Quasi-distributed sensing using enhanced sensing structures

Abstract

The invention discloses quasi-distributed sensing using an enhanced sensing structure. A system for distributed acoustic sensing may include a distributed acoustic sensing (DAS) station that transmits a DAS signal into a DAS fiber; and an enhanced sensing array, wherein the enhanced sensing array comprises at least one sensing part. The at least one sensing component may include a DAS fiber coil forming part of a DAS fiber; and a low elastic modulus housing surrounding the DAS fiber coil.

Description

Technical Field

[0001] Embodiments of the present disclosure relate to the field of optical communication systems. In particular, the present disclosure relates to techniques for extending and improving the sensitivity of distributed acoustic sensing (DAS) in underwater optical cables. Background Art

[0002] In a distributed acoustic sensing (DAS) system, an optical cable can be used to provide continuous real-time or near real-time monitoring of disturbances or anomalies in the vicinity of the cable. In other words, the cable itself can be used as a sensing element to detect or monitor different types of interruptions, interferences, irregularities, acoustic vibrations, natural or man-made activities occurring inside or outside an underwater environment, etc. in a DAS environment (e.g., a terrestrial environment, a marine environment). To this end, optoelectronic devices / equipment coupled to an optical cable of a DAS system can detect and process reflected optical signals (e.g., Rayleigh backscattered signals) within a certain distance range in the DAS environment.

[0003] Typically, a DAS system may include a DAS station that acts as an interrogator unit (IU) to probe an optical fiber cable using coherent laser pulses representing an outbound DAS signal, wherein changes in the phase of a returning optical backscattered signal are measured. The optical phase shift between pulses may be proportional to strain in the optical fiber, resulting in the ability to detect vibrations, etc., as measured by the effect of such perturbations on the phase. For example, a DAS system may be based on Rayleigh scattering, or more particularly, on Rayleigh backscattering (also referred to as a Rayleigh scattering-based DAS system).

[0004] In a known approach, distributed acoustic sensing employs an optical cable including optical fibers that are used as DAS sensors. The optical cable may also include steel wires, steel tapes, and an outer sheath. The DAS sensor fiber may sense sound waves along the length of the cable that affect the cable and, therefore, transmit vibrations into the DAS sensor fiber, resulting in a phase shift in the detected DAS signal (the returned DAS signal), which is returned to a detector and processor in the DAS station. One problem with this known DAS approach is the lack of acoustic sensitivity in the optical fiber encapsulated by a layer of steel material, where the intensity of the sound waves is significantly attenuated before reaching the DAS sensor fiber. Therefore, current technology for deploying DAS sensing in highly armored cables may exhibit less than ideal sensitivity for detecting acoustic disturbances.

[0005] It is with respect to these and other considerations that the present disclosure is provided. Summary of the invention

[0006] In one embodiment, a system for distributed acoustic sensing is provided. The system may include a distributed acoustic sensing (DAS) station that transmits a DAS signal into a DAS optical fiber; and an enhanced sensing array that includes at least one sensing component. The at least one sensing component may include a DAS optical fiber coil that forms a portion of the DAS optical fiber; and a low elastic modulus housing that surrounds the DAS optical fiber coil.

[0007] In another embodiment, a system for distributed acoustics may include a distributed acoustic sensing (DAS) station that transmits a DAS signal into a DAS optical fiber, and an enhanced sensing array that includes a plurality of sensing components arranged over a plurality of spans. A given sensing component of the plurality of sensing components may include a DAS optical fiber coil that forms a portion of the DAS optical fiber; and a low elastic modulus housing that surrounds the DAS optical fiber coil.

[0008] In a further embodiment, a method for distributed acoustic sensing is provided. The method may include transmitting an outbound distributed acoustic sensing (DAS) signal from a DAS station into a DAS optical fiber, and routing the outbound DAS signal through a sensing component formed of a low elastic modulus material, wherein the sensing component includes a DAS optical fiber coil forming a portion of the DAS optical fiber. The method may also include measuring a power, phase, frequency, or polarization of a reflected signal based on the outbound DAS signal after the outbound DAS signal passes through the DAS optical fiber coil. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a graph showing the phase responsivity of several different cable types;

[0010] Figure 2 A sensing component for enhanced DAS sensing according to an embodiment of the present disclosure is shown, the sensing component being implemented as a cable connector;

