optical cable

Abstract

An optical cable (31) includes: a stress wave detection optical cable (30) having an optical fiber (7) and a plurality of first steel wires (8), the plurality of first steel wires being spirally wound around the optical fiber (7), and the optical fiber and the plurality of first steel wires being surrounded by a flexible material (9); and a second steel wire (32) different from the first steel wire (8). The stress wave detection optical cable (30) and the plurality of second steel wires (32) are spirally wound to form a toroidal body as a whole, and a winding angle (α) of the stress wave detection optical cable (30) relative to an axis is determined by a characteristic value prescribed by a Lame constant of the flexible material (9).

Description

Technical Field

[0001] The present disclosure relates to an optical fiber cable. Background Art

[0002] Traditionally, in oil well development, the technology of using seismic waves to detect underground resources has been widely used. Among them, the technology of obtaining sound waves, that is, the amount of strain change, is propagated in a distributed manner at each point on the optical fiber through distributed acoustic sensing (DAS). In this case, the optical fiber is installed in an optical cable and embedded in a position near the oil well pipe that constitutes the oil well or another underground passage. Then, the vibration waves from the ground or ocean vibration source are received, and then, by using vertical seismic profiling (VSP) or microseismic (MS) research for analysis, the three-dimensional structure of the underground part can be grasped (for example, refer to non-patent document 1).

[0003] There are the following problems when using the above method to detect underground resources.

[0004] First, when the optical fiber is installed approximately along the axial direction of the cable (for example, see Non-Patent Document 1), the optical fiber will not respond to seismic waves incident perpendicular to the direction of the cable. Figure 18 In the case of the horizontal well shown, the likelihood of non-response is particularly high.

[0005] That is to say, if Figure 18 As shown, when the cable 103 is laid along the well 102 on the seabed, the longitudinal laying direction of the optical fiber (not shown) as the sensor provided in the cable 103 is approximately horizontal ( Figure 18 Middle Arrow D H direction), while the direction of travel of the vibration wave from the vibration source is perpendicular to the horizontal direction ( Figure 18 Middle Arrow D v The direction of displacement in the optical fiber due to the vibration wave, that is, the direction causing the strain in the optical fiber, is also perpendicular to the horizontal direction. Therefore, when the vibration wave moves in this direction, it is difficult for the optical fiber to respond to the seismic wave.

[0006] Secondly, when the optical fiber is installed in a helical manner in the optical cable, the torsional deformation has a significant impact on the received signal. In particular, its response to shear waves is nonlinear, making it difficult to use for quantitative analysis.

[0007] Third, it is desirable to have a larger cable body (excellent performance is achieved with a diameter of 14 mm to 24 mm). The resolution of the longitudinal sensor increases with increasing diameter of the optical fibers wound around the cable's outer circumference (the cable body) and increases with decreasing spacing (see, for example, Patent Document 2). However, as mentioned above, a larger cable body may create insufficient space for the oil well, while conversely, reducing the cable body's size reduces the sensor's resolution. Furthermore, since the cable body is made of plastic or rubber (see, for example, Patent Document 2), the cable itself lacks strength and is likely to be inadequate in heat resistance.

[0008] Fourth, although there is a method of installing optical fiber in a well using a spiral tube or other technology, this is a short-term temporary method and is not suitable for long-term installation.

[0009] Reference List

[0010] Patent Literature

[0011] Patent Document 1: US2018 / 0274954 A1

[0012] Patent Document 2: US2018 / 0245957 A1

[0013] Non-patent literature

[0014] Non-Patent Document 1: Andreas Wuestefeld et al., “How to Twist and Turn Fiber: Performance Modeling for Optimizing DAS Acquisition,” THE LEADING EDGE, March 2019, pp. 306–311.

[0015] Non-Patent Document 2: B.N. Kuvshinov, “Interaction of Helically Wound Fiber Optic Cables with Plane Seismic Waves,” Geophsical Prospecting, Vol. 64, 2016, pp. 671-688. Summary of the Invention

[0016] Problems to be solved by the present invention

[0017] The present invention is to solve the above problems, and an object of the present invention is to provide an optical cable whose structure can effectively solve the first to fourth problems mentioned above to detect underground resources using seismic waves.

[0018] Problem Solution

[0019] The optical cable of the present invention is an optical cable for measuring stress waves generated by vibration of a measurement target, the optical cable comprising: a stress wave detection optical cable, which includes an optical fiber arranged at an axial portion, a plurality of first steel wires spirally wound to surround the optical fiber, and a flexible material surrounding the optical fiber and the plurality of first steel wires; and a plurality of second steel wires different from the plurality of first steel wires, wherein the stress wave detection optical cable and the plurality of second steel wires are spirally wound to form an annular body as a whole, and a winding angle of the stress wave detection optical cable relative to the optical cable axis is associated with a characteristic value specified by a Lame constant derived from the elastic modulus and Poisson's ratio of the flexible material.

