Patent Publication Number: US-2022236106-A1

Title: Topside Distributed Acoustic Sensing Interrogation Of Subsea Wells With A Single Optical Waveguide

Description:
BACKGROUND 
     Boreholes drilled into subterranean formations may enable recovery of desirable fluids (e.g., hydrocarbons) using a number of different techniques. A number of systems and techniques may be employed in subterranean operations to determine borehole and/or formation properties. For example, Distributed Acoustic Sensing (DAS) along with a fiber optic system may be utilized together to determine borehole and/or formation properties. Distributed fiber optic sensing is a cost-effective method of obtaining real-time, high-resolution, highly accurate temperature and strain (acoustic) data along at least a portion of the wellbore. In examples, discrete 8sensors, e.g., for sensing pressure and temperature, may be deployed in conjunction with the fiber optic cable. Additionally, distributed fiber optic sensing may eliminate downhole electronic complexity by shifting all electro-optical complexity to the surface within the interrogator unit. Fiber optic cables may be permanently deployed in a wellbore via single- or dual-trip completion strings, behind casing, on tubing, or in pumped down installations; or temporally via coiled tubing, slickline, or disposable cables. 
     Distributed sensing can be enabled by continuously sensing along the length of the fiber, and effectively assigning discrete measurements to a position along the length of the fiber via optical time-domain reflectometry (OTDR). That is, knowing the velocity of light in fiber, and by measuring the time it takes the backscattered light to return to the detector inside the interrogator, it is possible to assign a distance along the fiber. 
     Distributed acoustic sensing has been practiced for dry-tree wells, but has not been attempted in wet-tree (or subsea) wells, to enable interventionless, time-lapse reservoir monitoring via vertical seismic profiling (VSP), well integrity, flow assurance, and sand control. A subsea operation may utilize optical engineering solutions to compensate for losses accumulated through long (˜5 to 100 km) lengths of subsea transmission fiber, 10 km of in-well subsurface fiber, and multiple wet- and dry-mate optical connectors, splices, and optical feedthrough systems (OFS). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of the preferred examples of the disclosure, reference will now be made to the accompanying drawings in which: 
         FIG. 1  illustrate an example of a well measurement system in a subsea environment; 
         FIG. 2  illustrates an example of a DAS system; 
         FIG. 3  illustrate the example of a DAS system with lead lines. 
         FIG. 4  illustrates a schematic of another example DAS system; 
         FIG. 5  illustrates an example of a remote circulator arrangement; 
         FIG. 6  illustrates a graph for determining time for a light pulse to travel in a fiber optic cable; 
         FIG. 7  illustrates another graph for determining time for a light pulse to travel in a fiber optic cable; 
         FIG. 8  illustrates an example of a remote circulator arrangement; 
         FIG. 9  illustrates another graph for determining time for a light pulse to travel in a fiber optic cable; 
         FIG. 10A  illustrates a graph of sensing regions in the DAS system; 
         FIG. 10B  illustrates a graph with an active proximal circulator using an optimized DAS sampling frequency of 12.5 kHz; 
         FIG. 10C  illustrates a graph with a passive proximal circulator using an optimized DAS sampling frequency of 12.5 kHz; 
         FIG. 11  illustrates a graph of optimized sampling frequencies in the DAS system; 
         FIG. 12  illustrates an example of a workflow for optimizing the sampling frequencies of the DAS system; 
         FIGS. 13-26  illustrate different examples of the DAS system; 
         FIG. 27  illustrates an example of an interrogator in the DAS system; 
         FIG. 28  illustrates a schematic of the interrogator with a single photon detector; 
         FIG. 29  illustrates an examples of a single photon detector; 
         FIG. 30  illustrates another example of the interrogator; 
         FIG. 31A-31D  illustrates examples of a downhole fiber deployed in a wellbore; and 
         FIG. 32  illustrates an example of the well measurement system in a land-based operation. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates generally to a system and method for using fiber optics in a DAS system in a subsea operation. Subsea operations may present optical challenges which may relate to the quality of the overall signal in the DAS system with a longer fiber optical cable. The overall signal may be critical since the end of the fiber contains the interval of interest, i.e., the well and reservoir sections. To prevent a drop in signal-to-noise (SNR) and signal quality, the DAS system described below may increase the returned signal strength with given pulse power, decrease the noise floor of the receiving optics to detect weaker power pulses, maintain the pulse power as high as possible as it propagates down the fiber, increase the number of light pulses that can be launched into the fiber per second, and/or increase the maximum pulse power that can be used for given fiber length. 
       FIG. 1  illustrates an example of a well system  100  that may employ the principles of the present disclosure. More particularly, well system  100  may include a floating vessel  102  centered over a subterranean hydrocarbon bearing formation  104  located below a sea floor  106 . As illustrated, floating vessel  102  is depicted as an offshore, semi-submersible oil and gas drilling platform, but could alternatively include any other type of floating vessel such as, but not limited to, a drill ship, a pipe-laying ship, a tension-leg platforms (TLPs), a “spar” platform, a production platform, a floating production, storage, and offloading (FPSO) vessel, and/or the like. Additionally, the methods and systems described below may also be utilized on land-based drilling operations. A subsea conduit or riser  108  extends from a deck  110  of floating vessel  102  to a wellhead installation  112  that may include one or more blowout preventers  114 . In examples, riser  108  may also be referred to as a flexible riser, flowline, umbilical, and/or the like. Floating vessel  102  has a hoisting apparatus  116  and a derrick  118  for raising and lowering tubular lengths of drill pipe, such as a tubular  120 . In examples, tubular  120  may be a drill string, casing, production pipe, and/or the like. 
     A wellbore  122  extends through the various earth strata toward the subterranean hydrocarbon bearing formation  104  and tubular  120  may be extended within wellbore  122 . Even though  FIG. 1  depicts a vertical wellbore  122 , it should be understood by those skilled in the art that the methods and systems described are equally well suited for use in horizontal or deviated wellbores. During drilling operations, the distal end of tubular  120 , for example a drill sting, may include a bottom hole assembly (BHA) that includes a drill bit and a downhole drilling motor, also referred to as a positive displacement motor (“PDM”) or“mud motor.” During production operations, tubular  120  may include a DAS system. The DAS system may be inclusive of an interrogator  124 , umbilical line  126 , and downhole fiber  128 . Without any limitation any optical fiber utilized in interrogator  124 , umbilical line  126 , or downhole fiber  128  may be an ultra-low loss transmission fiber that has higher power handling capability before non-linearity. This is captured in the optical budget, bit is also chosen to enable higher gain from co-propagating Raman amplification. An ultra-low loss transmission fiber does not include higher doping or embedded reflective features along the length of the ultra-low transmission, characteristics that may be found in current fiber optic cables. These characteristics increase light scattering within fiber optic cable. The more doping and embedded reflective features within the fiber optic cable, the larger a Rayleigh scattering coefficient of the fiber optic cable will be, and vice versa. 
     Downhole fiber  128  may be permanently deployed in a wellbore via single- or dual-trip completion strings, behind casing, on tubing, or in pumped down installations. In examples, downhole fiber  128  may be temporarily deployed via coiled tubing, wireline, slickline, or disposable cables.  FIGS. 31A-31D  illustrate examples of different types of deployment of downhole fiber  128  in wellbore  122  (e.g., referring to  FIG. 1 ). As illustrated in  FIG. 31A , wellbore  122  deployed in formation  104  may include surface casing  3100  in which production casing  3102  may be deployed. Additionally, production tubing  3104  may be deployed within production casing  3102 . In this example, downhole fiber  128  may be temporarily deployed in a wireline system in which a bottom hole gauge  3108  is connected to the distal end of downhole fiber  128 . Further illustrated, downhole fiber  128  may be coupled to a fiber connection  3106 . Without limitation, fiber connection  3106  may attach downhole fiber  128  to umbilical line  126  (e.g., referring to  FIG. 1 ). Fiber connection  3106  may operate with an optical feedthrough system (itself comprising a series of wet- and dry-mate optical connectors) in the wellhead that optically couples downhole fiber  128  from the tubing hanger, to umbilical line  126  on the wellhead instrument panel. Umbilical line  126  may include an optical flying lead, optical distribution system(s), umbilical termination unit(s), and transmission fibers encapsulated in flying leads, flow lines, rigid risers, flexible risers, and/or one or more umbilical lines. This may allow for umbilical line  126  to connect and disconnect from downhole fiber  128  while preserving optical continuity between the umbilical line  126  and the downhole fiber  128 . 