[0011] FIG. 3A to FIG. 3E illustrates alternative sensing components for enhanced DAS sensing according to various embodiments of the present disclosure;

[0012] Figure 4A An enhanced distributed acoustic sensing system according to an embodiment of the present disclosure is depicted;

[0013] Figure 4B depicts the geometry of a scene for enhanced distributed acoustic sensing according to an embodiment of the present disclosure;

[0014] Figure 5 Another enhanced distributed acoustic sensing system according to an embodiment of the present disclosure is depicted;

[0015] Figure 6A further enhanced distributed acoustic sensing system according to an embodiment of the present disclosure is depicted;

[0016] Figure 7 Yet another enhanced distributed acoustic sensing system according to an embodiment of the present disclosure is depicted; and

[0017] Figure 8 A process flow according to an embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0018] The present embodiment will now be described more fully below with reference to the accompanying drawings, in which exemplary embodiments are shown. The scope of the embodiments should not be construed as being limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the embodiments to those skilled in the art. In the accompanying drawings, similar reference numerals always refer to similar elements.

[0019] Before describing specific embodiments in detail with respect to the figures, general features with respect to the embodiments will be reviewed. Novel DAS devices, systems, architectures, and techniques are provided to improve DAS sensing capabilities.

[0020] In some embodiments, novel enhanced sensing components are provided, based on coiled optical fiber, low elastic modulus media, housings or enclosures, or combinations of these elements. In some embodiments, novel architectures are provided for DAS sensing, including arrays of multiple sensing components that perform what may be referred to as "quasi-distributed sensing".

[0021] By explaining, Figure 1 is a graph showing the phase responsivity of several different cable types. Note that currently deployed optical cables, including those used for submarine communication systems and for DAS systems, for example, are equipped with very strong protection measures to avoid damage to the optical fibers carrying optical signals used for communication and / or acoustic sensing. In deep water, what is called LW (lightweight) cable is often used. LW cable consists of an optical fiber that is housed in a PBT (polybutylene terephthalate) tube, which is then surrounded by multiple layers of steel wire in a concentric manner, followed by a copper conductor, and finally a polyethylene sheath, which serves as an insulator. For deployment in shallow water, additional protection measures are often necessary to prevent fishing activities, etc., including "single armored" or even "double armored" cables. These armored cables include additional layers of galvanized wire, steel tape, and are surrounded by nylon yarn on the outside. Steel wire, galvanized wire and steel strip are all hard structures and therefore exhibit a very large elastic modulus (wherein a very large elastic modulus may refer to a modulus greater than 100 GPa, and more particularly, a Young's modulus in the range from 150 GPa to 220 GPa).

[0022] exist Figure 1In Figure 1, five curves are shown, illustrating five different cable types, where the phase response of an optical signal is plotted against the intensity or pressure of an external source. The occurrence of an event that produces an acoustic wave will exert pressure on the fiber core in a way that changes the fiber's refractive index and, therefore, effectively compresses and expands the fiber, resulting in a corresponding change in the optical phase. By measuring the power spectral density of this optical phase at the acoustic frequency, one can determine the intensity of the acoustic wave. Figure 1 In Figure 1, the results show that the detected optical phase is heavily dependent on the cable (where different cable types exhibit different Young's moduli depending on the constituent materials making up a given cable). A higher phase response at a given pressure indicates a higher responsivity, which means that an acoustic signal applied to a given cable will produce a stronger phase response.

[0023] In particular, Figure 1 The detected phase power (in dB_rad / √Hz) of five different cables is compared as a function of the pressure that would be generated by the sound waves. The behavior shown in these curves can be summarized as follows: the stronger the protection provided by the cable, the weaker the phase response. Figure 1 It is suggested that the use of loose tube cables (soft structural cables with very low Young's modulus, ~3 GPa-4 GPa) provides substantially better response than other cables - for example, 26-dB higher sensitivity compared to double armored cables (such rigid cables have a large Young's modulus, ~72 GPa). However, loose tube cables cannot be deployed in submarine cables due to the lack of protection for the optical fibers therein. Therefore, for optical communications and for DAS sensing, cable technologies that produce lower phase responses are used, as shown by the four curves below.

