State measurement device and state measurement method

Abstract

To improve measurement accuracy of state measurement of a measurement object by an optical fiber cable.SOLUTION: A state measurement device 100 comprises: an optical fiber cable 50 which is embedded in a folded manner in a borehole 2; and a cover member 60 which covers a folded-back part 50a of the optical fiber cable 50. The optical fiber cable 50 comprises: a first optical fiber cable 51 which is embedded in a forward path; and a second optical fiber cable 52 which is embedded in a return path and is connected to the first optical fiber cable 51 in the cover member 60.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a state measurement device and a state measurement method. [Background technology]

[0002] Patent Document 1 discloses a condition measuring device that measures the condition of the ground using an optical fiber cable buried in the ground. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-156215 Summary of the Invention [Problem to be solved by the invention]

[0004] In the condition measurement device disclosed in Patent Document 1, a strain measurement optical fiber cable is provided as an optical fiber cable buried in the ground, and a temperature compensation optical fiber cable is also provided to correct the measurement values ​​measured by the strain measurement optical fiber cable. Because these optical fiber cables are separately connected to the scattered light measurement device, measurements are performed at different times. When measurements are performed at different times, for example, if the temperature changes between the time the strain measurement optical fiber cable is used for measurement and the time the temperature compensation optical fiber cable is used for measurement is completed, the amount of strain will be corrected at a temperature different from the temperature at which the strain measurement optical fiber cable was used for measurement. This may result in a decrease in the accuracy of the strain measurement.

[0005] An object of the present invention is to improve the measurement accuracy of measuring the state of an object to be measured using an optical fiber cable. [Means for solving the problem]

[0006] The present invention is a state measuring device for measuring the state of an object to be measured by an optical fiber cable, At the turning point Optical fiber cable folded back and buried of The optical fiber cable is provided with a first optical fiber cable buried in the outbound route and a second optical fiber cable buried in the inbound route. folding part a second optical fiber cable connected to the first optical fiber cable in the However, the first optical fiber cable and the second optical fiber cable each have a core, a clad surrounding the outer periphery of the core, and a coating surrounding the outer periphery of the clad, and the conformability of the clad to the coating of the first optical fiber cable is higher than the conformability of the clad to the coating of the second optical fiber cable. . [Effects of the Invention]

[0007] According to the present invention, it is possible to improve the measurement accuracy of measuring the state of an object to be measured using an optical fiber cable. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram illustrating a state measuring device according to an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view for explaining the configuration of the first optical fiber cable. [Figure 3] FIG. 4 is a cross-sectional view illustrating the configuration of a second optical fiber cable. [Figure 4] 5A and 5B are diagrams illustrating a cover member of the state measuring device according to the embodiment of the present invention. [Figure 5] 10A and 10B are diagrams illustrating modified examples of the cover member of the state measuring device according to the embodiment of the present invention. [Figure 6] FIG. 10 is a diagram showing a modified example of a state measuring device according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, a state measuring device according to an embodiment of the present invention will be described with reference to the drawings.

[0010] First, the configuration of a state measuring device 100 according to an embodiment of the present invention will be described with reference to Figures 1 to 4. The state measuring device 100 is a device that measures the state of ground 1, which is an object to be measured, using an optical fiber cable 50 buried in the ground 1. Note that the object to be measured is not limited to the ground 1, and may be anything in which the optical fiber cable 50 can be buried, such as a concrete structure such as a dam.

[0011] In the construction of civil engineering structures, it is important to understand the state of the ground, such as landslides, and also to understand the progress of loosened areas that occur in the ground as a result of excavating underground cavities such as tunnels, in order to proceed with excavation in a stable state. Therefore, in order to detect distortions and loosened areas in the ground, for example, as shown in Fig. 1, an optical fiber cable 50 is buried in a borehole 2 (insertion hole) that is drilled vertically upward in the ground 1. The borehole 2 is a bottomed hole whose one end is closed inside the ground or structure that is the object to be measured, and whose other end is open to the ground or structure that is the object to be measured.

[0012] The optical fiber cable 50 buried in the borehole 2 generates strain in response to strain in the ground and loosening of the ground 1, and therefore, as described below, by measuring the strain in the optical fiber cable 50, it is possible to measure strain in the ground and loosening of the ground 1.

[0013] The condition measuring device 100 shown in Figure 1 mainly comprises an optical fiber cable 50 that is buried by folding it back and forth along the axial direction of the borehole 2, a pipe material 10 to which the optical fiber cable 50 is attached, a cover member 60 that is attached to the tip side of the pipe material 10 and covers the folded portion 50a of the optical fiber cable 50, an expandable body 20 that is attached to the cover member 60 and can be expanded radially outward of the borehole 2, and a filling hose 30 that is provided along the pipe material 10.