       FIG. 31B  illustrates an example of permanent deployment of downhole fiber  128 . As illustrated in wellbore  122  deployed in formation  104  may include surface casing  3100  in which production casing  3102  may be deployed. Additionally, production tubing  3104  may be deployed within production casing  3102 . In examples, downhole fiber  128  is attached to the outside of production tubing  3104  by one or more cross-coupling protectors  3110 . Without limitation, cross-coupling protectors  3110  may be evenly spaced and may be disposed on every other joint of production tubing  3104 . Further illustrated, downhole fiber  128  may be coupled to fiber connection  3106  at one end and bottom hole gauge  3108  at the opposite end. 
       FIG. 31C  illustrates an example of permanent deployment of downhole fiber  128 . As illustrated in wellbore  122  deployed in formation  104  may include surface casing  3100  in which production casing  3102  may be deployed. Additionally, production tubing  3104  may be deployed within production casing  3102 . In examples, downhole fiber  128  is attached to the outside of production casing  3102  by one or more cross-coupling protectors  3110 . Without limitation, cross-coupling protectors  3110  may be evenly spaced and may be disposed on every other joint of production tubing  3104 . Further illustrated, downhole fiber  128  may be coupled to fiber connection  3006  at one end and bottom hole gauge  3008  at the opposite end. 
       FIG. 31D  illustrates an example of coiled tubing operation in which downhole fiber  128  may be deployed temporarily. As illustrated in  FIG. 31D , wellbore  122  deployed in formation  104  may include surface casing  3100  in which production casing  3102  may be deployed. Additionally, coiled tubing  3112  may be deployed within production casing  3102 . In this example, downhole fiber  128  may be temporarily deployed in a coiled tubing system in which a bottom hole gauge  3108  is connected to the distal end of downhole fiber. Further illustrated, downhole fiber  128  may be attached to coiled tubing  3112 , which may move downhole fiber  128  through production casing  3102 . Further illustrated, downhole fiber  128  may be coupled to fiber connection  3106  at one end and bottom hole gauge  3108  at the opposite end. During operations, downhole fiber  128  may be used to take measurements within wellbore  122 , which may be transmitted to the surface and/or interrogator  124  (e.g., referring to  FIG. 1 ) in the DAS system. 
     Additionally, within the DAS system, interrogator  124  may be connected to an information handling system  130  through connection  132 , which may be wired and/or wireless. It should be noted that both information handling system  130  and interrogator  124  are disposed on floating vessel  102 . Both systems and methods of the present disclosure may be implemented, at least in part, with information handling system  130 . Information handling system  130  may include any instrumentality or aggregate of instrumentalities operable to compute, estimate, 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  130  may be a processing unit  134 , a network storage device, or any other suitable device and may vary in size, shape, performance, functionality, and price. Information handling system  130  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  130  may include one or more disk drives, one or more network ports for communication with external devices as well as an input device  136  (e.g., keyboard, mouse, etc.) and video display  138 . Information handling system  130  may also include one or more buses operable to transmit communications between the various hardware components. 
     Alternatively, systems and methods of the present disclosure may be implemented, at least in par, with non-transitory computer-readable media  140 . Non-transitory computer-readable media  140  may include any instrumentality or aggregation of instrumentalities that may retain data and/or instructions for a period of time. Non-transitory computer-readable media  140  may include, for example, 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, optical fibers, microwaves, radio waves, and other electromagnetic and/or optical carriers; and/or any combination of the foregoing. 
     Production operations in a subsea environment present optical challenges for DAS. For example, a maximum pulse power that may be used in DAS is approximately inversely proportional to fiber length due to optical non-linearities in the fiber. Therefore, the quality of the overall signal is poorer with a longer fiber than a shorter fiber. This may impact any operation that may utilize the DAS since the distal end of the fiber actually contains the interval of interest (i.e., the reservoir) in which downhole fiber  128  may be deployed. The interval of interest may include wellbore  122  and formation  104 . For pulsed DAS systems such as the one exemplified in  FIG. 2 , an additional challenge is the drop-in signal to noise ratio (SNR) associated with the decrease in the number of light pulses that may be launched into the fiber per second (pulse rate) when interrogating fibers with overall lengths exceeding 10 km. As such, utilizing DAS in a subsea environment may have to increase the returned signal strength with given pulse power, increase the maximum pulse power that may be used for given fiber optic cable length, maintain the pulse power as high as possible as it propagates down the fiber optic cable length, and increase the number of light pulses that may be launched into the fiber optic cable per second. 
       FIG. 32  illustrates an example of a land-based well system  3200 , which illustrates a coiled tubing operation. Without limitation, while a coiled tubing operation is shown, a wireline operation and/or the like may be utilized. As illustrated interrogator  124  is attached to information handling system  130 . Further discussed below, lead lines may connect umbilical line  126  to interrogator  124 . Umbilical line  126  may include a first fiber optic cable  304  and a second fiber optic cable  308  which may be individual lead lines. Without limitation, first fiber optic cable  304  and a second fiber optic cable  308  may attach to coiled tubing  3202  as umbilical line  126 . Umbilical line  126  may traverse through wellbore  122  attached to coiled tubing  3202 . In examples, coiled tubing  3202  may be spooled within hoist  3204 . Hoist  3204  may be used to raise and/or lower coiled tubing  3202  in wellbore  122 . Further illustrated in  FIG. 20 , umbilical line  126  may connect to distal circulator  312 , further discussed below. Distal circulator  312  may connect umbilical line  126  to downhole fiber  128 . 
       FIG. 2  illustrates an example of DAS system  200 . DAS system  200  may include information handling system  130  that is communicatively coupled to interrogator  124 . Without limitation, DAS system  200  may include a single-pulse coherent Rayleigh scattering system with a compensating interferometer. In examples, DAS system  200  may be used for phase-based sensing of events in a wellbore using measurements of coherent Rayleigh backscatter or may interrogate a fiber optic line containing an array of partial reflectors, for example, fiber Bragg gratings. 
     As illustrated in  FIG. 2 , interrogator  124  may include a pulse generator  214  coupled to a first coupler  210  using an optical fiber  212 . Pulse generator  214  may be a laser, or a laser connected to at least one amplitude modulator, or a laser connected to at least one switching amplifier, i.e., semiconductor optical amplifier (SOA). 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. Pulse generator  214  may be coupled to optical gain elements (not shown) to amplify pulses generated therefrom. Example optical gain elements include, but are not limited to, Erbium Doped Fiber Amplifiers (EDFAs) or Semiconductor Optical Amplifiers (SOAs). 
     DAS system  200  may include an interferometer  202 . Without limitations, interferometer  202  may include a Mach-Zehnder interferometer. For example, a Michelson interferometer or any other type of interferometer  202  may also be used without departing from the scope of the present disclosure. Interferometer  202  may include a top interferometer arm  224 , a bottom interferometer arm  222 , and a gauge  223  positioned on bottom interferometer arm  222 . Interferometer  202  may be coupled to first coupler  210  through a second coupler  208  and an optical fiber  232 . Interferometer  202  further may be coupled to a photodetector assembly  220  of DAS system  200  through a third coupler  234  opposite second coupler  208 . Second coupler  208  and third coupler  234  may be a traditional fused type fiber optic splitter, a PLC fiber optic splitter, or any other type of optical splitter known to those with ordinary skill in the art. Photodetector assembly  220  may include associated optics and signal processing electronics (not shown). Photodetector assembly  220  may be a semiconductor electronic device that uses the photoelectric effect to convert light to electricity. Photodetector assembly  220  may be an avalanche photodiode or a pin photodiode but is not intended to be limited to such. 
     When operating DAS system  200 , pulse generator  214  may generate a first optical pulse  216  which is transmitted through optical fiber  212  to first coupler  210 . First coupler  210  may direct first optical pulse  216  through a fiber optical cable  204 . It should be noted that fiber optical cable  204  may be included in umbilical line  126  and/or downhole fiber  128  (e.g.,  FIG. 1 ). As illustrated, fiber optical cable  204  may be coupled to first coupler  210 . As first optical pulse  216  travels through fiber optical cable  204 , imperfections in fiber optical cable  204  may cause a portion of the light to be backscattered along fiber optical cable  204  due to Rayleigh scattering. Scattered light according to Rayleigh scattering is returned from every point along fiber optical cable  204  along the length of fiber optical cable  204  and is shown as backscattered light  228  in  FIG. 2 . This backscatter effect may be referred to as Rayleigh backscatter. Density fluctuations in fiber optical cable  204  may give rise to energy loss due to the scattered light, α scat , with the following coefficient: 
     