[0024] according to Figure 1 As a result, existing more robust cable structures typically employed for DAS sensing suffer from relatively insensitive sound pressure phase response. To address this problem, Figure 2 A sensing component 100 according to an embodiment of the present disclosure is shown, which can be used to significantly enhance DAS sensing sensitivity. The sensing component 100 in this example can be embodied as a cable connector, the structure of which will include a housing 10. In some embodiments, as detailed with respect to the following figures, the housing 10 can be formed using a low elastic modulus material (such as polyethylene). The sensing component 100 can also include an optical fiber coil 12, the elements of which can form a portion of an optical fiber coupled to a DAS station, discussed with respect to the following embodiments.

[0025] In operation, the sensing component 100 may be deployed in a subsea environment to detect acoustic signals, such as signals generated by disturbances including earthquakes, ships, and the like. The sensing component 100 may be deployed to perform "quasi-distributed acoustic sensing" in that the sensing component 100 may detect disturbances from acoustic sources that may be located anywhere over a large area, such as up to 50 km from the sensing component 100, depending on the conditions of the surrounding terrain and other factors. As noted, the fiber optic coil 12 may be a portion of an optical fiber 14 that performs distributed acoustic sensing by sensing acoustic signals that may impinge on the optical fiber 14 over the length of the optical fiber 14, which may span tens of kilometers. However, the sensing component 100 may be configured to detect acoustic signals by adjusting the sensitivity (such as with respect to the acoustic signals) to a desired level of acoustic sensitivity. Figure 1 The phase response discussed herein is focused at the location of the sensing component 100 to perform enhanced DAS sensing.

[0026] Go to FIG. 3A to FIG. 3E , shows alternative sensing components for enhancing DAS sensing according to different embodiments of the present disclosure. In these embodiments, each sensing component can be provided as a cable joint capable of detecting sound sources within 20km or so. FIG. 4A to FIG. 7 As described, these sensing components may be deployed for DAS sensing over an arbitrarily long sensing range, where a given sensing component may be deployed every tens of kilometers, as an example, to provide quasi-DAS sensing over the entire sensing range.

[0027] Special Go to Figure 3A , a sensing component 100A is shown, including a housing, shown as a housing 116, which housing may be formed of a low elastic modulus material, such as polyethylene. The housing 116 may contain a fiber coil 112 extending from the DAS station as part of the DAS fiber. In this embodiment, the fiber coil 112 may be surrounded by a steel enclosure 114. By providing the fiber coil 112 in the sensing component 100A, and a low modulus for the housing 116, the overall phase response of the structure may be improved at the location of the sensing component 100A. Note that for LW / SA cables, by excluding the steel wire or copper / steel tape, the acoustic response will be comparable to 0.25mm coated fiber, which provides ~11.8 / 12.7dB higher sensitivity.

[0028] The length of the fiber coil 112 (as well as other fiber coils to be discussed) can be tens of meters, such as 30m to 100m in some non-limiting embodiments. As a general rule, the length of the fiber coil 112 should be longer than the spatial resolution of the DAS instrument, where a 45m long coiled fiber is long enough when a 30m spatial resolution is required. By making the fiber coil 1.5 times the spatial resolution length, it is ensured that the response fully covers one spatial resolution location, regardless of where the spatial resolution boundary resides in the DAS receiver. The slightly longer (100m) length of the fiber coil 112 will be useful to combat the signal fading effects caused by the Rayleigh scattering effect. The fiber coil 112 can be relatively loosely deployed within the joint structure so that the fiber coil 112 can fully respond to changes in external sound pressure.

[0029] Note that Figure 3A In an embodiment, the coil can be arranged to lie flat in a plane. It is also noted that while the sensitivity of a fiber coil per unit length of fiber may be similar to the sensitivity per unit length of straight fiber, the use of coiled fiber facilitates providing longer fiber lengths (e.g., 30m to 100m) within a relatively small joint (e.g., 30cm to 50cm in length).

[0030] Special Go to Figure 3B , shows a sensing component 100B, including a housing 116, a steel package 114, an optical fiber coil 122, and a core shaft 124. In this embodiment, the optical fiber coil 122 is tightly wrapped around the core shaft 124, and the core shaft 124 can be formed using a low elastic modulus material (such as polyethylene, which is other solid). In the example of polyethylene with a Young's modulus of about 0.3GPa, the modulus can provide a 35dB pressure sensitivity improvement as compared to a stand-alone single-mode fiber (SMF). Note that the core shaft 124 can be a hollow cylinder to enhance the phase response to the acoustic signal. According to various embodiments, the sensitivity of the sensing component 100B depends on the ratio of the inner diameter to the outer diameter of the core shaft 124, and can therefore be adjusted to achieve better acoustic sensitivity by optimizing the inner diameter / outer diameter ratio. According to some non-limiting embodiments of the present disclosure, a fiber length of 30m to 100m of the optical fiber coil 122 wound on the core shaft 124 is suitable.