[0014] The pipe material 10 is a hollow, long member made of a resin such as polyvinyl chloride and having an outer diameter sufficiently smaller than the inner diameter of the borehole 2. It has an internal passage 11 formed through it in the axial direction, and a communication hole 12 which opens at one end on the inner surface of the internal passage 11 and connects the inside and outside of the pipe material 10.

[0015] The communication hole 12 is formed at the tip side of the pipe material 10 when the condition measuring device 100 is inserted into the borehole 2, and functions as an outlet for discharging air inside the borehole 2 to the outside of the borehole 2 when grout 40 (filling material) is filled into the borehole 2 through the filling hose 30.

[0016] The expansion body 20 has an expansion section 22 that expands radially outward in response to the pressure of the supplied fluid, a support section 21 that supports the expansion section 22 radially inward, and a supply pipe 24 that supplies the pressurized fluid. Figure 1 shows a state in which the expansion section 22 expands radially outward of the borehole 2 and presses against the inner wall surface of the borehole 2.

[0017] The expansion portion 22 is a rubber or metal member having a barrel-shaped or cylindrical outer shape, and is supported by the support portion 21 so as to expand radially outward in response to the pressure of the fluid supplied to the space (not shown) formed between the support portion 21 and the expansion portion 22.

[0018] The support portion 21 is fixed to the outer surface of the cover member 60 via a fixing tool (not shown).

[0019] The supply pipe 24 is attached to the pipe material 10 along its axial direction, and one end 24a thereof opens into the space formed between the support portion 21 and the expansion portion 22. The other end 24b of the supply pipe 24 is connected to a fluid supply source such as a pump (not shown) outside the borehole 2 before the pressurized fluid is supplied to the expansion portion 22 through the supply pipe 24, and once the supply of fluid to the expansion portion 22 is completed, the other end 24b is housed within the borehole 2 as shown in Figure 1. In order to maintain the expansion state of the expansion portion 22 once it has expanded, a check valve may be provided on the supply pipe 24 to prevent backflow of the supplied fluid.

[0020] The fluid supplied to the diameter expansion body 20 through the supply pipe 24 is, for example, pressurized water, and the expansion section 22 presses the inner wall surface of the borehole 2 with a load according to the water pressure. Note that the fluid supplied to the diameter expansion body 20 is not limited to water, and may be compressed air or pressurized hydraulic oil.

[0021] The filling hose 30 is a hose through which grout 40 to be filled in the borehole 2 flows, and is attached along the pipe 10 so that an outlet 30a for discharging the grout 40 opens into the borehole 2. As shown in Figure 1, the position of the outlet 30a is set vertically below the communicating hole 12 formed in the pipe 10. Note that the outlet 30a only needs to be located vertically below the communicating hole 12, and may be located, for example, near the open end 2b of the borehole 2. The other end of the filling hose 30 is connected to a grout delivery pump (not shown) outside the borehole 2.

[0022] As shown in FIG. 1, the optical fiber cable 50 is folded back at a folding back portion 50 a near the bottom surface 2 a of the borehole 2 and is laid in the ground 1 so as to travel back and forth within the borehole 2 .

[0023] The tip 50b of the optical fiber cable 50 is located near the open end 2b of the borehole 2 and is sealed with a sealant such as oil or silicone to prevent reflections at the tip surface. On the other hand, the base end 50c of the optical fiber cable 50 is connected to a measurement device 70 outside the borehole 2.

[0024] The optical fiber cable 50 is mainly composed of two different optical fiber cables: a first optical fiber cable 51 buried in the outbound path from the open end 2b side toward the bottom surface 2a of the borehole 2, and a second optical fiber cable 52 buried in the inbound path from the bottom surface 2a side toward the open end 2b. Note that the expressions "outbound path" and "inbound path" are used to distinguish the routes along which the optical fiber cables are laid, and the one buried in the outbound path may be referred to as the second optical fiber cable, and the one buried in the inbound path may be referred to as the first optical fiber cable.

[0025] The first optical fiber cable 51 is an optical fiber sensor used to measure strain occurring in the ground 1, and as shown in Fig. 2, includes a first optical fiber strand 51a, a tension member 51e which is a steel wire, and a resin coating 51d that covers these. The first optical fiber strand 51a is composed of a core 51b, a cladding 51c that surrounds the outer periphery of the core 51b, and an ultraviolet-curing resin (not shown) that coats the periphery of the cladding 51c. Fig. 2 is a cross-sectional view of the first optical fiber cable 51.

