer arms  222 ,  224  are then re-combined at a third coupler  234  to form an interferometric signal. The first, second, and third couplers  210 ,  208 , and  232  may be a traditional fused-type fiber optic splitter, a PLC fiber optic splitter, or any other type of splitter known to those with ordinary skill in the art having the benefit of this disclosure. 
     The interferometer  202  may be used to determine the relative phase shift variations between the light in the top interferometer arm  224  and the bottom interferometer arm  222  as they recombine. The interferometric signal, i.e., the relative phase shift, will vary over the distance of the distributed optical fiber  226 , and the location of the interferometric signal can be determined using time of flight for the optical pulse  216 . In the illustrative embodiment of  FIG. 2 , the interferometer is a Mach-Zehnder interferometer, but it is not intended to be limited to such. For instance, in certain implementations, a Michelson interferometer or any other type of interferometer known to those of skill in the art having the benefit of this disclosure may also be used without departing from the scope of the present disclosure. 
     The interferometer  202  may be coupled to a photodetector assembly  220 . The photodetector assembly  220  may include associated optics and signal processing electronics (not shown). The photodetector assembly  220  may be a semiconductor electronic device that uses the photoelectric effect to convert light to electricity. The photodetector assembly  220  may be an avalanche photodiode or a pin photodiode but is not intended to be limited to such. As the light from the top interferometer arm  224  and the bottom interferometer arm  222  reach the third coupler  234 , the photodetector assembly  220  may convert the optical signal (i.e., the interferometric signal) to an electronic signal proportional to the acoustic signal along the distributed optical fiber  226 . The photodetector assembly  220  may be coupled to an information handling system  230 . The photodetector assembly  220  and information handling system  230  may be communicatively and/or mechanically coupled. A first device may be communicatively coupled to a second device if it is connected to the second device through a wired or wireless communication network which permits the transmission of information. Thus, the information handling system  230  may be located uphole, downhole, or at a remote location. The information handling system  230  may also be communicatively or mechanically coupled to the pulse generator  214 . 
     In operation of the system  200 , the pulse generator  214  may generate a first optical pulse  216  which is transmitted through the optical fiber  212  to the first coupler  210 . In certain implementations, the pulse generator  214  may be a laser. The first coupler  210  may direct the first optical pulse  216  through the optical fiber  226 . At least a portion of the optical fiber  226  may be arranged in coils  218 . As the first optical pulse  216  travels through the optical fiber  226 , imperfections in the optical fiber  226  may cause a portion of the light to be backscattered along the optical fiber  226  due to Rayleigh scattering. Scattered light according to Rayleigh scattering is returned from every point along the optical fiber  226  along the length of the optical fiber  226  and is shown as backscattered light  228  in  FIG. 2 . This backscatter effect may be referred to as Rayleigh backscatter. Density fluctuations in the optical fiber  226  may give rise to energy loss due to the scattered light, with the following coefficient: 
               α   scat     =         8   ⁢           ⁢     π   3         3   ⁢     λ   4         ⁢     n   8     ⁢     p   2     ⁢     kT   f     ⁢   β           
where n is the refraction index, p is the photoelastic coefficient of the optical fiber  226 , k is the Boltzmann constant, and β is the isothermal compressibility. T f  is a fictive temperature, representing the temperature at which the density fluctuations are “frozen” in the material. The optical fiber  226  may be terminated with a low reflection device (not shown). In certain implementations, the low reflection device (not shown) may be a fiber coiled and tightly bent to violate Snell&#39;s law of total internal reflection such that all the remaining energy is sent out of the fiber. In other implementations, the low reflection device (not shown) may be an angle cleaved fiber. In still other implementations, the low reflection device (not shown) may be a coreless optical fiber with high optical attenuation. In still other implementations, the low reflection device (not shown) may be a termination, such as the AFL Endlight.
 
     The backscattered light  228  may travel back through the optical fiber  226 , until it reaches the second coupler  208 . The first coupler  210  may be mechanically coupled to the second coupler  208  on one side by the optical fiber  232  such that the backscattered light  228  may pass from the first coupler  210  to the second coupler  208  through the optical fiber  232 . The second coupler  208  may split the backscattered light  228  based on the number of interferometer arms so that one portion of any backscattered light  228  passing through the interferometer  202  travels through the top interferometer arm  224  and another portion travels through the bottom interferometer arm  222 . In other words, the second coupler  208  may split the backscattered light from the optical fiber  232  into a first backscattered pulse and a second backscattered pulse. The first backscattered pulse may be sent into the top interferometer arm  222 . The second backscattered pulse may be sent into the bottom interferometer arm  224 . These two portions may be re-combined at the third coupler  234 , and at that point, they may generate an interferometric signal. In an interferometric signal, two signals are superimposed from points separated by a distance of L, where L is the difference in length between the top interferometer arm  224  and bottom interferometer arm  222 . The output from the compensating interferometer  202 , or the interferometric signal, includes backscattered interfered light from two positions. This interferometric signal may reach the photodetector assembly  220 , where it may be converted to an electrical signal. The photodetector assembly  220  may integrate or add up the number of photons received in a given time period. The photodetector assembly  220  may provide output relating to the backscattered light  228  to the information handling system  230 , which may convey the data to a display and/or store it in computer-readable media. 
