Optical Cable

Abstract

The optical cable (30) for detecting stress waves comprises an optical fiber (7) covered with a plurality of spirally wound first steel wires (8) of an optical cable (31), the plurality of first steel wires (8) covered with a flexible body (9), and a second steel wire (32) different from the first steel wire (8), wherein the optical cable (30) for detecting stress waves and the plurality of second steel wires (32) are each spirally wound to form a circular body as a whole, and the winding angle (α) of the optical cable for detecting stress waves with respect to the axis of the optical cable (30) is determined by a characteristic value defined by the Lame's constant of the flexible body (9).

Description

[Technical field]

[0001] This application relates to optical cables. [Background technology]

[0002] In the past, when developing oil wells, a technology for detecting underground resources using seismic waves has been widely adopted. Among them, a technology for acquiring acoustic waves at each point on an optical fiber, i.e., the amount of strain change, distributedly by DAS (Distributed Acoustic Sensing) is widely used. In this case, an optical fiber is mounted on an optical cable and buried in a place close to the oil well tubular goods that constitute the oil well or in another underground route to receive waves from a vibration source on the ground or sea, and then an analysis using a VSP (Vertical Seismic Profiling) method or a study on MS (Micro Seismic) is performed to grasp the three-dimensional structure of the underground (see, for example, Non-Patent Document 1).

[0003] When detecting underground resources using the above-mentioned techniques, the following problems arise. First, when the optical fiber is installed almost in the axial direction of the cable (see, for example, Non-Patent Document 1), there is a problem that the optical fiber does not respond to seismic waves that are incident perpendicularly to the optical cable. In particular, in the case of a horizontal well as shown in Figure 18, the possibility of the optical fiber not responding is particularly high.

[0004] That is, as shown in FIG. 18, when a cable 103 is laid along a borehole 102 under the seabed, the longitudinal direction of an optical fiber (not shown) that is a sensor provided in the cable 103 is arranged in a substantially horizontal direction (arrow D in the figure). H The direction of the vibration waves from the earthquake source is perpendicular to the horizontal direction (arrow D in the figure). VThe direction of displacement caused by the vibration waves in the optical fiber, i.e., the direction that causes distortion in the optical fiber, is also perpendicular to the horizontal direction. Therefore, when the vibration waves are in this direction, the optical fiber has a hard time responding to seismic waves.

[0005] Secondly, when the optical fiber is mounted in a cable in a spiral direction, the received signal is significantly affected by torsional deformation, and the response to transverse waves in particular is nonlinear, making it difficult to use for quantitative analysis. Thirdly, the larger the cable body is, the better (a diameter of 14mm to 24mm shows good performance), and the resolution of the sensor in the longitudinal direction is better the larger the winding diameter (cable body) of the fiber wound around the cable and the smaller the pitch (see Patent Document 2, for example), but if the cable body is this large, there is a risk that there will not be enough space in the oil well, and conversely, if the cable body is made smaller, the resolution of the sensor will deteriorate. Also, since the cable body is made of plastic or rubber (see Patent Document 2, for example), there is a high possibility that the strength of the cable itself will be insufficient and that its heat resistance will also be insufficient. Fourth, while there are methods for installing optical fiber in a borehole using techniques such as coiled tubing, this method is a temporary method and is not suitable for long-term installation. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] US Patent Application Publication No. 2018 / 0274954 [Patent Document 2] US Patent Application Publication No. 2018 / 0245957 [Non-patent literature]

[0007] [Non-Patent Document 1] Andreas Wuestefeld, et al., “How to twist and turn a fiber: Performance modeling for optimal DAS acquisitions”, THE LEADING EDGE, March 2019, pp.306-311. [Non-Patent Document 2] BNKuvshinov, “Interaction of helically wound fiber-optic cables with plane seismic waves”, Geophsical Prospecting, 2016, 64, pp.671-688. Summary of the Invention [Problem to be solved by the invention]

[0008] The present application discloses technology for solving the above-mentioned problems, and aims to provide an optical cable having a structure that can effectively solve the above first to fourth problems in order to detect underground resources using seismic waves. [Means for solving the problem]

[0009] The optical cable disclosed in the present application comprises: An optical cable for measuring stress waves generated by vibration of a measurement object, an optical cable for detecting stress waves, the optical cable having an optical fiber disposed in an axial center portion, a plurality of first steel wires wound in a spiral shape so as to cover the optical fiber, and a flexible body covering the optical fiber and the plurality of first steel wires; a second steel wire different from the first steel wire; Equipped with The optical cable for detecting stress waves and the plurality of second steel wires are each wound in a spiral shape to form a circular body as a whole, and the winding angle of the optical cable for detecting stress waves with respect to the axis of the optical cable is related to a characteristic value defined by Lame's constant derived from the elastic coefficient of the flexible body and the Poisson's ratio. Effect of the Invention