[0026] The surface of the coating 51d is embossed to improve adhesion with the grout 40 filled in the borehole 2. This makes it easier for strain generated in the ground 1 to be transmitted to the first optical fiber 51a via the grout 40.

[0027] The second optical fiber cable 52 is an optical fiber sensor used to measure the temperature inside the ground 1, and as shown in Figure 3, has a second optical fiber strand 52a and a metal tube 52d (coating) arranged to cover the second optical fiber strand 52a.

[0028] Like the first optical fiber 51a, the second optical fiber 52a is made up of a core 52b, a cladding 52c surrounding the outer periphery of the core 52b, and an ultraviolet curing resin (not shown) that coats the periphery of the cladding 52c.

[0029] The metal tube 52d is a steel tube made of highly corrosion-resistant stainless steel or nickel alloy, and has an inner diameter larger than the outer diameter of the second optical fiber 52a. That is, a small gap is formed between the second optical fiber 52a and the metal tube 52d, allowing the second optical fiber 52a to freely expand and contract without being restricted by the metal tube 52d. Note that Fig. 3 is a cross-sectional view of the second optical fiber cable 52.

[0030] Comparing the conformability of the cladding 51c (first optical fiber 51a) to the coating 51d of the first optical fiber cable 51 used for strain measurement with the conformability of the cladding 52c (second optical fiber 52a) to the metal tube 52d (coating) of the second optical fiber cable 52 used for temperature measurement, the former is higher and the latter is lower. In other words, comparing the friction coefficient between the coating 51d and cladding 51c of the first optical fiber cable 51 with the friction coefficient between the coating (metal tube 52d) and cladding 52c of the second optical fiber cable 52, the former is higher than the latter. In other words, comparing the conformability of the first optical fiber 51a (cladding 51c) to the grout 40 (filler) in which the respective coatings are embedded with the conformability of the second optical fiber 52a (cladding 52c), the former is higher and the latter is lower.

[0031] In particular, the surface of the coating 51d of the first optical fiber cable 51 is embossed as described above, which makes it easier for the coating 51d to adhere to the grout 40, thereby further enhancing the ability of the first optical fiber 51a (clad 51c) to conform to the grout 40. Furthermore, a small gap is provided between the second optical fiber 52a and the metal tube 52d (coating) of the second optical fiber cable 52 as described above, which makes it difficult for the second optical fiber 52a to conform to the metal tube 52d (coating), thereby further reducing the ability of the second optical fiber 52a (clad 52c) to conform to the grout 40.

[0032] In other words, if distortion occurs in the ground 1, in the first optical fiber cable 51, the distortion is easily transmitted to the first optical fiber strand 51a via the coating 51d, while in the second optical fiber cable 52, the distortion is less likely to be transmitted to the second optical fiber strand 52a via the metal tube 52d.

[0033] In addition, in the second optical fiber cable 52 for temperature measurement, a metal tube 52d with high thermal conductivity acts as a coating covering the second optical fiber strand 52a, making it easier for the temperature of the ground 1 to be transmitted to the second optical fiber strand 52a.

[0034] The first optical fiber cable 51 and the second optical fiber cable 52 are connected by fusing the first optical fiber 51a and the second optical fiber 52a together inside a cover member 60 that covers the folded-back portion 50a of the optical fiber cable 50. Specifically, the first optical fiber 51a and the second optical fiber 52a are connected by fusing the cores 51b, 52b together and the claddings 51c, 52c together. The coating 51d of the first optical fiber cable 51 and the metal tube 52d (coating) of the second optical fiber cable 52, which interfere with fusing the first optical fiber 51a and the second optical fiber 52a, are removed in advance from the ends over a predetermined length. The fused portion of the first optical fiber 51a and the second optical fiber 52a is protected by a protective sleeve 54, which will be described later. The method of connecting the first optical fiber cable 51 and the second optical fiber cable 52 is not limited to fusion splicing, but may be mechanical splicing or connector connection. However, in order to reduce connection loss at the connection part, fusion splicing is preferable.

[0035] Furthermore, the first optical fiber cable 51 and the second optical fiber cable 52, which are arranged along the axial direction on the outer peripheral surface of the pipe material 10, are laid on the pipe material 10 together with the supply pipe 24 and the filling hose 30 by the fixing tape 14 and fixed so as to be integrated. The pipe material 10 and the first optical fiber cable and the second optical fiber cable only need to be integrated when the pipe material 10 and the first optical fiber cable and the second optical fiber cable are inserted into the borehole 2. A plurality of fixing tapes 14 are provided at predetermined intervals in the axial direction.