     Referring now to  FIG. 3 , an exemplary system for performing Distributed Acoustic Sensing (DAS) is referenced generally by reference numeral  300 . A DAS interrogation unit  310  includes the information handling system  230 , the pulse generator  214  coupled to the information handling system  230 , the photodetector assembly  220  coupled to the information handling system  230 , and an interferometer  302  coupled to the photodetector assembly  220 . As shown in  FIG. 3 , the optical fiber  226  may be disposed between the interferometer  302  and the pulse generator  214  but other configurations are possible. The optical fiber  226  may be lowered downhole but the DAS interrogation unit  310  may be located at the surface. Specifically, the optical fiber  226  may be coupled to a casing or tubing. 
     Still referring to  FIG. 3 , the system  300  may include the interferometer  302 . The interferometer  302  may include three or more interferometer arms  304   a -N that may be selectively engaged. Each interferometer arm  304   a -N may be coupled to an optical gain element  306   a -N, and each optical gain element  306   a -N may be coupled to the information handling system  230 . The interferometer arms  304   a -N may each be of a different length. The interferometer arms  304   a -N may be arranged in coils. However, the disclosure is not intended to be limited to any number or combination of coils. An optical gain element  306   a -N may include any amplifier of optical transmissions that uses any suitable means to achieve desired gains and/or any desired attenuation element that may prohibit light from passing through the selected interferometer arms. An example of an attenuation element is a Variable Optical Attenuator (VOA). For instance, in certain implementations, a semiconductor optical amplifier or rare earth doped fiber or any other optical amplification medium known to those with skill in the art may be used to achieve gains. In some embodiments, the optical amplification medium may be replaced with VOAs that may be used to attenuate selected interferometer arms while allowing light to pass through other interferometer arms with minimum attenuation. 
     Still referring to  FIG. 3 , the interferometer  302  may be communicatively and/or mechanically coupled to a photodetector assembly  220 . The photodetector assembly  220  may include associated optics and signal processing electronics. The photodetector assembly  220  may be coupled to an information handling system  230 . The information handling system  230  may be located downhole, uphole, or at a remote location. A second coupler  208  may be part of the interferometer  302 . A first coupler  210  may be coupled at one side to the second coupler  208  and at the other side, to an optical fiber  212 . A pulse generator  214  may be coupled to the first coupler  210  using the optical fiber  212 . The pulse generator  214  may include associated opto-electronics and a laser but is not intended to be limited to such. The pulse generator  214  may be located at any suitable location when performing subterranean operations. For instance, in some embodiments, the pulse generator  214  may be located at the surface of the wellbore  103 . 
     In operation of the system  300 , the pulse generator  214  may generate a first optical pulse  216  which is transmitted through the optical fiber  212  to the first coupler  210 . The optical pulse may be optically amplified using optical gain elements, for example, Erbium Doped Fiber Amplifiers (EDFAs) or Semiconductor Optical Amplifiers (SOAs). The first coupler  210  may direct the first optical pulse  216  through the optical fiber  226 . At least a portion of the optical fiber  226  may be arranged in coils  218 . As the pulse  216  travels through the optical fiber  226 , imperfections in the optical fiber  226  may cause light to be reflected back along the optical fiber  226 . Backscattered light  228  according to Rayleigh scattering may be returned from every point along the optical fiber  226  along the length of the optical fiber  226 . This backscatter effect may be referred to as Rayleigh backscatter. The optical fiber  226  may be terminated with a low reflection device (not shown). In certain implementations, the low reflection device (not shown) may be a fiber coiled and tightly bent to violate Snell&#39;s law of total internal reflection such that all the remaining energy is sent out of the fiber. In other implementations, the low reflection device (not shown) may be an angle cleaved fiber. In still other implementations, the low reflection device (not shown) may be a coreless optical fiber with high optical attenuation. In still other implementations, the low reflection device (not shown) may be a termination, such as the AFL Endlight. 
     Still referring to  FIG. 3 , the backscattered light  228  may travel back through the optical fiber  226  until it reaches the second coupler  208 . The second coupler  208  may be coupled to an interferometer  302 . The second coupler  208  may split the backscattered light  228  from the optical fiber  232  into various paths along the interferometer arms  304   a -N. Two of the optical gain elements  306   a -N may be active and turned on to allow light to pass through, and they may provide gain on two selected interferometer arms (for example,  304   a  and  304   b ) while all the other optical gain elements may be turned off to provide high attenuation. Thus, there may be high optical attenuation in the remaining interferometer arms (for example,  304   c -N). The two active optical paths will form an interferometer, and the difference in path length will be dependent on which optical gain elements  306   a -N are active and which optical gain elements  306   a -N are turned off. Thus, the second coupler  208  may split the backscattered light from the optical fiber  232  into a number of backscattered pulses, based on the number of interferometer arms in the interferometer  302 . A first backscattered pulse may be sent into a top interferometer arm. A second backscattered pulse may be sent into a bottom interferometer arm. The interferometer arms  304   a -N may then be re-combined at a third coupler  234 , and the first and second backscattered pulses from the selected active interferometer arms may be re-combined to form an interferometric signal. The interferometric signal is comprised of backscattered interfered light. In an interferometric signal, two signals are superimposed from points separated by a distance of L, where L is the difference in length between the top interferometer arm and bottom interferometer arm. The interferometric signal (i.e., the backscattered interfered light) may be representative of a downhole condition. For example, the downhole condition may include, but is not limited to: perforating, operating downhole hardware, monitoring downhole pumps, sensing acoustic signals during fracturing and in-flow stimulation, water injection, production monitoring, flow regimes, reflection seismic, micro-seismic, and acoustic events related to wellbore integrity (e.g., leaks, cross-flow, and formation compaction). The interferometric signal may also be representative of a condition on pipelines, flow-lines and risers related to flow, leaks, integrity, pigging and maintenance. Further, the interferometric signal may also be representative of conditions on subsea equipment where rotating equipment may cause vibration and/or acoustic noise. Similarly, the interferometric signal may be representative of a condition on infrastructure and security monitoring where it may be beneficial to dynamically vary the optical path length in the system  300 . 
