A broadband enhanced acoustic wave sensing optical cable

Abstract

The present invention discloses a broadband enhanced acoustic wave sensing optical cable, comprising an outer sheath; one or more reinforcement members are arranged in the outer sheath; the outer sheath is covered on the outside of one or more sensing units; the sensing unit comprises a sensing optical fiber, a loose tube and a plurality of support members; the sensing optical fiber is sheathed with a plurality of support members; the support members are arranged in the loose tube to form a plurality of resonant cavities. The precise control of the number and spacing of the support segments enables the resonant frequency of the optical fiber in the optical cable to cover a specific range with high density, thereby realizing broadband enhanced perception of sound waves. Through the discrete distribution of the sensing units, segmented sensitivity of different frequencies (and regions) is achieved. The multi-core design ensures that the broadband response is enhanced while ensuring high spatial resolution. The sensing optical cable of the present invention greatly improves the sensitivity of acoustic wave perception, has a flexible design, and takes into account system parameters such as broadband response, high sensitivity, and high spatial resolution for distributed optical fiber acoustic wave perception technology, and has significant application value.

Description

Technical Field

[0001] The present invention belongs to the field of optical cables and sensing technologies, and in particular relates to a broadband enhanced acoustic wave sensing optical cable. Background Art

[0002] Distributed acoustic fiber sensing (DAS) is widely used in oil and gas exploration, pipeline monitoring, and perimeter security. By demodulating the phase of the Rayleigh scattering signal of light transmitted in the optical fiber, this technology can detect and locate the sound source / vibration source. The phase change of the Rayleigh scattering signal can be expressed as Calculated, where ΔL and Δn are the changes in fiber length and refractive index, respectively. To improve the cable's response to acoustic waves / vibration, significant axial or radial deformation of the fiber is typically attempted to achieve larger ΔL and Δn values.

[0003] Currently, the optical cables used for distributed fiber optic sensing are primarily commercial communications cables, which are not specifically optimized for acoustic wave sensing. Consequently, their sound and vibration detection capabilities still need to be significantly improved. Furthermore, in specialized applications such as pipeline leak monitoring, a significant portion of the vibrations generated by leaks fall within the mid- to high-frequency range of 500Hz to 2000Hz. These mid- and high-frequency vibrations decay faster in the medium than low-frequency signals, making them difficult for sensing systems to effectively detect.

[0004] To this end, some vibration sensitivity enhancement measures are used in the design and manufacture of optical fiber cables, with the aim of increasing the coupling effect of the sensing cable to vibration and increasing the response degree of the optical fiber cable material to vibration. For the former, the technical approach is usually to design special-shaped optical cables to increase the fitting area between the sensing cable and the object to be measured, thereby increasing the proportion of vibration coupling into the optical cable; for example, the butterfly-shaped cross-section optical cable given in Example 5 of the patent (CN201510913919.4) is designed to improve the fitting effect between the optical cable and the vibration source. For the latter, there are two main mainstream technical approaches: one is to use polymer materials such as thermoplastic elastomers as the coating layer, tight cladding layer or sheath of the optical fiber on the basis of the conventional optical cable structure, and to achieve optical fiber sensing performance enhancement by appropriately optimizing the structure of the reinforcing element. For example, Han Bing and others from the University of Electronic Science and Technology of China designed several polymer materials and reinforcing elements in the article "Distributed Acoustic Sensing With Sensitivity-Enhanced Optical Cable, IEEE Sensors Journal, 21: (4), 4644-4651". After actual measurement, they found that the soft optical cable made of PVC as the outer layer material has the best acoustic sensitivity enhancement effect; the second method is to use an elastomer material as the bobbin and tightly wind the optical fiber around the bobbin in the form of a spiral wire. The sound waves cause the bobbin to produce radial deformation while driving the optical fiber to produce axial deformation, thereby achieving phase change. The former is mostly used in land-based fields such as pipeline monitoring and perimeter security, while the latter is mostly used in the field of underwater acoustic detection. In addition, there are also patent reports on achieving vibration sensitivity enhancement by inducing air resonance around the optical fiber. For example, the patent (CN202022002730.9) designs a C-shaped metal belt and skeleton to reflect the sound waves multiple times, thereby causing resonance and amplifying the sound waves. However, the optical cable has a complex structure, and theoretically, the vibration frequency must reach hundreds of kilohertz to achieve the conditions for superposition of reflection peaks for sound waves with a wavelength of millimeters. This frequency band is far higher than the vibration frequency range of typical application scenarios.