[0020] Effects of the Invention

[0021] The optical cable according to the present invention can achieve a significant effect, that is, it can provide an optical cable having a structure that can effectively solve the first to fourth problems mentioned above, so as to detect underground resources using seismic waves. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1A and Figure 1B This is a model diagram for considering the shape of the optical fiber cable according to the first embodiment.

[0023] Figure 2A and Figure 2B is Figure 1A and Figure 1B Model diagram of the optical cable shown when it is subjected to axial stress.

[0024] Figure 3A 、 Figure 3B and Figure 3C is Figure 1A and Figure 1B Model diagram of the optical cable shown when subjected to radial stress.

[0025] Figure 4 This is a static load model diagram taking into account the specifications of the optical fiber cable according to the first embodiment.

[0026] Figure 5 The relationship between the optical fiber installation angle and the strain ratio parameter that satisfies the optical fiber cable specifications of the first embodiment is shown.

[0027] Figure 6 Specific numerical examples of the optical fiber installation angle, the strain ratio parameter, and the strain relative sensitivity ratio that satisfy the specifications of the optical fiber cable according to the first embodiment are shown.

[0028] Figure 7 This is a dynamic load model diagram taking into account the specifications of the optical fiber cable according to the first embodiment.

[0029] Figures 8A to 8D The configuration of a sample model for tentatively considering the specifications of the optical fiber cable according to the first embodiment is shown.

[0030] Figure 9A and Figure 9B A DAS measurement model for tentatively considering the specifications of the optical fiber cable according to Embodiment 1 is shown.

[0031] 10A to 10C The arrangement and connection method of optical fibers are shown, which experimentally considers the specifications of the optical fiber cable according to Embodiment 1.

[0032] Figures 11A to 11C This is a graph showing the relationship between the optical fiber installation angle and the normalized relative sensitivity, which is calculated by analyzing the core specifications of the optical fiber cable according to the first embodiment.

[0033] Figure 12 The configuration of experimental equipment used for an experiment considering the specifications of the optical fiber cable according to Embodiment 1 is shown.

[0034] Figure 13A and Figure 13B Shown with Figure 12 The experimental setup shown is an example of experimental results measured by DAS.

[0035] Figure 14 Shown with Figure 12 The experimental equipment shown shows the experimental results measured by DAS and the experimental results measured by frequency shift of Brillouin scattered light.

[0036] Figure 15A and Figure 15B Shown in Figure 12 An example of comparison between the experimental results of the experimental apparatus shown, measured by DAS, and the results obtained by analysis regarding normalized relative sensitivity.

[0037] Figure 16 The influence of the optical cable material on the relative sensitivity of the optical cable of Embodiment 1 is expressed by specific numerical values.

[0038] 17A to 17D An example of the optical fiber cable according to Embodiment 1 is shown.

[0039] Figure 18 It is a diagram for explaining a problem to be solved in this embodiment. DETAILED DESCRIPTION

[0040] Implementation Method 1

[0041] [Analysis Considerations - Static Load]

[0042] First, this embodiment considers optical cables that are affected by the direction of earthquake-induced vibration waves, or are likely to be affected in this way. The cable model considered is described below. Assuming a cylindrical cable, the specifications that the helically installed optical fibers must meet when a static load is applied to the cable are described below.

[0043] Figure 1A and Figure 1B These are model diagrams for a case where an optical fiber is helically installed in a cylindrical cable. These diagrams consider an optical cable in which the strain detected by the optical fiber is constant regardless of the incident direction of the stress wave, which serves as a model for seismic waves. Among the various optical fiber parameters considered, the fiber lay angle (also called the winding angle, hereinafter referred to as the fiber lay angle) and the strain ratio parameters between various different materials in the uniform stress model are particularly important (the details of these parameters are described below).

[0044] Figure 1A A model of an optical cable is shown. Figure 1A In the figure, the optical cable 3 is modeled as a core 1 and an optical fiber 2 spirally wound around the core 1. The three arrows represent the three-dimensional coordinate axes x, y, and z, and α represents the laying angle of the optical fiber 2 (hereinafter referred to as the optical fiber laying angle). The optical fiber laying angle α is defined as Figure 1B The angle between fiber 2 in and the x-axis (negative direction) on the xz plane, Figure 1B for Figure 1A The expanded pattern.

[0045] Here, for the optical cable 3, a ξηζ coordinate system is introduced, which is rotated by an angle of -θ about the y-axis relative to the xyz coordinate system. The reason is as follows. In the ξηζ coordinate system, when the strain ε0 occurs only in the ζ-axis direction, the relationship between strain and stress can be simply expressed by the Lame constants λ and μ as shown below. That is, if ε is satisfied ξ =0,ε η =0,ε ζ =ε0, then we get σ ξ =λε0、σ η =λε0、σ ζ =(λ+2μ)ε0.