     Still referring to  FIG. 3 , the photodetector assembly  220  may convert the interferometric signal (i.e., an optical signal) to an electrical signal proportional to the acoustic signal along the distributed optical fiber  226 . The photodetector assembly  220  may be an avalanche photodiode or a pin photodiode but is not intended to be limited to such. The photodetector assembly  220  may include associated optics and signal processing electronics that may be used to measure the voltage of the light incoming from the interferometer  202 . The photodetector assembly  220  may be coupled to an information handling system  230 . The photodetector assembly  220  and information handling system  230  may be communicatively and/or mechanically coupled. Thus, the information handling system  230  may be located uphole, downhole, or at a remote location. The information handling system  230  may also be communicatively and/or mechanically coupled to the pulse generator  214 . The photodetector assembly  220  may integrate or add up the number of photons received in a given time period. The photodetector assembly  220  may provide output relating to the back reflected light to the information handling system  230 , which may convey the data to a display and/or store it in computer-readable media. 
     The optical pulse  216  may travel down the length of the optical fiber  226  while generating backscattered light  228  from various positions along the length of the optical fiber  226 . The time at which the optical pulse  216  is sent from the pulse generator  214 , and the time it takes for the backscattered light  228  to travel to the photodetector assembly  220  may be measured accurately. The velocity of the optical pulse  216  as it travels down the optical fiber  226  may be well-known. The location of any backscattered light  228  may then simply be calculated by measuring the time at which it reaches the photodetector assembly  220 , i.e., a time of flight measurement. Using contiguous readings over the time it takes for the backscattered light  228  to traverse the optical fiber  218 , a measurement may be collected at the photodetector assembly  220  relating to how the back reflected light varies over the length of the optical fiber  226 . 
     The interferometer arms  304   a -N may each be of a different length. Thus, various combinations of optical gain element  306   a -N may be selectively activated such that the backscattered light  228  may travel through them and the interferometer arm  304   a -N coupled to them, thereby varying the distance over which the reflected optical pulse  228  may travel. Each optical gain element  306   a -N may be communicatively coupled to a control unit (not shown) such that a user may select which optical gain elements  306   a -N may be engaged at any given time. In certain implementations, the control unit may be an information handling system. Alternatively, the optical gain elements  306   a -N may be engaged according to an automated program. Thus, the sensitivity and spatial resolution of the system  300  may be changed in-situ depending on the needs of the system  300 . Applications where active sources are used may generate strong acoustic signals, and users may prefer to have the system settings selected to provide higher spatial resolution with good signal-to-noise ratios. The well depth as well as the associated signal paths may vary. Thus, shallow applications may have a stronger signal, whereas signals in deep wells may experience higher signal attenuation due to the longer travel path for acoustic signals. It may therefore be beneficial to change the difference in path length to optimize the signal-to-noise ratio dependent on the attenuation of the acoustic signals or on the application. Other applications may include micro-seismic sensing and/or passive sensing where small micro-seismic events in the formation may generate noise, and it may be beneficial to record these events and use them for reservoir characterization and optimization. 
     The term “spatial resolution” as used herein refers to the ability to discriminate between two adjacent acoustic events along an optical fiber. It is generally desirable to have a fine spatial resolution in a system to allow for detection of events that are spatially near each other, like perforations in a hydrocarbon well, for example. The spatial resolution of the system  300  is a function of the width of the first optical pulse  216  and the difference in length between the top interferometer arm, which may be any of  304   a - 304 (N−1) and the bottom interferometer arm, which may be any of  304   b - 304 N, depending on which of the arms in the system have activated optical gain elements  306   a -N. The sensitivity of the system  300  is a function of the difference in length between the top interferometer arm and the bottom interferometer arm, and a greater difference in length between these two fibers improves the system&#39;s sensitivity to acoustic and/or vibrational energy. In other words, greater sensitivity allows the system  300  to detect acoustic and/or vibrational events with smaller signal amplitude. 
     Additional optical pulses may be sent into the optical fiber  226  from the pulse generator  214  in close succession and at a fixed rate. By measuring the backscattered interfered light from each of these optical pulses at the photodetector assembly  220 , a discrete representation of the change in acoustic energy in the wellbore may be measured as a function of time. The changes in acoustic energy may then be correlated with sub-surface events. For example, a change in acoustic energy may be related to a change in flow, a change in solids in a fluid, or a change in the oil/water/gas ratio present in the wellbore  103 . The pulse generator  214  may be operable to vary the pulse width of optical pulses it generates. Further, the differential path length difference between two selected interferometer arms may be varied. In this way, the spatial resolution of the system  300  may be varied. 