       
         
           
             
               
                 
                   
                     α 
                     scat 
                   
                   = 
                   
                     
                       
                         8 
                         ⁢ 
                         
                           π 
                           3 
                         
                       
                       
                         3 
                         ⁢ 
                         
                           λ 
                           4 
                         
                       
                     
                     ⁢ 
                     
                       n 
                       8 
                     
                     ⁢ 
                     
                       p 
                       2 
                     
                     ⁢ 
                     k 
                     ⁢ 
                     
                       T 
                       f 
                     
                     ⁢ 
                     β 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where n is the refraction index, p is the photoelastic coefficient of fiber optical cable  204 , 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. Fiber optical cable  204  may be terminated with a low reflection device (not shown). In examples, 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 fiber optical cable  204 . 
     Backscattered light  228  may travel back through fiber optical cable  204 , until it reaches second coupler  208 . First coupler  210  may be coupled to second coupler  208  on one side by optical fiber  232  such that backscattered light  228  may pass from first coupler  210  to second coupler  208  through optical fiber  232 . Second coupler  208  may split backscattered light  228  based on the number of interferometer arms so that one portion of any backscattered light  228  passing through interferometer  202  travels through top interferometer arm  224  and another portion travels through bottom interferometer arm  222 . Therefore, second coupler  208  may split the backscattered light from optical fiber  232  into a first backscattered pulse and a second backscattered pulse. The first backscattered pulse may be sent into top interferometer arm  224 . The second backscattered pulse may be sent into bottom interferometer arm  222 . These two portions may be re-combined at third coupler  234 , after they have exited interferometer  202 , to form an interferometric signal. 
     Interferometer  202  may facilitate the generation of the interferometric signal through the relative phase shift variations between the light pulses in top interferometer arm  224  and bottom interferometer arm  222 . Specifically, gauge  223  may cause the length of bottom interferometer arm  222  to be longer than the length of top interferometer arm  224 . With different lengths between the two arms of interferometer  202 , the interferometric signal may include backscattered light from two positions along fiber optical cable  204  such that a phase shift of backscattered light between the two different points along fiber optical cable  204  may be identified in the interferometric signal. The distance between those points L may be half the length of the gauge  223  in the case of a Mach-Zehnder configuration, or equal to the gauge length in a Michelson interferometer configuration. 
     While DAS system  200  is running, the interferometric signal will typically vary over time. The variations in the interferometric signal may identify strains in fiber optical cable  204  that may be caused, for example, by seismic energy. By using the time of flight for first optical pulse  216 , the location of the strain along fiber optical cable  204  and the time at which it occurred may be determined. If fiber optical cable  204  is positioned within a wellbore, the locations of the strains in fiber optical cable  204  may be correlated with depths in the formation in order to associate the seismic energy with locations in the formation and wellbore. 
     To facilitate the identification of strains in fiber optical cable  204 , the interferometric signal may reach photodetector assembly  220 , where it may be converted to an electrical signal. The photodetector assembly may provide an electric signal proportional to the square of the sum of the two electric fields from the two arms of the interferometer. This signal is proportional to: 
         P ( t )= P 1+ P 2+2*√{square root over (( P 1 P 2)cos(ϕ1−ϕ2))}  (2)
 