[0031] Special Go to Figure 3C , shows a sensing component 100C, including a housing 116, a steel layer 134 (which layer may be formed of wire and / or ribbon), and a fiber coil 132 (which coil may be arranged differently from the fiber coil 112). Fiber Acoustic Sensitivity vs. cos 2(θ), where θ is the incident angle between the acoustic wave from the moving object and the sensing fiber. When θ = 90° (the acoustic wave is perpendicular to the sensing section), no acoustic signal can be detected. Figure 3A In the case of sensing fibers, the sensing fibers are arranged on a surface or plane, so when the sound waves propagate perpendicular to the sensing fibers, the sensitivity is less. By arranging the sensing fibers in a circular or even spherical manner, e.g. Figure 3C As suggested in for the fiber coil 132, the angle θ defined by the fiber coil and the incident acoustic signal can inherently vary from 0 to 90° at different locations along the length of the fiber coil 132, thus providing uniform sensitivity that is independent of the direction of the incident acoustic wave.

[0032] Special Go to Figure 3D , shows a sensing component 100D, including a housing 116, a steel layer 144 (which layer may be formed of wire and / or ribbon), and a fiber coil 142. In this example, a portion of the steel layer 144 may be removed, such as adjacent to the fiber coil 142, to provide enhanced acoustic wave penetration, and thus enhanced optical phase response. In some embodiments, the fiber coil 142 may utilize an enhanced fiber structure that provides a higher Rayleigh backscattering response.

[0033] Special Go to Figure 3E , shows a sensing component 100E, including a steel package 154, an optical fiber coil 152, and a low viscosity gel 156, surrounding the optical fiber coil 152. In this configuration, the optical fiber coil 152 can be arranged in a spiral manner in the gel, with FIG. 3A to FIG. 3D The wrapped coil structure of the embodiment is slightly different. Through appropriate design, the length of the optical fiber projected parallel to the steel tube can be similar to the length of the optical fiber projected perpendicular to the steel tube. Figure 3C As a result of the embodiment of the present invention, the spiral design can also provide uniform sensitivity that is independent of the angle of the incident sound waves emitted from the moving object. In addition, the low viscosity gel 156 can provide an enhanced phase response.

[0034] Figure 4A An enhanced distributed acoustic sensing (DAS) system according to an embodiment of the present disclosure is depicted, shown as DAS system 200. In this example, DAS sensing is facilitated over a 50 km range using four sensing components arranged as cable joints equipped with coiled optical fibers. For example, sensing component 204 may be arranged in a manner similar to FIG. 3A to FIG. 3E. The DAS system 200 includes a DAS station 202, which is arranged to transmit an outbound signal, which may be referred to as an outbound DAS signal, for example using a DAS transmitter. DAS stations are known in the art, and the details of the DAS station 202 will not be discussed further herein. In this example, the distance (S) between each pair of adjacent sensing components (meaning sensing components 204) is different. The different distances are set to accommodate the fact that the input power of each sensing component 204 is different due to fiber loss. The first of the sensing components 204, which is closest to the DAS station, may have the highest power; therefore, the first of the sensing components 204 can in principle detect acoustic signals within the longest distance (in one example, the distance is estimated to be 21.7 km); the second of the sensing components 204 may be determined to be able to detect acoustic signals only within a range of ±7.0 km, which means from a distance of about 7 km closer to the DAS station 202 relative to the sensing components 204 to a distance of 7 km farther from the DAS station 202 relative to the sensing components 204. The third of the sensing components 204 may be assumed to be capable of detecting acoustic signals within a range of ±4.2 km, while the last of the sensing components 204 may be assumed to be capable of detecting acoustic signals within a range of only ±3.0 km - assuming that all sensing components 204 exhibit the same minimum responsivity (assuming that the acoustic source may be located at the same depth as the sensing components 204). The total maximum sensing distance provided by the DAS system 200 may therefore be calculated to be 28.7 km + 11.1 km + 7.2 km + 3 km = 50 km. In this example, we assume that the vessel is directly above the sensing cable.

