Strain measurement method and sensing cable used therefor

Abstract

To provide a strain measurement method capable of measuring strain caused by shear waves in two-dimensional geophysical exploration using an optical fiber distributed acoustic sensing method, and a sensing cable used therefor.SOLUTION: An optical fiber is disposed along a linear measurement line set on a ground surface to measure strain on the ground surface. The optical fiber is disposed to form a series of meandering patterns that cross the measurement line alternately to the left and right within a two-dimensional plane along the ground surface including the measurement line. The meandering pattern consists of a repeating pattern of alternately including a non-detection part that perpendicularly intersects with the linear measurement line of a prescribed length at a center position, and a detection part that obliquely intersects with the linear measurement line at an interior angle θ at the center position. The sensing cable includes the optical fiber embedded within a flat bar-shaped mold having a central axis between two parallel main planes and exhibiting flexibility, where the optical fiber is disposed to follow a similar meandering pattern within a plane including the central axis and parallel to the main planes.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a strain measurement method capable of measuring strain caused by shear waves in two-dimensional geophysical exploration using optical fiber distributed acoustic sensing, and a sensing cable used therefor. [Background technology]

[0002] The distributed acoustic sensing (DAS) method using optical fibers is a method in which a sensing cable with an optical fiber inside is attached to a measurement target such as a structure or the ground, and an interrogator unit that sends and receives optical signals is connected to measure the strain of the measurement target. The strain generated in the measurement target is measured by the interrogator unit as strain in the longitudinal direction of the optical fiber. Here, by meandering the optical fiber in a predetermined zigzag pattern relative to the axis of the sensing cable, i.e., the measurement line of the measurement target, strain measurements can be obtained in each direction.

[0003] For example, Patent Document 1 discloses a method for detecting deformation and collapse of the ground in a ground soundness assessment system for civil engineering and construction works and buildings of thermal and nuclear power plants, etc., in which a stretchable optical fiber is laid out in a serpentine pattern within a two-dimensional plane or three-dimensional space of the ground to detect deformation and collapse of the ground. When pulsed light is incident on one end of the optical fiber, Brillouin scattered light and Rayleigh scattered light based on strain in the optical fiber caused by deformation of the ground return after a time period corresponding to the distance, and therefore the magnitude of the strain can be measured by measuring the frequency shift distribution, and the return time of the incident light can be measured to identify the location where the strain occurred. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 11-166869 Summary of the Invention [Problem to be solved by the invention]

[0005] When a sensing cable containing coaxial optical fibers is installed linearly on the ground surface, it can measure vibrations along that line, but this geophysical exploration method cannot measure shear waves (SH: Shear Horizontal Waves) that vibrate parallel to the ground surface, as measured by a vibrometer.On the other hand, as mentioned above, a sensing cable containing optical fibers that meanders in a predetermined zigzag pattern around its axis can measure strain components in several directions, but when the cables are arranged equivalently and in antiphase with respect to the axis in units of repeated zigzags (see Figure 3 of Patent Document 1), the signals cancel each other out, making it impossible to measure strain.

[0006] The present invention has been made in consideration of the above-mentioned circumstances, and its object is to provide a strain measurement method that can measure strain caused by shear waves in two-dimensional geophysical exploration using optical fiber distributed acoustic sensing, and a sensing cable used therein. [Means for solving the problem]

[0007] The method according to the present invention is a method for measuring strain occurring on the ground surface by arranging optical fibers along a linear measurement line set on the ground surface, wherein the optical fibers are arranged in a two-dimensional plane along the ground surface that includes the measurement line and describes a series of meandering patterns that cross the measurement line alternately to the left and right, and the meandering pattern is characterized by a repeating pattern that alternates between non-detection sections that are linear and have a length L1 and intersect the measurement line at a right angle at a center position, and detection sections that are linear and have a length L2 and intersect the measurement line at an oblique angle θ at a center position with the measurement line.

[0008] According to this feature, in two-dimensional geophysical exploration using optical fiber distributed acoustic sensing, it is possible to measure strain in the measurement line direction as well as strain due to shear waves generated on the earth's surface.

[0009] In the above-described invention, the oblique intersection points of the measurement line and the detection unit may be provided at regular intervals along the measurement line, thereby enabling detection at all points along the measurement line.

[0010] In the above invention, the interior angle θ may be 27 to 51 degrees. The meandering pattern may also include a curved connecting portion connecting the adjacent ends of the non-detection portion and the detection portion. This feature provides a clearer detection signal and enables highly accurate strain measurement.

[0011] The above invention may further include a second optical fiber in the two-dimensional plane, the second optical fiber being arranged so as to trace a series of meandering patterns at positions symmetrical to the meandering pattern with respect to the measurement line. This feature makes it possible to provide a clearer detection signal and achieve highly accurate strain measurement.