[0005] It can be seen that the various sensitivity enhancement measures introduced above are often limited to optimizing the structure and materials of the optical cable cross-section, and do not change the uniformity of the optical cable structure along the axial direction, resulting in a single vibration sensitivity performance of the entire optical cable. For example, the frequency range that can respond with high sensitivity is narrow, which makes it difficult to cover the measured vibration frequency band in the application scenario.

[0006] In summary, in view of the deficiencies in the prior art, it is necessary to provide a broadband enhanced acoustic wave sensing optical cable for pipeline leak detection to improve the existing technical problems. Summary of the Invention

[0007] The purpose of the present invention is to address the deficiencies of the existing technology and provide a broadband enhanced acoustic wave sensing optical cable to effectively improve the acoustic sensitivity of the sensing optical cable used for pipeline leak detection, especially to enhance the response capability and response range of the optical cable to medium and high frequency vibrations, thereby improving the sensitivity of the pipeline leak detection system and reducing the system's missed alarm rate and false alarm rate.

[0008] The objective of the present invention is achieved through the following technical solutions: a broadband enhanced acoustic wave sensing optical cable, comprising an outer sheath; one or more reinforcement members are arranged in the outer sheath; the outer sheath is coated on the outside of one or more sensing units; the sensing units include a sensing optical fiber, a loose tube and a plurality of support members; the sensing optical fiber is sheathed with a plurality of support members; the support members are arranged in the loose tube and form a plurality of resonant cavities together with the sensing optical fiber.

[0009] Preferably, the loose tube is filled with an acoustic impedance matching medium to enhance the coupling between external sound waves or vibration signals and the sensing optical fiber.

[0010] Preferably, the acoustic impedance matching medium includes grease, silicone oil, water and air.

[0011] Preferably, the length of the sensing optical fiber segment between two adjacent support members, that is, the length of the resonant cavity, is 0.05m-2m.

[0012] Preferably, the length of the resonant cavity increases and / or decreases gradually.

[0013] Preferably, the cross-sectional shape of the loose tube is circular, square or triangular.

[0014] Preferably, the thickness of the support member is 2 mm-10 mm.

[0015] Preferably, the sensing units are discretely distributed.

[0016] Preferably, when multiple sensing units are arranged both near the outer circle of the optical cable cross section and near the center of the cross section, the resonant frequency of the sensing units near the outer circle is higher than the resonant frequency near the center, ensuring that high-frequency sound waves are attenuated to a minimum when entering the high-frequency resonant cavity.

[0017] Preferably, the resonant frequency Wherein, n is the order of the resonant frequency, u is the linear density of the sensing fiber, l is the length of the sensing fiber segment between two adjacent supports, that is, the length of the resonant cavity, and T is the tension on the sensing fiber.

[0018] The beneficial effects of the present invention are:

[0019] 1. The present invention achieves matching of the optical fiber resonant frequency and the detection frequency through the axial resonant cavity design, greatly enhancing the acoustic wave sensitivity and having high sensitivity characteristics;

[0020] 2. The present invention can achieve resonance enhancement of broadband response and frequency response enhancement of narrowband acoustic signals by tuning the fiber segment length, meeting the requirements of different application scenarios for high-sensitivity frequency response range of optical cables and having the characteristics of simple structure and flexible design.