[0035] Also refer to Figure 4A , Figure 4B Depicts the geometry of an enhanced distributed acoustic sensing scene according to an embodiment of the present disclosure. In this example, a portion of a variation of a DAS system 200 is shown, including four sensing components 204, shown as sensing component 204A, sensing component 204B, sensing component 204C, and sensing component 204D. Although only four sensing components are shown in this example, it is understood that a DAS system, such as DAS system 200, may include up to dozens of such sensing components.

[0036] Example determination of appropriate configuration of sensing components

[0037] In the context of deploying a DAS sensing system, such as the DAS system 200, in a subsea environment, an acoustic source, such as the vessel 222, located above the sea surface 224 may not be positioned directly above the sensing cable including the sensing components 204. Furthermore, not all of the sensing components 204 may be positioned at the same depth below the sea surface 224. Therefore, the spacing between the sensing components may be globally optimized to ensure a similar minimum responsivity for the sensing components 204 within the coverage of each one of the sensing components 204.

[0038] In one approach, to establish a suitable configuration of the sensing components 204, a design concept is to set the optimal connector (sensing component 204) spacing (see distance S) so that the DAS signal loss (round trip loss) between two adjacent connectors is equal to the transmission loss (TL) difference of the acoustic wave between the vessel 222 and the two adjacent connectors. The transmission loss of the acoustic wave can be modeled as a simple r 2 The relationship between the distance and the load is given by the optical fiber. The relationship between the optical fiber and the load is given by the optical fiber. The relationship between the optical fiber and the load is given by the optical fiber. 2 law, one can set the following equation to optimize the joint spacing:

[0039]

[0040] d1+2*d2+2*d3+2*d4=D (4)

[0041] like Figure 4B As shown, different parameters, shown as r1, r2, r3 and r4, represent the shortest distances from the vessel 222 at three different shown positions relative to the sensing component 204A, the sensing component 204B, the sensing component 204C and the sensing component 204D, respectively. Figure 4B ) to represent the shortest horizontal distance from the vessel 222 to the sensing cable, and v1 to v4 ( Figure 4B (not shown) can be set to the shortest vertical depth of the cable to the vessel 222. Thus, for a given set of h and v values, a corresponding value p can be determined that represents the shortest distance from the vessel 222 to a given sensing component, where h 2 +v 2 =ρ 2. In other words, when a given one of the sensing components 204 is located directly below the vessel 222 in the XZ plane shown, at a depth equal to v and displaced a distance h in the YZ plane, the value of p represents the hypotenuse of the right triangle. For each different sensing component, a set of parameters p1 to p4 then represents the shortest distance from the vessel 222 to the corresponding sensing component 204A to 204D. In addition, a set of parameters d1, d2, d3 and d4 represent the corresponding maximum sensing distances along the optical fiber for sensing component 204A, sensing component 204B, sensing component 204C and sensing component 204D, respectively. Parameter (D) can be set to represent the overall maximum sensing distance (D = d1 + 2*d2 + 2*d3 + 2*d4), where the total sensing fiber length is equal to D-d4.

[0042] With respect to Eq(1), when r2 or d2 is too large (too much fiber loss), the equation will not converge. In this case, one can simply minimize the difference between the DAS signal loss in the fiber (assumed to be 0.2 dB / km in Eq(5)) and the difference in acoustic loss between the two connectors in water:

[0043]

[0044] According to additional embodiments of the present disclosure, the sensing component may be implemented in a connector integrated into a multi-span amplification system having an amplified-filtered loop back (AFLB). Figure 5 One implementation of such a system according to various embodiments of the present disclosure is shown. Figure 5 The system 250 includes a DAS station 202 and a set of components forming an outbound path 252 and another set of components forming a return path 254. Figure 5 In the example of FIG. 1 , three spans are depicted as span 260A, span 260B, and span 260C. However, it is understood that Figure 5 The system can be implemented in N spans, where N represents any suitable integer. In this example, only one sensing component (joint) may be needed for each span covering a distance of 50 km, because the corresponding sensing components 262A, 262B, and 262C can be configured to detect acoustic signals within a range of + / - 25 km. Note that these sensing components can be generally as described with respect to FIG. 3A to FIG. 3E The embodiment is configured as described above.