[0036] The cover member 60 is a resin housing formed by injection molding, and as shown in Fig. 4, has a housing portion 61 formed in a shape capable of housing the folded portion 50a of the optical fiber cable 50, and a lid member (not shown) attached to the housing portion 61 so as to prevent the optical fiber cable 50 routed inside the housing portion 61 from being exposed to the outside. Note that Fig. 4 shows the shape of the inside of the housing portion 61 with the lid member removed from the housing portion 61.

[0037] The accommodating section 61 has a plate-shaped bottom wall 61a and a side wall 61b formed along the outer edge of the bottom wall 61a. Inside the accommodating section 61, there is provided a curvature holding section 63 capable of holding the folded portion 50a of the optical fiber cable 50 at a predetermined curvature, as well as a first holding section 64 capable of holding the first optical fiber cable 51, a second holding section 65 capable of holding the second optical fiber cable 52, a sleeve holding section 66 capable of holding a protective sleeve 54 that protects the fused portion between the first optical fiber strand 51a and the second optical fiber strand 52a, and an excess length holding section 67 capable of holding an excess length portion.

[0038] The curvature retaining portion 63 has a bulging portion 63a that bulges out in a circular shape from the bottom wall 61a, and a groove portion 63b formed between the bulging portion 63a and the side wall 61b. The outer peripheral surface of the bulging portion 63a is an arcuate surface 63c with a predetermined curvature, and the optical fiber cable 50 inserted into the groove portion 63b is held along the arcuate surface 63c. By being held in this manner by the curvature retaining portion 63, the extension direction of the optical fiber cable 50 is changed. In other words, the portion held by the curvature retaining portion 63 becomes the folded portion 50a of the optical fiber cable 50.

[0039] The magnitude of the curvature of the arc surface 63c is set according to the minimum bending radius of the optical fiber cable 50 to avoid an increase in bending loss of the optical fiber cable 50. For example, as shown in Fig. 4, when the second optical fiber strand 52a is provided at the turning portion 50a, the magnitude of the curvature of the arc surface 63c is set according to the minimum bending radius of the second optical fiber strand 52a so as not to increase bending loss.

[0040] It should be noted that in order to reduce bending loss at the turning portion 50a, it is better to reduce the curvature of the arcuate surface 63c, but if the curvature of the arcuate surface 63c is reduced, the width of the cover member 60 increases, and therefore the inner diameter of the borehole 2 into which the cover member 60 is inserted must be increased. On the other hand, the cost and time required to drill the borehole 2 increase as the inner diameter of the borehole 2 increases, so it is preferable to make the inner diameter of the borehole 2 as small as possible.

[0041] In addition, generally, when comparing cables such as the first optical fiber cable 51 and the second optical fiber cable 52 with strands such as the first optical fiber strand 51a and the second optical fiber strand 52a, the optical fiber strands are easier to bend and have a smaller minimum bending radius.

[0042] For this reason, in this embodiment, by arranging a strand such as the second optical fiber strand 52a in the folded-back portion 50a, the curvature of the arc surface 63c is increased and the width of the cover member 60 is made as small as possible. This makes it possible to insert the cover member 60 even when the inner diameter of the borehole 2 is small.

[0043] The shape of the bulge 63a on which the arc surface 63c is provided is not limited to a circular shape, and may be any shape as long as it has an arc surface 63c formed convexly toward the tip end of the cover member 60 that abuts against the bottom surface 2a of the borehole 2 when inserted into the borehole 2, for example, it may be semicircular or crescent-shaped.

[0044] The first holding portion 64 is a groove 64a formed in a shape capable of holding the first optical fiber cable 51 by fitting the first optical fiber cable 51 from which the coating 51d has not been removed, and the second holding portion 65 is a groove 65a formed in a shape capable of holding the second optical fiber cable 52 from which the metal tube 52d has not been removed by fitting the second optical fiber cable 52. Each of the grooves 64a, 65a is formed by cutting out a portion of the side wall 61b so as to communicate between the inside and outside of the accommodating portion 61.

[0045] The sleeve holding portion 66 is a groove 66b formed between a linearly extending side wall 61b and an inner wall 66a extending parallel thereto, and the width of the groove 66b is set to be slightly smaller than the outer diameter of the protective sleeve 54. By fitting the protective sleeve 54 into the groove 66b, the fused portion of the first optical fiber 51a and the second optical fiber 52a is fixed to the cover member 60 via the protective sleeve 54.

[0046] The protective sleeve 54 is a tubular member provided to protect the fused portion between the first optical fiber strand 51a and the second optical fiber strand 52a, which is a fragile portion, and has a rod-shaped hard member provided inside that straddles the first optical fiber strand 51a and the second optical fiber strand 52a.