     Referring now to  FIG. 4 , an exemplary system for performing Distributed Acoustic Sensing (DAS) according to an alternative embodiment of the present disclosure is referenced generally by reference numeral  400 . As shown in  FIG. 4 , the interferometer  402  may be disposed between the pulse generator  214  and the optical fiber  226 , although other configurations are possible. The pulse generator  214  may generate a single pulse that may be split in the first coupler  420  into N paths according to the number of active arms in the interferometer  402  (i.e., those arms of interferometer  402  that allow light transmission). For example, two of the N paths may be active. In this example, a first optical pulse may be split into a number of portions, according to the number of arms in the interferometer  402 . A first portion of the first optical pulse may be sent into a first active arm of the interferometer  402 . A second portion of the first optical pulse may be sent into a second active arm of the interferometer  402 . The first portion and the second portion may then both reenter the optical fiber  408  at the second coupler  422 . The two portions may be separated in time by a delay proportional to the difference in path length between the selected interferometer arms. Both portions may generate backscattered light as they travel down the optical fiber  226 . The backscattered light from the first portion may then interfere with the backscattered light from the second portion. The two portions of backscattered light may interfere in the optical fiber  226 , and they may travel in the optical fiber  226  to the photodetector assembly  220 , where the backscattered interfered light may be converted to an electrical signal. As discussed with respect to  FIG. 3 , the backscattered interfered light may be representative of a downhole condition. The downhole condition may include, for example, perforating, operating downhole hardware, monitoring downhole pumps, sensing acoustic signals during fracturing and in-flow stimulation, water injection, production monitoring, flow regimes, reflection seismic, micro-seismic, and acoustic events related to wellbore integrity, like, e.g., leaks, cross-flow, formation compaction. The interferometric signal may also be representative of a condition on pipelines, flow-lines and risers related to flow, leaks, integrity, pigging and maintenance. The interferometric signal may also be representative of conditions on subsea equipment where rotating equipment cause vibration and/or acoustic noise. Similarly, the interferometric system signal may be representative of a condition on infrastructure and security monitoring where it may be beneficial to dynamically vary the optical path length in the system  400 . The spatial resolution and sensitivity of the system  400  may be tuned by changing which optical gain elements  406   a -N are active. As discussed with respect to  FIG. 3 , the pulse generator  214  may be operable to vary the optical pulse width. Further, the differential path length difference between two selected interferometer arms may be varied. In this way, the spatial resolution of the system  400  may be varied. 
     Referring now to  FIG. 5 , an exemplary system for performing Distributed Acoustic Sensing (DAS) according to an alternative embodiment of the present disclosure is referenced generally by reference numeral  500 . A DAS interrogation unit  310 , such as that shown in  FIG. 3 , includes the information handling system  230 , the pulse generator  214  coupled to the information handling system  230 , the photodetector assembly  220  coupled to the information handling system  230 , and an interferometer  302  coupled to the photodetector assembly  220 . As shown in  FIG. 5 , the optical fibers  226  may be disposed between the interferometer  302  and the pulse generator  214 , but other configurations are possible. The optical fibers  226  may be lowered downhole but the DAS interrogation unit  310  may be located at the surface. Specifically, the optical fibers  226  may be coupled to a casing or tubing. 
     Like system  300  of  FIG. 3 , system  500  may include the interferometer  302 . The interferometer  302  may include three or more interferometer arms  304   a -N that may be selectively engaged. Each interferometer arm  304   a -N may be coupled to an optical gain element  306   a -N, and each optical gain element  306   a -N may be coupled to the information handling system  230 . The interferometer arms  304   a -N may each be of a different length. The interferometer arms  304   a -N may be arranged in coils. However, the disclosure is not intended to be limited to any number or combination of coils. An optical gain element  306   a -N may include any amplifier of optical transmissions that uses any suitable means to achieve desired gains and/or any desired attenuation element that may prohibit light from passing through the selected interferometer arms. An example of an attenuation element is a Variable Optical Attenuator (VOA). For instance, in certain implementations, a semiconductor optical amplifier or rare earth doped fiber or any other optical amplification medium known to those with skill in the art may be used to achieve gains. In some embodiments, the optical amplification medium may be replaced with VOAs that may be used to attenuate selected interferometer arms while allowing light to pass through other interferometer arms with minimum attenuation. 
     Referring to  FIG. 5 , the interferometer  302  may be communicatively and/or mechanically coupled to a photodetector assembly  220 . The photodetector assembly  220  may include associated optics and signal processing electronics. The photodetector assembly  220  may be coupled to an information handling system  230 . The information handling system  230  may be located downhole, uphole, or at a remote location. A second coupler  208  may be part of the interferometer  302 . A first coupler  210  may be coupled at one side to the second coupler  208  and at the other side, to an optical fiber  212 . A pulse generator  214  may be coupled to the first coupler  210  using the optical fiber  212 . The pulse generator  214  may include associated opto-electronics and a laser but is not intended to be limited to such. The pulse generator  214  may be located at any suitable location when performing subterranean operations. For instance, in some embodiments, the pulse generator  214  may be located at the surface of the wellbore  103 . 
     In operation of the system  500 , the pulse generator  214  may generate a first optical pulse  216  which is transmitted through the optical fiber  212  to the first coupler  210 . The optical pulse may be optically amplified using optical gain elements, for example, Erbium Doped Fiber Amplifiers (EDFAs) or Semiconductor Optical Amplifiers (SOAs). The first coupler  210  may direct the first optical pulse  216  through the optical fibers  226   a  and  226   b . Before traveling through optical fibers  226   a  and  226   b , the light from the first optical pulse  216  may be directed through attenuator or gain elements such as VOAs  510   a  and  510   b  (corresponding to optical fibers  226   a  and  226   b , respectively). VOAs  510  may each be coupled to and controlled by information handling system  230  such that only one of VOAs  510  is activated. In other words, through activation of a particular VOA  510 , information handling system  230  may determine and select which of optical fibers  226   a  and  226   b  that the light from the first optical pulse  216  may travel through. As the pulse  216  travels through the selected optical fiber  226 , imperfections in the optical fiber may cause light to be reflected back along the optical fiber. Backscattered light according to Rayleigh scattering may be returned from every point along the optical fiber  226  along the length of the optical fiber  226 . This backscatter effect may be referred to as Rayleigh backscatter. Optical fibers  226   a  and  226   b  may be terminated with a low reflection device (not shown). In certain implementations, the low reflection device (not shown) may be a fiber coiled and tightly bent to violate Snell&#39;s law of total internal reflection such that all the remaining energy is sent out of the fiber. In other implementations, the low reflection device (not shown) may be an angle cleaved fiber. In still other implementations, the low reflection device (not shown) may be a coreless optical fiber with high optical attenuation. In still other implementations, the low reflection device (not shown) may be a termination, such as the AFL Endlight. 