[0045] In addition to loop 263, system 250 also includes a plurality of repeaters, shown as repeater 264A, repeater 264B, and repeater 264C, wherein these repeaters can each be configured with an amplification filter loop (AFLB), including a coupler, an erbium doped fiber amplifier (EDFA), a switch, a filter, a circulator, as is known in the art. In system 250, DAS sensing of an outbound DAS signal transmitted from DAS station 202 occurs on outbound path 252, while return path 254 is used to carry the sensed signal back to DAS station 202 via loops at each repeater. Note that in this and other embodiments, a first fiber of a fiber pair can be used to conduct an outbound DAS signal through outbound path 252, while a second fiber of the fiber pair is used to conduct a return signal via return path 254.

[0046] Another advantage provided by system 250 is that no transmission fiber is required for the very last span, since the (n-0.5)*50km link can be sensed with only n splices (given a 50-km repeater spacing). Additionally, the signal to noise ratio (SNR) of the last sensing element (furthest from DAS station 202) is increased due to the absence of additional noise reflections from the 'missing' span.

[0047] Figure 6 A further enhanced distributed acoustic sensing system according to an embodiment of the present disclosure is depicted. System 270 includes DAS station 202 and a set of two sensing components, shown as sensing component 272 and sensing component 274, which sensing components may include enhanced fiber coil segments formed using enhanced coiled fiber material. Note that these sensing components may be generally as described with respect to FIG. 3A to FIG. 3E As described in the embodiments of the present invention, the invention is configured. For reference, recently, optical fibers with relatively high Rayleigh backscatter coefficients have been developed for distributed sensing. In particular, enhanced scattering fibers with an OSNR improvement of 15 dB compared to standard SMFs have been developed. However, this special fiber can preferably be used locally to increase the DAS signal power. If this special fiber is used for the entire link, the noise power will increase by the same 15 dB, resulting in no overall OSNR improvement due to the higher losses in the special fiber.

[0048] Figure 6 Embodiments take advantage of the advantages of using a fiber optic sensor coil with enhanced scattering properties of higher Rayleigh backscatter, while avoiding the disadvantages of increased noise that would occur if sensing components with enhanced scattering fiber optic coils were deployed throughout the link.

[0049] Only 37.5 km of fiber length and 2 cable joints (sensing component 272 and sensing component 274) are used to cover acoustic detection over a single span of ~55 km. The first joint, sensing component 272 (whose components can be considered as having unenhanced fiber coil segments) uses a coil made of SMF, and the second joint, sensing component 274 uses a coil made of enhanced scattering fiber. The 15 dB improvement from enhanced scattering offsets the round-trip loss (15 dB) from the 37.5 km fiber, so the DAS signal power is at the same level as sensing component 272 and sensing component 274. Therefore, each of the sensing components can cover the same sensing distance (±18.25 km) with the same responsiveness.

[0050] Figure 7 Yet another enhanced distributed acoustic sensing system according to an embodiment of the present disclosure is depicted. In this example, system 280 is depicted as covering two spans, span 290A and span 290B. However, it is understood that Figure 7 The system may be implemented in N spans, where N represents any suitable integer. Figure 7 The embodiment of the present invention adopts a heterogeneous arrangement of sensing components, wherein, within a given span, sensing components having fiber coils formed of standard SMF (sensing components 282A, sensing components 282B) are deployed closest to the DAS station 202, while sensing components having fiber coils made of enhanced scattering fiber (sensing components 284A, sensing components 284B) are deployed farther from the DAS station 202. Note that these sensing components can be generally arranged as described with respect to FIG. 3A to FIG. 3E The embodiment is configured as described above.

[0051] In addition to loop 283, loops are provided at repeaters 286A and 286B to route the outbound DAS signal on outbound path 292 back to the DAS station on return path 294. Again, sensing components are provided on outbound path 292 only.

[0052] exist Figure 7 In the example shown, the repeater spacing is increased from the previous example of 50 km to 75 km by using standard fiber optic material splices (sensing components 282A, 282B) and enhanced scattering fiber coil splices (sensing components 284A, 284B) in a given sensing span. Figure 6 With the aforementioned embodiment, one can manage the accumulated amplified spontaneous emission (ASE) to ensure the same responsivity in all sensing elements. By increasing the repeater spacing to 75 km, Figure 5This implementation only requires 2 repeaters compared to 3 repeaters in the configuration of FIG. 1 , thereby reducing costs.