[0010] According to the optical cable disclosed in the present application, since underground resources are detected using seismic waves, it is possible to obtain the remarkable effect of providing an optical cable having a structure that can effectively solve the above first to fourth problems. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a model diagram for examining the shape of the optical cable according to the first embodiment. [Diagram 2] FIG. 2 is a model diagram of the optical cable of FIG. 1 when stress is applied from the axial direction. [Diagram 3] FIG. 2 is a model diagram of the optical cable of FIG. 1 when stress is applied from a radial direction. [Figure 4] FIG. 2 is a diagram showing a static load model for examining the specifications of the optical cable according to the first embodiment. [Diagram 5] 4 is a diagram showing the relationship between the optical fiber installation angle and the distortion ratio parameter that satisfies the specifications of the optical cable according to the first embodiment. FIG. [Figure 6] 4 is a diagram showing specific numerical examples of optical fiber installation angles, distortion ratio parameters, and relative sensitivity ratios to distortion that satisfy the specifications of the optical cable according to the first embodiment. FIG. [Figure 7] FIG. 2 is a diagram showing a dynamic load model for examining the specifications of the optical cable according to the first embodiment. [Figure 8] FIG. 2 is a diagram showing a configuration of a test model for experimentally examining the specifications of the optical cable according to the first embodiment. [Figure 9] FIG. 2 is a model diagram for DAS measurement for experimentally examining the specifications of the optical cable according to the first embodiment. [Figure 10] FIG. 2 is a diagram showing the arrangement and coupling of optical fibers for experimentally examining the specifications of the optical cable according to the first embodiment. [Figure 11] 4 is a diagram showing the relationship between the optical fiber installation angle and the normalized relative sensitivity obtained by analysis in order to consider the specifications of the core material of the optical cable according to the first embodiment. FIG. [Figure 12] FIG. 2 is a diagram showing a configuration of an experimental device for experimentally examining the specifications of the optical cable according to the first embodiment. [Figure 13] FIG. 13 is a diagram showing an example of an experimental result measured by a DAS using the experimental apparatus of FIG. 12. [Figure 14] FIG. 13 shows experimental results measured by DAS using the experimental apparatus of FIG. 12 and experimental results measured by frequency shift of Brillouin scattered light. [Figure 15] FIG. 13 is a diagram showing an example of a comparison between the experimental results measured by DAS using the experimental apparatus of FIG. 12 and the results obtained by analysis regarding normalized relative sensitivity. [Figure 16] 4 is a diagram showing an example of numerical values ​​of the influence of the cable material on the relative sensitivity of the optical cable according to the first embodiment. FIG. [Figure 17] FIG. 1 is a diagram showing an example of an optical cable according to a first embodiment. [Figure 18] FIG. 1 is a diagram for explaining a problem to be solved according to the present embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Embodiment 1 [Analytical study - static load] First, we will consider optical cables that are affected or susceptible to the direction of vibration waves caused by earthquakes. We will first explain the model of the optical cable used in this study. Assuming that the optical cable is cylindrical, we will explain the specifications that the helically attached optical fiber must satisfy when a static load acts on this cable.

[0013] Figures 1(a) and 1(b) are model diagrams of a cylindrical cable with an optical fiber attached in a spiral shape. In these figures, we consider an optical cable in which the strain detected by the optical fiber is constant regardless of the direction of incidence of stress waves, which serve as a model of seismic waves. As parameters of the optical fiber, we focus here on the laying angle of the optical fiber (also called the winding angle; hereafter, abbreviated as the optical fiber laying angle) and the strain ratio parameter between different materials in the uniform stress model (details of these will be described later).

[0014] Fig. 1(a) is a model of an optical cable. In this figure, an optical cable 3 is modeled as being composed of a core material 1 and an optical fiber 2 wound in a spiral shape around the core material 1. Three arrows indicate three-dimensional coordinate axes x, y, and z, and α indicates the installation angle of the optical fiber 2 (hereinafter referred to as the optical fiber installation angle). This optical fiber installation angle α is defined as the angle between the optical fiber 2 and the x-axis (negative direction) in the xz plane in Fig. 1(b), which is an expanded view of Fig. 1(a).