[0021] 3. The broadband enhanced acoustic wave sensing optical cable of the present invention has a structure with discrete sensing units distributed, which can realize segmented sensitivity enhancement of specific frequency bands at specific locations as needed;

[0022] 4. The broadband enhanced acoustic wave sensing optical cable of the present invention adopts the design of a multi-core optical cable in structure. Each optical fiber core covers a different resonant frequency band at the same axial position. It can achieve broadband acoustic wave sensitivity enhancement in the axial direction of the short-distance optical cable and has the characteristics of high spatial resolution.

[0023] 5. The broadband enhanced acoustic wave sensing optical cable of the present invention has a multi-core optical cable design in which each optical fiber is close to the object to be measured, thereby achieving high-efficiency acoustic wave coupling;

[0024] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, the following preferred embodiments are specifically cited and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the cross-sectional structure and the axial cross-sectional structure of the preferred embodiment 1 of the present invention;

[0026] Figure 2 Schematic diagram of the distribution of the resonant frequency in the axial direction of the optical cable according to the preferred embodiment 1 of the present invention;

[0027] Figure 3 Schematic diagram of the axial cross-sectional structure of the loose tube and its internal components according to the preferred embodiment 2 of the present invention;

[0028] Figure 4 Schematic diagram of the distribution of the resonant frequency in the axial direction of the optical cable according to the preferred embodiment 2 of the present invention;

[0029] Figure 5 Schematic diagram of the axial cross-sectional structure of three loose tubes and their internal components according to preferred embodiments 3 and 4 of the present invention;

[0030] Figure 6 Schematic diagram of the distribution of the resonant frequencies in the axial direction of the optical cable according to the preferred embodiments 3 and 4 of the present invention.

[0031] Among them, sensing optical fiber 1, acoustic impedance matching medium 2, loose tube 3, outer sheath 4, reinforcement 5, support 6 DETAILED DESCRIPTION

[0032] Exemplary embodiments will be described in detail herein, examples of which are illustrated in the accompanying drawings. In the following description, when referring to the drawings, like numbers in different figures represent like or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments are not intended to represent all possible embodiments consistent with the present invention. Rather, they are merely examples of apparatus and methods consistent with certain aspects of the present invention, as detailed in the appended claims.

[0033] The terms used in this invention are for the purpose of describing specific embodiments only and are not intended to limit the invention. The singular forms "a," "the," and "the" used in this invention and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise. It should also be understood that the term "and / or" as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0034] It should be understood that although the terms "first," "second," "third," etc. may be used in the present invention to describe various information, such information should not be limited to these terms. These terms are merely used to distinguish information of the same type from one another. For example, first information may also be referred to as second information, and similarly, second information may also be referred to as first information, without departing from the scope of the present invention. Depending on the context, the term "if" as used herein may be interpreted as "when," "when," or "in response to determining."

[0035] The present invention will be described in detail below with reference to the accompanying drawings. Unless there is any conflict, the features of the following embodiments and implementations may be combined with each other.

[0036] The invention relates to a broadband vibration sensing optical cable for pipeline leak detection, such as Figure 1 and Figure 2 As shown, it comprises a sensing optical fiber (1), an acoustic impedance matching medium (2), a loose tube (3), an outer sheath (4), a reinforcement member (5) and a support member (6). The sensing unit is composed of the sensing optical fiber (1), the acoustic impedance matching medium (2), the loose tube (3) and the support member (6); a section of the sensing optical fiber (1) between two adjacent support members (6) forms a string, and a series of strings of different lengths have a series of high and low resonance frequencies.

[0037] The sensing optical fiber (1) is usually a tight-buffered optical fiber. The acoustic impedance matching medium (2) can be grease, silicone oil, water or air. The loose tube (3) is made of stainless steel, aluminum alloy, copper or hard plastic. The outer sheath (4) is made of polyethylene or polyurethane. The reinforcing member (5) is made of steel wire or fiber-reinforced composite plastic. The supporting member (6) is made of plastic or metal. The portion of the sensing optical fiber (1) in contact with the supporting member (6) is non-vibratory. The length of the sensing optical fiber segment between two adjacent supporting members is 0.05m to 2m.

