Quasi-distributed sensing using enhanced sensing structures

Abstract

To provide quasi-distributed sensing using enhanced sensing structures.SOLUTION: A system for distributed acoustic sensing may comprise: a distributed acoustic sensing (DAS) station to launch a DAS signal into a DAS fiber; and an enhanced sensing array comprising at least one sensing component. The at least one sensing component may comprise: a DAS fiber coil forming a portion of the DAS fiber; and a low elastic modulus outer shell surrounding the DAS fiber coil.SELECTED DRAWING: Figure 2

Description

[Technical field]

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

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

[0003] Typically, a DAS system may include a DAS station, which acts as an interrogator unit (IU) and probes a fiber optic cable with coherent laser pulses representing an outbound DAS signal, where the change in phase of the returned optical backscattered signal is measured. The optical phase shift between the pulses may provide the ability to detect vibrations and the like proportional to strains in the fiber, as measured by the effect on phase due to such disturbances. For example, the DAS system may be based on Rayleigh scattering, and more specifically, on Rayleigh backscattering (also referred to as a Rayleigh scattering based DAS system).

[0004] In a known method, distributed acoustic sensing employs an optical cable that includes a fiber used as a DAS sensor. The optical cable may further include a steel wire, a steel belt, and an outer jacket. The DAS sensor fiber can sense acoustic waves along the cable length, which impinge on the cable and thereby transmit vibrations to the DAS sensor fiber, causing a phase shift in the DAS detected DAS signal (returned DAS signal) and returned to a detector and processor at the station. One problem with such known DAS methods is that the fiber sealed with a steel material layer lacks acoustic sensitivity, where the intensity of the acoustic wave is significantly attenuated before it reaches the DAS sensor fiber. Thus, current techniques for deploying DAS sensing on armored cables may exhibit unfavorable sensitivity to the detection of acoustic interference.

[0005] It is with respect to these considerations and others that the present disclosure is provided. Summary of the Invention [Means for solving the problem]

[0006] In one embodiment, a system for distributed acoustic sensing is provided, which may include a distributed acoustic sensing (DAS) station for transmitting a DAS signal to a DAS fiber, and an enhanced sensing array including at least one sensing member, which may include a DAS fiber coil forming a portion of the DAS fiber, and a low modulus outer shell surrounding the DAS fiber coil.

[0007] In another embodiment, a system for distributed acoustic sensing may include a distributed acoustic sensing (DAS) station for transmitting a DAS signal to a DAS fiber and an enhanced sensing array including a plurality of sensing elements arranged in a plurality of spans. A given one of the plurality of sensing elements may include a DAS fiber coil forming a portion of the DAS fiber and a low modulus outer shell surrounding the DAS fiber coil.

[0008] In yet another 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 to a DAS fiber and routing the outbound DAS signal through a sensing member formed of a low modulus material, the sensing member including a DAS fiber coil forming a portion of the DAS fiber. The method may further 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 fiber coil. [Brief description of the drawings]

[0009] [Figure 1] 1 shows a graph of the phase response of several different cable types. [Diagram 2] 1 illustrates a sensing element for enhanced DAS sensing according to an embodiment of the present disclosure, the sensing element being implemented as a cable joint. [Figure 3A] 13 illustrates an alternative sensing element for enhanced DAS sensing according to different embodiments of the present disclosure. [Figure 3B] 13 illustrates an alternative sensing element for enhanced DAS sensing according to different embodiments of the present disclosure. [Figure 3C] 13 illustrates an alternative sensing element for enhanced DAS sensing according to different embodiments of the present disclosure. [Figure 3D]13 illustrates an alternative sensing element for enhanced DAS sensing according to different embodiments of the present disclosure. [Figure 3E] 13 illustrates an alternative sensing element for enhanced DAS sensing according to different embodiments of the present disclosure. [Figure 4A] An enhanced distributed acoustic sensing system according to an embodiment of the present disclosure is described. [Figure 4B] 1 describes the geometry of an enhanced distributed acoustic sensing scenario according to an embodiment of the present disclosure. [Diagram 5] Another enhanced distributed acoustic sensing system according to an embodiment of the present disclosure is described. [Figure 6] A more enhanced distributed acoustic sensing system according to an embodiment of the present disclosure is described. [Figure 7] A further enhanced distributed acoustic sensing system according to an embodiment of the present disclosure is described. [Figure 8] 1 illustrates a process flow according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The present embodiments will now be described more fully with reference to the drawings, which show exemplary embodiments. The scope of the embodiments should not be construed as being limited to the embodiments described 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 drawings, the same reference numerals always refer to the same components.