[0012] Furthermore, a sensing cable according to the present invention is a sensing cable used in a method of measuring strain occurring on the ground surface by arranging an optical fiber along a linear measurement line set on the ground surface, the optical fiber being embedded in a flexible flat rod-shaped mold having a central axis between two parallel main surfaces, the optical fiber being arranged in a plane that includes the central axis and is parallel to the main surfaces, so as to describe a series of meandering patterns that cross the central axis alternately to the left and right, the meandering pattern being a repeating pattern that alternately includes non-detecting portions that are linear and have a length L1 and are perpendicular to the central axis at a central position, and detecting portions that are linear and have a length L2 and are obliquely intersecting the central axis at an interior angle θ with the central axis at a central position.

[0013] According to this feature, when laying a sensing cable along the measurement line and conducting two-dimensional physical exploration using the optical fiber distributed acoustic sensing method, it is possible to measure not only the strain in the longitudinal direction (central axis direction) of the sensing cable, but also the strain due to shear waves generated on the ground surface.

[0014] In the above invention, the interior angle θ may be 27 to 51 degrees. The meandering pattern may also include a curved connecting portion connecting the adjacent ends of the non-detection portion and the detection portion. This feature provides a clearer detection signal and enables highly accurate strain measurement.

[0015] The above-described invention may further include a second optical fiber in the two-dimensional plane, arranged so as to trace a series of meandering patterns at positions symmetrical to the meandering pattern across the central axis. This feature provides a clearer detection signal and enables highly accurate strain measurement. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a diagram illustrating the arrangement of optical fibers showing a strain measurement method according to an embodiment of the present invention. [Figure 2] FIG. 2 is a partially enlarged plan view showing the arrangement of optical fibers. [Figure 3] FIG. 1 is a perspective view of a sensing cable in which an optical fiber is arranged. [Figure 4] FIG. 10 is a partially enlarged plan view of another arrangement of optical fibers. DETAILED DESCRIPTION OF THE INVENTION

[0017] A strain measurement method and a sensing cable used therein as one embodiment of the present invention will be described with reference to Figures 1 to 4. The strain measurement uses a distributed acoustic sensing (DAS) method.

[0018] 1, in the strain measurement method, an optical fiber 10 is placed along the ground surface including a linear measurement line M set at a zero depth position. At this time, the optical fiber 10 is placed in a two-dimensional plane S along the ground surface so as to trace a series of meandering patterns that alternately cross the measurement line M on the left and right. The optical fiber 10 includes at least a core and a cladding, and may further include a coating.

[0019] Referring also to FIG. 2, this meandering pattern alternates between non-detection sections 1, each having a linear length L1 and intersecting the measurement line M at a right angle at its center, and detection sections 2, each having a linear length L2 and intersecting the measurement line M at an internal angle θ at its center. Specifically, the orthogonal point N1, which is the center of non-detection section 1, is located at a position along the length L1 / 2, and the oblique intersection point N2, which is the center of detection section 2, is located at a position along the length L2 / 2. The internal angle θ indicates the smaller of the angles formed between measurement line M and detection section 2. A value of 27 to 51 degrees for the internal angle θ is preferable because the signal obtained is clear. While it is preferable for detection section 2 to intersect the measurement line M at a right angle at its center, deviations between the "orthogonal" and "center" positions are acceptable within the acceptable ranges for measurement accuracy and sensitivity. Similarly, it is preferable for non-detection section 1 to intersect the measurement line M at an oblique angle at its center, but deviations from the "center" position are acceptable.

[0020] Furthermore, this meandering pattern preferably includes a curved connection portion 3 that connects the proximal ends of the non-detection portion 1 and the detection portion 2. The connection portion 3 is curved with a bending radius equal to or greater than the allowable bending radius of the optical fiber 10 to suppress signal attenuation so that a signal of sufficient strength can be obtained in the DAS method. If the shape of the connection portion 3 is constant throughout the meandering pattern, then the oblique intersection points N2 where the measurement line M and the detection portion 2 obliquely intersect will be arranged at regular intervals D along the measurement line M.

[0021] This arrangement creates a series of meandering patterns that alternately cross the measurement line on the left and right, but does not result in an arrangement that is out of phase with and equivalent to the measurement line M. In other words, the signals do not cancel each other out. This arrangement makes it possible to measure strain in the direction of the measurement line M as well as strain due to shear waves generated on the ground surface in two-dimensional geophysical exploration using the DAS method.

[0022] 3, a sensing cable 20 can also be used to easily achieve the above-described arrangement of the optical fibers 10. The sensing cable 20 has a central axis C between two parallel principal surfaces 21a and 21b, and the optical fibers 10 are embedded in a flexible flat rod-shaped mold 21. The optical fibers 10 are arranged in a meandering pattern similar to that described above within the flat rod-shaped mold 21. In other words, this meandering pattern is a series of patterns that lie within a plane that includes the central axis C and is parallel to the principal surfaces 21a and 21b, and that alternately cross the central axis C on the left and right, and similarly to the above, includes non-detection portions 1 and detection portions 2.

[0023] By using such a sensing cable 20, the optical fiber 10 can be easily arranged as described above. For example, the sensing cable 20 is placed or buried in the ground so that one of the main surfaces 21b is in contact with a plane along the ground surface and the central axis C is arranged along the measurement line M. Then, the optical fiber 10 can be arranged in a two-dimensional plane along the ground surface so as to trace a series of meandering patterns that alternately cross the above-mentioned measurement line M on the left and right.

[0024] Using such a sensing cable 20, it is possible to measure strain in the direction of the measurement line M as well as strain due to shear waves generated on the earth's surface in two-dimensional geophysical exploration using the DAS method, as described above.

[0025] 4, it is also preferable to arrange a second optical fiber 11 in addition to the arrangement of the optical fiber 10. Here, the second optical fiber 11 is arranged in a two-dimensional plane along the ground surface so as to trace a series of meandering patterns at positions that are line-symmetrical to the meandering pattern of the optical fiber 10 across the measurement line M. In other words, the second optical fiber 11 is arranged at a position obtained by rotating the optical fiber 10 by 180° around the measurement line M as the central axis. As a result, the detection unit 2 of the optical fiber 10 and the detection unit 2 of the second optical fiber 11 are equivalent and have opposite phases to each other, but are independent of each other and do not cancel each other out in signal.

[0026] By arranging the optical fiber 10 and the second optical fiber 11 in this way, it is possible to increase the number of directions in which strain can be detected without reducing sensitivity. In other words, it is possible to provide a clearer detection signal and achieve highly accurate strain measurement. As with the sensing cable 20 described above, the optical fiber 10 and the second optical fiber 11 may be embedded in a flat rod-shaped mold 21 to achieve this arrangement, thereby forming a sensing cable.

[0027] Although the embodiments of the present invention and modifications based thereon have been described above, the present invention is not necessarily limited to these examples. Furthermore, those skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims. [Explanation of symbols]

[0028] 1 Non-detection part 2. Detection unit 3 Connection 10 Optical Fiber 11 Second Optical Fiber 20 Sensing cable

Claims

1. A method for measuring strain occurring on the earth's surface by arranging an optical fiber along a linear measurement line set on the earth's surface, comprising: the optical fiber is arranged in a two-dimensional plane along the ground surface, including the measurement line, so as to describe a series of meandering patterns that cross the measurement line alternately to the left and right, The meandering pattern is a non-detection portion having a linear length L1 and perpendicular to the measurement line at a central position; a linear detection unit having a length L2, positioned at the center of the measurement line and obliquely intersecting the measurement line at an interior angle θ; A strain measurement method characterized in that the strain measurement is performed using a repeating pattern that alternately includes

2. 2. The strain measuring method according to claim 1, wherein oblique intersection points where said measurement line and said detection portion intersect obliquely are provided at regular intervals along said measurement line.

3. 3. The strain measuring method according to claim 2, wherein the interior angle θ is 27 to 51 degrees.

4. 2. The strain measuring method according to claim 1, wherein the meandering pattern includes a curved connecting portion connecting adjacent ends of the non-detecting portion and the detecting portion.

5. 5. A strain measurement method according to claim 1, further comprising a second optical fiber within the two-dimensional plane, the second optical fiber being arranged so as to trace a series of meandering patterns at positions symmetrical to the meandering pattern across the measurement line.

6. A sensing cable used in a method for measuring strain occurring on the earth's surface by arranging an optical fiber along a linear measurement line set on the earth's surface, The optical fiber is embedded in a flexible flat rod-shaped mold having a central axis between two parallel main surfaces, the optical fiber is arranged in a two-dimensional plane that includes the central axis and is parallel to the main surface, so as to describe a series of meandering patterns that alternately cross the central axis to the left and right, The meandering pattern is a non-detection portion having a linear length L1 and perpendicular to the central axis at a central position; a linear detection portion having a length L2, located at a center position relative to the central axis and obliquely intersecting the central axis at an interior angle θ; A sensing cable characterized by comprising a repeating pattern including alternating

7. 7. The sensing cable according to claim 6, wherein the interior angle θ is 27 to 51 degrees.

8. 7. The sensing cable according to claim 6, wherein the meandering pattern includes a curved connecting portion that connects adjacent ends of the non-sensing portion and the sensing portion together.

9. The sensing cable according to any one of claims 6 to 8, further comprising a second optical fiber within the two-dimensional plane, the second optical fiber being arranged to describe a series of meandering patterns at positions symmetrical to the meandering pattern across the central axis.

Images