[0038] Example 1 uses Figure 1 The cross-sectional structure and axial cross-sectional structure of the optical cable are shown. The cross-sectional shape of the loose tube is circular, where the circular cross-section is conducive to the coupling of sound waves in any direction. The length of the sensing optical fiber after being separated by the support is a structure that alternates between periodic increasing and decreasing phases. The length of a single sensing optical fiber segment ranges from 0.15m to 0.08m, with a decreasing and increasing step of 10mm. 14 segments are formed by 15 support members with a thickness of 2mm. The optical cable length corresponding to the monotonically decreasing and increasing period of a single segment length is approximately 1.64m.

[0039] According to the string vibration theory, when the optical fiber has a certain tension, each section of the optical fiber can be regarded as a string with a specific resonant frequency. According to the string vibration theory model, the resonant frequency of the optical fiber is Where n is the order of the resonant frequency, u is the linear density of the sensing fiber, l is the length of the sensing fiber segment between two adjacent supports (i.e., the length of the resonant cavity), and T is the tension on the sensing fiber. When n=1, the corresponding resonance is the fundamental frequency vibration, which is the most important vibration mode. In this embodiment, the linear density of the sensing fiber is 0.231g / m, the tension T is 20N, and l is substituted into the formula from 0.15m to 0.08m in steps of 0.01m to calculate the first-order resonant frequency from 982Hz (segment length 0.15m) to 1841Hz (segment length 0.08m), which is covered by 8 frequencies. Figure 2 The following is a schematic diagram of the resonant frequency distribution of the optical cable along the axial direction of this embodiment. The main parameters of the optical cable are shown in Table 1.

[0040] Example 2 uses Figure 1The cross-sectional structure and axial cross-sectional structure of the optical cable are shown. The lengths of the segments of the sensing optical fiber after being separated by the support members are arranged in a periodic decreasing manner. The lengths of the individual sensing optical fiber segments are 2m, 1m, 0.8m, 0.6m, 0.4m and 0.2m respectively. Six segments are formed by seven supports with a thickness of 10mm. The length of the optical cable corresponding to the decreasing period of a single segment length is about 5.07m. The linear density of the sensing optical fiber is 0.231g / m, and the tension T is 10N. By substituting it into the formula, it can be calculated that the first-order resonant frequency is from 52Hz (segment length 2m) to 2082Hz (segment length 0.2m), covering six frequencies of 52Hz, 104Hz, 130Hz, 174Hz, 260Hz and 521Hz. The main parameters of the optical cable are shown in Table 1.

[0041] Example 3 uses Figure 3 The axial cross-sectional structure of the optical cable is shown. The optical cable adopts discrete distribution of sensing units (that is, the optical cable section between the sensing unit and the adjacent sensing unit has no sensing function and only maintains light continuity. From the perspective of the entire sensing optical cable, the sensing units are not continuously distributed along the entire length, but only have sensing units at the position (area) that needs to be monitored. Therefore, the distribution of the sensing units is segmented and the sensing units are separated from each other, so it is called discrete distribution.) The corresponding resonant frequency distribution along the optical cable is as follows Figure 4As shown. The three areas of segmented sensitivity A, B, and C are the areas where the sensing units are distributed. The optical cable lengths corresponding to the areas without sensing units between the three areas are all about 8m. For the sensing units in area A, the segment lengths of the sensing optical fibers after being separated by the supports are a structure in which periodic increasing and decreasing phases are arranged alternately. The length of a single sensing optical fiber segment is 0.15m to 0.08m, with a decreasing and increasing step length of 10mm. 14 segment lengths are formed by 15 supports with a thickness of 2mm, so that the first-order resonant frequency is covered by 8 frequencies from 982Hz (segment length 0.15m) to 1841Hz (segment length 0.08m). The optical cable length corresponding to the monotonically decreasing and increasing periods of a single segment length is about 1.61m. For the sensing units in area B, the segment lengths of the sensing optical fibers after being separated by the supports are a periodically decreasing structure. The length of a single sensing fiber segment ranges from 0.24m to 0.17m, decreasing in 10mm increments. Nine 2mm-thick supports create eight different segment lengths, covering the first-order resonant frequency range from 614Hz (0.24m segment length) to 866Hz (0.17m segment length). The cable length corresponding to the monotonically decreasing and increasing periods of a single segment length is approximately 1.658m. For the sensing units in region C, the segment lengths of the sensing fibers, separated by supports, are periodically decreasing. The length of a single sensing fiber segment ranges from 0.32m to 0.25m, with a decreasing step of 10mm. Nine 2mm-thick support members form eight segments, achieving a first-order resonant frequency range from 460Hz (segment length 0.32m) to 589Hz (segment length 0.25m) across eight frequencies. The cable length corresponding to the monotonically decreasing and increasing periods of a single segment length is approximately 2.298m. The main cable parameters are shown in Table 1.

[0042] Example 4 uses Figure 5 (a) shows the cross-sectional structure of the optical cable. The lengths of the sensing optical fibers in the three sensing units (M, N, and R) separated by the support members are arranged in a periodic alternating pattern of increasing and decreasing phases. The cross-section of the sensing unit is triangular. The three sensing units are distributed in a triangular shape on the cross-section. When the sides of the triangle are perpendicular to the direction of sound wave propagation, it is beneficial to the coupling of sound waves. In this embodiment, the sides of the triangle are facing the sound source, so that each sensing unit can respectively and well receive the sound waves from the nearest sound source on its outside. The resonant frequencies of the three sensing units are distributed along the axial direction as shown in FIG. Figure 6As shown. In the sensing unit M, each resonant frequency is continuously arranged by a single segment of sensing optical fiber with a segment length of 0.068m to 0.052m, and the decreasing step length is 0.002m. A single cycle of alternating increase and decrease forms 16 segment lengths through 17 supports with a thickness of 10mm, so that the first-order resonant frequency is covered by 9 frequencies from 1531Hz (segment length 0.068m) to 2002Hz (segment length 0.052m), and the corresponding optical cable length is approximately 1.176m. In the sensing unit N, each resonant frequency is continuously arranged by a single segment of sensing optical fiber with a segment length of 0.1m to 0.07m, and the decreasing step length is 0.005m. A single cycle of alternating increase and decrease is formed into 12 segments through 13 supports with a thickness of 10 mm, so that the first-order resonant frequency is covered by 7 frequencies from 1041 Hz (segment length 0.1 m) to 1487 Hz (segment length 0.07 m), and the corresponding optical cable length is approximately 1.252 m. In the sensing unit R, each resonant frequency is continuously arranged by a single segment of sensing optical fiber with a segment length of 0.25 m to 0.11 m and a decreasing step length of 0.02 m. A single cycle of alternating increase and decrease is formed into 14 segments through 15 supports with a thickness of 10 mm, so that the first-order resonant frequency is covered by 8 frequencies from 416 Hz (segment length 0.25 m) to 947 Hz (segment length 0.11 m), and the corresponding optical cable length is approximately 2.782 m. The average number of resonant frequencies per meter of the sensing optical fibers in sensing units M, N, and R is 8.6, 6.1, and 3.0, respectively. By stacking the three-core sensing optical fibers in parallel in this embodiment, approximately 17.7 resonant frequencies per meter of optical cable can be achieved. The main parameters of the optical cable are shown in Table 1.

[0043] Example 5 uses Figure 5 In the optical cable cross-sectional structure shown in (b), the three sensing units (M, N, and R) are distributed in an arc shape on the cross section; the loose tube cross section is square, and the square side is perpendicular to the direction of sound wave propagation, which is conducive to the coupling of sound waves. In this way, when the optical cable is close to the measured pipeline on the side close to the sensing unit, the square side is facing the vibration sound source, and the resonant cavity in each sensing unit can well receive the sound waves emitted by the pipeline leakage. The concave arc cross-section of the optical cable is to better couple the sound waves transmitted in the direction opposite the arc. When the concave arc cross-section of the optical cable is changed to a plane, the three sensing units are distributed in a straight line, and one surface of each sensing unit is parallel to the outer plane of the optical cable. The distribution structure of the resonant cavity in the three sensing units is the same as that in Example 4, and the frequency distribution of the resonant cavity in the sensing unit is also the same as that in Example 4. The main parameters of the optical cable are shown in Table 1. It can be understood that the distribution of the sensing units can be adjusted according to the actual application and is not necessarily limited to linear distribution, triangular distribution, and arc distribution.

[0044] In combination with Examples 1-5, when the resonant frequency distribution is relatively uniform, broadband acoustic wave sensing enhancement can be achieved, taking into account both the spatial resolution of the resonant frequency and the intensity of the resonance (ie, the energy of the acoustic wave absorbed by a single resonant cavity).

[0045] Table 1: Parameters of the optical cable in this embodiment

[0046]

[0047]

[0048] The above embodiments are intended only to illustrate the design concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. The scope of protection of the present invention is not limited to the above embodiments. Therefore, any equivalent changes or modifications made based on the principles and design concepts disclosed in the present invention are within the scope of protection of the present invention.

[0049] Those skilled in the art will readily appreciate other embodiments of the present application after considering the specification and practicing the contents disclosed herein. This application is intended to cover any variations, uses, or adaptations of the present application that follow the general principles of this application and include common knowledge or customary techniques in the art not disclosed herein. The description and examples are to be considered as exemplary only.

[0050] It will be understood that the present application is not limited to the exact construction that has been described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof.

Claims

1. A broadband enhanced acoustic wave sensing optical cable, characterized in that: The invention comprises an outer sheath (4); one or more reinforcing members (5) are arranged in the outer sheath (4); the outer sheath (4) is coated on the outside of one or more sensing units; the sensing units comprise a sensing optical fiber (1), a loose tube (3) and a plurality of supporting members (6); the sensing optical fiber (1) is sheathed with a plurality of supporting members (6); the supporting members (6) are arranged in the loose tube (3) and form a plurality of resonant cavities together with the sensing optical fiber (1).

2. A broadband enhanced acoustic wave sensing optical cable according to claim 1, characterized in that: The loose tube (3) is filled with an acoustic impedance matching medium (2).

3. A broadband enhanced acoustic wave sensing optical cable according to claim 2, characterized in that: The acoustic impedance matching medium (2) includes grease, silicone oil, water and air.

4. A broadband enhanced acoustic wave sensing optical cable according to claim 1, characterized in that: The length of the resonant cavity is 0.05m-2m.

5. The broadband enhanced acoustic wave sensing optical cable according to claim 1, characterized in that: The length of the resonant cavity increases or / and decreases gradually.

6. The broadband enhanced acoustic wave sensing optical cable according to claim 1, characterized in that: The cross-sectional shape of the loose tube (3) is circular, square or triangular.

7. The broadband enhanced acoustic wave sensing optical cable according to claim 1, characterized in that: The thickness of the support member is 2mm-10mm.

8. The broadband enhanced acoustic wave sensing optical cable according to claim 1, characterized in that: The sensing units are discretely distributed.

9. The broadband enhanced acoustic wave sensing optical cable according to claim 1, characterized in that: When multiple sensing units are arranged both near the outer circle and near the center of the optical cable cross section, the resonant frequency of the sensing units near the outer circle is higher than the resonant frequency near the center, ensuring that high-frequency sound waves are attenuated to a minimum when entering the high-frequency resonant cavity.

10. The broadband enhanced acoustic wave sensing optical cable according to claim 9, characterized in that: The resonant frequency Wherein, n is the order of the resonant frequency, u is the linear density of the sensing fiber, l is the length of the sensing fiber segment between two adjacent supports, that is, the length of the resonant cavity, and T is the tension on the sensing fiber.

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