[0011] Before describing specific embodiments in detail with reference to the drawings, let us review some general features of the embodiments: New DAS devices, systems, architectures and techniques are provided for improving DAS sensing capabilities.

[0012] In some embodiments, new enhanced sensing elements are provided based on coiled fiber, low modulus media, housings or outer shells or combinations of these elements. In some embodiments, new architectures are provided for DAS sensing, including arrays of multiple sensing elements performing what may be referred to as "quasi-distributed sensing."

[0013] By way of illustration, FIG. 1 shows a graph of the phase response of several different cable types. It should be noted that currently deployed optical cables, including those for undersea communication systems and DAS systems, all have very strong protection measures to avoid damage to the fibers carrying the optical signals for communication and / or acoustic sensing, for example. In deep water, they usually use what is called LW (lightweight) cables. LW cables are made of fibers housed in PBT (polybutylene terephthalate) tubes, then concentrically surrounded by multiple layers of steel wires, then copper conductors, and finally a polyethylene jacket used as an insulator. When deployed in shallow water, additional protection measures are usually required to prevent fishing activities, etc., and include "single armored" or "double armored" cables. These armored cables include an additional layer of galvanized wire, steel belts, and are surrounded by nylon threads on the outside. The steel wire, galvanized wire and steel belt all have a rigid structure and therefore exhibit a very large elastic modulus (wherein a very large elastic modulus may mean an elastic modulus greater than 100 GPa, more specifically, the Young's modulus is in the range of 150 GPa to 220 GPa).

[0014] In FIG. 1, five curves are shown illustrating five different cable types, where the phase response of the optical signal versus the strength of an external source or pressure is plotted. The occurrence of an acoustic event applies pressure to the core of the fiber to change the refractive index of the fiber, and this effectively compresses and expands the fiber, correspondingly changing the optical phase. By measuring the power spectral density of the optical phase at acoustic frequencies, the intensity of the acoustic wave can be determined. In FIG. 1, the results show that the detected optical phase is highly cable-dependent (where different cable types exhibit different Young's moduli depending on the construction materials that make up a given cable). A higher phase response at a given pressure indicates a higher responsivity, which means that an acoustic signal acting on a given cable will generate a stronger phase response.

[0015] In particular, Figure 1 compares the detected phase power (in dB_rad / √Hz) of five different cables as a function of pressure, where pressure is generated by acoustic waves. The behavior shown in these curves can be summarized as the stronger the protection of the cable, the weaker the phase response. As proposed in Figure 1, the use of loose tube cables (soft structured cables with very low Young's modulus, such as about 3 GPa to 4 GPa) provides substantially better response compared to other cables, for example, providing 26-dB more sensitivity compared to double armored cables (such rigid cables with large Young's modulus, such as about 72 GPa). However, loose tube cables cannot be deployed in undersea cables due to the lack of protection for the fibers therein. Therefore, optical communications and DAS sensing use cable technologies that generate low phase response, for example, as shown in the following four curves.

[0016] According to the results of FIG. 1, in DAS sensing, the existing more rigid cable structure typically adopted suffers from a relatively insensitive acoustic pressure phase response. To solve this problem, FIG. 2 shows a sensing member 100 according to an embodiment of the present disclosure, which can be used to significantly enhance DAS sensing sensitivity. The sensing member 100 in this example can be embodied as a cable joint, the structure including a housing 10. In some embodiments, the housing 10 can be formed of a low modulus material (e.g., polyethylene), as will be described in detail in the following figures. The sensing member 100 can further include a fiber coil 12, which can form a portion of a fiber that is coupled to a DAS station, as will be discussed in the following embodiments.