     where P n  is the power incident to the photodetector from a particular arm (1 or 2) and ϕ n  is the phase of the light from the particular arm of the interferometer. Photodetector assembly  220  may transmit the electrical signal to information handling system  130 , which may process the electrical signal to identify strains within fiber optical cable  204  and/or convey the data to a display and/or store it in computer-readable media. Photodetector assembly  220  and information handling system  130  may be communicatively and/or mechanically coupled. Information handling system  130  may also be communicatively or mechanically coupled to pulse generator  214 . 
     Modifications, additions, or omissions may be made to  FIG. 2  without departing from the scope of the present disclosure. For example,  FIG. 2  shows a particular configuration of components of DAS system  200 . However, any suitable configurations of components may be used. For example, pulse generator  214  may generate a multitude of coherent light pulses, optical pulse  216 , operating at distinct frequencies that are launched into the sensing fiber either simultaneously or in a staggered fashion. For example, the photo detector assembly is expanded to feature a dedicated photodetector assembly for each light pulse frequency. In examples, a compensating interferometer may be placed in the launch path (i.e., prior to traveling down fiber optical cable  204 ) of the interrogating pulse to generate a pair of pulses that travel down fiber optical cable  204 . In examples, interferometer  202  may not be necessary to interfere the backscattered light from pulses prior to being sent to photo detector assembly. In one branch of the compensation interferometer in the launch path of the interrogating pulse, an extra length of fiber not present in the other branch (a gauge length similar to that of gauge  223 ) may be used to delay one of the pulses. To accommodate phase detection of backscattered light using DAS system  200 , one of the two branches may include an optical frequency shifter (for example, an acousto-optic modulator) to shift the optical frequency of one of the pulses, while the other may include a gauge. This may allow using a single photodetector receiving the backscatter light to determine the relative phase of the backscatter light between two locations by examining the heterodyne beat signal received from the mixing of the light from different optical frequencies of the two interrogation pulses. 
     In examples, DAS system  200  may generate interferometric signals for analysis by the information handling system  130  without the use of a physical interferometer. For instance, DAS system  200  may direct backscattered light to photodetector assembly  220  without first passing it through any interferometer, such as interferometer  202 . Alternatively, the backscattered light from the interrogation pulse may be mixed with the light from the laser originally providing the interrogation pulse. Thus, the light from the laser, the interrogation pulse, and the backscattered signal may all be collected by photodetector assembly  220  and then analyzed by information handling system  130 . The light from each of these sources may be at the same optical frequency in a homodyne phase demodulation system, or may be different optical frequencies in a heterodyne phase demodulator. This method of mixing the backscattered light with a local oscillator allows measuring the phase of the backscattered light along the fiber relative to a reference light source. 
       FIG. 3  illustrates an example of DAS system  200  system, which may be utilized to overcome challenges presented by a subsea environment. DAS system  200  may include interrogator  324 , umbilical line  126 , and downhole fiber  128 . As illustrated, interrogator  324  may include pulse generator  214  and photodetector assembly  220 , both of which may be communicatively coupled to information handling system  130 . Additionally, interferometers  202  may be placed within interrogator  324  and operate and/or function as described above.  FIG. 3  illustrates an example of DAS system  200  in which lead lines  300  may be used. As illustrated, an optical fiber  212  may attach pulse generator  214  to an output  302 , which may be a fiber optic connector. Umbilical line  126  may attach to output  302  with a first fiber optic cable  304 . First fiber optic cable  304  may traverse the length of umbilical line  126  to a remote circulator  306 . Remote circulator  306  may connect first fiber optic cable  304  to second fiber optic cable  308 . In examples, remote circulator  306  functions to steer light unidirectionally between one or more input and outputs of remote circulator  30 . Without limitation, remote circulators  306  are three-port devices wherein light from a first port is split internally into two independent polarization states and wherein these two polarization states are made to propagate two different paths inside remote circulator  306 . These two independent paths allow one or both independent light beams to be rotated in polarization state via the Faraday effect in optical media Polarization rotation of the light propagating through free space optical elements within the circulator thus allows the total optical power of the two independent beams to uniquely emerge together with the same phase relationship from a second port of remote circulator  306 . 
     Conversely, if any light enters the second port of remote circulator  300  in the reverse direction, the internal free space optical elements within remote circulator  306  may operate identically on the reverse direction light to split it into two polarizations states. After appropriate rotation of polarization states, these reverse in direction polarized light beams, are recombined, as in the forward propagation case, and emerge uniquely from a third port of remote circulator  306  with the same phase relationship and optical power as they had before entering remote circulator  306 . Additionally, as discussed below, remote circulator  306  may act as a gateway, which may only allow chosen wavelengths of light to pass through remote circulator  36  and pass to downhole fiber  128 . Second fiber optic cable  308  may attach umbilical line  126  to input  309 . Input  309  may be a fiber optic connector which may allow backscatter light to pass into interrogator  324  to interferometer  202 . Interferometer  202  may operate and function as described above and further pass back scatter light to photodetector assembly  220 . 
       FIG. 4  illustrates another example of DAS system  400 . As illustrated, interrogator  424  may include one or more DAS interrogator units  401 , each emitting coherent light pulses at a distinct optical wavelength, and a Raman Pump  402  connected to a wavelength division multiplexer  404  (WDM) with fiber stretcher. Without limitation, WDM  404  may include a multiplexer assembly that multiplexes the light received from the one or more DAS interrogator units  401  and a Raman Pump  402  onto a single optical fiber and a demultiplexer assembly that separates the multi-wavelength backscattered light into its individual frequency components and redirects each single-wavelength backscattered light stream back to the corresponding DAS interrogator unit  401 . In an example, WDM  404  may utilize an optical add-drop multiplexer to enable multiplexing the light received from the one or more DAS interrogator units  401  and a Raman Pump  402  and demultiplexing the multi-wavelength backscattered light received from a single fiber WDM  404  may also include circuitry to optically amplify the multi-frequency light prior to launching it into the signal optical fiber and/or optical circuitry to optically amplify the multi-frequency backscattered light returning from the single optical fiber, thereby compensating for optical losses introduced during optical (de-)multiplexing Raman Pump  402  may be a co-propagating optical pump based on stimulated Raman scattering, to feed energy from a pump signal to a main pulse from one or more DAS interrogator units  401  as the main pulse propagates down one or more fiber optic cables. This may conservatively yield a 3 dB improvement in SNR. As illustrated, Raman Pump  402  is located in interrogator  424  for co-propagation. In another example. Raman Pump  402  may be located topside after one or more remote circulators  306  either in line with first fiber optic cable  304  (co-propagation mode) and/or in line with second fiber optic cable  308  (counter-propagation). In another example, Raman Pump  402  is murinized and located after distal circulator  312  configured either for co-propagation or counter-propagation. In still another example, the light emitted by the Raman Pump  402  is remotely reflected by using a wavelength-selective filter beyond a remote circulator in order to provide amplification in the return path using a Raman Pump  402  in any of the topside configurations outlined above. 
     Further illustrated in  FIG. 4 , WDM  404  with fiber stretcher may attach proximal circulator  310  to umbilical line  126 . Umbilical line  126  may include one or more remote circulators  306 , a first fiber optic cable  304 , and a second fiber optic cable  308 . As illustrated, a first fiber optic cable  304  and as second fiber optic cable  308  may be separate and individual fiber optic cables that may be attached at each end to one or more remote circulators  306 . In examples, first fiber optic cable  304  and second fiber optic cable  308  may be different lengths or the same length and each may be an ultra-low loss transmission fiber that may have a higher power handling capability before non-literarily. This may enable a higher gain, co-propagation Raman amplification from interrogator  124 . 
     Deploying first fiber optic cable  304  and as second fiber optic cable  308  from floating vessel  102  (e.g., referring to  FIG. 1 ) to a subsea environment to a distal-end passive optical circulator arrangement, enables downhole fiber  128 , which is a sensing fiber, to be below a remote circulator  306  (e.g., well-only) that may be at the distal end of DAS system  400 . Higher (2-3×) pulse repetition rates, and non-saturated (non-back reflected) optical receivers may also be adjusted such that their dynamic range is optimized for downhole fiber  128 . This may approximately yield a 3.5 dB improvement in SNR. Additionally, downhole fiber  128  may be a sensing fiber that has higher Rayleigh scattering coefficient (i.e., higher doping) which may be result in a ten times improvement in backscatter, which may yield a 7-dB improvement in SNR. In examples, remote circulators  306  may further be categorized as a proximal circulator  310  and a distal circulator  312 . Proximal circulator  310  is located closer to interrogator  424  and may be located on floating vessel  102  or within umbilical line  126 . Distal circulator  312  may be further away from interrogator  424  than proximal circulator  310  and may be located in umbilical line  126  or within wellbore  122  (e.g., referring to  FIG. 1 ). As discussed above, a configuration illustrated in  FIG. 3  may not utilize a proximal circulator  310  with lead lines  300 . 
       FIG. 5  illustrates another example of distal circulator  312 , which may include two remote circulators  306 . As illustrated, each remote circulator  306  may function and operate to avoid overlap, at interrogator  124 , of backscattered light from two different pulses. For example, during operations, light at a first wavelength may travel from interrogator  124  down first fiber optic cable  304  to a remote circulator  306 . As the light passes through remote circulator  306  the light may encounter a Fiber Bragg Grating 500. In examples, Fiber Bragg Grating 500 may be referred to as a filter mirror that may be a wavelength specific high reflectivity filter mirror or filter reflector that may operate and function to recirculate unused light back through the optical circuit for “double-pass” co/counter propagation induced DAS signal gain at 1550 nm. In examples, this wavelength specific “Raman light” mirror may be a dichroic thin film interference filter, Fiber Bragg Grating 500, or any other suitable optical filter that passes only the 1550 nm forward propagating DAS interrogation pulse light while simultaneously reflecting most of the residual Raman Pump light. 
     Without limitation, Fiber Bragg Grating 500 may be set-up, fabricated, altered, and/or the like to allow only certain selected wavelengths of light to pass. All other wavelengths may be reflected back to the second remote circulator, which may send the reflected wavelengths of light along second fiber optic cable  308  back to interrogator  124 . This may allow Fiber Bragg Grating 500 to split DAS system  200  (e.g., referring to  FIG. 4 ) into two regions. A first region may be identified as the devices and components before Fiber Bragg Grating 500 and the second region may be identified as downhole fiber  128  and any other devices after Fiber Bragg Grating 500. 
     Splitting DAS system  200  (e.g., referring to  FIG. 4 ) into two separate regions may allow interrogator  124  (e.g., referring to  FIG. 1 ) to pump specifically for an identified region. For example, the disclosed system of  FIG. 4  may include one or more pumps, as described above, placed in interrogator  124  or after proximal circulator  310  at the topside either in line with first fiber optic cable  304  or second fiber optic cable  308  that may emit a wavelength of light that may travel only to a first region and be reflected by Fiber Bragg Grating 500. A second pump may emit a wavelength of light that may travel to the second region by passing through Fiber Bragg Grating 500. Additionally, both the first pump and second pump may transmit at the same time. Without limitation, there may be any number of pumps and any number of Fiber Bragg Gratings 500 which may be used to control what wavelength of light travels through downhole fiber  128 .  FIG. 5  also illustrates Fiber Bragg Gratings 500 operating in conjunction with any remote circulator  306 , whether it is a distal circulator  312  or a proximal circulator  310 . Additionally, as discussed below, Fiber Bragg Gratings 500 may be attached at the distal end of downhole fiber  218 . Other alterations to DAS system  200  (e.g., referring to  FIG. 4 ) may be undertaken to improve the overall performance of DAS system  200 . For example, the lengths of first fiber optic cable  304  and second fiber optic cable  308  selected to increase pulse repetition rate (expressed in terms of the time interval between pulses t rep ). 
       FIG. 6  illustrates an example of fiber optic cable  600  in which no remote circulator  306  may be used. As illustrated, at least a portion of fiber optic cable  600  is a sensor and the pulse interval may be greater than the time for the pulse of light to travel to the end of fiber optic cable  600  and its backscatter to travel back to interrogator  124  (e.g., referring to  FIG. 1 ). This is so, since in DAS systems  200  at no point in time, backscatter from more than one location along sensing fiber (i.e., downhole fiber  128 ) may be received. Therefore, the pulse interval t rep  may be greater than twice the time light takes to travel “one-way” down the fiber. Let t s  be the “two-way” time for light to travel to the end of fiber optic cable  600  and back, which may be written as t rep &gt;t s . 
       FIG. 7  illustrates an example of fiber optic cable  600  with a remote circulator  306  using the configuration shown in  FIG. 3 . When a remote circulator  306  is used, only the light traveling in fiber optic cable  600  that is allowed to go beyond remote circulator  306  and to downhole fiber  128  may be returned to interrogator  124  (e.g., referring to  FIG. 1 ), thus, the interval between pulses is dictated only by the length of the sensing portion, downhole fiber  128 , of fiber optic cable  600 . It should be noted that all light must travel “to” and “from” the sensing portion, downhole fiber  128 , with respect to pulse timing, what matters is the total length of fiber “to” and “from” remote circulator  306 . Therefore, first fiber optic cable  304  or second fiber optic cable  308  may be longer than the other, as discussed above. 
       FIG. 8  illustrates an example remote circulator arrangement  800  which may allow, as described above, configurations that use more than one remote circulator  306  close together at the remote location. Although remote circulator arrangement  800  may have any number of remote circulators  306 , remote circulator arrangement  800  may be illustrated as a single remote circulator  306 . 
       FIG. 9  illustrates an example first fiber optic cable  304  and second fiber optic cable  308  attached to a remote circulator  306  at each end. As discussed above, each remote circulator may be categorized as a proximal circulator  310  and a distal circulator  312 . When using a proximal circulator  310  and a distal circulator  312 , light from the fiber section before proximal circulator  310 , and light from the fiber section below the remote circulator  306  are detected, which is illustrated in  FIGS. 10 and 11 . There is a gap  1000  between them of “no light” that depends on the total length of fiber (summed) between proximal circulator  310  and a distal circulator  312 . 
     Referring back to  FIG. 9 , with t s1  the duration of the light from fiber sensing section before proximal circulator  310 , t sep  the “dead time” separating the two sections (and due to the cumulative length of first fiber optic cable  304  and second fiber optic cable  308  between proximal circulator  310  and a distal circulator  312 ), and t s2  the duration of the light from the sensing fiber, downhole fiber  128 , beyond distal circulator  312 , the constraints on fiber lengths and pulse intervals may be identified as: 
       ( i ) t   rep   &lt;t   sep   (3)
 