[0053] In review, the quasi-distributed fiber optic sensing method as disclosed herein provides sensing of sound sources not only in the immediate vicinity, but also for detection of sound sources up to 50 km away, depending on the prevailing conditions of the surrounding terrain. Thus, given the deployment of sensing components, such as splices along the optical fiber as detailed herein, at intervals of about a few kilometers to tens of kilometers, a fully distributed sensing system is not necessary in order to monitor sound sources within any desired coverage area.

[0054] Note that the relative spacing of the sensing elements in a quasi-distributed sensing system can be selected based on the intended use. For example, to monitor a ship and determine the position, speed, and direction of the ship, it is best to collect received phase information from 3 to 5 different locations. Therefore, for ship monitoring in a single-span or multi-span communication system, the deployment of 3 to 5 sensing elements may be necessary.

[0055] Furthermore, according to additional embodiments of the present disclosure, a sensing system may be implemented using a combination of a) several discrete joints comprising optical fiber coils spaced several kilometers apart from one another as primary sensors and b) distributed optical fiber sensors along the length of the optical fiber as auxiliary sensors, so as to achieve a hybrid quasi-distributed sensing system.

[0056] Figure 8 A process flow 800 is shown in accordance with an embodiment of the present disclosure. At block 802, an outbound distributed acoustic sensing (DAS) signal is transmitted from a DAS station into a DAS optical fiber.

[0057] At block 804, an outbound DAS signal is conducted through a sensing component formed of a low elastic modulus material, the sensing component including a DAS optical fiber coil forming a portion of a DAS optical fiber.

[0058] At block 806, a Rayleigh backscatter signal is monitored, which is generated in the DAS fiber coil, such as based on the outbound DAS signal. At block 808, the power, phase, frequency, or polarization of the Rayleigh backscatter signal is measured, processed, and analyzed.

[0059] The quasi-distributed sensing method as disclosed herein can significantly reduce the amount of digital signal processing required to extract the necessary sensing information. As an example, assuming a 50-km span length with a 30m spatial resolution, all phase information on the 50-km link (1667 x 30m) should be processed if a regular fully distributed sensing system is employed. Using a single sensing component deployed in a cable joint (assuming a 90m coiled fiber length and still a 30m spatial resolution), only 3m x 30m of information needs to be processed to monitor the same 50km link. However, information from auxiliary sensors may be required during the initial stages of phase calibration. Overall, the amount of digital signal processing can be reduced to 1 / 100 for a quasi-distributed sensing system compared to a standard fully distributed sensing system as disclosed in the embodiments mentioned earlier herein. th .

[0060] Novel and inventive devices, systems, structures and techniques are provided herein for providing less complex methods of distributed acoustic sensing.

[0061] The present disclosure is not limited in scope by the specific embodiments described herein. In fact, various other embodiments and modifications of the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and the accompanying drawings. Therefore, such other embodiments and modifications are intended to fall within the scope of the present disclosure. In addition, although the present disclosure is described herein in the context of a specific embodiment, for a specific purpose in a specific environment, a person of ordinary skill in the art will recognize that its usefulness is not limited thereto, and the present disclosure can be beneficially implemented for any number of purposes in any number of environments. Therefore, the claims set forth below should be interpreted according to the full breadth and spirit of the present disclosure as described herein.

Claims

1. A system for distributed acoustic sensing, comprising: a distributed acoustic sensing (DAS) station for transmitting a DAS signal into a DAS optical fiber; as well as An enhanced sensing array, the enhanced sensing array comprising at least one sensing component, the at least one sensing component comprising: a DAS fiber coil forming a portion of the DAS fiber; as well as A low elastic modulus housing surrounds the DAS fiber optic coil.

2. The system of claim 1, wherein the at least one sensing component is formed in a cable connector.

3. The system of claim 1, the low elastic modulus housing comprising polyethylene.

4. The system according to claim 1, wherein: The at least one sensing component includes at least three packages, wherein a first separation between a first sensing component located closest to the DAS station and a second sensing component located second closest to the DSS station is greater than a second separation between the second sensing component and a third sensing component located third closest to the DAS station.

5. The system according to claim 1, wherein: The at least one sensing component includes a plurality of enclosures respectively arranged in a plurality of cable joints, each cable joint including a local DAS fiber optic coil, wherein at least one cable joint is arranged within one or more spans of a multi-span communication system.