[0017] In operation, the sensing member 100 can be deployed in an undersea environment to detect acoustic signals such as signals from interference, including earthquakes, ships. The sensing member 100 can be deployed to perform "quasi-distributed acoustic sensing" because it can detect interference from sources, which may be located anywhere over a large area, for example, 50 km away from the sensing member 100, depending on the surrounding terrain conditions and other factors. As pointed out, the fiber coil 12 may be a portion of a fiber 14, which performs distributed acoustic sensing by sensing acoustic signals that may impinge on the fiber 14 over a length of the fiber 14, which may span tens of kilometers. However, the sensing member 100 can perform enhanced DAS sensing by concentrating sensitivity (e.g., phase response discussed in FIG. 1) to the location of the sensing member 100.

[0018] 3A-3E, alternative sensing elements for enhanced DAS sensing according to different embodiments of the present disclosure are shown. In these embodiments, each sensing element may be provided as a cable joint, each capable of detecting sound sources within 20 km or so. As described below in FIGS. 4A-7, these sensing elements may be deployed for use in DAS sensing within any length of sensing range, where a given sensing element may be deployed, by way of example, every tens of kilometers to provide quasi-DAS sensing within the entire sensing range.

[0019] 3A, the sensing member 100A is shown including a housing and shown as an outer shell 116 formed of a low modulus material (e.g., polyethylene). The outer shell 116 may include 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. Providing the sensing member 100A with a low modulus for the fiber coil 112 and the outer shell 116 may improve the overall phase response of the structure at the location of the sensing member 100A. Note that for the LW / SA cable, by eliminating the steel wire or copper / steel belt, the acoustic response is equivalent to a 0.25 mm coated fiber, providing a sensitivity higher than about 11.8 / 12.7 dB.

[0020] The length of the fiber coil 112 (and other fiber coils discussed) may be tens of meters, for example, 30 m to 100 m in some non-limiting examples. As a general rule, the length of the fiber coil 112 should be longer than the spatial resolution of the DAS device, where a 45 m long coiled fiber is long enough if a spatial resolution of 30 m is required. Making the fiber coil 1.5 times the length of the spatial resolution ensures that the response fully covers one spatial resolution location, regardless of where the boundary of the spatial resolution is in the DAS receiver. The rather long (100 m) length of the fiber coil 112 is useful in combating the signal fading effect due to the Rayleigh scattering effect. Because the fiber coil 112 can be deployed relatively loosely within the joint structure, the fiber coil 112 can respond well to changes in external sound pressure.

[0021] Note that in the embodiment of Figure 3A, the coil may be laid flat in a plane, and the sensitivity of the fiber coil per unit length of fiber may be similar to the sensitivity of a straight fiber per unit length, but the use of a coiled fiber helps provide longer fiber lengths (e.g., 30m to 100m) in a relatively small joint (e.g., 30cm to 50cm in length).

[0022] 3B, the sensing member 100B includes an outer shell 116, a steel enclosure 114, a fiber coil 122, and a mandrel 124. In this embodiment, the fiber coil 122 is tightly wound around the mandrel 124, which may be made of a low modulus material, such as polyethylene, which is otherwise solid. In the example of polyethylene having a Young's modulus of about 0.3 GPa, this modulus may provide a 35 dB improvement in pressure sensitivity compared to a standalone single-mode fiber (SMF). Additionally, the mandrel 124 may be a hollow cylinder to enhance the phase response to the acoustic signal. According to each embodiment, the sensitivity of the sensing member 100B depends on the ratio of the inner diameter to the outer diameter of the mandrel 124, and may be adjusted to achieve better acoustic sensitivity at an optimized inner diameter / outer diameter ratio. For example, a fiber length of 30 m to 100 m of fiber coil 122 wound around mandrel 124 is suitable according to some non-limiting embodiments of the present disclosure.