[0047] The slack holding portion 67 is a portion around which the slack portion of the first optical fiber strand 51 a from which the coating 51 d has been removed with a slack and the slack portion of the second optical fiber strand 52 a from which the metal tube 52 d has been removed with a slack are routed when splicing the first optical fiber cable 51 and the second optical fiber cable 52, and has a bulging portion 67 a that bulges out in a semicircular shape from the bottom wall 61 a. A part of the outer peripheral surface of the bulging portion 67 a forms an arcuate surface 67 b with a predetermined curvature, and the optical fiber cable 50 is held along the arcuate surface 67 b.

[0048] The shape of the bulge portion 67a on which the arc surface 67b is provided is not limited to a semicircular shape, and may be any shape as long as it has the arc surface 67b, and may be, for example, a circular shape similar to the bulge portion 63a of the curvature maintaining portion 63. Note that if the excess length portion is short, the optical fiber cable 50 may be held along a groove portion 67c formed between the bulge portion 67a and the side wall 61b, as indicated by the dashed line. Also, if the excess length portion is long, the optical fiber cable 50 may be held by wrapping around the bulge portion 63a of the curvature maintaining portion 63.

[0049] Furthermore, the surplus length holding portion 67 is not limited to holding the surplus portion of the second optical fiber strand 52a as shown in Figure 4, but instead of or in addition to this, it may be formed in a position where it can hold the surplus portion of the first optical fiber strand 51a.

[0050] The cover member 60 with the optical fiber cable 50 routed therein is attached to the tip end of the pipe 10 so that the portion where the curvature retaining portion 63 that retains the folded portion 50a is provided faces the bottom surface 2a of the borehole 2. In other words, when inserting the pipe 10 or the like into the borehole 2, first the cover member 60 is inserted into the borehole 2 until the tip of the cover member 60 abuts against the bottom surface 2a of the borehole 2.

[0051] The condition measuring device 100 further has a flange 16 at the open end 2 b of the borehole 2 for fixing the base end of the pipe material 10 to the borehole 2 .

[0052] The flange 16 is a plate-like member provided to support the base end of the pipe 10 protruding outside the borehole 2, and has an insertion hole 16a through which the pipe 10 and the optical fiber cable 50 and filling hose 30 provided around the pipe 10 can be inserted, and a plurality of bolt holes 16b through which anchor bolts 4 embedded in the ground 1 so as to surround the opening of the borehole 2 can be inserted. The flange 16 is fixed to the borehole 2 by tightening nuts 5 threaded onto the anchor bolts 4.

[0053] In addition, the flange 16 has a sealant injection hole (not shown) formed in it, one end of which opens into the borehole 2. By injecting a sealant 18 such as a urethane material through this sealant injection hole into the vicinity of the flange 16 inside the borehole 2, the open end 2b of the borehole 2 is closed and the space inside the borehole 2 is sealed off from the outside.

[0054] The space within the borehole 2, which has been sealed off by injecting the sealing material 18 into the open end 2b, is filled with grout 40 (filler) such as cement bentonite through the filling hose 30, and the optical fiber cable 50 is buried within the borehole 2 together with the pipe material 10, becoming substantially integrated with the ground 1. When the grout 40 is filled, the air within the borehole 2 is discharged to the outside of the borehole 2 through the communicating holes 12 and internal passage 11 of the pipe material 10.

[0055] Next, a method for measuring the state of the ground 1, which is the object to be measured, using the state measuring device 100 configured as described above will be described.

[0056] Generally, optical fiber cables have the property of slightly scattering incident pulsed light backward, and by utilizing this property, it is possible to measure strain at multiple positions in the optical fiber cable. Since the frequency of the scattered light depends on the strain of the optical fiber cable, the strain of the optical fiber cable can be measured by injecting pulsed light into the optical fiber cable and measuring the frequency of the scattered light. In addition, by measuring the time it takes for the scattered light generated within the optical fiber cable to return to the incident position after pulsed light is injected into the optical fiber cable, the position where the scattered light occurred, i.e., the position where strain occurred in the optical fiber cable, can be measured.

[0057] On the other hand, the frequency of scattered light changes not only with distortion but also with changes in ambient temperature, so if distortion is simply calculated from the frequency of scattered light, the calculated distortion will also include the change in frequency due to temperature changes. In other words, to accurately measure distortion, it is necessary to compensate for the amount of frequency change corresponding to temperature changes.