       ( ii )(2 t   rep )&gt;( t   s1   +t   sep   +t   s2 )  (4)
 
     Criterion (i) ensures that “pulse n” light from downhole fiber  128  does not appear while “pulse n+1” light from fiber before proximal circulator  310  is being received at interrogator  124  (e.g., referring to  FIG. 1 ). Criterion (ii) ensures that “pulse n” light from downhole fiber  128  is fully received before “pule n+2” light from fiber before proximal circulator  310  is being received at interrogator  124  is received. It should be noted that the two criteria given above only define the minimum and maximum t rep  for scenarios where two pulses are launched in the fiber before backscattered light below the remote circulator  306  is received. However, it should be appreciated that for those skilled in the art these criteria maybe generalized to cases where n∈{1,2,3, . . . } light pulses may be launched in the fiber before backscattered light below the remote circulator  306  is received. 
     The use of remote circulators  306  (e.g., referring to  FIG. 3 ) may allow for DAS system  200  (e.g., referring to  FIG. 3 ) to increase the sampling frequency.  FIG. 12  illustrates workflow  1200  for optimizing sampling frequency when using a remote circulator  306  in DAS system  200 . Workflow  1200  may begin with block  1202 , which determines the overall fiber length in both directions. For example, a 17 km of first fiber optic cable  304  (e.g., referring to  FIG. 3 ) and 17 km of second fiber optic cable  308  (e.g., referring to  FIG. 3 ) before distal circulator  312  (e.g., referring to  FIG. 3 ) and 8 km of sensing fiber, downhole fiber  128  (e.g., referring to  FIG. 3 ), after distal circulator  312 , the overall fiber optic cable length in both directions would be 50 km. Assuming a travel time of the light of 5 ns/m, the following equation may be used to calculate a first DAS sampling frequency f s   
     
       
         
           
             
               
                 
                   
                     f 
                     s 
                   
                   = 
                   
                     
                       1 
                       
                         t 
                         s 
                       
                     
                     = 
                     
                       1 
                       
                         5 
                         · 
                         
                           10 
                           
                             - 
                             9 
                           
                         
                         · 
                         z 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     where t s  is the DAS sampling interval and z is the overall two-way fiber length. Thus, for an overall two way fiber length of 50 km the first DAS sampling rate f s  is 4 kHz. In block  1204  regions of the fiber optic cable are identified for which backscatter is received. For example, this is done by calculating the average optical backscattered energy for each sampling location followed by a simple thresholding scheme. The result of this step is shown in  FIG. 10A  where boundaries  1002  identify two sensing regions  1004 . As illustrated in  FIG. 10 , optical energy is given as: 
         I   2   +Q   2   (6)
 
     where I and Q correspond to the in-phase (I) and quadrature (Q) components of the backscattered light. In block  1206 , the sampling frequency of DAS system  200  is optimized. To optimize the sampling frequency a minimum time interval is found that is between the emission of light pulses such that at no point in time backscattered light arrives back at interrogator  124  (e.g., referring to  FIG. 1 ) that corresponds to more than one spatial location along a sensing portion of the fiber-optic line. Mathematically, this may be defined as follows. Let S be the set of all spatial sample locations x along the fiber for which backscattered light is received. The desired light pulse emission interval t s  is the smallest one for which the cardinality of the two sets S and {mod(x, t s ):x∈S} is still identical, which is expressed as: 
     
       
         
           
             
               
                 
                   
                     
                       min 
                       
                         t 
                         s 
                       
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           t 
                           s 
                         
                         ) 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         s 
                         . 
                         t 
                         . 
                         
                             
                         
                         ⁢ 
                         
                            
                           S 
                            
                         
                       
                     
                   
                   = 
                   
                      
                     
                       { 
                       
                         
                           mod 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ( 
                             
                               x 
                               , 
                               
                                 t 
                                 s 
                               
                             
                             ) 
                           