[0046] In this case, the vertical component of the strain in the xyz coordinate system can be expressed as shown in the following calculation formula (1).

[0047]

[0048] Then, if Figure 2A and Figure 2B As shown, it can be assumed that the optical cable 3 is stressed in the z direction and a strain ε is generated therein. z In the case. Figure 2A and Figure 2B In the case shown, according to formula (1), the strain ε is used z and the fiber laying angle α, the strain of the optical fiber 2 is expressed as shown in equation (2).

[0049]

[0050] Then, if Figure 3A 、 Figure 3B 、 Figure 3C As shown, the optical cable is subjected to stress in the x direction and a strain ε is generated therein. x In the case of (See Figure 3C Specifically, the strain can be expressed by, for example, calculation formula (3) and calculation formula (4).

[0051]

[0052] Furthermore, a general formula including the above calculation formula is represented by calculation formula (5).

[0053]

[0054] In addition, when the optical cable is long enough and the full-circle throw of the optical cable is sufficiently smaller than the spatial resolution of the measuring instrument, the strain can be taken as the average value along the length of the optical cable as shown in formula (6).

[0055]

[0056] As described above, when the strain ε0 is generated only in the z-axis direction, the strain sensed by the optical fiber can be expressed as formula (7).

[0057]

[0058] Here, A is the strain ratio parameter, which is expressed in formula (8) and is used in Figure 4 A total of four Lame constants λ0, μ0, λ, μ for two different materials Z0, Z are shown in the one-dimensional uniform stress model including the two different materials Z0, Z. Here, the different materials Z0, Z correspond to, for example, bedrock and an optical cable as objects surrounding the optical cable.

[0059]

[0060] It should be noted that the above A is equal to the distance between two different materials. Figure 4The strain ratio calculated from the relationship between stress and strain in the one-dimensional uniform stress model shown. Specifically, the stress in the x-direction is constant across the surfaces of different materials. The constant stress value is represented by σ0, which is expressed as σ0 = (λ0 + 2μ0)ε0 = (λ + 2μ)ε. Therefore, ε / ε0 = (λ0 + 2μ0) / (λ + 2μ), with the value on the right equal to the value of A mentioned above.

[0061] From formula (7), we can know that the strain ε of the optical fiber is f Independent of the angular position of the fiber circumference That is, even if the direction of the strain is rotated around the z-axis, the result will not change. In addition, from the second term in equation (7), it can be seen that the lateral stiffness of the optical cable affects the sensitivity of the optical fiber, while the axial stiffness of the optical cable does not affect the sensitivity of the optical fiber.

[0062] Then, the optical fiber laying angle α and the strain ratio parameter A are set to satisfy the following equation (9).

[0063] A=2tan 2 α (9)

[0064] When α and A are set to satisfy the calculation formula (9), the strain detected by the optical fiber has a constant value expressed by the following calculation formula (10) regardless of the incident direction of the stress wave.

[0065] ε f =ε0sin 2 α (10)

[0066] Wherein, the optical fiber laying angle α and the strain ratio parameter A under the condition of satisfying formula (10) are respectively denoted as α opt and A opt , α opt and A opt The relationship as Figure 5 As shown, where the horizontal axis represents α opt , the left vertical axis represents A opt It should be noted that the right vertical axis represents the relative sensitivity of strain ε f / ε0. Figure 5 In the figure, the curve connecting the diamonds represents A opt Relative to α opt , and the curve connecting the squares represents ε f / ε0 relative to α opt The above α opt 、A opt and ε f The relationship between / ε0 is given by Figure 6 The specific numerical value in .

[0067] Specifically, for example, in αopt =73 degrees, if Figure 5 The dashed line parallel to the vertical axis is shown by Figure 6 It can be seen that when A opt When set to 21.4, the optical fiber can detect the stress waves of the earthquake, and the relative sensitivity ratio εf / ε0 is 0.91, which is close to 1. It can be seen from this that no matter what the incident direction of the stress wave is, the stress wave with almost no attenuation can be detected. In other words, it can be said that if the optical fiber laying angle α is set to satisfy the above relationship, the optical fiber can respond to the seismic wave regardless of the incident angle of the seismic wave. It should be noted that when using a hard outer fiber optic cable as the optical cable structure, A<1 can generally be satisfied, and thus it has a lower sensitivity. Therefore, in this embodiment, a part of the optical cable uses a flexible material to increase the sensitivity (the structure of the optical cable will be described in detail below).

[0068] In practice, due to various conditions, it is often difficult to set the relationship between α and A to satisfy equation (10), so the evaluation based on equation (7) is quite reasonable. Therefore, the influence of the incident angle θ of the sound wave (hereinafter referred to as the incident angle θ) as a parameter also needs to be considered.