[0023] In particular, referring to FIG. 3C, a sensing member 100C is shown, which includes an outer shell 116, a steel layer 134 (which can be formed of wire and / or tape) and a fiber coil 132 (which can be arranged differently from the fiber coil 112). The fiber acoustic sensitivity is proportional to cos2(θ), where θ is the angle of incidence between the sound wave from the moving object and the sensing fiber. If θ=90° (the sound wave is perpendicular to the sensing segment), no acoustic signal can be detected. In FIG. 3A, the sensing fiber is arranged on a surface or plane, so that the sensitivity is small when the sound wave propagates perpendicular to the sensing fiber. By arranging the sensing fiber in a circular or spherical manner, as proposed for the fiber coil 132 in FIG. 3C, the angle θ defined by the fiber coil and the incident acoustic signal can be uniquely varied from 0 to 90° at different positions along the length of the fiber coil 132, thus providing a uniform sensitivity that is independent of the incident sound wave direction.

[0024] 3D, sensing member 100D is shown including an outer shell 116, a steel layer 144 (which may be formed of wire and / or tape), and a fiber coil 142. In this example, a portion of steel layer 144, such as adjacent fiber coil 142, may be removed to provide enhanced acoustic transmission and thus enhanced optical phase response. In some embodiments, fiber coil 142 may use an enhanced fiber structure that provides a higher Rayleigh backscattering response.

[0025] With particular reference to FIG. 3E, a sensing member 100E is shown, including a steel enclosure 154, a fiber coil 152, and a low viscosity gel 156 surrounding the fiber coil 152. In such a configuration, the fiber coil 152 can be arranged within the gel in a helical fashion, somewhat different from the wrapped coil structure of the embodiment of FIGS. 3A-3D. With proper design, the fiber length projected parallel to the steel tube can be similar to the fiber length projected perpendicular to the steel tube. Similar to the results of the embodiment of FIG. 3C, such a helical design can also provide uniform sensitivity that is independent of the angle of the incident sound wave from the moving object. Additionally, the low viscosity gel 156 can provide an enhanced phase response.

[0026] FIG. 4A illustrates an enhanced distributed acoustic sensing (DAS) system according to an embodiment of the present disclosure, shown as DAS system 200. In the example, DAS sensing is facilitated within a 50 km range and uses four sensing elements, which are arranged as a cable joint with a coiled fiber. For example, the sensing elements 204 can be arranged in a manner similar to any of the above-described embodiments shown in FIG. 3A-3E. The DAS system 200 includes a DAS station 202 arranged to transmit an outbound signal, for example, using a DAS transmitter, which may be referred to as an outbound DAS signal. DAS stations are known in the art, and details of the DAS station 202 will not be discussed further in this document. In the example, the distance (S) between adjacent sensing elements (referred to as sensing elements 204) of each pair is different. The different distances are set to accommodate the fact that the input power of each sensing element 204 is different due to fiber loss. The first of the sensing members 204 may be closest to the DAS station and have the highest power, therefore the first of the sensing members 204 may 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 members 204 may be determined to only be able to detect acoustic signals within a range of ±7.0 km, meaning from a distance of about 7 km closer to the DAS station 202 than the sensing member 204 to a distance of 7 km further from the DAS station 202 than the sensing member 204. The third of the sensing members 204 may be assumed to be able to detect acoustic signals within a range of ±4.2 km, and the last of the sensing members 204 is assumed to only be able to detect acoustic signals within a range of ±3.0 km, assuming that all of the sensing members 204 exhibit the same minimum responsivity (assuming that the sound source may be located at the same depth as the sensing members 204). The total maximum sensing range provided by the DAS system 200 can be calculated as 28.7km+11.1km+7.2km+3km=50km. In this example, it is assumed that the vessel is directly above the sensing cable.

[0027] 4A, FIG. 4B illustrates the geometry of an enhanced distributed acoustic sensing scenario according to an embodiment of the present disclosure. In the example, a portion of a variation of DAS system 200 is shown, including four sensing members 204, shown as sensing member 204A, sensing member 204B, sensing member 204C, and sensing member 204D. Although only four sensing members are shown in the example, it can be understood that a DAS system (e.g., DAS system 200) may include dozens of such sensing members.