[0015] Here, a ξηζ coordinate system is introduced to the optical cable 3, which is rotated by −θ around the y axis with respect to the xyz coordinate system. In this ξηζ coordinate system, the strain ε 0 If we assume that the strain is generated, the relationship between the strain and the stress can be expressed simply as follows using Lame's constants λ and μ: ξ = 0, ε η = 0, ε ζ =ε 0 If so, then σ ξ = λε 0 , σ η = λε 0 , σ ζ =(λ+2μ)ε 0 It can be expressed as:

[0016] In this case, the vertical component of the distortion in the xyz coordinate system can be expressed as the following equation (1).

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[0017] As shown in Fig. 2(a) and Fig. 2(b), the optical cable 3 is subjected to stress in the z direction and has a strain ε z In the case of Fig. 2(a) and Fig. 2(b), the strain in the optical fiber 2 is ε z and the optical fiber installation angle α, it can be expressed as formula (2).

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[0018] Next, as shown in Fig. 3(a), Fig. 3(b), and Fig. 3(c), the optical cable is subjected to stress in the x direction, resulting in strain ε x When the optical fiber generates a strain, the strain sensed by the optical fiber varies depending on the angular position φ (see FIG. 3(c)) in the circumferential direction of the optical fiber. Specifically, the equations are expressed as (3) and (4), for example.

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[0019] Furthermore, when the optical cable is sufficiently long and the helical pitch of the cable is sufficiently smaller than the spatial resolution of the measuring instrument, the distortion can be expressed as equation (6) by taking the average in the longitudinal direction of the optical cable.

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[0020] As a result, the strain ε 0 When the optical fiber is in a state where It is expressed as equation (7).

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[0021] The above A is equal to the ratio of strain between two dissimilar materials, which is calculated from the relationship between stress and strain in the one-dimensional uniform stress model shown in Figure 4. In other words, the stress in the x direction is the same on the material surfaces of these dissimilar materials. The value of this equal stress is called σ 0 Then, σ 0 =(λ 0 +2μ 0 )ε 0 =(λ+2μ)ε, so ε / ε 0 =(λ 0 +2μ 0 ) / (λ+2μ), and the value of the right hand side of this is equal to the value of A above.

[0022] Then, from equation (7), the strain of the optical fiber, ε f It can be seen that does not depend on the angular position φ of the optical fiber in the circumferential direction. In other words, the result does not change even if the direction of strain rotates around the z-axis. In addition, from the second term of equation (7), it can be seen that the stiffness of the optical cable in the lateral direction affects the sensitivity of the optical fiber, but the stiffness of the optical cable in the axial direction does not affect the sensitivity of the optical fiber.

[0023] Therefore, the optical fiber installation angle α and the distortion ratio parameter A are set so as to satisfy the following equation (9).

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[0024] When α and A are set to satisfy equation (9), the strain detected by the optical fiber becomes a constant value expressed by the following equation (10), regardless of the incident direction of the stress wave.

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[0025] The optical fiber installation angle α that satisfies equation (10) is α opt The distortion ratio parameter A is A opt When we set opt and A opt The horizontal axis shows the relationship between opt Take A on the left vertical axis. opt The vertical axis on the right shows the relative sensitivity ratio of strain, ε f / ε 0 In this figure, the curve connecting the diamonds is α opt A for opt The curve connecting the squares shows the change in α opt for ε f / ε 0 The above α opt and A opt and ε f / ε 0 The relationship between these values ​​is shown in Figure 6.

[0026] Specifically, for example, in FIG. 5, α opt In the case of =73 degrees, from Figure 6, A opt is set to 21.4, the optical fiber has a relative sensitivity ratio ε f / ε 0It can be seen that the stress waves of an earthquake can be detected when A is 0.91, which is close to 1. In other words, it can be seen that the stress waves can be detected with almost no attenuation regardless of the incident direction of the stress waves. In other words, if the optical fiber installation angle α, etc. is set so as to satisfy this relationship, it can be said that the optical fiber can respond to seismic waves regardless of the incident angle of the seismic waves. Note that when an armored cable is used as the optical cable structure, A generally satisfies A<1, so the sensitivity is low. Therefore, in this embodiment, a flexible body is used in part of the cable to increase the sensitivity (the structure of the optical cable will be explained in detail later). In reality, it is often difficult to determine the relationship between α and A so as to satisfy formula (10) under various conditions, and the evaluation using formula (7) is appropriate. Therefore, it is necessary to consider the effect of the parameter, the incident angle θ of the acoustic wave (hereinafter sometimes abbreviated as the incident angle θ).

[0027] [Analytical study - dynamic load] In the above, we have considered the case where the load is static. Next, we will consider the case where the load is dynamic, using the model shown in Figure 7. In this figure, x on the horizontal axis indicates position, and t on the vertical axis indicates time. Also, ρ 0 , ρ is the density of each material, C 0 , C is the sound velocity in each material. P and Q are symbols indicating the interface between different materials. In this case, the particle velocity ν ∞ , stress σ ∞ Assume that a plane wave of the above is incident normally on the interface of these dissimilar materials and is multiple-reflected at interfaces P and Q.

[0028] In Fig. 7, ρ 0 , ρ is the density of each material, C 0 , C is the sound velocity in each material. In the uniaxial strain state, C 0 , C are expressed by equations (11) and (12), respectively.

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[0029] Let the particle velocity in the initial state be ν 0 , stress σ 0 Then, point P in Figure 7 1 Incident wave at σ i 1 , transmitted wave σ t 1 , reflected wave σ r 1 are respectively, σ i 1 =-ρ 0 C 0 ν ∞ = σ ∞ , σ t 1 =-ρC(ν 1 -ν 0 )=σ 1 , σ r 1 =ρ 0 C 0 (ν 1 -ν 0 ) From the balance of forces at the interface, σ i 1 +σ r 1 = σ t 1 Therefore, by expressing k as in equation (13), the following equations (14) and (15) hold.

[0030]

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[0031] In FIG. 7, the incident wave σ at points Qn (n=2m, m is an integer equal to or greater than 1) and Pn (n=2m+1, m is an integer equal to or greater than 1) i n , transmitted wave σ t n , reflected wave σ r n The balance of forces at the interface between in +σ r n = σ t n ), the following recurrence formulas (16) and (17) can be obtained in a similar manner.

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[0032] By solving these recurrence equations, ν n , σ n We obtain equations (18) and (19).

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[0033] By the way, equation (20) holds for all positive numbers k.

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[0034] This means that if the wavelength of the incident wave is sufficiently larger than the size of the different materials, the stress will be the incident stress σ ∞This means that the stress approaches the same value as in the case of static load. In other words, this shows that the same uniform stress model holds true for dynamic load as for static load. In other words, in the case of dynamic load, as in the case of static load, if α and A are set to satisfy equation (9), the optical fiber can detect the strain caused by the stress wave regardless of the direction of incidence of the stress wave. Here, since the stress in each layer becomes uniform as described above, it is clear that in order to improve the sensitivity of the measured strain, it is necessary to lower the rigidity of the layer in which the optical fiber is embedded (it can be inferred that the sensitivity of the sensor that measures strain can be increased by lowering the rigidity).

[0035] As will be described later, in this embodiment, the cable diameter is smaller and a harder material can be used (specifically, an armored cable can be used) compared to the cable of Patent Document 2, so that the speed is faster, the above n is effectively larger, and the response range is also wider. The ease of propagation of a plane sound wave is expressed by the acoustic impedance I, which is expressed by the density ρr of the measurement object, which is the medium through which the plane sound wave propagates, and the sound speed Cr specific to the medium. Z (I Z =ρrCr). Here, if the bulk modulus of the medium is Kr, then Cr=(Kr / ρr) 1 / 2 Therefore, the acoustic impedance I Z is (ρr × Kr) 1 / 2 It is generally known that the ratio of the amplitude of the sound wave in the measurement object to the amplitude of the sound wave transmitted through the optical fiber is expressed by the ratio of the acoustic impedances of the two, according to the multiple reflection theory.

[0036] [Experimental study] Next, to clarify the problems that may arise when implementing the analytical results, we will conduct an experimental study. When measuring seismic waves, the level of the incident stress wave input to the optical cable is considered to be very small because the seismic waves are usually attenuated due to the large distance from the epicenter. Therefore, to detect seismic waves, we used a distributed acoustic sensing (DAS) with excellent characteristics in terms of sensitivity. Below, we will explain the experiment conducted using this DAS measurement (hereinafter referred to as DAS measurement). The spatial resolution of DAS is generally 20 cm or more, which is sufficiently larger than the diameter of the optical cable. Therefore, the measurement of strain using DAS can be treated in the same way as the static load measurement described above.

[0037] First, the experimental setup for the DAS measurement is explained using the diagram. Figures 8(a), 8(b), 8(c), and 8(d) show the configuration of the test model. Optical fibers 5a and 5b for detecting stress waves 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, rocks, etc., which are the surrounding coverings around which the optical cable is placed, are replaced with this mortar for the experiment). In order to try to measure the strain detected by the simulated cable 4 using a semiconductor strain gauge 6, three types of semiconductor strain gauges shown in the figure, namely, a trigger gauge 6a, an incident wave / reflected wave measurement gauge 6b, and a transmitted wave measurement gauge 6c, are arranged, and detection using these semiconductor strain gauges is also tried at the same time (see Figure 8(a)).

[0038] The length L of the mortar block 10 is 1 is 1200mm, section size L 3 ×L 4 (See Fig. 8(b)) is 200 mm × 200 mm, and the length L of the stress wave trap block 11 2 is 350 mm and the cross-sectional size is the same as that of mortar block 10.

[0039] In addition, multiple simulated cables 4 with different optical fiber installation angles α were embedded in one mortar block 10 (see Fig. 8(c) and Fig. 8(d) shown below the white arrow in the figure). The embedding locations were the same cross-sectional positions of this mortar block 10, and about three simulated cables 4 were embedded at the same time. In addition, in order to study the effect of the incident angle of the acoustic wave on the simulated cable 4, experiments were carried out with three different incident angles. Furthermore, in order to examine the influence of the material of the core material of the simulated cable 4 in which the optical fiber is installed, the materials of the core material of the simulated cable 4 used in the experiment are also changed. Specifically, the optical fiber installation angle α is three types, 65 degrees, 73 degrees, and 90 degrees, the incidence angle is three types, 60 degrees, 75 degrees, and 90 degrees, and the core material is two types, aluminum and polyacetal resin (hereinafter sometimes abbreviated as POM material), and both are rod-shaped. In the case of aluminum, in addition to the rod-shaped core material, experiments were also conducted using a circular core material (pipe) in order to examine the influence of the elastic modulus.

[0040] Furthermore, to simulate the incident stress, a projection block was used, which was made by bonding together multiple plates of polyvinyl chloride (hereinafter sometimes abbreviated as PVC) etc. To operate this projection block, a projection device was also made at the same time and used.

[0041] Next, an experimental model for DAS measurement will be described with reference to the drawings. FIG. 9(a) is a three-dimensional schematic diagram for explaining this experimental model. FIG. 9(b) is a top view of the three-dimensional schematic diagram shown in FIG. 9(a) as seen from the direction of the white arrow. As shown in FIG. 9(a), the above-mentioned shot block is collided with the center position R of the left side of the buried rectangular parallelepiped mortar block 10 in which the simulated cable 4 is buried (the center position R corresponds to the epicenter of the earthquake), and an acoustic wave (corresponding to the stress wave caused by the earthquake) is generated inside the mortar block 10. At this time, the acoustic wave travels in the direction of the arrows in FIG. 9(a) and FIG. 9(b), and propagates at an incident angle θ with respect to the longitudinal direction (axial direction) of the simulated cable 4 (see FIG. 9(b)).

[0042] Next, the arrangement and coupling of the optical fibers in the experiment are shown in Fig. 10(a), Fig. 10(b), and Fig. 10(c). Fig. 10(a) is a three-dimensional model diagram showing the appearance, Fig. 10(b) is a diagram seen in the direction of the arrow D in Fig. 10(a), and Fig. 10(c) is a diagram showing the details of the installation state of the optical fibers. In the actual measurement, as shown in Fig. 10(a) and Fig. 10(b), in addition to the simulated cable 4, the acoustic wave detection fibers are fixed to the surfaces of the two sides, and the other fibers are embedded in the longitudinal direction inside the mortar block. Here, as shown in Fig. 10(c), all the fibers are usually continuously coupled and arranged to detect the acoustic waves. As shown in Fig. 10(b) and Fig. 10(c), the three simulated cables 4a, 4b, and 4c are connected in layers in the thickness direction of the rectangular parallelepiped mortar block, respectively, from the top. In each simulated cable, if any of the cables in the multi-layered configuration breaks during assembly, the broken cable must be omitted.

[0043] Here, we explain the theoretical analysis results that were obtained in advance for comparison with the experimental results. In this analysis, the relationship between the theoretical optical fiber installation angle α and the normalized relative sensitivity was obtained for each core material, which was an aluminum rod, an aluminum tube, and a POM rod (polyacetal resin rod). The results are shown in Figure 11(a), Figure 11(b), and Figure 11(c), respectively. These figures show theoretical analysis values ​​for three cases where the parameter is the incident angle θ: 60 degrees, 75 degrees, and 90 degrees. In each figure, it can be seen that for values ​​of α smaller than the optical fiber installation angle at which the normalized relative sensitivity is 1, the larger the incident angle θ, the larger the change in normalized relative sensitivity with respect to the optical fiber installation angle α.

[0044] Here, the normalized relative sensitivity is a value calculated by equation (22) based on equation (21) when S(θ, α, A) is defined by equation (21), and α A= 65 degrees. The strain ratio parameter A is given as 0.233 for the aluminum rod, 0.695 for the aluminum tube, and 6.211 for the POM rod.

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[0045] The experimental setup is shown in Fig. 12. The electromagnet enclosed in the dashed line frame, the damping box, and the damping device, which is comprised of a processor (not shown) built into the distortion measuring device 20, are used to damp the steel ball to the support point P at the required timing. 0 Distance L indicated by the dotted line from 5 From the standby position on the braking device side, which is a distance away from the support point P 0 The horizontal distance is calculated by the dotted line with an arrow around 6 The steel ball is moved by a distance L and struck against the side of a mortar block 10. Three types of dummy cables 4 are attached to the mortar block 10, and a measuring optical fiber 5 having a length of about 500 m is also provided. The acoustic waves generated in the mortar block 10 by the impact of the steel ball cause distortion in the measuring optical fiber 5, and the distortion generated in the measuring optical fiber 5 is measured by a distortion measuring device 20. 5 Size: 90.5cm, L 6 is about 13cm, and the L of the steel ball is 6 The travel time for this part is about 0.4 seconds.

[0046] Next, the results measured using the experimental apparatus shown in FIG. 12 will be explained with reference to the following figures. FIG. 13(a) shows an example of the measurement results in the time domain, and FIG. 13(b) shows an example of the measurement results in the frequency domain. In FIG. 13(a), the curve shown by the solid line is the measurement result, and the vertical axis shown on the left and the horizontal axis shown on the bottom are the reference axes. The vertical axis of this reference axis represents strain in nε, and the horizontal axis represents elapsed time in milliseconds. The measurement position is 559 m. It can be seen that this vibration waveform decays in about 100 milliseconds. On the other hand, the curve shown by the dotted line is the corresponding FFT (Fast Fourier Transform) of the above time signal. The vertical axis shown by the dotted line on the right and the horizontal axis shown by the dotted line on the top are the reference axes. The vertical axis of this reference axis represents strain in nε, and the horizontal axis represents frequency in Hz. From the FFT analysis, the peak frequency was found to be 1605.2Hz. Therefore, the speed of the acoustic wave is 3.85km / s (=1.2m×2×1605.2Hz) since the block length is 1.2m.

[0047] Next, Figure 13(b) shows an example of the measurement results obtained by DAS measurement of acoustic waves (1605 Hz component) using a simulated cable whose core material is an aluminum rod, with an incident angle θ of 60 degrees on the simulated cable. In this figure, the horizontal axis indicates the measurement distance on the fiber (position on the simulated cable) in meters, and the vertical axis indicates the amount of strain generated in nε units. The curves in this figure are the measurement results, and in particular, for the frequency component of approximately 1605 Hz, show the amount of strain caused by the acoustic wave (peak component) generated in the fiber or the cable being measured at the measurement position (distance) along the simulated cable.

[0048] From this figure, it can be seen that the largest strain peak appears near a distance of 560 m. This is the strain that occurred in the stress wave detection optical fiber attached to the surface of the sample in Figure 8 described above. In addition, it can be seen that a second strain peak appears near a distance of 580 m. This is the strain that occurred in the stress wave detection optical fiber embedded inside the sample in Figure 8 described above. Using these strains that occurred in the stress wave detection optical fiber as markers, we will analyze the strain that occurred in the simulated cable, that is, the strain that occurred at the simulated cable position indicated by the double arrow in the figure.

[0049] However, since the level (magnitude) of the strain generated in the simulated cable is smaller than the level of strain generated in the stress wave detection optical fiber, as shown in Figure 13 above, we will next use another diagram, Figure 14, to explain how the strain generated in this simulated cable can certainly be detected.

[0050] Figure 14 shows the strain generated in the above-mentioned simulated cable together with data measured by another strain measurement method, the Brillouin frequency shift using backscattered light. From this figure, it can be seen that the Brillouin frequency shift occurs at three positions on the simulated cable, A, B, and C (A, B, and C correspond to the simulated cables 4a, 4b, and 4c shown in Figure 10, respectively) (the strain in this case corresponds to the vertical axis scale on the right side of Figure 14), and it can be seen that it is possible to measure acoustic waves using this simulated cable.

[0051] Next, to confirm the repeatability of this measurement and the accuracy of the magnitude (amount of strain) of the measured strain, we show the measurement results for a different core material and compare them with the theoretical value of normalized relative sensitivity shown above. The results are explained below with reference to Figure 15(a), which shows the strain distribution in the cable's longitudinal direction, and Figure 15(b), which shows the measurement results compared with the theoretical value obtained by analysis.

[0052] Figures 15(a) and 15(b) are graphs showing the results of measurements of the strain generated by acoustic waves three times using a simulated cable with a POM rod as the core material, measured by DAS. The reason for taking the results of measurements using a POM rod here is that it was assumed that the sensitivity of the measurement using a POM rod was the highest compared to the measurements using other core materials. Of these figures, Figure 15(a) shows the results of measurements of the generated strain measured by DAS, with the horizontal axis representing the measurement distance on the fiber (in m) and the vertical axis representing the amount of generated strain (in nε), when the incident angle θ is 60 degrees, as in Figure 13.

[0053] In addition, in order to compare the measurement results with the theoretical values, Figure 15(b) plots the optical fiber installation angle α on the horizontal axis and the normalized relative sensitivity on the vertical axis to compare the measurement results with the theoretical values. As shown in the graph in Figure 15(b), it can be seen that almost all of the measurement data except for one data point agrees well with the theoretical value.

[0054] Furthermore, specific values ​​of relative sensitivity due to differences in core materials are summarized in Figure 16. It is known that the elastic modulus of a POM rod is more than 10 times smaller than that of an aluminum rod, in other words, that a POM rod is softer than an aluminum rod. For this reason, conversely, the strain ratio parameter A of a POM rod is more than an order of magnitude larger than that of an aluminum rod, and the relative sensitivity is also higher for a POM rod than an aluminum rod at almost all laying angles, regardless of the angle of incidence. In other words, it was found that the softer the core material, the better the relative sensitivity.

[0055] As described above, the optical fiber as the sensor is laid at a predetermined optical fiber installation angle α opt It was found that by using the optical cable installed in the above, it is possible to measure the generated seismic waves using the DAS even when the seismic waves are incident perpendicular or close to the longitudinal direction of the optical cable.

[0056] Representative forms of optical cables that satisfy the above conditions are shown in Figures 17(a) to 17(d). Figure 17(a) is a perspective view showing the overall configuration of such an optical cable, and Figure 17(b) is a cross-sectional view showing a cross section perpendicular to the longitudinal axis of the optical cable shown in Figure 17(a). Figure 17(c) is a perspective view of the cross-sectional view shown in Figure 17(b) with all of the components (such as multiple steel wires) at the outermost periphery removed, and Figure 17(d) is a cross-sectional view showing a cross section perpendicular to the longitudinal axis of Figure 17(c).

[0057] In Fig. 17(a), the optical fiber 7a serving as the sensor is covered with a plurality of steel wires 8 (hereinafter, these steel wires are also referred to as first steel wires) twisted in a spiral shape, and is further protected by a flexible body 9 made of a soft material compared to the covering (hereinafter, referred to as the cable covering) that covers the optical cable at the location where the optical cable is installed (the flexible body may include, for example, plastic). The cable covering 33 (not shown) includes rock, etc.

[0058] As shown in FIG. 17(b), the optical cable 30 for detecting stress waves is disposed in the outermost layer of an optical cable 31 having a multi-layer structure in which a spiral wire is wound in multiple layers. The outermost layer is formed into a ring shape by twisting a plurality of steel wires 32 (hereinafter also referred to as second steel wires) having an outer diameter similar to that of the optical cable 30 for detecting stress waves in a spiral shape. Therefore, the second steel wire has a larger outer diameter than the first steel wire. As shown in FIG. 17(b), a flexible body 9a, which is a buffer layer made of a flexible body that does not allow liquid to pass through and covers the entire circumference, may be disposed between the outermost layer and the layer immediately inside it. Furthermore, an optical fiber 7b having a pressure sensor function other than the optical fiber 7a is disposed in the axial center of the optical cable.

[0059] Also, as shown in Fig. 17(c) and Fig. 17(d), the above-mentioned relaxation layer may be replaced with a flexible body 9b (for example, a mesh-like layer through which liquid can enter from the outside) which is a relaxation layer having water permeability. In this case, even if the optical fiber 7b arranged in the central axis portion of the optical cable 31 is a pressure measurement sensor, there is no influence on the pressure measurement. Therefore, this optical fiber 7b can also function as a sensor for detecting stress waves. This optical fiber 7b is arranged corresponding to the central axis of the optical cable 31, and the optical cable 30 for detecting stress waves is arranged at a specific winding angle with respect to this central axis. Therefore, when the two are operated simultaneously, two types of incident angles are provided for acoustic waves (or seismic waves), and it is expected that the optical cable will function as a more sensitive optical cable.

[0060] In the above, some of the steel wires 32 may be replaced with a metal tube covered optical fiber cable, i.e., FIMT (Fiber In Metallic Tube) 32a. Also, instead of covering the entire wire with the flexible body 9, a Kevlar fiber (softer than the steel wire) with an outer diameter of about 5 μm may be wound in a spiral shape.

[0061] Here, the winding angle β of the stress wave detection optical cable 30 with respect to the longitudinal axis of the optical cable is the optical fiber installation angle α opt The winding angle β is determined based on the physical properties of the flexible body 9 and the cable covering 33 (for example, rock). Specifically, the Lame constant λ of each material is calculated from the elastic coefficient E and Poisson's ratio ν of each of the flexible body 9 and the cable covering 33. 9 , μ 9 , and λ 33 , μ 33 (see equations (23) and (24)) using the above equations (8) and (9). It is more preferable that the flexible body 9 is disposed not only on the outer periphery of the steel wire 8 as described above, but also in the gap between the optical fiber 7a and the steel wire 8.

[0062]

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[0063] Although exemplary embodiments are described herein, the various features, aspects, and functions described in the embodiments are not limited to application to a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless modified examples not exemplified are assumed within the scope of the technology disclosed in the present specification. For example, this includes cases where at least one component is modified, added, or omitted. For example, in the above-mentioned first embodiment, a case where an optical cable is arranged in the outermost layer of a multilayer cable has been described, but this is not limited to this, and the same effect can be obtained even when an optical cable is arranged in a layer inside the outermost layer. [Explanation of symbols]

[0064] 1 Core material 2, 7a, 7b Optical fiber 3, 31 Optical cable 4, 4a, 4b, 4c Simulation Cable 5. Measurement optical fiber 5a, 5b Optical fiber for detecting stress waves 6 Semiconductor strain gauge 6a Trigger Gauge 6b Gauge for measuring incident and reflected waves 6c Gauge for measuring transmitted waves 8, 32 Steel wire 9, 9a, 9b Flexible body 10 Mortar Block 11 Stress wave trap block 20 Distortion Measuring Instrument 30 Optical cable for detecting stress waves 32a FIMT 33 Cable Cover A Distortion Ratio Parameter α Optical fiber installation angle θ Incident angle of acoustic wave

Claims

1. An optical cable for measuring stress waves generated by vibration of a measurement object, an optical cable for detecting stress waves, the optical cable having an optical fiber disposed in an axial center portion, a plurality of first steel wires wound in a spiral shape so as to cover the optical fiber, and a flexible body covering the optical fiber and the plurality of first steel wires; a second steel wire different from the first steel wire; Equipped with the optical cable for detecting stress waves and the plurality of second steel wires are each wound in a spiral shape to form a ring-shaped body as a whole, and a winding angle (α) of the optical cable for detecting stress waves with respect to an axis of the optical cable is a function of a distortion ratio parameter (A), and the distortion ratio parameter (A) is a function of Lame's constants (λ, μ) of the flexible body and Lame's constants (λ 0, μ 0 ) of a cable covering that covers the optical cable, Each of Lame's constants (λ, μ, λ 0 , μ 0 ) is a function of the elastic modulus (E) and Poisson's ratio (ν) of the flexible body and the cable covering that covers the optical cable at a location where the optical cable is installed, A, α, λ, μ, λ 0 , μ 0 , E, and ν have a relationship represented by the following formula: [0010] Optical cable.

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

3. a second optical fiber capable of measuring pressure, the second optical fiber being disposed in an axial center portion of the optical cable; 3. The optical cable according to claim 1, further comprising a second flexible body which is a water permeability mitigation layer having water permeability, the second flexible body being opposed to the inner annular surface of the one annular body.

4. 4. The optical cable according to claim 1, wherein gaps between the optical fiber and the plurality of first steel wires are covered with the flexible body.

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

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