[0069] [Analysis Considerations - Dynamic Loads]

[0070] In the above description, the static load situation has been considered. Figure 7 The model shown in the figure considers the case where the load is dynamic. Figure 7 In this example, x on the horizontal axis represents position, and t on the vertical axis represents time. In addition, ρ0 and ρ are the densities of the respective materials, and C0 and C are the speeds of sound in the respective materials. Furthermore, P and Q are symbols representing interfaces between different materials. In this case, we assume that the particle velocity ν ∞ and stress σ ∞ A plane wave is incident vertically on the interface of different materials and is reflected at interfaces P and Q in a multiple reflection manner.

[0071] exist Figure 7 Where ρ0 and ρ are the densities of the respective materials, C0 and C are the sound velocities in the respective materials. Under uniaxial strain, C0 and C are expressed by equations (11) and (12), respectively.

[0072]

[0073] The particle velocity and stress in the initial state are represented by v0 and σ0 respectively. Figure 7 The incident wave σ at the midpoint P1 i 1 , transmitted wave σ t 1 and the reflected wave σ r 1Respectively expressed as σ i 1 =-ρ0C0ν ∞ =σ ∞ , σ t 1 =-ρC(ν1-ν0)=σ1,σ r 1 =ρ0C0(ν1-ν0). From the force balance in the interface, σ i 1 +σ r 1 =σ t 1 Therefore, for k expressed in equation (13), the following equations (14) and (15) can be satisfied.

[0074]

[0075]

[0076] According to the incident wave σ i n The balance of forces at the interface (σ i n +σ r n =σ t n ),exist Figure 7 The transmitted wave σ at point Qn (n=2m, m is an integer not less than 1) and point Pn (n=2m+1, m is an integer not less than 1) in the t n and the reflected wave σ r n Similarly, the following recursive relations (16) and (17) are obtained.

[0077]

[0078] σ n =σ n-1 +(-1) n ρC(v n -v n-1 ) (17)

[0079] By solving the above recursive relation, we can get the value of v n and σ n Calculation formula (18) and calculation formula (19).

[0080]

[0081] Here, the calculation formula (20) can be satisfied for any positive value k.

[0082]

[0083] Therefore, according to equations (18) and (19), when n approaches ∞, v n Close to v ∞ And σ n Close to σ ∞ .

[0084] As described below, in this embodiment, the cable diameter is smaller than that of the cable in Patent Document 2, and a harder material can be used (specifically, a hard-shell cable can be used). This increases the speed, significantly increasing the aforementioned n, and also widening the response range.

[0085] This means that if the wavelength of the incident wave is large enough to be larger than the dimensions of many different materials, the stress will approach the incident stress σ through multiple reflections at the interface. ∞ That is, the same uniform stress model as that for static load can be established in the case of dynamic load. Therefore, it can be found that in the case of dynamic load, if α and A are set to satisfy the calculation formula (9), as in the case of static load, the optical fiber can detect the strain caused by the stress wave, regardless of the incident direction of the stress wave.

[0086] Here, as described above, since the stress in each layer is uniform, it can be found that the stiffness of the inner layer embedded with the optical fiber needs to be reduced to improve the sensitivity of strain measurement (it can be inferred that the sensitivity of the sensor in measuring strain can be improved by reducing the stiffness).

[0087] As described below, in this embodiment, the cable diameter is smaller than that of the cable in Patent Document 2, and a harder material can be used (specifically, a hard outer cable can be used). As a result, the speed can be increased, the aforementioned n can be significantly increased, and the response range can be wider.

[0088] The propagation difficulty of plane acoustic waves can be measured using the acoustic impedance I represented by the density ρr of the measurement object. z (I z =ρrCr) to evaluate, ρr is the medium that transmits plane acoustic waves, Cr is the sound velocity specific to the medium. Here, the volume elastic modulus Kr of the medium is used, and Cr is expressed as Cr = (Kr / ρr) 1 / 2 , so the acoustic impedance I z Equal to (ρr×Kr) 1 / 2 According to the multiple reflection theory, it is well known that the ratio between the amplitude of an acoustic wave in a measurement object and the amplitude of an acoustic wave transmitted to an optical fiber can be expressed by the ratio of their acoustic impedances.

[0089] [Consider it as an experiment]

[0090] Next, with respect to the results of the above analytical considerations, considerations conducted in the form of experiments are added in order to clarify the issues when implementing them. When measuring seismic waves, the distance from the source of the vibration is usually very far, so the seismic waves are often attenuated. Therefore, we believe that the degree of incident stress waves input to the optical cable becomes very small. Therefore, in order to detect seismic waves, distributed acoustic sensing (DAS) with excellent characteristics in terms of sensitivity is used. Hereinafter, experiments using DAS-based measurements (hereinafter, may be referred to as DAS measurements) will be described. Generally speaking, the spatial resolution of DAS is 20 cm or greater, which is much larger than the diameter of the optical cable. Therefore, strain measurements performed by DAS can be processed in the same manner as the above-mentioned static load measurements.

[0091] First, the experimental equipment for performing DAS measurements will be described with reference to the accompanying drawings. Figure 8A 、 Figure 8B 、 Figure 8C 、 Figure 8D The structure of the sample model is shown in FIG. Here, stress wave detection optical fibers 5a and 5b for DAS measurement are laid on the surface of a rectangular mortar block 10, in which a simulated cable 4 for verification is embedded (in the actual field, the experiment is conducted by replacing objects placed around the optical cable, such as mortar, bedrock, etc.). Regarding the strain detected by the simulated cable 4, in order to attempt to detect it using a semiconductor strain gauge 6, three types of semiconductor strain gauges are provided as shown in the figure, namely a trigger gauge 6a, an incident wave / reflected wave measuring instrument 6b, and a transmitted wave measuring instrument 6c. Therefore, these semiconductor strain gauges are used simultaneously for detection (see FIG. Figure 8A ).

[0092] The length L1 of the mortar block 10 is 1200 mm, and the cross-sectional dimensions are L3×L4 (see Figure 8B The length L2 of the stress wave catching block 11 is 350 mm, and its cross-sectional dimensions are the same as those of the mortar block 10 .

[0093] In addition, a plurality of simulated cables 4 with different optical fiber laying angles α are buried in one mortar block 10 (see the lower side of the hollow arrow in the figure). Figure 8C and Figure 8D The burying position is the same cross-sectional position of the mortar block 10 , and about three simulated cables 4 are buried at the same time.

[0094] Furthermore, in order to examine the influence of the incident angle of the sound wave on the simulated cable 4 , three values ​​were set as the incident angle in the experiment.

[0095] Furthermore, to investigate the influence of the core material of simulated cable 4, optical fibers were provided to simulated cable 4, and the core material of the simulated cable 4 used in the experiment was varied. Specifically, the fiber placement angle α was set to three values: 65 degrees, 73 degrees, and 90 degrees; the incident angle was set to three values: 60 degrees, 75 degrees, and 90 degrees; and the core material was made of aluminum and polyacetal resin (hereinafter referred to as POM material), both in a rod-like shape. In the case of aluminum, the experiment used not only strip cores but also ring cores (tubes) to examine the influence of the elastic modulus.

[0096] In addition, a shooting block was used to simulate the incident stress. The shooting block was made by bonding multiple sheets of polyvinyl chloride (PVC) or the like. A shooting device was also prepared and used to operate the shooting block.

[0097] Next, referring to the drawings, an experimental model for DAS measurement will be described. Figure 9A is a three-dimensional schematic diagram illustrating the experimental model. Figure 9B yes Figure 9A The top view when viewed along the direction of the hollow arrow. Figure 9A As shown, the shooting block collides with the center position R of the left side of the rectangular mortar block 10 where the simulated cable 4 is buried (corresponding to the center position R of the earthquake source) to generate sound waves (corresponding to the stress waves caused by the earthquake) inside the mortar block 10. At this time, the sound waves will travel along the Figure 9A and Figure 9B The direction of the arrow in FIG. 4 is forward and propagates at an incident angle θ relative to the longitudinal (axial) direction of the simulated cable 4 (see Figure 9B ).

[0098] Next, the arrangement and connection of optical fibers in the experiment are as follows Figure 10A 、 Figure 10B and Figure 10C shown. Figure 10A This is a 3D model diagram showing its appearance. Figure 10B It is from Figure 10A The view is viewed in the direction of arrow D, and Figure 10C The installation details of the optical fiber are shown. In actual measurement, Figure 10A and Figure 10B As shown, in addition to the simulated cable 4, the acoustic wave detection optical fiber is fixed on two sides, and the other multiple optical fibers are embedded in the mortar block along the longitudinal direction. Figure 10C To detect sound waves, all optical fibers are usually arranged in a continuous connection. Figure 10B and Figure 10C As shown, three simulated cables 4a, 4b, and 4c are arranged in layers from the upper side of the rectangular mortar block along its thickness direction. It should be noted that if the simulated cables arranged in multiple layers are partially broken during the assembly process, the broken cables need to be removed.

[0099] Here, we describe the results of a previously obtained theoretical analysis for comparison with experimental results. This analysis used aluminum bars, aluminum tubes, and POM rods (polyacetal resin rods) as core materials, and calculated the theoretical relationship between the fiber placement angle α and the normalized relative sensitivity for each material. Figure 11A 、 Figure 11B and Figure 11C The results are shown.

[0100] In each of these figures, theoretical analysis values ​​for three cases, θ = 60 degrees, 75 degrees, and 90 degrees, are shown, using the incident angle θ as a parameter. In each of these figures, it can be seen that in the region where the fiber laying angle α is less than the value at which the normalized relative sensitivity is 1, the change in normalized relative sensitivity with respect to the fiber laying angle α increases as the incident angle θ increases.

[0101] Here, in S(θ, α, A) defined by equation (21), the normalized relative sensitivity is A =65 degrees, calculated using equation (22) based on equation (21). Furthermore, the strain ratio parameter A is 0.233 for the aluminum bar, 0.695 for the aluminum tube, and 6.211 for the POM rod.

[0102]

[0103]

[0104] Figure 12 The configuration of the experimental equipment is shown. Using a damping device (not shown) including an electromagnet, a damping box, and a processor (not shown) within a strain gauge 20, which is enclosed by a dashed box, a steel ball is moved from a standby position on the damping device side, separated from support point P0 by a dashed distance L5, as indicated by the dashed line, along the dashed arrows surrounding support point P0, at the desired time, a distance equivalent to a horizontal distance L6, thereby colliding with the side of a mortar block 10. Three dummy cables 4 are provided within the mortar block 10, and a measuring optical fiber 5 approximately 500 meters in length is attached thereto. The acoustic waves generated by the collision of the steel ball with the mortar block 10 induce strain in the measuring optical fiber 5. The strain generated in the measuring optical fiber 5 is measured by the strain gauge 20. It should be noted that distance L5 is 90.5 cm, distance L6 is approximately 13 cm, and the steel ball's motion period at distance L6 is approximately 0.4 seconds.

[0105] Next, the following will describe the Figure 12 Results of measurements performed with the experimental equipment shown. Figure 13A An example of time domain measurement results is shown. Figure 13B An example of the measurement results in the frequency domain is shown. Figure 13AIn the figure, the solid curve represents the measurement results, and the vertical axis on the left and the horizontal axis on the bottom are the reference axes. In the reference axes, the vertical axis represents strain (unit: nε) and the horizontal axis represents the elapsed time (unit: ms). The measurement position is 559m. It can be found that the vibration waveform decays within about 100ms. On the other hand, the dotted curve represents the calculation result of the corresponding fast Fourier transform (FFT) of the time signal. The vertical axis represented by the dotted line on the right and the horizontal axis represented by the dotted line on the top are the reference axes. In the reference axes, the vertical axis represents strain (unit: nε) and the horizontal axis represents frequency (unit: Hz). According to the analysis results of FFT, the peak frequency is 1605.2Hz. Therefore, since the length of the mortar block is 1.2m, the speed of the sound wave is 3.85km / s (=1.2m×2×1605.2Hz).

[0106] Next, when a simulated cable having an aluminum core wire is used and the incident angle θ on the simulated cable is 60 degrees, Figure 13B This figure shows an example of DAS measurement results for acoustic waves (frequency component 1605 Hz). In this figure, the horizontal axis represents the measurement distance (position on the simulated cable) on the optical fiber (unit: m), and the vertical axis represents the amount of strain (unit: nε). The curve in the figure represents the measurement results, specifically, for a frequency component around 1605 Hz, indicating the amount of strain caused by the acoustic wave (peak component) generated in the optical fiber or measurement target cable at each measurement position (distance) along the simulated cable.

[0107] It can be seen from the figure that the maximum peak of strain occurs at a distance of about 560m. Figures 8A to 8D The strain generated in the stress wave detection optical fiber connected to the sample surface. In addition, it can be found that the second strain peak appears at a distance of about 580m. This is in the above Figures 8A to 8D The strain in the stress wave detection fiber embedded in the sample is detected. Using the strain in the stress wave detection fiber as an indicator, the strain in the simulated cable is analyzed, indicating the strain occurring at the simulated cable location indicated by the double arrows in the figure.

[0108] like Figure 13A and 13B As shown in the figure, the strain level (magnitude) in the simulated cable is smaller than the strain level in the stress wave detection fiber. Therefore, please refer to another figure. Figure 14 , which shows that the strain generated in the simulated cable can also be reliably detected.

[0109] Figure 14The figure shows the strain in the simulated cable and the data of Brillouin frequency shift measurement based on backscattered light, which is another strain measurement method. As can be seen from the figure, the Brillouin frequency shift appears at three locations A, B, and C on the simulated cable (A, B, and C correspond to the above Figures 10A to 10C The simulated cables 4a, 4b, 4c shown in FIG) (the strain in this case corresponds to Figure 14 ), it can be seen that sound waves can be measured even with analog cables.

[0110] Next, in order to confirm the reproducibility of the measurement and the accuracy of the measured strain magnitude (strain amount), the measurement results using another core wire will be displayed and compared with the theoretical value of the standardized relative sensitivity mentioned above. The results will be described below, with reference to Figure 15A shows the strain distribution of the cable in the longitudinal direction, and Figure 15B A comparison between the measured results and the theoretical values ​​based on the analysis is shown.

[0111] Figure 15A and Figure 15B The graph shows the measurement results of three times of strain caused by acoustic waves using a simulated cable with a POM rod as the core wire using DAS. The measurement results using the POM rod are used here because it is speculated that the use of the POM rod has the highest measurement sensitivity compared to the use of other core wires. In these graphs, Figure 15A The figure shows the measurement results of the strain induced by DAS when the incident angle θ is 60 degrees. The horizontal axis represents the measurement distance on the optical fiber (unit: m), and the vertical axis represents the induced strain (unit: nε). Figure 13A and 13B shown.

[0112] Furthermore, in order to compare the measured results with the theoretical values, Figure 15B The figure shows the comparison between the measured results and the theoretical values, where the horizontal axis represents the fiber laying angle α and the vertical axis represents the standardized relative sensitivity. Figure 15B As shown in the graphs in , it can be found that almost all the measured data are in good agreement with the theoretical values, except for one.

[0113] Furthermore, Figure 16 The specific values ​​of relative sensitivity depending on core wire differences are uniformly shown. It is well known that the elastic modulus of a POM rod is more than ten times smaller than that of an aluminum bar, meaning that the POM rod is softer than the aluminum bar. Consequently, the strain ratio parameter A of the POM rod is one or more orders of magnitude greater than that of the aluminum bar. Furthermore, it can be seen that for almost all laying angles, regardless of the angle of incidence, the relative sensitivity of the POM rod is greater than that of the aluminum bar. In other words, the softer the core wire material, the higher the relative sensitivity.

[0114] As described above, it has been found that even when seismic waves are incident in a direction perpendicular to or almost perpendicular to the longitudinal direction of the optical cable, by using an optical cable in which the optical fiber serving as the sensor is laid at a predetermined optical fiber laying angle α, opt Provided to the fiber optic cable, the generated seismic waves can be measured by DAS.

[0115] The representative structure of the optical cable that meets the above conditions is as follows 17A to 17D shown. Figure 17A This is a three-dimensional diagram showing the overall structure of this optical cable. Figure 17B Is along with Figure 17A The cross-sectional view of the optical cable is taken in a direction perpendicular to the longitudinal axis. Figure 17C is Figure 17B The cross-sectional view shown is a perspective view after all the components (several steel wires, etc.) of the outermost periphery are removed. Figure 17D is with Figure 17C A cross-sectional view of a section taken in a direction perpendicular to the longitudinal axis.

[0116] exist Figure 17A In the embodiment, the optical fiber 7a serving as a sensor is arranged at the central axis of the stress wave detection optical cable 30. The stress wave detection optical cable 30 has a structure in which the outer periphery of the optical fiber 7a is surrounded by a plurality of spirally twisted steel wires 8 (these steel wires may be referred to as first steel wires hereinafter). In addition, the outer periphery of the optical fiber 7a is protected by a flexible material 9. This flexible material is softer than the surrounding material surrounding the optical cable at the location where the optical cable is arranged (hereinafter referred to as the cable enclosure) (for example, the flexible material includes plastic). In addition, the cable enclosure 33 (not shown) includes bedrock or the like.

[0117] like Figure 17B As shown, the stress wave detection optical cable 30 is located at the outermost layer of the optical cable 31, and the optical cable 31 has a multi-layer structure in which the wires are wound in a multi-layer spiral manner. The outermost layer is formed into a ring shape by spirally twisting the stress wave detection optical cable 30 and a plurality of steel wires 32 (hereinafter referred to as the second steel wires) with approximately the same outer diameter. Therefore, the outer diameter of the second steel wire is larger than the outer diameter of the first steel wire. In addition, as shown in FIG. Figure 17B As shown, a flexible material 9a can be provided between the outermost layer and the adjacent inner layer to surround the entire circumference. The flexible material 9a is a slack layer formed of a flexible material that does not allow liquid to pass through. In addition, an optical fiber 7b having a pressure sensor function, which is different from the optical fiber 7a, is provided in the shaft portion of the optical cable.

[0118] like Figure 17C and Figure 17DAs shown, the relaxation layer can be replaced by a flexible material 9b, which is a water-permeable relaxation layer (the flexible material 9b is, for example, a layer formed into a mesh to allow liquid to enter from the outside). In this case, even if the optical fiber 7b provided in the central axis portion of the optical cable 31 is a pressure measurement sensor, it will not affect the pressure measurement. Therefore, the optical fiber 7b can also function as a sensor for stress wave detection. The optical fiber 7b is provided in a manner corresponding to the central axis of the optical cable 31, and the stress wave detection optical cable 30 is provided at a specific winding angle relative to the central axis. Therefore, when the two components are operated simultaneously, two incident angles can be obtained relative to the sound wave (or seismic wave), so it can be expected that the optical cable functions as a cable with higher sensitivity.

[0119] In the above configuration, the plurality of steel wires 32 can be partially replaced by an optical fiber cable encased in a metal tube, i.e., a fiber in metal tube (FIMT, abbreviation for "Fiber In Metallic Tube") 32a. As an alternative to the uniformly surrounding flexible material 9, a Kevlar fiber (softer than steel wire) having an outer diameter of approximately 5 μm can be spirally wound.

[0120] Here, the winding angle β of the stress wave detection optical cable 30 relative to the longitudinal axis of the optical cable is based on the above-mentioned optical fiber laying angle α. opt The winding angle β can be set and determined by the physical property values ​​of the flexible material 9 and the cable enclosure 33 (for example, bedrock). Specifically, based on the Lame constants λ9, μ9 and λ 33 、μ 33 (See calculation formula (23) and calculation formula (24)). The above calculation formula (8) and calculation formula (9) can be used to determine the winding angle β, which can be calculated from its elastic modulus E and Poisson's ratio ν.

[0121] Preferably, the flexible material 9 is provided not only on the outer periphery of the plurality of steel wires 8 , but also in the gaps between the optical fibers 7 a and the plurality of steel wires 8 .

[0122]

[0123] Although the present invention has been described above based on exemplary embodiments, it should be understood that the various features, aspects, and functions described in the embodiments are not limited to their applicability to the specific embodiments described, but can be applied alone or in various combinations in the embodiments of the present invention.

[0124] Therefore, it will be understood that many modifications not illustrated herein may be devised without departing from the scope of this disclosure. For example, at least one component may be modified, added, or eliminated. For example, in the above-described embodiment 1, the optical cable is described as being located in the outermost layer of a multilayer cable. However, this is not limiting, and the optical cable may be located in a layer inside the outermost layer to achieve the same effect.

[0125] Description of Reference Numerals

[0126] 1 core wire

[0127] 2,7a,7b optical fibers

[0128] 3,31 optical cable

[0129] 4,4a,4b,4c analog cables

[0130] 5. Measuring Fiber

[0131] 5a, 5b Stress wave detection optical fiber

[0132] 6 Semiconductor strain gauge

[0133] 6a Trigger

[0134] 6b Incident wave / reflected wave measuring instrument

[0135] 6c Transmission Wave Meter

[0136] 8,32 steel wire

[0137] 9,9a,9b Flexible materials

[0138] 10 mortar blocks

[0139] 11 Stress wave capture block

[0140] 20 Strain gauges

[0141] 30 Stress wave detection optical cable

[0142] 32a Optical fiber in metal tube

[0143] 33 Cable enclosure

[0144] A Strain ratio parameter

[0145] α Fiber laying angle

[0146] θ: angle of incidence of sound waves

Claims

1. An optical cable for measuring stress waves generated by vibration of a measurement target, the optical cable comprising: a stress wave detection optical cable comprising an optical fiber provided at an axial portion, a plurality of first steel wires wound in a spiral manner to surround the optical fiber, and a flexible material surrounding the optical fiber and the plurality of first steel wires; as well as A plurality of second steel wires different from the plurality of first steel wires, wherein The stress wave detection optical cable and the plurality of second steel wires are wound in a spiral manner to form a ring-shaped body as a whole; and The winding angle (α) of the stress wave detection optical cable relative to the axis of the optical cable is a function of a strain ratio parameter (A), wherein the strain ratio parameter (A) is a function of two Lame constants (λ, μ; λ0, μ0), Each of the Lame constants (λ, μ; λ0, μ0) is a function of the elastic modulus (E) and Poisson's ratio (ν) of the flexible material and the cable enclosure surrounding the cable at a location where the cable is arranged; in: A=2tan 2 a, Wherein, λ and μ represent the Lame constants of the flexible material, and λ0 and μ0 represent the Lame constants of the cable enclosure.

2. The optical cable according to claim 1, wherein The stress wave detection optical cable is located at the outermost periphery of the optical cable.

3. The optical cable according to claim 1, further comprising: a second optical fiber disposed at an axial portion of the optical cable and capable of measuring pressure; as well as The second flexible material is a water-permeable loose layer having a water-permeable property, and the second flexible material is opposite to the inner annular surface of the one annular body.

4. The optical cable according to any one of claims 1 to 3, wherein The gap between the optical fiber and the plurality of first steel wires is surrounded by the flexible material.

5. The optical cable according to claim 1, wherein The value of the strain ratio parameter of the flexible material is greater than the value of the strain ratio parameter of the cable enclosure.

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