     In particular implementations, one or both of optical fibers  226  may comprise a coating that, when exposed to an electromagnetic signal (e.g., electromagnetic signal  560  from electromagnetic source  550 ), causes the fiber to expand or contract. The electromagnetic signal  560  may come from any electromagnetic source  550 , which may be located adjacent to system  500 . For example, one or more electromagnetic sources may be deployed in adjacent wells to system  500 , on the surface of a well in which system  500  is deployed, or on the ocean floor near a well in which system  500  is deployed. Electromagnetic sources  550  may operate at a range of 0.1 Hz to 30 kHz in particular implementations. In some implementations, the coating may include a magnetostrictive material that may surround the optical fiber(s)  226  or may be bonded to the optical fiber(s)  226 . Some examples of magnetostrictive materials include Cobalt, Iron, Nickel, alloys of the preceding, METGLAS, and Terfenol-D. Examples of magnetostrictive materials that may expand when exposed to an electromagnetic signal (i.e., negative magnetostriction) include METGLAS, Iron, Permalloy, and Terfenol-D. Examples of magnetostrictive materials that may contract when exposed to an electromagnetic signal (i.e., negative magnetostriction) include Nickel, Cobalt, ferrites, Nickel ferrites, and Cobalt-doped Nickel ferrites. 
     In some implementations, only one fiber of optical fibers  226  may comprise a coating that causes it to expand when exposed to an electromagnetic signal (e.g., electromagnetic signal  560 ). For example, as shown in  FIG. 5 , optical fiber  226   a  may comprise a coating that causes it to expand when exposed to electromagnetic signal  560  (as shown by the dotted line extending the length of optical fiber  226   a  in  FIG. 5 ), while optical fiber  226   b  may not comprise any coating and may not expand or contract when exposed to the same electromagnetic signal  560  as optical fiber  226   a . In other implementations, one fiber of optical fibers  226  may comprise a coating that causes it to expand when exposed to electromagnetic signal  560 , while the other fiber of optical fibers  226  may comprise a coating that causes it to contract when exposed to the same electromagnetic signal  560 . For example, although not shown in  FIG. 7 , optical fiber  226   a  may comprise a coating that causes it to expand when exposed to an electromagnetic signal, while optical fiber  226   b  may comprise a coating that causes it to contract when exposed to the same electromagnetic signal as optical fiber  226   a.    
     In operation of system  500 , an electromagnetic signal  560  from electromagnetic source  550  may be applied to or received at each of optical fibers  226  simultaneously, causing optical fibers  226   a  and  226   b  to have different lengths (due to one or both of optical fibers  226  having a magnetostrictive coating as described above). For example, as shown in  FIG. 5 , optical fiber  226   a  may be coated with a magnetostrictive coating causing it to expand and lengthen when exposed to electromagnetic signal  560  (as shown by the dotted line in  FIG. 5 ), while optical fiber  226   b  may have no magnetostrictive coating and may not expand or contract in the presence of the electromagnetic signal  560 . Backscattered light from the first optical pulse  216  may then return back through the selected optical fiber  226  and through a second coupler  208 . The second coupler  208  may be coupled to an interferometer  302 . The second coupler  208  may split the backscattered light from the optical fiber  232  into various paths along the interferometer arms  304   a -N. As described above with respect to  FIG. 3 , two of the optical gain elements  306   a -N may be active and turned on to allow light to pass through, and they may provide gain on two selected interferometer arms (for example,  304   a  and  304   b ) while all the other optical gain elements may be turned off to provide high attenuation. Thus, there may be high optical attenuation in the remaining interferometer arms (for example,  304   c -N). The two active optical paths will form an interferometer, and the difference in path length will be dependent on which optical gain elements  306   a -N are active and which optical gain elements  306   a -N are turned off. Thus, the second coupler  208  may split the backscattered light from the optical fiber  232  into a number of backscattered pulses, based on the number of interferometer arms in the interferometer  302 . A first backscattered pulse may be sent into a top interferometer arm. A second backscattered pulse may be sent into a bottom interferometer arm. The interferometer arms  304   a -N may then be re-combined at a third coupler  234 , and the first and second backscattered pulses from the selected active interferometer arms may be re-combined to form an interferometric signal comprised of backscattered interfered light. 