[0028] Determining suitable configurations of sensing components When a DAS sensing system (e.g., DAS system 200) is deployed in a subsea environment, a sound source (e.g., vessel 222) located at the ocean surface 224 may not be located directly above the sensing cable that includes sensing members 204. Also, not all sensing members 204 may be located at the same depth below the ocean surface 224. Therefore, the spacing between sensing members 204 may be globally optimized to ensure similar minimum responsivity to the sensing members 204 within the coverage area of ​​each of the sensing members.

[0029] In one method, to establish a suitable configuration of the sensing members 204, one design principle is to set the optimum joint (sensing member 204) spacing (referred to as distance S) such that the DAS signal loss (round trip loss) between two adjacent joints is equal to the difference in transmission loss (TL) of the acoustic waves between the vessel 222 and the two adjacent joints. The transmission loss of the acoustic waves can be expressed as a simple formula: 2 It may be modelled as a function of distance or it may be based on more complex models including water loss, Lloyd's mirror effect, seabed reflection and absorption etc. Assuming the fibre loss is 0.2 dB / km and using the simple r2 law, the following equation can be set to optimise the joint spacing:

[0030]

number

[0031] As shown in FIG. 4B, different parameters are denoted as r1, r2, r3 and r4, which respectively represent the shortest distance from the vessel 222 to the sensing member 204A, the sensing member 204B, the sensing member 204C and the sensing member 204D at the three different depicted positions. For example, other parameters h1-h4 (not shown in FIG. 4B) can be set to represent the shortest horizontal distance from the vessel 222 to the sensing cable, and v1-v4 (not shown in FIG. 4B) can be set to the shortest vertical depth from the cable to the vessel 222. Thus, for a given set of h and v values, a corresponding value ρ can be determined, which represents the shortest distance from the vessel 222 to the given sensing member, where h 2 +v 2 =ρ 2 In other words, when a given one of the sensing members 204 is located directly beneath 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 ρ represents the hypotenuse of a right triangle. For each different sensing member, a set of parameters ρ1-ρ4 then represents the shortest distance from the vessel 222 to the corresponding sensing member 204A-204D. Also, a set of parameters d1, d2, d3, and d4 represent the corresponding maximum sensing distances along the fiber of the sensing member 204A, the sensing member 204B, the sensing member 204C, and the sensing member 204D, respectively. A parameter (D) can be configured to represent an overall maximum sensing distance (D=d1+2*d2+2*d3+2*d4), where the total length of the sensing fiber is equal to D-d4.

[0032] For Eq(1), if r2 or d2 is too large (too much fiber loss), the equation will not converge. In this case, we can only minimize the loss difference between the DAS signal loss in the fiber (assumed to be 0.2 dB / km in Eq(5)) and the acoustic loss difference between the two joints in the water.

[0033]

number

[0034] According to additional embodiments of the present disclosure, the sensing elements can be implemented in joints integrated into a multi-span amplification system with amplified-filtered loop back (AFLB). FIG. 5 illustrates one embodiment of such a system according to embodiments of the present disclosure. The system 250 of FIG. 5 includes a DAS station 202, a set of elements forming an outbound path 252, and another set of elements forming a return path 254. In the example of FIG. 5, three spans are depicted as span 260A, span 260B, and span 260C. However, it can be understood that the system of FIG. 5 can be implemented with N spans, where N represents any suitable integer. In the example, each span covering a distance of 50 km may require only one sensing element (joint) since the corresponding sensing element 262A, sensing element 262B, and sensing element 262C can be configured to detect acoustic signals within a range of + / - 25 km. It should be noted that these sensing members may generally be arranged as described in the embodiment of FIGS. 3A to 3E.

[0035] In addition to loopback 263, system 250 further includes a number of repeaters, shown as repeater 264A, repeater 264B, and repeater 264C, where each of these repeaters may be configured with an amplification, filtering, and loopback (AFLB) including couplers, Erbium-doped fiber amplifiers (EDFAs), switches, filters, and circulators, as known in the art. In system 250, DAS sensing of outbound DAS signals transmitted from DAS station 202 occurs on outbound path 252, and return path 254 is used to transmit the sensing signals back to DAS station 202 via loopbacks at each repeater location. Note that in this and other embodiments, a first fiber in the fiber pair is used to conduct the outbound DAS signal via outbound path 252, and a second fiber in the fiber pair is used to conduct the return signal via return path 254.

[0036] Another advantage provided by system 250 is that no transmission fiber is required in the last span since a (n-0.5)*50 km link can be sensed with only n joints (given the 50-km repeater spacing), and the signal to noise ratio (SNR) of the last sensing element (farthest from DAS station 202) is increased since there is no additional noise reflection from the "missing" span.

[0037] FIG. 6 illustrates a more enhanced distributed acoustic sensing system according to an embodiment of the present disclosure. The system 270 includes a DAS station 202 and a set of two sensing elements, denoted as sensing element 272 and sensing element 274, which may include enhanced fiber coil segments formed using enhanced coiled fiber material. These sensing elements may be generally configured as described in the embodiment of FIG. 3A-3E. For reference, recently, fibers with relatively high Rayleigh backscattering coefficients have been developed and used for distributed sensing. In particular, enhanced scattering fibers have been developed with about 15 dB OSNR improvement compared to standard SMF. However, such special fibers are most preferably used locally to improve DAS signal power. If such special fibers are used for the entire link, the noise power increases by the same 15 dB, and the OSNR is not improved overall due to the high loss in the special fibers.

[0038] The embodiment of FIG. 6 takes advantage of the benefits of a fiber sensor coil with enhanced scattering characteristics having higher Rayleigh backscattering, while avoiding the drawback of increased noise that would occur if a sensing element with an enhanced scattering fiber coil were deployed throughout the link.

[0039] To cover acoustic detection in a single span of approximately 55 km, only 37.5 km of fiber and two cable joints (sensing member 272 and sensing member 274) are used. The first joint, sensing member 272 (which may be considered to have a non-enhanced fiber coil segment), uses a coil made of SMF, and the second joint, sensing member 274, uses a coil made of enhanced scattering fiber. The DAS signal power is at the same level for sensing member 272 and sensing member 274, since the 15 dB improvement from enhanced scattering offsets the round trip loss (15 dB) from the 37.5 km of fiber. Thus, each of the sensing members can cover the same sensing distance (±18.25 km) with the same responsivity.

[0040] FIG. 7 illustrates a further enhanced distributed acoustic sensing system according to an embodiment of the present disclosure. In the example, system 280 is described covering two spans, span 290A and span 290B. However, it is understood that the system of FIG. 7 may be implemented with N spans, where N represents any suitable integer. The embodiment of FIG. 7 employs a heterogeneous arrangement of sensing elements, where in a given span, the sensing elements (sensing element 282A, sensing element 282B) with fiber coils made of standard SMF are deployed in a position closest to DAS station 202, and the sensing elements (sensing element 284A, sensing element 284B) with fiber coils made of enhanced scattering fiber are deployed in a position farther from DAS station 202. It should be noted that these sensing elements may be generally arranged as described in the embodiment of FIG. 3A-3E.

[0041] In addition to loopback 283, a loopback is further provided at repeater 286A and repeater 286B to route the outbound DAS signal on outbound path 292 back to the DAS station on return path 294. Again, sensing material is provided only on outbound path 292.

[0042] In the example shown in FIG. 7, by using standard fiber material joints (sensing elements 282A, 282B) and enhanced scattering fiber coil joints (sensing elements 284A, 284B) for a given sensing span, the repeater spacing is increased to 75 km from 50 km in the previous example. According to the above-mentioned embodiment of FIG. 6, the same responsivity in all sensing elements can be ensured by managing the accumulated amplified spontaneous emission (ASE). By increasing the repeater spacing to 75 km, such an embodiment requires only two repeaters compared to three repeaters in the configuration of FIG. 5, reducing costs.

[0043] In retrospect, the quasi-distributed fiber sensing method disclosed herein not only provides sensing for sound sources in nearby areas, but also provides detection for sound sources up to 50 km away, depending on the general conditions of the surrounding terrain. Thus, a given sensing element, such as a joint along the fiber detailed herein, can be deployed at intervals of about several kilometers to tens of kilometers to monitor sound sources within any desired coverage area, without the need for a fully distributed sensing system.