                           ⁢ 
                           
                             : 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           x 
                         
                         ∈ 
                         S 
                       
                       } 
                     
                      
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     where |⋅| is the cardinality operator, measuring the number of elements in a set.  FIG. 11  shows the result of optimizing the sampling frequency from  FIG. 10  with workflow  1200 . Here, the DAS sampling frequency may increase from 4 kHz to 12.5 kHz without causing any overlap in backscattered locations, effectively increasing the signal to noise ratio of the underlying acoustic data by more than 5 dB due to the increase in sampling frequency. 
     Variants of DAS system  200  may also benefit from workflow  1200 . For example,  FIG. 13  illustrates DAS system  1300  in which proximal circulator  310  is placed within interrogator  1324 . This system set up of DAS system  1300  may allow for system flexibility on how to implement during measurement operations and the efficient placement of Raman Pump  402  in another illustrated example of DAS system  1400 , referring to  FIG. 14 . As illustrated in  FIGS. 13 and 14 , first fiber optic cable  304  and second fiber optic cable  308  may connect interrogator  124  to umbilical line  126 , which is described in greater detail above in  FIG. 3 . 
       FIG. 14  illustrates another example of DAS system  1400  having an interrogator  1424  in which Raman Pump  402  is operated in co-propagation mode and is attached to first fiber optic cable  304  after proximal circulator  310 . For example, if the first sensing region before proximal circulator  310  should not be affected by Raman amplification. Moreover, Raman Pump  402 , may also be attached to second fiber optic cable  308  which may allow the Raman Pump  402  to be operated in counter-propagation mode. In examples, the Raman Pump may also be attached to fiber  1401  between WDM  404  and proximal circulator  310  in interrogator  1424 . 
       FIG. 15  illustrates another example of DAS system  1500  in which an optical amplifier assembly  1501  (i.e., an Erbium doped fiber amplifier (EDFA)+Fabry-Perot filter) may be attached to proximal circulator  310 , which may also be identified as a proximal locally pumped optical amplifier. In examples, a distal optical amplifier assembly  1502  may also be attached at distal circulator  312  on first fiber optical cable  304  or second fiber optical cable  308  as an inline or “mid-span” amplifier. In examples, optical amplifier assembly  1502  located in-line with fiber optical cable  304  and above distal circulator  312  may be used to boost the light pulse before it is launched into the downhole fiber  128 . Referring to  FIGS. 10B and 10C , the effect of using an optical amplifier assembly  1501  in-line with a second fiber optic cable  308  prior to proximal circulator  310  and/or using an distal optical amplifier assembly  1502  located in line with second fiber optical cable  308  above distal circulator  312  may allow for selectively amplifying the backscattered light originating from downhole fiber  128  which tends to suffer from much stronger attenuation as it travels back along downhole fiber  128  and second fiber optical cable  308  than backscattered light originating from shallower sections of fiber optic cable that may also perform sensing functions.  FIG. 10B  illustrates measurements where proximal circulator  310  is active (optical amplifier assembly  1501  in-line with a second fiber optic cable  308  prior to proximal circulator  310  and/or distal optical amplifier assembly  1502  located in line with second fiber optical cable  308  above distal circulator  312  is used).  FIG. 10C  illustrates measurements where proximal circulator  310  is passive (no optical amplification is used in-line with second fiber optic cable  308 ). In  FIGS. 10B and 10C , boundaries  1002  identify two sensing regions  1004 . Additionally, in  FIGS. 10B and 10C  the DAS sampling frequency is set to 12.5 kHz using workflow  1200 . Further illustrated Fiber Bragg Grating 500 may also be disposed on first fiber optical cable  304  between distal optical amplifier assembly  1502  and distal circulator  312 . 
       FIG. 16  illustrates another example of DAS system  1600  in which proximal circulator  310  and distal circulator  312  are disposed within interrogator  1624 . Without limitation, proximal circulator  310  and distal circulator  312  may be disposed outside of interrogator  1624  as a separate device but position on but may still be disposed on floating vessel  102  (e.g., referring to  FIG. 1 ) and/or above the water or earth surface. Similar to  FIG. 4  above, interrogator  1624  may include one or more DAS interrogator units  401 , a Raman Pump  402 , and a WDM  404  all of which may operate and function according to the description above. As described above, interrogator  1624  is attached to umbilical line  126 , which is attached to downhole fiber  128 . Additionally, first fiber optic cable  304  and second fiber optic cable  308  may connect proximal circulator  310  and distal circulator  312 . As illustrated in  FIG. 16 , second fiber optic cable  308  may be connected to an optical shutter  1601  which protects an erbium-doped fiber amplifier (EDFA)  1602 . The output from EDFA  1602  may connect second fiber optic cable  308  to proximal circulator  310 . This example may allow for selective amplification, which may allow for the separation of the optical path into a discrete down and up going paths. The up going path being second fiber optic cable  308  and the down going path being first fiber optic cable  304 . In examples optical shutter  1601  is closed a down going pulse of light traverses through distal circulator  312  and may remain close until such time that a backscattered light from downhole fiber  128  approaches optical shutter  1601  in second fiber optic cable  308 . This may allow for selective amplification of light from downhole fiber  128  and may prevent all backscattered light, unless specifically chosen, from reaching EDFA  1602 . 
       FIG. 17  illustrates another example DAS system  1700  in which optical shutter  1601  is disposed in interrogator  1724  and between distal circulator  312  and umbilical line  126 . Additionally, EDFA  1602  is disposed on second fiber optic cable  308  between proximal circulator  310  and distal circulator  312 . Optical shutter  1601  and EDFA  1602  may still operate and function as described above in  FIG. 16 . 
       FIG. 18  illustrates another example DAS system  1800  in which, as illustrated in  FIG. 16 , optical shutter  1601  and EDFA  1602  are disposed between proximal circulator  310  and distal circulator  312  on second fiber optic cable  308 . In addition, Raman Pump  402  may be attached to WDM pump  1801  which is disposed on first fiber optic cable  304 . Raman Pump  402  and WDM pump  1801  may operate and function as described in  FIG. 14  above. 
       FIG. 19  illustrates an example including an optical amplifier  1900 . Optical amplifier  1900  may function and operate by stimulated optical emission within a semiconductor optical amplifier (SOA) using a material such as InGsAsP or by stimulated emission of excited erbium ions within an Erbium doped fiber amplifier (EDFA) or via non-linear optical energy conversion using stimulated Raman processes, whereby optical energy is added to the signal light in the optical domain. Optical amplifiers may be pumped or excited via direct electron injection in the case of the SOA or by local optical pumping, via laser diode, or remote all-optical pumping of an EDFA or Distributed Raman Amplification along the fiber itself. In example, optical amplifier  1900  is disposed in umbilical line  126 . Umbilical line  126  is attached to interrogator  124  at one end and downhole fiber  128  at the opposite end. As illustrated, optical amplifier  1900  may be attached to shortwave optical Pump laser  402  at 1480 nm by pump laser fiber  1902 , which may also be disposed in umbilical line  126 . As illustrated optical Pump laser  402  may operate and function to excite the optical gain medium within the optical amplifier  1900  by providing additional optical energy into the amplifier gain medium. 
       FIG. 20  illustrates an example of two optical amplifiers  1900  disposed in umbilical line  126 . In examples, each optical amplifier  1900  may be disposed in series or in parallel with each other. As illustrated, optical amplifiers  1900  are disposed in series, which may allow for reduction in non-linear amplification at each stage whereby the effective signal gain is provided via summing of multiple gain stages. As illustrated, each optical amplifier  1900  may be powered by a Raman Pump  402  connected by an individual pump laser fiber  1902 . In additional examples, a single Raman Pump  402  may be connected to each optical amplifier  1900  by a single pump laser fiber  1902  or two pump laser fibers  1902 . 
       FIG. 21  illustrates another example in which optical amplifier  1900  is disposed in umbilical line  126  and a proximal circulator  310  and a distal circulator  312  are disposed between interrogator  2124  and umbilical liner  126  on surface  2100 . Surface  2100  is defined as on vessel  102  (e.g., referring to  FIG. 1 ) or in any suitable place above a body of water. As discussed above in  FIGS. 16-18 , distal circulator  312  and proximal circulator  310  may be connected by a first fiber optic cable  304  and a second fiber optic cable  308 . As illustrated, optical shutter  1601  and EDFA  1602  are disposed between proximal circulator  310  and distal circulator  312  on second fiber optic cable  308 . In this example, optical shutter  1600 , EDFA  1602 , and optical amplifier  1900  operate and function as described above. 
       FIG. 22  illustrates an example DAS system  2200  that includes two optical amplifiers  1900  disposed in umbilical line  126  with proximal circulator  310  and distal circulator  312 . In examples distal circulator  312  and proximal circulator  310  may be connected by a first fiber optic cable  304  and a second fiber optic cable  308 . As illustrated, an optical amplifier  1900  is disposed in first fiber optic cable  304  and second fiber optic cable  308 . As noted above, each optical amplifier  1900  is powered by an optical Pump laser  402  that is connected to optical amplifier  1900  by a pump laser fiber  1902 . In this example, optical amplifiers  1900  may operate and function to provide quasi distributed signal gain at multiple locations along said fiber.  FIG. 23  illustrates the same setup as  FIG. 22  but in this example DAS system  2300  each optical amplifier  1900  is connected to a single pump laser fiber  1902 , which is connected to a single Raman Pump  402 .  FIG. 24  illustrates another example DAS system  2400  with the same setup as  FIG. 23 , however in this example proximal circulator  310  (e.g., referring got  FIG. 23 ) has been removed, leaving distal circulator  312 . In this example, the removal of proximal circulator  310  may allow for both forward and return amplification with the proximal circulator replaced with a second return fiber  2401 . 