     After the interferometric signal has passed through the third coupler  234 , the photodetector assembly  220  may convert the interferometric signal (i.e., an optical signal) to an electrical signal proportional to the acoustic signal along the distributed optical fibers  226   a  and  226   b . The photodetector assembly  220  may be an avalanche photodiode or a pin photodiode but is not intended to be limited to such. The photodetector assembly  220  may include associated optics and signal processing electronics that may be used to measure the voltage of the light incoming from the interferometer  202 . The photodetector assembly  220  may be coupled to an information handling system  230 . The photodetector assembly  220  and information handling system  230  may be communicatively and/or mechanically coupled. Thus, the information handling system  230  may be located uphole, downhole, or at a remote location. The information handling system  230  may also be communicatively and/or mechanically coupled to the pulse generator  214 . The photodetector assembly  220  may integrate or add up the number of photons received in a given time period. The photodetector assembly  220  may provide output relating to the back reflected light to the information handling system  230 , which may convey the data to a display and/or store it in computer-readable media. 
     The backscattered interfered light may be representative of a downhole condition. The downhole condition may include, for example, perforating, operating downhole hardware, monitoring downhole pumps, sensing acoustic signals during fracturing and in-flow stimulation, water injection, production monitoring, flow regimes, reflection seismic, micro-seismic, and acoustic events related to wellbore integrity, like, e.g., leaks, cross-flow, formation compaction. The interferometric signal may also be representative of a condition on pipelines, flow-lines and risers related to flow, leaks, integrity, pigging and maintenance. The interferometric signal may also be representative of conditions on subsea equipment where rotating equipment cause vibration and/or acoustic noise. Similarly, the interferometric system signal may be representative of a condition on infrastructure and security monitoring where it may be beneficial to dynamically vary the optical path length in the system  500 . The spatial resolution and sensitivity of the system  500  may be tuned by changing which optical gain elements  306   a -N are active. The pulse generator  214  may be operable to vary the optical pulse width. Further, the differential path length difference between two selected interferometer arms may be varied. In this way, the spatial resolution of the system  500  may be varied. 
     As described above with respect to  FIG. 3 , measuring the backscattered interfered light at the photodetector assembly  220  may allow a user to determine a discrete representation of the change in acoustic or vibrational energy in the wellbore as a function of time. Using system  500  as described above, a user may be able to measure backscattered interfered light coming from each of optical fibers  226   a  and  226   b  when each is exposed to an electromagnetic signal such as electromagnetic signal  560 , with each path being selected at different times (but still substantially close in time to each other such that acoustic or vibrational energy measurements are not substantially different from one another). For instance, a first path through one of the optical fibers  226  may be selected first with backscatter light measured from that path first, and a second path through the other optical fiber  226  may be selected second with backscatter light measured from that path second. Because of the difference in length of these paths when exposed to an electromagnetic signal, the backscattered light measured from the first path (e.g., through optical fiber  226   a  of  FIG. 5 ) and the backscattered light measured from the second path (e.g., through optical fiber  226   b  of  FIG. 5 ). The measurements from each path may comprise the same measure of acoustic and/or vibrational energy, but may comprise differing measures of the electromagnetic energy due to one or both fibers expanding or contracting due to the applied electromagnetic signal. 
     By varying the spatial resolution of system  500  as described above, a user may be able to measure resistivity of a formation at different distances along the optical fibers  226  and thus may be able to detect flooding of a well. In addition, by varying the spatial resolution as described above, a user may be able to vary the sensitivity of measurements and may thus measure resistivity of signals with varying strength. This is especially true if there are multiple electromagnetic sources adjacent to system  500  having varying distances between the positive and negative poles. 
     Referring now to  FIG. 6 , an exemplary system for performing Distributed Acoustic Sensing (DAS) according to an alternative embodiment of the present disclosure is referenced generally by reference numeral  600 . As shown in  FIG. 6 , the interferometer  402  may be disposed between the pulse generator  214  and the optical fibers  226   a  and  226   b , although other configurations are possible. The pulse generator  214  may generate a single pulse that may be split in the first coupler  420  into N paths according to the number of active arms in the interferometer  402  (i.e., those arms of interferometer  402  that allow light transmission). For example, two of the N paths may be active. In this example, a first optical pulse may be split into a number of portions, according to the number of arms in the interferometer  402 . A first portion of the first optical pulse may be sent into a first active arm of the interferometer  402 . A second portion of the first optical pulse may be sent into a second active arm of the interferometer  402 . The first portion and the second portion may then both reenter the optical fiber  408  at the second coupler  422 . The two portions may be separated in time by a delay proportional to the difference in path length between the selected interferometer arms. 
     As described with respect to  FIG. 5 , the light from the two portions of the first optical pulse  216  may be directed through attenuator or gain elements such as VOAs  510   a  and  510   b  (corresponding to optical fibers  226   a  and  226   b , respectively) before traveling through optical fibers  226   a  and  226   b . VOAs  610  may each be coupled to and controlled by information handling system  230  such that only one of VOAs  610  is activated. Thus, through activation of a particular VOA  610 , information handling system  230  may determine and select which of optical fibers  226   a  and  226   b  that the portions of the first optical pulse  216  may travel through. Similar to optical fibers  226  in  FIG. 5 , one or both of optical fibers  226  of system  600  may comprise a coating that, when exposed to an electromagnetic signal (e.g., electromagnetic signal  660  from electromagnetic source  650 ), causes the fiber to expand or contract. The electromagnetic signal  660  may come from any electromagnetic source  650 , which may be located adjacent to system  600 . In some implementations, the coating may include a magnetostrictive material that may surround the optical fiber(s)  226  or may be bonded to the optical fiber(s)  226 . In some implementations, only one fiber of optical fibers  226  may comprise a coating that causes it to expand when exposed to an electromagnetic signal (e.g., electromagnetic signal  660 ). In other implementations, one fiber of optical fibers  226  may comprise a coating that causes it to expand when exposed to electromagnetic signal  660 , while the other fiber of optical fibers  226  may comprise a coating that causes it to contract when exposed to the same electromagnetic signal  660 . 
     In operation of system  600 , an electromagnetic signal  660  from electromagnetic source  650  may be applied to or received at each of optical fibers  226  simultaneously, causing optical fibers  226   a  and  226   b  to have different lengths (due to one or both of optical fibers  226  having a magnetostrictive coating as described above). For example, as shown in  FIG. 6 , optical fiber  226   a  may be coated with a magnetostrictive coating causing it to expand and lengthen when exposed to electromagnetic signal  660  (as shown by the dotted line in  FIG. 6 ), while optical fiber  226   b  may have no magnetostrictive coating and may not expand or contract in the presence of the electromagnetic signal  660 . As the portions of the optical pulse travel down the selected optical fiber  226 , they may generate backscattered light. The backscattered light from the first portion may then interfere with the backscattered light from the second portion. The two portions of backscattered light may interfere in the selected optical fiber  226  and may travel toward the photodetector assembly  220 , where the backscattered interfered light may be converted to an electrical signal. 
     As discussed with respect to  FIG. 5 , the backscattered interfered light may be representative of a downhole condition, such as, for example, perforating, operating downhole hardware, monitoring downhole pumps, sensing acoustic signals during fracturing and in-flow stimulation, water injection, production monitoring, flow regimes, reflection seismic, micro-seismic, and acoustic events related to wellbore integrity, like, e.g., leaks, cross-flow, formation compaction. The interferometric signal may also be representative of a condition on pipelines, flow-lines and risers related to flow, leaks, integrity, pigging and maintenance. The interferometric signal may also be representative of conditions on subsea equipment where rotating equipment cause vibration and/or acoustic noise. Similarly, the interferometric system signal may be representative of a condition on infrastructure and security monitoring where it may be beneficial to dynamically vary the optical path length in the system  600 . The spatial resolution and sensitivity of the system  600  may be tuned by changing which optical gain elements  406   a -N are active. In addition, the pulse generator  214  may be operable to vary the optical pulse width. Further, the differential path length difference between two selected interferometer arms may be varied. In this way, the spatial resolution of the system  400  may be varied. 
     Using system  600  as described above, a user may be able to measure backscattered interfered light coming from each of optical fibers  226   a  and  226   b  when each is exposed to an electromagnetic signal such as electromagnetic signal  660 , with each path being selected at different times (but still substantially close in time to each other such that acoustic or vibrational energy measurements are not substantially different from one another). For instance, a first path through one of the optical fibers  226  may be selected first with backscattered light measured from that path first, and a second path through the other optical fiber  226  may be selected second with backscattered light measured from that path second. Because of the difference in length of these paths when exposed to an electromagnetic signal, the backscattered light measured from the first path (e.g., through optical fiber  226   a  of  FIG. 6 ) and the backscattered light measured from the second path (e.g., through optical fiber  226   b  of  FIG. 6 ). The measurements from each path may comprise the same measure of acoustic and/or vibrational energy, but may comprise differing measures of the electromagnetic energy due to one or both fibers expanding or contracting due to the applied electromagnetic signal. 
     By varying the spatial resolution of system  600  as described above, a user may be able to measure resistivity of a formation at different distances along the optical fibers  226 , and thus may be able to detect flooding of a well. In addition, by varying the spatial resolution as described above, a user may be able to vary the sensitivity of measurements and may thus measure resistivity of signals with varying strength. This is especially true if there are multiple electromagnetic sources adjacent to system  600  having varying distances between the positive and negative poles. 
     Referring now to  FIG. 7 , an exemplary system for performing Distributed Acoustic Sensing (DAS) according to an alternative embodiment of the present disclosure is referenced generally by reference numeral  700 . A DAS interrogation unit  710  includes the information handling system  230 , the pulse generator  214  coupled to the information handling system  230 , and the photodetector assembly  220  coupled to the information handling system  230 . DAS interrogation unit  710  may be coupled to optical fibers  726  as shown in  FIG. 7 . The optical fibers  726  may be lowered downhole but the DAS interrogation unit  710  may be located at the surface. Specifically, the optical fibers  726  may be coupled to a casing or tubing. 
     In particular implementations, pulse generator  214  may generate a pulse that travels through coupler  708 , creating pulses  716   a  and  716   b . Pulses  716   a  and  716   b  may then travel down optical fibers  726   a  and  726   b , respectively, and may each have the same pulse width. As pulses  716  travel through the optical fibers  726 , imperfections in the optical fibers  726  may cause a portion of the light to be backscattered along the optical fibers  726  due to Rayleigh scattering. Scattered light according to Rayleigh scattering is returned from every point along the optical fibers  726  along the length of the optical fibers  726 . This backscatter effect may be referred to as Rayleigh backscatter. The optical fibers  726  may be terminated with a low reflection device (not shown). In certain implementations, the low reflection device (not shown) may be a fiber coiled and tightly bent to violate Snell&#39;s law of total internal reflection such that all the remaining energy is sent out of the fiber. In other implementations, the low reflection device (not shown) may be an angle cleaved fiber. In still other implementations, the low reflection device (not shown) may be a coreless optical fiber with high optical attenuation. In still other implementations, the low reflection device (not shown) may be a termination, such as the AFL Endlight. 