[0058] It is possible to provide a separate optical fiber cable for temperature compensation in order to compensate for changes in frequency due to temperature, but if the timing of measurements by the optical fiber cable for measuring strain and the optical fiber cable for temperature compensation differs, for example, if the temperature changes between the time when measurement is made by the optical fiber cable for measuring strain and the time when measurement is completed by the optical fiber cable for temperature compensation, the amount of strain will be corrected at a temperature different from the temperature at which measurement was made by the optical fiber cable for measuring strain.

[0059] Therefore, simply providing an optical fiber cable for temperature compensation cannot completely eliminate the influence of temperature changes, and as a result, there is a risk that the accuracy of measuring the amount of strain will decrease.

[0060] Therefore, in this embodiment, as described above, a single optical fiber cable 50 formed by connecting a first optical fiber cable 51 used to measure the strain occurring in the ground 1 and a second optical fiber cable 52 used to measure the temperature within the ground 1 is folded back and buried within the borehole 2, making it possible to measure the strain in the ground 1 and the temperature of the ground 1 simultaneously, thereby reliably eliminating the effect of temperature changes on the measurement of the amount of strain.

[0061] Specifically, first, a predetermined pulse light is incident on the optical fiber cable 50 from a light source (not shown) in the measuring device 70 (light incident step).

[0062] Next, the strain of the ground 1 is measured along the borehole 2 based on the scattered light (reflected light) returning from the first optical fiber cable 51 of the optical fiber cables 50 (strain measurement process). The strain is measured by, for example, the well-known BOTDR (Brillouin Optical Time Domain Reflectometry) method that uses Brillouin scattering.

[0063] At approximately the same time, the temperature of the ground 1 is measured along the borehole 2 based on scattered light (reflected light) returning from the second optical fiber cable 52 of the optical fiber cables 50 (temperature measurement step). The temperature measurement is performed by the well-known BOTDR method, as with the strain measurement.

[0064] In this way, when the strain distribution and temperature distribution in the axial direction within the borehole 2 are measured simultaneously, the strain measured in the first optical fiber cable 51 is corrected using the temperature measured in the second optical fiber cable 52 (correction process).

[0065] Here, the frequency difference (frequency shift) acquired by the first optical fiber cable 51 to calculate the strain also includes a difference depending on temperature, as described above. On the other hand, the frequency difference (frequency shift) acquired by the second optical fiber cable 52 includes only a difference depending on temperature, and does not include a difference depending on strain.

[0066] Therefore, the frequency difference due to strain alone can be determined by subtracting the frequency difference obtained by the second optical fiber cable 52 from the frequency difference obtained by the first optical fiber cable 51. Based on the frequency difference due to strain alone determined in this way, the strain occurring at a predetermined position in the borehole 2 can be measured with high accuracy.

[0067] It should be noted that the measurement of strain and temperature is not limited to the above method, and may be performed by other methods as long as the measurement can be performed based on the backscattered light of the optical fiber cable 50. For example, the measurement may be performed by the well-known OFDR (Optical Frequency Domain Reflectometry) method, or may be performed by combining the above method with the OFDR method.

[0068] According to the above embodiment, the following effects are achieved.

[0069] According to the condition measurement device 100 of this embodiment, the optical fiber cable 50 buried in the borehole 2 is integrated by connecting the first optical fiber cable 51 buried in the outbound path and the second optical fiber cable 52 buried in the return path.

[0070] Therefore, measurements by the first optical fiber cable 51 and measurements by the second optical fiber cable 52 are performed at the same time. This makes it possible to correct the measurement value measured by the first optical fiber cable 51 with the measurement value measured by the second optical fiber cable 52 at the same time, and as a result, it is possible to improve the measurement accuracy of the strain of the ground 1 (the state of the object to be measured). Furthermore, because measurements by the first optical fiber cable 51 and measurements by the second optical fiber cable 52 are performed simultaneously, it is possible to shorten the time required to measure the strain of the ground 1.

[0071] Furthermore, the optical fiber cable 50 is held at a predetermined curvature at the turn-back portion 50a by the cover member 60. This prevents bending loss from occurring at the turn-back portion 50a, and as a result, it is possible to maintain measurement accuracy over the entire area of ​​the optical fiber cable 50 even when the optical fiber cable 50 has the turn-back portion 50a.

[0072] Furthermore, the connection portion between the first optical fiber cable 51 and the second optical fiber cable 52 is disposed inside the cover member 60. Therefore, the connection portion, which is a weak portion, can be protected from the outside.

[0073] The following modified examples are also within the scope of the present invention, and it is possible to combine the configuration shown in the modified example with the configuration described in the above embodiment, or to combine the configurations described in the different modified examples below.

[0074] In the above embodiment, the second optical fiber 52a is arranged in the folded-back portion 50a. Alternatively, as in a modified example shown in Fig. 5, a third optical fiber 53 (third optical fiber cable) separate from the first optical fiber 51a and the second optical fiber 52a may be arranged in the folded-back portion 50a, and this third optical fiber 53 may be held by the curvature holding portion 63. Note that Fig. 5 is a view corresponding to Fig. 4.

[0075] 5, the first optical fiber cable 51 and the second optical fiber cable 52 are connected via a third optical fiber strand 53. Specifically, one end 53a of the third optical fiber strand 53 is fused to the first optical fiber strand 51a, and the other end 53b of the third optical fiber strand 53 is fused to the second optical fiber strand 52a, and each fused portion is protected by a first protective sleeve 154 and a second protective sleeve 155, respectively.

[0076] The third optical fiber 53 is an optical fiber specialized for bending resistance, such as a holey fiber, and has a smaller minimum bending radius than the first optical fiber 51a and the second optical fiber 52a.

[0077] In this modified example, as shown in FIG. 5, the accommodating portion 161 of the cover member 160 is provided with a curvature retaining portion 63 similar to that of the above embodiment, as well as a first sleeve retaining portion 166 capable of retaining the first protective sleeve 154 and a second sleeve retaining portion 167 capable of retaining the second protective sleeve 155.

[0078] The first sleeve holding portion 166 is a groove 166b formed between a linearly extending side wall 61b and an inner wall 166a extending parallel thereto, and the width of the groove 166b is set to be slightly smaller than the outer diameter of the first protective sleeve 154. By fitting the first protective sleeve 154 into the groove 166b, the fused portion of the first optical fiber 51a and the third optical fiber 53 is fixed to the cover member 160 via the first protective sleeve 154.

[0079] Similar to the first sleeve holding portion 166, the second sleeve holding portion 167 has a groove 167b formed between a linearly extending side wall 61b and an inner wall 167a extending parallel thereto, and the width of the groove 167b is set to be slightly smaller than the outer diameter of the second protective sleeve 155. By fitting the second protective sleeve 155 into the groove 167b, the fused portion of the second optical fiber 52a and the third optical fiber 53 is fixed to the cover member 160 via the second protective sleeve 155.

[0080] Since the third optical fiber 53 arranged in the turning portion 50a has a smaller minimum bending radius than the first optical fiber 51a and the second optical fiber 52a, in this modification, the curvature of the arc surface 63c of the curvature retaining portion 63 can be made larger than in the above embodiment, and the width of the cover member 160 can be made even smaller. This allows the optical fiber cable 50 to be buried in a borehole 2 with an even smaller inner diameter.

[0081] The portion of the third optical fiber strand 53 that is held by the curvature holding portion 63 may be in the form of an optical fiber cable covered with a resin coating, similar to the first optical fiber cable 51. Also, the housing portion 161 of the cover member 160 may be provided with a slack holding portion 67 that can hold a slack portion, similar to the above embodiment.

[0082] In the above embodiment, the optical fiber cable 50 is buried in a borehole 2 drilled vertically upward in the ground 1. Alternatively, the optical fiber cable 50 may be buried in a borehole 2 drilled horizontally, or may be buried in a borehole 2 drilled vertically downward as in the modified example shown in Figure 6.

[0083] In the variation shown in FIG. 6, the fiber optic cable 50 is inserted into the borehole 2 with a steel rope 264 rather than with a pipe 10 .

[0084] Specifically, the condition measurement device 200 shown in Figure 6 includes an optical fiber cable 50 that is folded back and forth along the axial direction of the borehole 2 and buried, a cover member 260 that covers the folded back portion 50a of the optical fiber cable 50, a weight 262 attached to the cover member 260, and a rope 264 having one end connected to the cover member 260.

[0085] Like the cover member 60 of the above embodiment, the cover member 260 has a configuration that can hold the folded portion 50a of the optical fiber cable 50 at a predetermined curvature, and also has a configuration that can hold the connection portion between the first optical fiber cable 51 and the second optical fiber cable 52.

[0086] The weight 262 is a metal member attached to the side of the cover member 260, and functions as a weight to stabilize the position of the cover member 260 when the cover member 260 is inserted into the borehole 2. The weight 262 may be a cylindrical metal member having a space formed therein capable of accommodating the cover member 260. The weight 262 may also be connected to the tip of the cover member 260 via a wire or the like, and inserted into the borehole 2 before the cover member 260.