       FIG. 25  illustrates an example DAS system  2500  that an optical amplifier  1900  disposed in umbilical line  126  with proximal circulator  310  and distal circulator  312 . As discussed above, optical amplifier  1900  is connected to the optical Pump laser  402  by pump laser fiber  1902 . In examples distal circulator  312  and proximal circulator  310  may be connected by a first fiber optic cable  304  and a second fiber optic cable  308 . As illustrated, an optical amplifier  1900  is disposed in first fiber optic cable  304 . In this example, optical amplifier  1900  may operate and function as a single stage remotely-pumped EDFA or other lumped optical amplifier along the outgoing transmission fiber. Remote pumping of lumped in-line amplifiers allows tailoring of signal strengths for optimum signal-to-noise ratios.  FIG. 26  illustrates another example DAS system  2600 , with a similar setup of  FIG. 25  with optical amplifier  1900  disposed in second fiber optic cable  308  and not in first fiber optic cable  304 . As discussed above, optical amplifier  1900  is connected to Raman Pump  402  by pump laser fiber  1902 . In this example, optical amplifier  1900  may operate and function as a single stage remotely-pumped EDFA or other lumped optical amplifier along the returning transmission fiber. Remote pumping of lumped in-line amplifiers allows tailoring of signal strengths for optimum signal-to-noise ratios. 
       FIG. 27  illustrates an example schematic view of interrogator  2724 . As illustrated interrogator  2724  may be connected to umbilical line  126  and downhole fiber  128  to form DAS system  2700 . As illustrated, umbilical line  126  may include any number of distal circulators  312  and downhole fiber  128  may include an optional Raman Mirror, which may also be referred to as Fiber Bragg Grating 500. 
     Interrogator  2724  may include one or more lasers  2701 . Lasers  2701  may be multiplexing laser, which may operate by multiplexing a plurality coherent laser sources via a WDM  404 . One or more lasers  2701  may emit a light pulse  2702 , which may be of a modified pulse shape. Optical pulse shaping and pre-distortion methods may be employed to increase overall optical power that may be launched into a fiber string  2704 , which may connect one or more lasers  2701  to proximal circulator  310 . Light pulse  2702  may travel from proximal circulator  310  through first fiber optic cable  304  to WDM  404 , which may be attached to a Raman Pump  402  at the opposite end, and to umbilical line  126 . Light pulse  2702  may travel to distal circulator  312  in umbilical line  126  and the length of downhole fiber  128 . Any residual Raman amplification may be reflected back by Fiber Bragg Grating 500 that has been constructed to reflect the particular wavelengths used by the Raman Pump and transmit all others. The backscattered light from the downhole fiber  128  may travel back to distal circulator  312  and then up second fiber optic cable  308  to a dedicated interrogator receiver arm. 
     In examples, the dedicated interrogator receiver arm may allow interrogator  2724  to selectively receive backscattered light from different portions along the length of a fiber optic cable, as seen in  FIGS. 10 and 11 . For example, interrogator receiver arm may include a dedicated amplifier  2708  that may selectively amplify the backscattered light from downhole fiber  128 , a second region of the fiber optic cable, using a higher amplification factor than the dedicated amplifier  2708  used to selective amplify the backscattered light received from first fiber optic cable  304 , a first region of the fiber optic cable. Gauges  2712  may have gauge lengths employed in the two dedicated interrogator receiver arms may differ (e.g., also described in  FIG. 2 ). Finally, each dedicated interrogator receiver arm may be equipped with receivers  2706  that are optimized according to certain characteristics of the interferometric signals corresponding to the backscattered light received from the two fiber sensing regions. Note that although  FIG. 27  only shows two dedicated interrogator receiver arms for each sensing fiber regions, it is not intended to be limited to such and may be extended to an arbitrary number of dedicated interrogator receiver arms, where each receiver arm receives and processes the backscattered light signal of a single sensing fiber region of downhole fiber  128 . 
       FIG. 27  further illustrates example inputs  2710  for piezoelectric (PZT) devices. In examples, PZT devices functionally allow dynamic stretching (straining) of optical fibers, which may be embodied in coiled form around the PZT, attached thereto, resulting in optical phase modulation of light propagating along the attached optical fiber. The PZT elements are excitable via electrical signals from any electronic signal information generating source thus allowing information to be converted from electrical signals to optical phase modulated signals along the optical fiber attached thereto. Without limitation, PZT devices attached to input  2710  may be a GPS receiver, seismic controller, hydrophone, and/or the like. 
       FIG. 28  illustrates an example of a schematic view of another example of interrogator  2824  with a single photon detector (SPD)  2800 . SPD  2800  replaces receivers  2706  (e.g., referring to  FIG. 27 ) within interrogator  2824 . This allows for the removal of Raman Pump  402 , dedicated amplifier  2708 , and WDM  404  (e.g., referring to  FIG. 27 ) from interrogator  2824 . Utilization of SPD  2800  alters DAS system  2700  (e.g., referring to  FIG. 27 ) by reducing the noise floor with DAS system  200  to increase SNR. The noise floor is the average energy over a spectral range generated by background processes in the detection system. For an optical device, these may include thermal noise (due to fluctuations caused by heat), pink noise (due to fluctuations caused by changing defects), burst noise (due to fluctuations caused by static defects), and shot noise (due to intrinsic fluctuations of the electromagnetic field with the detector). An SPD  2800  may eliminate (through reduction or compensation) all sources of noise except shot noise and may lead to a reduction in the noise floor by up to 100 dB, directly increasing SNR. 
     In examples, SPD  2800  may be used in subsea operation or land operations utilizing Rayleigh DAS, Raman Distributed Temperature Sensing (DTS), and Brillouin Distributed Strain Sensing (DSS). DTS operates and functions when a light pulse generates backscattered signals due to inelastic scattering within optical fiber. This inelastic scattering, which is strongly temperature dependent, results in a frequency shift to lower frequency (Stokes Raman Scattering) or higher frequency (Anti-Stokes Raman Scattering), both of which are temperature dependent (and usually around ˜13 THz). By detecting these two shifted back-scattered signals, and appropriate math, the temperature may be determined. DSS operates and functions on a photon inelastically interacting with an acoustic phonon in an optical fiber. During the interaction, momentum is transferred with the phonon and the backscattered photon is frequency shifted (˜9-11 GHz) compared to the incident light frequency. The extent of frequency shift is dependent on the strain and the temperature of the fiber. 
     SPD  2800  may be cyro-cooled and operate and function utilizing superconducting nanowire technology. In examples SPD  2800  does not require boosting of optical power but rather lowers the noise floor of signal detection by up to a factor of 100 dB. The detector in SPD  1700  may be designed to multiplex multiple wavelengths or polarizations into the same detector system and may have very narrow wavelength selectivity or larger optical linewidths. These allow both strong wavelength selectivity without the need of optical filters or enables detection of multiple backscatter pulse types (Raman, Brillouin, Rayleigh) on the same detector system. An SPD  2800  may include superconducting nanowire single-photon detector, Photomultiplier tubes, Avalanche photodiodes, Frequency up-conversion, Visible light photon counter, Transition edge sensor, Quantum dots, and Perovskite/Graphene phototransistors (for room temperature operation). In examples, Multiple SPDs and beam-splitters may be used, such as in a homodyne configuration comparing the sum and differences of two SPD signals after a beam path is split by the beamsplitter), to determine the extent of the contribution of shot noise of the overall signal. In examples, the quantum efficiency of SPDs may range from ˜20% up to 99.99% 
       FIG. 29  illustrates an example of a schematic drawing of SPD  2800 . As illustrated, SPD  2800  may include a housing  2900  for enclosing the optical detector  2902  and for providing an optical shield for optical detector  2902 . Housing  2900  may include an aperture  2904  for passage of the fiber optic cable, which is identified as second fiber optic cable  308 . However, examples are not limited thereto, and in some examples, a coupler may be mounted so that second fiber optic cable  308  terminates at a boundary of the housing  2900 . 
     In examples, SPD  2800  may include a cooling mechanism  2906  having the housing  2900  mounted thereto. Cooling mechanism  2906  is configured to maintain the temperature of a light-sensitive region of optical detector  2902  within a temperature range below 210 degrees Kelvin. In some examples, cooling mechanism  2906  operates using liquid helium (He) or liquid nitrogen (N2). In some examples, cooling mechanism  2906  maintains the temperature of the light-sensitive region of optical detector  2902  at a temperature at or below 80 degrees Kelvin. In some examples, cooling mechanism  2906  maintains the temperature of the light-sensitive region of the optical detector  2902  at a temperature at or below 5 degrees Kelvin (e.g., when sealed helium systems are used). In some examples, cooling mechanism  2906  may be of one or more of a variety of configurations, including Dilutio-Magnetic, Collins-Helium Liquefier, Joule-Thomson, Stirling-cycle cryocooler, self-regulated Joule-Thomson, Closed-Cycle Split-Type Stirling, Pulse Tube, a two-stage Gifford-McMahon cryogenic cooler or multi-stage Gifford-McMahon cryogenic cooler, or a cooler using magnetocaloric effect, by way of example. Lowering the temperature of optical detector  2902  improves the SNR of optical detector  2902  by decreasing dark current, by increasing sensitivity, and by reducing resistive loss by causing optical detector  2902  to enter a superconducting regime of operation. In some embodiments or configurations non-SPD optical detectors  2902  may not enter a superconducting regime, while still having little to no thermal noise. 
     In some examples, SPD  2800  includes a cold head  2908  between the optical detector  2902  and cooling mechanism  2906 . However, some embodiments do not include cold head  2908 . In examples, housing  2900  is mounted to cooling mechanism  2906  such that moisture is prevented from entering the housing. For example, housing  2900  may be mounted such that a vacuum seal is formed with the cooling mechanism  2906  or the cold head  2908 . Additionally, housing  2900  may have a non-reflective inner surface. 