6. The system according to claim 5, wherein: the DAS optical fiber forms a first optical fiber of an optical fiber pair, wherein the first optical fiber is coupled to a DAS transmitter at the DAS station to conduct an outbound signal, and Wherein a second optical fiber of the optical fiber pair is arranged to conduct a return signal to a receiver at the DAS station, the return signal being derived from the outbound signal after being routed through a loop located at the one or more spans.

7. The system according to claim 5, wherein: A plurality of cable splices are arranged within a given span of the multi-span communication system.

8. The system according to claim 7, wherein: A first cable joint among the plurality of cable joints located relatively closer to the DAS station includes an unreinforced optical fiber coil segment, and a second cable joint among the plurality of cable joints located relatively farther from the DAS station includes a reinforced optical fiber coil segment, wherein the reinforced optical fiber coil segment has a relatively higher Rayleigh backscattering coefficient than the unreinforced optical fiber coil segment.

9. The system according to claim 1, wherein: The enhanced sensing array includes at least two sensing components respectively arranged as at least two cable joints, wherein a first cable joint of the at least two cable joints located relatively closer to the DAS station includes an unreinforced optical fiber coil segment, and wherein a second cable joint of the at least two cable joints located relatively farther away from the DAS station includes an enhanced optical fiber coil segment, wherein the enhanced optical fiber coil segment has a relatively higher Rayleigh backscattering coefficient than the unreinforced optical fiber coil segment.

10. A system for distributed acoustic sensing, comprising: Distributed acoustic sensing (DAS) stations that transmit DAS signals into the DAS optical fiber; An enhanced sensing array comprising a plurality of sensing elements arranged on a plurality of spans, wherein a given sensing element of the plurality of sensing elements comprises: a DAS fiber coil forming part of the DAS fiber; and A low elastic modulus housing surrounds the DAS fiber optic coil.

11. The system according to claim 10, wherein the plurality of sensing components are respectively formed in a plurality of cable joints.

12. The system of claim 11, each of the plurality of cable joints comprising a local DAS fiber optic coil, wherein a given cable joint is disposed within a given span of a multi-span communication system.

13. The system of claim 10, the low elastic modulus housing comprising polyethylene.

14. The system according to claim 10, wherein: the DAS optical fiber forms a first optical fiber of an optical fiber pair, wherein the first optical fiber is coupled to a DAS transmitter at the DAS station to conduct an outbound signal, and Wherein a second optical fiber of the optical fiber pair is arranged to conduct a return signal to a receiver at the DAS station, the return signal being derived from the outbound signal after being routed through a loop located at one or more spans.

15. The system of claim 12, wherein: Each span of the multi-span communication system includes only one cable connector having a DAS fiber optic coil.

16. The system of claim 12, wherein: A given span of the multi-span communication system comprises: a first cable joint located relatively closer to the DAS station, the first cable joint comprising an unreinforced optical fiber coil segment; and A second cable joint is located relatively farther from the DAS station, the second cable joint comprising an enhanced optical fiber coil segment, wherein the enhanced optical fiber coil segment has a relatively higher Rayleigh backscatter coefficient than the unenhanced optical fiber coil segment.

17. A method for distributed acoustic sensing, comprising: transmitting an outbound distributed acoustic sensing (DAS) signal from a DAS station into a DAS optical fiber; conducting the outbound DAS signal through a sensing component formed of a low elastic modulus material, the sensing component comprising a DAS optical fiber coil forming a portion of the DAS optical fiber; and After the outbound DAS signal passes through the DAS fiber coil, a power, phase, frequency, or polarization of a reflected signal is measured based on the outbound DAS signal.

18. The method according to claim 17, wherein: The sensing element is part of a sensing array comprising a plurality of sensing elements arranged in a linear manner, wherein each sensing element of the plurality of sensing elements comprises a local DAS fiber coil, wherein the outbound DAS signal is conducted through the plurality of sensing elements.

19. The method according to claim 18, wherein: The sensing component is one of a plurality of sensing components respectively arranged in a plurality of cable joints, each cable joint comprising a local DAS optical fiber coil, wherein at least one cable joint is arranged in a plurality of spans of a multi-span communication system.

20. The method according to claim 19, wherein: the DAS optical fiber forms a first optical fiber of an optical fiber pair, wherein the first optical fiber is coupled to a DAS transmitter at the DAS station to conduct an outbound signal, and Wherein a second optical fiber of the optical fiber pair is arranged to conduct a return signal to a receiver at the DAS station, the return signal being derived from the outbound signal after being routed through a loop located at the plurality of spans.

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