[0087] The rope 264 is a steel wire rope that suspends and supports the cover member 260 together with the sinker 262 when they are inserted into the borehole 2, and the other end is attached to a winding device such as a winch (not shown). The rope 264 is not limited to a steel wire rope, and may be a resin rope made of polyethylene or the like. It is also preferable that the rope 264 is a non-rotating rope. One end of the rope 264 may be connected to the sinker 262 instead of the cover member 260.

[0088] The optical fiber cable 50, which is folded back and installed inside the borehole 2, is fixed to the rope 264 by a plurality of fixing tapes 14 arranged at predetermined intervals in the axial direction, and is inserted into the borehole 2 in this state.

[0089] Then, as shown in Figure 6, when the tip of the cover member 260 abuts against the bottom surface 2a of the borehole 2 and the tip 50b of the optical fiber cable 50 is positioned near the open end 2b of the borehole 2, grout 40 is filled into the borehole 2.

[0090] In this way, even when the optical fiber cable 50 is buried in a borehole 2 drilled vertically downward, the measurement accuracy of the strain in the ground 1 (the state of the object to be measured) can be improved, as in the above embodiment.

[0091] In the above embodiment, the second optical fiber strand 52a is disposed in the turn-back portion 50a. Alternatively, the first optical fiber cable 51 from which the coating 51d has not been removed or the second optical fiber cable 52 from which the metal tube 52d has not been removed may be disposed in the turn-back portion 50a. However, as described above, in order to increase the curvature of the arcuate surface 63c of the curvature holding portion 63 that holds the turn-back portion 50a, it is preferable to dispose the optical fiber strands 51a, 52a in the turn-back portion 50a.

[0092] In the above embodiment, the first optical fiber cable 51 is buried in the outward path from the open end 2b side toward the bottom surface 2a of the borehole 2, and the second optical fiber cable 52 is buried in the return path from the bottom surface 2a side toward the open end 2b. Alternatively, the second optical fiber cable 52 may be buried in the outward path, and the first optical fiber cable 51 may be buried in the return path.

[0093] Although the embodiments of the present invention have been described above, the above embodiments merely illustrate some of the application examples of the present invention, and it is not intended that the technical scope of the present invention be limited to the specific configurations of the above embodiments. [Explanation of symbols]

[0094] 100,200... Condition measuring device 1. Ground (object to be measured) 2. Borehole (insertion hole) 40 Grout (filler) 50···Fiber optic cable 50a...Folding section 51···First optical fiber cable 51a: First optical fiber 51b Core 51c···Clad 51d···Coating 52···Second optical fiber cable 52a: Second optical fiber 52b Core 52c···clad 52d Metal pipe (coated) 53 Third optical fiber strand (third optical fiber cable) 60, 160, 260... Cover member 63...curvature holding part

Claims

1. A state measuring device that measures the state of an object to be measured using an optical fiber cable, the optical fiber cable is folded back at a folded portion and buried in the object to be measured so as to travel back and forth within the object to be measured, the optical fiber cable includes a first optical fiber cable buried in the outgoing path and a second optical fiber cable buried in the return path and connected to the first optical fiber cable at the turning-back portion, the first optical fiber cable and the second optical fiber cable each have a core, a clad surrounding the outer periphery of the core, and a coating surrounding the outer periphery of the clad; the conformability of the clad to the coating of the first optical fiber cable is higher than the conformability of the clad to the coating of the second optical fiber cable; State measuring device.

2. Further comprising a cover member for covering the folded portion of the optical fiber cable. The state measuring device according to claim 1 .

3. the cover member has a curvature holding portion that can hold the folded portion of the optical fiber cable at a predetermined curvature. The state measuring device according to claim 2 .

4. the first optical fiber cable and the second optical fiber cable are connected via a third optical fiber cable having a smaller allowable bending radius than the first optical fiber cable and the second optical fiber cable; The third optical fiber cable is held by the curvature holding portion. The state measuring device according to claim 3 .

5. the optical fiber cable is inserted into an insertion hole provided in the object to be measured, A gap between the optical fiber cable and the insertion hole is filled with a filler material.

5. A state measuring device according to claim 1.

6. a conformability of the clad of the first optical fiber cable to the filler material is higher than a conformability of the clad of the second optical fiber cable to the filler material; The state measuring device according to claim 5 .

7. A state measurement method for measuring the state of the object to be measured by the state measurement device according to any one of claims 1 to 4, comprising: injecting light into the optical fiber cable; measuring the distortion of the object to be measured based on scattered light returning from the first optical fiber cable among the optical fiber cables; measuring the temperature of the object to be measured based on scattered light returning from the second optical fiber cable among the optical fiber cables; and correcting the measured distortion using the measured temperature. State measurement method.

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