     As further illustrated in  FIG. 29 , SPD  2800  may further include a switching or splitting mechanism  2910  to direct optical signals to optical detector  2902 , or a non-SPD optical detector  2912 . Splitting mechanism  2910  may split optical signals based at least in part on wavelength of the optical signal, power of the optical signal, polarization, or any other parameter or criterion. For example, high-power optical signals may be routed to non-SPD optical detector  2912 , and away from optical detector  2902  and low-powered optical signals may be routed to optical detectors  2902 . This routing may be performed to prevent damage to optical detector  2902  while still taking full advantage of LLD and ELLD capabilities of optical detector  2902 . Without limitation, high-power optical signals may cause saturation in optical detector  2902 , leading to damage to optical detector  2902  or to inaccurate results. In some examples, saturation of optical detector  2902  may occur with optical signal inputs having a power of about 100 microwatts, and damage may occur at about 10 milliwatts. The noise floor that may be detected by optical detector  2902  may be at a level slightly below saturation level but is typically at least 20-30 dB. The saturation level and noise floors for non-SPD optical detectors  2912  may be different from the saturation level and noise floors for optical detector  2902 . The saturation levels and noise floors also may or may not overlap, and thus multiple types of detectors may be used that may cover the full power range for system measurements. For at least these reasons, to measure a larger range of possible optical signals, optical detector  2902  are used in a system with non-SPD optical detectors  2912 . Splitting mechanisms  2910  may direct or reroute optical signals based on power level or other criteria, to take advantage of the different power ranges measurable by optical detector  2902  versus non-SPD optical detectors  2912 . 
     In addition to or instead of a splitting mechanism  2910 , SPD  2800  may include a coupling mechanism or other mechanism to split the light with optical couplers (with or without feedback). These mechanisms may be multi-stage (e.g., the light may be split in one stage, then split again in a second stage), and may split light based on power, wavelength, or phase. Processor or computation-based systems may also be used in some embodiments to dynamically direct or reroute light signals among any available optical path as power increases or based on any other criteria. 
     In examples, SPD  2800  may be connected to information handling system  130  (e.g., referring to  FIG. 1 ) through interrogator  2824  to obtain measurement data. In some examples, some portions of the interrogator  2824  may be positioned at a surface of the Earth, while some portions to interrogator  2824  may be placed downhole. When more than one optical detector  1802  is used, for example, some of the optical detectors  2902  or  2912  may be placed downhole, and some may be placed at the surface. In some examples, one or more cooling mechanisms  2906  may be placed downhole proximate one or more optical detectors  2902  although power and geometry considerations should be considered with such configurations to provide power for cooling in an appropriately sized borehole. 
     In production and/or measurement operations, the use of SPD  2800  may be safer than using a Raman Pump  402  (e.g., referring to  FIG. 27 ). Raman Pump  402  may increase the power moving through DAS system  200 , which may lead to explosions and damage from high power increased by Raman Pump  402 . Using SPD  2800  removes Raman Pump  402  and protects against explosions from hazardous gas used with Raman Pump  402 , increases eye safety by prevent high energy light pulses from contacting the human eye, and may further prevent connector damage and failure from high power densities. Additionally, as lower optical pulse powers are used, non-linear distortion of the optical pulse shape is negligible, allowing for minimal to no pulse forming. 
     Utilizing SPD  2800  may improve current technology by allowing greater lengths of fiber with greatly attenuated signals, high transmission loss interconnects (such as used offshore) may be used, even though the attenuation is high. An SPD  2800  may have selective frequency, reducing background noise contribution of other optical sources or devices (such as from a Raman pump or scatter from a grating), and distortion of the optical pulse shape is negligible. Additionally, an SPD  2800  may be gated extremely fast, detect very few photons, and the spatial resolution can be extremely high. 
       FIG. 30  illustrates another example DAS system  3000  having an interrogator  3024  with optical shutter  1601  and EDFA  1602  are disposed between proximal circulator  310  and distal circulator  312  on second fiber optic cable  308 , as described in  FIG. 16  above. In addition, as described in  FIG. 16 , Raman Pump  402  may be attached to WDM  1801  which is disposed on first fiber optic cable  304 . Raman Pump  402  and WDM  1801 . In this example, distal circulator  312  is disposed in umbilical line  126 . In this example, interrogator  3024  may be configured to combine the optical pump laser for remote amplification and signal laser from the interrogator onto one common fiber. 
     The systems and methods for using a distributed acoustic system in a subsea environment may include any of the various features of the systems and methods disclosed herein, including one or more of the following statements. Additionally, the systems and methods for a DAS system within a subsea environment may include any of the various features of the systems and methods disclosed herein, including one or more of the following statements. 
     Statement 1. A distributed acoustic system (DAS) may comprise an interrogator and an umbilical line attached at one end to the interrogator, a downhole fiber attached to the umbilical line at the end opposite the interrogator. The interrogator may further include a proximal circulator, a distal circulator connected to the proximal circulator by a first fiber optic cable, and a second fiber optic cable connecting the proximal circulator and the distal circulator. 
     Statement 2. The DAS of statement 1, further comprising an erbium doped fiber amplifier (EDFA) disposed between the proximal circulator and the distal circulator on the second fiber optic cable. 
     Statement 3. The DAS of statement 2, further comprising an optical shutter disposed between the EDFA and the distal circulator on the second fiber optic cable. 
     Statement 4. The DAS of statement 3, further comprising a wavelength division multiplexer (WDM) pump disposed on the first fiber optic cable between the proximal circulator and the distal circulator. 
     Statement 5. The DAS of statement 4, further comprising a Raman Pump connected to the WDM pump. 
     Statement 6. The DAS of statement 2, further comprising an optical shutter disposed between the distal circulator and the umbilical line. 
     Statement 7. The DAS of statements 1 or 2, wherein the first fiber optic cable and the second fiber optic cable are different lengths. 
     Statement 8. The DAS of statements 1, 2, or 7, further comprising at least one Fiber Bragg Grating attached to the proximal circulator or the distal circulator. 
     Statement 9. The DAS of statements 1, 2, 7, or 8, wherein the interrogator is configured to receive backscattered light from a first sensing region and a second sensing region. 
     Statement 10. The DAS of statements 1, 2, or 7-9, wherein the DAS is disposed in a subsea system operation of one or more wells and the umbilical line attaches to the downhole fiber at a fiber connection. 
     Statement 11. A distributed acoustic system (DAS) may comprise an interrogator, an umbilical line attached to the interrogator at one end, and a downhole fiber attached to the umbilical line at the end opposite the interrogator. 
     Statement 12. The DAS of statement 11, further comprising an optical amplifier disposed in the umbilical line that is connected to a Raman Pump by a pump laser fiber. 
     Statement 13. The DAS of statements 11 or 12, further comprising two optical amplifiers disposed in series in the umbilical line that are each connected to individual Raman Pumps by individual pump laser fibers. 
     Statement 14. The DAS of statement 13, wherein the two optical amplifiers are connected to a Raman Pump by a pump laser fiber. 
     Statement 15. The DAS of statements 11-14, further comprising a proximal circulator, a distal circulator connected to the proximal circulator by a first fiber optic cable, a second fiber optic cable connecting the proximal circulator and the distal circulator, and wherein the proximal circulator, the distal circulator, the first fiber optic cable, and the second fiber optic cable are disposed in the umbilical line. 
     Statement 16. The DAS of statement 15, further comprising a first optical amplifier disposed on the first fiber optic cable between the proximal circulator and the distal circulator. 
     Statement 17. The DAS of statement 16, further comprising a second optical amplifier disposed on the second fiber optic cable between the proximal circulator and the distal circulator. 
     Statement 18. The DAS of statement 17, wherein the first optical amplifier is connected to a first Raman Pump by a first pump laser fiber and the second optical amplifier is connected to a second Raman Pump by a second pump laser fiber. 
     Statement 19. The DAS of statement 17, wherein the first optical amplifier and the second optical amplifier is connected to a Raman Pump by a pump laser fiber. 
     Statement 20. The DAS of statement 15, further comprising an optical amplifier disposed on the second fiber optic cable between the proximal circulator and the distal circulator. 
     Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations may be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. The preceding description provides various examples of the systems and methods of use disclosed herein which may contain different method steps and alternative combinations of components. It should be understood that, although individual examples may be discussed herein, the present disclosure covers all combinations of the disclosed examples, including, without limitation, the different component combinations, method step combinations, and properties of the system. It should be understood that the compositions and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions and methods can also “consist essentially of” or “consist of” the various components and steps. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces. 
     For the sake of brevity, only certain ranges are explicitly disclosed herein. However, ranges from any lower limit may be combined with any upper limit to recite a range not explicitly recited, as well as, ranges from any lower limit may be combined with any other lower limit to recite a range not explicitly recited, in the same way, ranges from any upper limit may be combined with any other upper limit to recite a range not explicitly recited. Additionally, whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range are specifically disclosed. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values even if not explicitly recited. Thus, every point or individual value may serve as its own lower or upper limit combined with any other point or individual value or any other lower or upper limit, to recite a range not explicitly recited. 
     Therefore, the present examples are well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular examples disclosed above are illustrative only, and may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Although individual examples are discussed, the disclosure covers all combinations of all of the examples. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and dearly defined by the patentee. It is therefore evident that the particular illustrative examples disclosed above may be altered or modified and all such variations are considered within the scope and spirit of those examples. If there is any conflict in the usages of a word or term in this specification and one or more patent(s) or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted.