Fiber optic strain sensor and earthquake monitoring system

Abstract

The earthquake monitoring system comprises a fiber optic cable that defines a central axis and includes an axial fiber optic cable arranged along the central axis and three helical fiber optic cables arranged spirally around the central axis, the three helical fiber optic cables being arranged at equal intervals, a strain detection unit configured to measure the axial strain distribution in the axial fiber optic cable and the three helical fiber optic cables, and a processing server configured to calculate the axial strain distribution, pressure distribution, bending distribution in a first direction, and bending distribution in a second direction perpendicular to the first direction of the fiber optic cable from the measured axial strain distribution in the axial fiber optic cable and the three helical fiber optic cables.

Description

[Technical Field]

[0001] <Cross-reference with related applications> This disclosure asserts the interests of U.S. Provisional Patent Application No. 63 / 489,002, filed on 8 March 2023, the entire contents of which are incorporated herein by reference for all purposes.

[0002] This disclosure relates to surface earthquake fiber optic cables and monitoring systems. [Background technology]

[0003] Distributed fiber optic sensing (DFOS) technology is being developed for a wide range of applications, including monitoring seismic activity (e.g., earthquake early warning systems). Ideally, such systems should indicate the location and magnitude of seismic activity. However, existing systems employing DFOS technology for this application are either impractical or inaccurate. [Overview of the project]

[0004] The earthquake monitoring system comprises an optical fiber cable that defines a central axis and includes an axial optical fiber arranged along the central axis and three helical optical fibers arranged spirally around the central axis; a strain detection unit configured to measure the axial strain distribution in the axial optical fiber and the three helical optical fibers; and a processing server configured to calculate the axial strain distribution of the optical fiber cable, the pressure distribution of the optical fiber cable, the bending distribution of the optical fiber cable in a first direction, and the bending distribution of the optical fiber cable in a second direction perpendicular to the first direction, from the measured axial strain distributions in the axial optical fiber and the three helical optical fibers. [Brief explanation of the drawing]

[0005] Other features and advantages disclosed herein will become more apparent upon reading the following detailed description of exemplary embodiments in conjunction with the accompanying drawings.

[0006] [Figure 1] This is a perspective view of a partial cutout of an optical fiber cable according to one embodiment.

[0007] [Figure 2] This is a cross-sectional view of the optical fiber cable shown in Figure 1, with the spiral optical fiber removed.

[0008] [Figure 3] This is a partially enlarged view of the cross-sectional view in Figure 2.

[0009] [Figure 4] This is a schematic diagram of an embodiment of an earthquake monitoring system.

[0010] [Figure 5] This is a schematic diagram of an embodiment of a hardware processor.

[0011] [Figure 6] This is a schematic diagram of an optical fiber cable.

[0012] <Detailed explanation> A detailed description of embodiments of the surface seismic exploration fiber optic cable and monitoring system of this disclosure is given below with reference to the attached drawings. These embodiments illustrate examples of the surface seismic exploration fiber optic cable and monitoring system of this disclosure.

[0013] The optical fiber cable 1 according to the embodiment of the present disclosure is shown in Figures 1 to 3 and comprises an axial optical fiber 2 arranged along a central axis and three helical optical fibers 3a, 3b, and 3c arranged spirally around the central axis at equal intervals. In this embodiment, the axial optical fiber 2 and the three helical optical fibers 3a, 3b, and 3c are each coated with a coating 4 made of, for example, thermoplastic copolyester.

[0014] In this embodiment, multiple wires 5a to 5c are spirally wound around an axial optical fiber, and multiple wire ropes 6a to 6f are spirally wound around the multiple wires. In this embodiment, the wires and wire ropes are made of steel. In this embodiment, the multiple wire ropes 6a to 6f are fitted with sheaths made of elastomer material such as rubber (each consisting of a tubular inner sheath 7a and an outer sheath 7b). Three spiral grooves 8a to 8c are formed on the outer surface of the inner sheath 7a, through which the three spiral optical fibers 3a to 3c extend. The outer sheath 7b is fitted around the outer surface of the inner sheath 7a and the three spiral optical fibers 3a to 3c. With this arrangement, pressure, bending, and longitudinal strain applied to the entire optical fiber cable 1 cause corresponding movements in the three spiral optical fibers 3a to 3c. Furthermore, the axial optical fiber 2 is subjected only to longitudinal strain.

[0015] In the optical fiber strain sensor of this embodiment, the axial strain rate at each position of the axial optical fiber 2 and the helical optical fibers 3a, 3b, and 3c is measured using a known method. For example, in known optical fiber sensing techniques, an optical time-domain reflectometer (hereinafter referred to as "OTDR" as an example of an "optical interrogator") is used to inject optical pulses from the OTDR's tunable wavelength laser into the optical fiber, and the axial strain in the longitudinal direction of the optical fiber is measured using the travel time and length of the Rayleigh backscattered light detected by the OTDR's photodetector. Furthermore, by comparing these measured values ​​with the measured axial strain in the longitudinal direction of the optical fiber in its initial state, the change in the longitudinal strain of the optical fiber over time can be determined, and this strain distribution can be recorded in time series. In this embodiment, as shown in FIG. 4, the strain detection unit 9 is composed of optical interrogators 10a, 10b, 10c, and 10d corresponding to the axial optical fiber 2 and the helical optical fibers 3a, 3b, and 3c, respectively. As described above, in this embodiment, each of the optical interrogators 10a, 10b, 10c, and 10d is composed of an OTDR including a wavelength-variable laser and a photodetector. An example of a suitable optical time domain reflectometer (OTDR) is the Neubrescope NBX-7000 manufactured by Neubrex.

[0016] In the monitoring system of this embodiment, the processing server 11 receives the strain rate data of each part of the optical fiber in each section of the optical fiber cable acquired by the strain detection unit 9, and uses this to obtain the axial strain distribution of the optical fiber cable, the pressure distribution of the optical fiber cable, the bending distribution in the first direction of the optical fiber cable, and the bending distribution in the second direction perpendicular to the first direction in a time series manner. In earthquake monitoring applications, the optical fiber cable 1 of this embodiment is installed in a shallow trench, and by monitoring the above parameters regarding the optical fiber cable 1 and their changes over time in real time, the occurrence, scale, and origin of an earthquake can be detected in real time, and information regarding the earthquake including the above parameters can be displayed on a display or stored in the memory of the processing server 11. In particular, the forces of various types of seismic waves transmitted through the ground are transmitted to the buried optical fiber cable 1 having a length in kilometers, and thus it is also possible to perform triangulation using the above parameters to determine, for example, the intensity, direction, and origin of the seismic waves.

[0017] FIG. 5 shows a computer system 500 in which an embodiment of the present disclosure, or a part thereof, can be implemented as computer-readable code. For example, the processing server 11 in FIG. 4 can be implemented in the computer system 500 using hardware, software, firmware, a non-transitory computer-readable medium storing instructions, or a combination thereof, and can be implemented in one or more computer systems or other processing systems. Hardware, software, or any combination thereof can embody the modules and components used to execute the methods discussed herein.

[0018] When using programmable logic, such logic can be executed on a commercially available processing platform configured to be a special-purpose computer or a dedicated device (e.g., a programmable logic array, an application-specific integrated circuit, etc.) by executable software code. Those skilled in the art will understand that embodiments of the disclosed subject matter can be implemented in various computer system configurations, such as multi-core multiprocessor systems, minicomputers, mainframe computers, computers linked or clustered with distributed functions, and even pervasive computers or miniature computers that can be incorporated into virtually any device. For example, at least one processor device and memory can be used to implement the above-described embodiments.

[0019] The processor unit or device described herein may be a single processor, multiple processors, or a combination thereof. The processor device may have one or more processor "cores". The terms "computer program medium", "non-transitory computer-readable medium", and "computer-usable medium" described herein are generally used to refer to tangible media such as a removable storage unit 518, a removable storage device 522, and a hard disk mounted on a hard disk drive 512.

[0020] Various embodiments of this disclosure will be described using this exemplary computer system 500. Those skilled in the art will be able to see how the disclosure can be implemented using other computer systems and / or computer architectures. While operations may be described as sequential processes, in practice some operations are performed in parallel, concurrent, and / or distributed environments, and program code is stored locally or remotely and accessed from one or more processor machines. Furthermore, in some embodiments, the order of operations can be changed without departing from the spirit of the disclosed subject matter.

[0021] The processor device 504 may be a dedicated or general-purpose processor device specifically configured to perform the functions discussed herein. The processor device 504 may be connected to a communication infrastructure 506 such as a bus, message queue, network, or multicore message passing scheme. The network may be any network suitable for performing the functions disclosed herein and may include a local area network (LAN), a wide area network (WAN), a wireless network (e.g., Wi-Fi), a mobile communication network, a satellite network, the Internet, optical fiber, coaxial cable, infrared, radio frequency (RF), or any combination thereof. Other suitable network types and configurations will be apparent to those skilled in the art. The computer system 500 may also include a main memory 508 (such as random access memory or read-only memory) and a secondary storage device 510. The secondary storage device 510 may include a hard disk drive 512 and removable storage drives 514 such as a floppy disk drive, magnetic tape drive, optical disk drive, or flash memory.

[0022] The removable storage drive 514 can read data from and write data to the removable storage unit 518 in a well-known manner. The removable storage unit 518 may include removable storage media that can be read from and written to by the removable storage drive 514. For example, if the removable storage drive 514 is a floppy disk drive or a universal serial bus port, the removable storage unit 518 is a floppy disk drive or a portable flash drive, respectively. In one embodiment, the removable storage unit 518 is a non-temporary computer-readable recording medium.

[0023] In some embodiments, the secondary storage device 510 may include alternative means, such as removable storage units 522 and interfaces 520, that enable loading computer programs or other instructions into the computer system 500. Examples of such means may include program cartridges and cartridge interfaces (e.g., those found in video game systems), removable memory chips (e.g., EEPROM, PROM, etc.) and associated sockets, as well as other removable storage devices 522 and interfaces 520 that would be apparent to those skilled in the art.

[0024] The data stored in the computer system 500 (e.g., main memory 508 and / or secondary memory 510) can be stored in any suitable computer-readable medium, such as optical memory (e.g., compact discs, digital multipurpose discs, Blu-ray discs, etc.) or magnetic tape memory (e.g., hard disk drives). The data can be configured in any suitable database configuration, such as relational databases, structured query language (SQL) databases, distributed databases, or object databases. Suitable configurations and storage types should be obvious to those skilled in the art.

[0025] The computer system 500 may also include a communication interface 524. The communication interface 524 may be configured to transfer software and data between the computer system 500 and external devices. Examples of the communication interface 524 include modems, network interfaces (e.g., Ethernet cards), communication ports, PCMCIA slots and cards, etc. The software and data transferred via the communication interface 524 may be in the form of signals, which may be electronic signals, electromagnetic signals, optical signals, or other signals, as will be obvious to those skilled in the art. The signals may be transmitted via a communication path 526, which may be configured to carry signals and may be implemented using wires, cables, optical fibers, telephone lines, mobile phone links, radio frequency links, etc.

[0026] The computer system 500 may further include a display interface 502. The display interface 502 may be configured to transfer data between the computer system 500 and an external display 530. Examples of display interfaces 502 include high-definition multimedia interfaces (HDMI), digital visual interfaces (DVI), and video graphics arrays (VGA). The external display 530 is any suitable type of display for displaying data transmitted via the display interface 502 of the computer system 500, and includes cathode ray tube (CRT) displays, liquid crystal displays (LCDs), light-emitting diode (LED) displays, capacitive touch displays, and thin-film transistor (TFT) displays.

[0027] The computer program medium and computer-usable medium refer to memory, such as main memory 508 or secondary memory 510, and may be memory semiconductors (such as DRAM). These computer program products can serve as means for providing software to the computer system 500. The computer program (e.g., computer control logic) may be stored in the main memory 508 and / or secondary memory 510. The computer program may be received via the communication interface 524. When executed, such a computer program enables the computer system 500 to perform the methods described herein. In particular, when executed, the computer program enables the processor device 504 to perform the methods described herein. Thus, such a computer program can represent a controller of the computer system 500. If the present disclosure is implemented using software, the software may be stored in the computer program product and loaded into the computer system 500 using the removable storage drive 514, interface 520, and hard disk drive 512, or the communication interface 524.

[0028] The processor device 504 may comprise one or more modules or engines configured to perform functions of the computer system 500. Each module or engine may be implemented using hardware, but in some cases, program code and / or software corresponding to the program stored in main memory 508 or secondary memory 510 may also be available. In such cases, the program code may be compiled by the processor device 504 (for example, by a compilation module or engine) before being executed by the hardware of the computer system 500. For example, the program code may be source code written in a programming language that is translated into a low-level language such as assembly language or machine code so that it can be executed by the processor device 504 and / or any additional hardware components of the computer system 500. The compilation process may include lexical analysis, preprocessing, syntactic analysis, semantic analysis, syntax-based translation, code generation, code optimization, and the use of other techniques suitable for translating the program code into a low-level language suitable for controlling the computer system 500 to perform the functions disclosed herein. It will be apparent to those familiar with the relevant art that such processing results in the computer system 500 becoming a specially configured computer system 500, independently programmed to perform the functions discussed herein.

[0029] To determine the axial strain distribution, pressure distribution, bending distribution in the first direction, and bending distribution in the second direction perpendicular to the first direction of the optical fiber cable, the processing server 11 performs the following calculations: The strain of optical fiber 2 is ε f 0 Let the strain of the helical optical fibers 3a, 3b, and 3c be ε f 1 , ε f 2 , ε f 3Then, the axial strain ε of the optical fiber cable 1 axial and the corresponding strain of each optical fiber are represented by the following formula. TIFF2026512789000002.tif2581 Here, as shown in Fig. 6, α is the helical pitch angle (i.e., the winding angle) of the helical optical fibers 3a, 3b, and 3c.

[0030] Furthermore, the strain (hoop strain ε hoop ) due to the radial pressure in a specific cross-section of the optical fiber cable 1 and the corresponding strain of the optical fiber in that specific cross-section are represented by the following formula. TIFF2026512789000003.tif2581 Here, as shown in the following formula, the hoop strain ε hoop and the pressure P are in a proportional relationship. TIFF2026512789000004.tif2055 Here, k is a force constant that depends on the material and shape of the optical fiber cable.

[0031] Furthermore, the vertical bending curvature κ x , κ y in a specific cross-section of the optical fiber cable 1 and the corresponding strain of the optical fiber in that specific cross-section are represented by the following formula. TIFF2026512789000005.tif26118 Here, r is the helical winding radius of the helical optical fibers 3a, 3b, and 3c, and φ i is the azimuth angle of the optical fiber i in a specific cross-section as shown in Fig. 3. For example, in the cross-section shown in Fig. 3, the azimuth angle φ i of the optical fiber in the groove 7b is 0 degrees, the optical fiber in the groove 7c is 120 degrees, and for the optical fiber in the groove 7c is 240 degrees.

[0032] Combining these, the strain of each optical fiber at a specific cable cross-section / coordinate is as follows. TIFF2026512789000006.tif25155 Furthermore, from the above formula, the solution vector s (axial strain, pressure, and bending in two directions) and the strain ε of the optical fiber fThe relationship can be expressed using the coefficient matrix as follows: TIFF2026512789000007.tif30134

[0033] The inverse of this coefficient matrix is ​​the transfer matrix T, and the following relationship holds between the solution vector and the fiber strain. TIFF2026512789000008.tif43106

[0034] Furthermore, considering a moving average window l with a weight profile w(s), the fiber strain is calculated as follows: TIFF2026512789000009.tif29155 Here, the weight (w) represents the shape of the window in the weighted moving average equation. For example, in the case of a boxcar, the moving average w(s) = 1, and in the case of a Gaussian distribution, w(s) represents a bell curve. Furthermore, s is the integral variable of the distance along the optical cable, and l is the range of integration. By calculating these strains as a moving average rather than instantaneously at each point, physical phenomena can be reproduced more accurately.

[0035] Finally, the total cable length is discretized into nodes using the least squares method. Each node is represented by the solution vector s. i It has. TIFF2026512789000010.tif2966 is calculated as follows: (ε f m -ε f We obtain the least squares objective function for the minimum absolute value of ). TIFF2026512789000011.tif5970 Here, A is the coefficient matrix after taking the moving average over the integration range l. TIFF2026512789000012.tif5274 Applying the least squares method to moving-averaged strain is not feasible or practical because obtaining an accurate inverse analytical solution is impossible.

[0036] As described above, in this embodiment, the hardware processor 11 is programmed to perform the aforementioned calculations to determine the axial strain distribution of the optical fiber cable, the pressure distribution of the optical fiber cable, the bending distribution of the optical fiber cable in a first direction, and the bending distribution of the optical fiber cable in a second direction perpendicular to the first direction, in a time series, output the results to a display such as a corresponding graph or chart, and further analyze them to monitor for earthquake occurrences.

[0037] Those skilled in the art will understand that the disclosure herein can be implemented in other specific forms without departing from its spirit or essential features. Therefore, the embodiments disclosed herein are considered illustrative and not restrictive in all respects. The scope of this disclosure is defined by the appended claims rather than by the foregoing description, and their meaning and scope, as well as any modifications arising within the scope of equivalents, are encompassed within the claims.

Claims

1. An optical fiber cable having a defined central axis and axial optical fibers arranged along the central axis, and three helical optical fibers arranged spirally around the central axis, wherein the three helical optical fibers are arranged at equal intervals, An optical fiber strain sensor comprising: a strain detection unit configured to measure the axial strain distribution in the axial optical fiber and the three helical optical fibers.

2. The optical fiber strain sensor according to claim 1, characterized in that the axial optical fiber and the three helical optical fibers are each coated with thermoplastic copolyester.

3. The optical fiber strain sensor according to claim 1, characterized in that the optical fiber cable further comprises a plurality of wires spirally wound around the axial optical fiber.

4. The optical fiber strain sensor according to claim 3, characterized in that the optical fiber cable further comprises a plurality of wire ropes spirally wound around the plurality of wires.

5. The optical fiber strain sensor according to claim 1, characterized in that the optical fiber cable further comprises a sheath to which the three helical optical fibers are attached.

6. The optical fiber strain sensor according to claim 5, characterized in that the sheath is made of an elastomer material.

7. An earthquake monitoring system comprising an optical fiber strain sensor as described in claim 1, An earthquake monitoring system comprising: a processing server configured to calculate the axial strain distribution in the optical fiber cable, the pressure distribution in the optical fiber cable, the bending distribution in the first direction in the optical fiber cable, and the bending distribution in the second direction perpendicular to the first direction in the optical fiber cable, from the axial strain distribution measured by the axial optical fiber and the three helical optical fibers.

8. The earthquake monitoring system according to claim 7, characterized in that the axial optical fiber and the three helical optical fibers are each coated with thermoplastic copolyester.

9. The earthquake monitoring system according to claim 7, characterized in that the optical fiber cable further comprises a plurality of wires spirally wound around the axial optical fiber.

10. The earthquake monitoring system according to claim 9, characterized in that the optical fiber cable further comprises a plurality of wire ropes spirally wound around the plurality of wires.

11. The earthquake monitoring system according to claim 7, wherein the optical fiber cable further comprises a sheath to which three helical optical fibers are attached.

12. The earthquake monitoring system according to claim 11, characterized in that the sheath is made of an elastomer material.

13. A process for measuring the axial strain distribution in the axial optical fiber and three helical optical fibers of an embedded optical fiber cable, The process involves calculating the axial strain distribution of the optical fiber cable, the pressure distribution within the optical fiber cable, the bending distribution in a first direction of the optical fiber cable, and the bending distribution of the optical fiber cable in a second direction perpendicular to the first direction, based on the axial strain distribution of the measured axial optical fiber and three helical optical fibers. A method for monitoring earthquake activity, characterized by including the following:

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