     In particular implementations, one or both of optical fibers  726  may comprise a coating that, when exposed to an electromagnetic signal (e.g., electromagnetic signal  760  from electromagnetic source  750 ), causes the fiber to expand or contract. The electromagnetic signal  760  may come from any electromagnetic source  750 , which may be located adjacent to system  700 . For example, one or more electromagnetic sources may be deployed in adjacent wells to system  500 , on the surface of a well in which system  500  is deployed, or on the ocean floor near a well in which system  500  is deployed. In some implementations, the coating may include a magnetostrictive material that may surround the optical fiber(s)  726  or may be bonded to the optical fiber(s)  726 . Some examples of magnetostrictive materials include Cobalt, Iron, Nickel, alloys of the preceding, METGLAS, and Terfenol-D. Examples of magnetostrictive materials that may expand when exposed to an electromagnetic signal (i.e., negative magnetostriction) include METGLAS, Iron, Permalloy, and Terfenol-D. Examples of magnetostrictive materials that may contract when exposed to an electromagnetic signal (i.e., negative magnetostriction) include Nickel, Cobalt, ferrites, Nickel ferrites, and Cobalt-doped Nickel ferrites. 
     In some implementations, only one fiber of optical fibers  726  may comprise a coating that causes it to expand when exposed to an electromagnetic signal (e.g., electromagnetic signal  760 ). For example, as shown in  FIG. 7 , optical fiber  726   a  may comprise a coating that causes it to expand when exposed to electromagnetic signal  760  (as shown by the dotted line extending the length of optical fiber  726   a  in  FIG. 7 ), while optical fiber  726   b  may not comprise any coating and may not expand or contract when exposed to the same electromagnetic signal  760  as optical fiber  726   a . In other implementations, one fiber of optical fibers  726  may comprise a coating that causes it to expand when exposed to electromagnetic signal  760 , while the other fiber of optical fibers  726  may comprise a coating that causes it to contract when exposed to the same electromagnetic signal  760 . For example, although not shown in  FIG. 7 , optical fiber  726   a  may comprise a coating that causes it to expand when exposed to an electromagnetic signal, while optical fiber  726   b  may comprise a coating that causes it to contract when exposed to the same electromagnetic signal as optical fiber  726   a.    
     In particular implementations of system  700 , such as the one shown in  FIG. 7 , an electromagnetic signal  760  from electromagnetic source  750  may be applied to or received at each of optical fibers  726  simultaneously, causing optical fibers  726   a  and  726   b  to have different lengths (due to one or both of optical fibers  726  having a magnetostrictive coating as described above; as shown in  FIG. 7 , optical fiber  726   a  may be coated with a magnetostrictive coating while optical fiber  726   b  may have no magnetostrictive coating). Pulses  716   a  and  716   b  may be sent down optical fibers  726   a  and  726   b , respectively, through couplers  720   a  and  720   b , respectively. Backscattered light from pulses  716   a  and  716   b  may then return back through optical fibers  726   a  and  726   b , respectively, and through couplers  720   a  and  720   b , respectively, on its way toward coupler  730 . Coupler  730  may then combine the backscattered light from pulses  716   a  and  716   b , generating an interferometric signal that is then sent to photodetector  220 . The interferometric signal therefore includes backscattered, interfered light from two positions along the length of optical fibers  726 . The photodetector assembly  220  may then convert the interferometric signal to an electrical signal, which may be done in certain implementations by integrating or adding up the number of photons received in a given time period. The photodetector assembly  220  may then provide output relating to the interferometric signal to the information handling system  230 , which may convey the data output to a display and/or store it in computer-readable media. 
     In operation of system  700 , the information handling system  230  may compare the interferometric signal to determine a phase difference between the backscattered light coming from optical fiber  726   a  and the backscattered light coming from optical fiber  726   b . Because of the difference in distance caused by the electromagnetic signal, the phase difference between the backscattered light coming from optical fiber  726   a  and the backscattered light coming from optical fiber  726   b  may be proportional to the electromagnetic signal. This is because the backscattered light coming from optical fiber  726   a  and the backscattered light coming from optical fiber  726   b  may each comprise information about acoustic energy and/or vibrational energy in the near the optical fibers  726 , but may be out of phase because of the difference in length between the optical fibers  726   a  and  726   b . Thus, information handling system  230  may be able to determine a magnitude of the electromagnetic signal  760  applied to or received at the optical fibers  726  by determining the phase difference between the backscattered light coming from optical fiber  726   a  and the backscattered light coming from optical fiber  726   b.    
     In particular implementations of system  700 , spatial resolution of system  700  may be varied by varying the pulse width of optical pulses  716   a  and  716   b . For example, using lower pulse widths in pulses  716   a  and  716   b  may allow for shorter spatial resolution. However, lower pulse widths may cause a lower signal level in the backscattered light from optical fibers  726 . This may not be an issue at higher wellbore depths, but may be an issue at lower wellbore depths. Accordingly, information handling system  230  may increase the pulse width in order to have higher spatial resolution and better visibility into the lower wellbore depths. 
     Therefore, the present disclosure is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present disclosure may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present disclosure. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee.

Summary:
Systems and methods for distributed acoustic sensing based on coherent Rayleigh scattering are disclosed herein. A system comprises a pulse generator, optical fibers coupled to the pulse generator, an interferometer coupled to the optical fibers, a photodetector assembly coupled to the interferometer, and an information handling system configured to detect a difference in backscattered light from the optical fibers. A method comprises sending an optical pulses down optical fibers, receiving backscattered light from the optical pulses, combining the backscattered light from the optical pulses to form an interferometric signal, receiving the interferometric signal at a photodetector assembly, and determining a difference in the interferometric signal at an information handling system.