[0044] It should be noted that the relative spacing of the sensing elements in a quasi-distributed sensing system may be selected depending on the anticipated application. For example, to monitor a vessel and determine the vessel's position, speed and direction, it is most preferable to collect phase information received from 3-5 different locations. Thus, for vessel monitoring in a single-span or multi-span communication system, deployment of 3-5 sensing elements may be necessary.

[0045] Also, according to additional embodiments of the present disclosure, the sensing system can be implemented using a combination of a) several discrete joints containing fiber coils spaced apart from each other by several kilometers as primary sensors and b) distributed fiber sensors along the fiber length as secondary sensors to achieve a hybrid quasi-distributed sensing system.

[0046] 8 illustrates a process flow 800 according to an embodiment of the present disclosure. In block 802, an outbound distributed acoustic sensing (DAS) signal is transmitted from a DAS station to a DAS fiber.

[0047] At block 804, the outbound DAS signal is conducted by a sensing member formed of a low modulus material, the sensing member including a DAS fiber coil forming a portion of the DAS fiber.

[0048] In block 806, a Rayleigh backscatter signal generated by the DAS fiber coil based on, for example, the outbound DAS signal is monitored. In block 808, the power, phase, frequency, or polarization of the Rayleigh backscatter signal is measured, processed, and analyzed.

[0049] The quasi-distributed sensing method disclosed in this paper is The amount of digital signal processing required to extract the required sensing information can be significantly reduced. As an example, assuming a 50-km span length with a spatial resolution of 30 m, if a regular fully distributed sensing system is used, all phase information in a 50-km link (1667 x 30 m) needs to be processed. If a single sensing element (with a coiled fiber length of 90 m and assuming a spatial resolution of still 30 m) is used deployed at a cable joint, the same 50 km link needs to be monitored by processing information of 3 m x 30 m. However, the initial phase of phase calibration may require information from an auxiliary sensor. In short, compared with the standard fully distributed sensing system disclosed in the above embodiment, the quasi-distributed sensing system requires 1 / 100th the amount of digital signal processing. th can be reduced to.

[0050] Provided herein are new and inventive devices, systems, structures and techniques for providing an uncomplicated method of distributed acoustic sensing.

[0051] The present disclosure is not limited in scope by the specific examples described herein. Indeed, in addition to those described herein, various other embodiments and improvements of the present disclosure will be apparent to those skilled in the art from the above description and drawings. Thus, such other embodiments and improvements are intended to be included within the scope of the present disclosure. Also, although the present disclosure has been described in the context of specific embodiments for specific purposes in specific environments, those skilled in the art will recognize that its usefulness is not limited thereto, and the present disclosure may be advantageously implemented in any number of environments and for any number of purposes. Accordingly, the claims set forth below should be construed in accordance with the full breadth and spirit of the present disclosure as described herein.

Claims

1. a distributed acoustic sensing (DAS) station for transmitting a DAS signal to the DAS fiber; an enhanced sensing array including at least one sensing element; The at least one sensing member comprises: a DAS fiber coil forming a portion of the DAS fiber; and a low modulus outer shell surrounding the DAS fiber coil. A system for distributed acoustic sensing.

2. the at least one sensing member is formed at a cable joint; 10. A system for distributed acoustic sensing as recited in claim 1.

3. The low modulus outer shell comprises polyethylene.

10. A system for distributed acoustic sensing as recited in claim 1.

4. the at least one sensing member includes at least three enclosures, and a first spacing between a first sensing member closest to the DAS station and a second sensing member second closest to the DAS station is greater than a second spacing between the second sensing member and a third sensing member third closest to the DAS station; 10. A system for distributed acoustic sensing as recited in claim 1.

5. the at least one sensing member includes a plurality of enclosures respectively disposed at a plurality of cable joints, each cable joint including a local DAS fiber coil, and the at least one cable joint is disposed at one or more spans of a multi-span communication system; 10. A system for distributed acoustic sensing as recited in claim 1.

6. the DAS fiber forms a first fiber in a fiber pair, the first fiber being coupled to a DAS transmitter in the DAS station to conduct outbound signals; and a second fiber in the fiber pair is positioned 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 loopback located in the one or more spans.

6. A system for distributed acoustic sensing as recited in claim 5.

7. a plurality of cable joints are disposed in predetermined spans of the multi-span communication system; 6. A system for distributed acoustic sensing as recited in claim 5.

8. a first cable joint of the plurality of cable joints located relatively closer to the DAS station includes a non-enhanced fiber coil segment, and a second cable joint of the plurality of cable joints located relatively farther from the DAS station includes an enhanced fiber coil segment, the enhanced fiber coil segment having a relatively higher Rayleigh backscattering coefficient than the non-enhanced fiber coil segment.

8. A system for distributed acoustic sensing as recited in claim 7.

9. the enhanced sensing array includes at least two sensing elements respectively arranged as at least two cable joints, a first cable joint of the at least two cable joints located relatively closer to the DAS station includes a non-enhanced fiber coil segment, and a second cable joint of the at least two cable joints located relatively farther from the DAS station includes an enhanced fiber coil segment, the enhanced fiber coil segment having a relatively higher Rayleigh backscattering coefficient than the non-enhanced fiber coil segment. A system for distributed acoustic sensing according to any one of claims 1 to 8.

10. a distributed acoustic sensing (DAS) station for transmitting a DAS signal to the DAS fiber; an enhanced sensing array including a plurality of sensing elements arranged in a plurality of spans; A predetermined sensing member among the plurality of sensing members is a DAS fiber coil forming a portion of the DAS fiber; and a low modulus outer shell surrounding the DAS fiber coil. A system for distributed acoustic sensing.

11. The plurality of sensing members are formed at a plurality of cable joints, respectively.

11. A system for distributed acoustic sensing as recited in claim 10.

12. each of the plurality of cable joints includes a local DAS fiber coil, a given cable joint being disposed in a given span of a multi-span communication system; 12. A system for distributed acoustic sensing as recited in claim 11.

13. The low modulus outer shell comprises polyethylene.

11. A system for distributed acoustic sensing as recited in claim 10.

14. the DAS fiber forms a first fiber in a fiber pair, the first fiber being coupled to a DAS transmitter in the DAS station to conduct outbound signals; and a second fiber in the 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 loopback located in one or more spans; A system for distributed acoustic sensing according to any one of claims 10 to 13.

15. each span of the multi-span communication system includes only one cable joint having a DAS fiber coil; 13. A system for distributed acoustic sensing as recited in claim 12.

16. A given span of the multi-span communication system comprises: a first cable joint located relatively close to the DAS station and including a non-enhanced fiber coil segment; a second cable joint located relatively far from the DAS station and including an enhanced fiber coil segment having a relatively higher Rayleigh backscattering coefficient than the non-enhanced fiber coil segment.

13. A system for distributed acoustic sensing as recited in claim 12.

17. transmitting an outbound distributed acoustic sensing (DAS) signal from a DAS station to a DAS fiber; conducting the outbound DAS signal through a sensing member formed of a low modulus material, the sensing member including a DAS fiber coil forming a portion of the DAS fiber; and 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 fiber coil. A method for distributed acoustic sensing.

18. the sensing element is part of a sensing array, the sensing array being a plurality of sensing elements arranged in a linear fashion, each of the plurality of sensing elements including a local DAS fiber coil, and the outbound DAS signal being conducted by the plurality of sensing elements; 20. A method for distributed acoustic sensing as claimed in claim 17.

19. the sensing member being one of a plurality of sensing members each disposed at a plurality of cable joints, each cable joint including a local DAS fiber coil, and at least one cable joint disposed at a plurality of spans of a multi-span communication system; 20. A method for distributed acoustic sensing as claimed in claim 18.

20. the DAS fiber forms a first fiber in a fiber pair, the first fiber being coupled to a DAS transmitter in the DAS station to conduct outbound signals; and a second fiber in the 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 loopbacks located in the plurality of spans.

20. A method for distributed acoustic sensing as claimed in claim 19.

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