Water ingress detection device and water ingress detection method

Abstract

The present disclosure pertains to a water ingress detection device that calculates a temperature change at each point in the longitudinal direction of an optical fiber, and determines a water ingress point in the longitudinal direction of the optical fiber on the basis of the calculated temperature change.

Description

Water immersion detection device and water immersion detection method

[0001] The present disclosure relates to a water immersion detection method using optical fiber sensing.

[0002] Water immersion detection techniques for optical fibers have been proposed (see, for example, Non-Patent Documents 1 and 2). In Non-Patent Document 1, water immersion is detected by measuring different characteristics of forward Brillouin scattering (shift amount and spectral width) depending on the substance covering the optical fiber. In Non-Patent Document 1, in order to perform water immersion detection using the characteristics of forward Brillouin scattering, a dedicated device is required in which the outer skin and cladding of the optical fiber are shaved so that the core of the optical fiber is directly in contact with water.

[0003] In Non-Patent Document 2, water immersion is detected by detecting deformation of the optical fiber. In Non-Patent Document 2, a dedicated device that expands when immersed in water is required to deform the optical fiber when immersed in water.

[0004] Chow, Desmond M., et al. “Distributed forward Brillouin sensor based on local light phase recovery.” Nature communications 9.1 (2018): 2990. NTT Technical Journal, November 2009

[0005] It is required to be able to identify where the laid optical fiber is immersed in water. However, in Non-Patent Documents 1 and 2, there was a problem that water immersion could only be detected at the location where the dedicated device was installed.

[0006] Therefore, an object of the present disclosure is to enable water immersion detection at an arbitrary point in an optical fiber.

[0007] By using distributed acoustic sensing (DAS: Distributed Acoustic Sensing), it is possible to measure temperature changes at each point in the longitudinal direction of an optical fiber. Therefore, in the present disclosure, this function of DAS is used to detect water immersion of the optical fiber.

[0008] Specifically, the flood detection system of this disclosure comprises a distributed acoustic sensing device capable of measuring temperature changes at various points along the longitudinal direction of an optical fiber, and a flood detection device of this disclosure.

[0009] The flood detection device and flood detection method of this disclosure calculate temperature changes at each point along the longitudinal direction of the optical fiber and determine flood locations along the longitudinal direction of the optical fiber based on the calculated temperature changes.

[0010] The flood detection device may determine a flooded location as a point where the temperature fluctuation is smaller than that of the surrounding area when comparing temperature changes along the longitudinal direction of the optical fiber. Alternatively, the flood detection device may determine a flooded location as a point where the temperature fluctuation is smaller than that of the surrounding area when comparing temperature changes over time at the same point along the longitudinal direction of the optical fiber.

[0011] Furthermore, the above disclosures can be combined as much as possible.

[0012] According to this disclosure, it is possible to enable water ingress detection at any point in an optical fiber.

[0013] This document shows an example of an embodiment of the flood detection system of this disclosure. It also shows a measurement system comparing temperature changes between flooded and non-flooded areas. A comparative example of temperature changes between flooded and non-flooded areas is shown. Finally, an example of the configuration of the flood detection device of this disclosure is presented. And an example of the flood detection method of this disclosure is also presented.

[0014] Embodiments of this disclosure will be described in detail below with reference to the drawings. However, this disclosure is not limited to the embodiments shown below. These examples are illustrative, and this disclosure can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. In this specification and in the drawings, components with the same reference numerals refer to the same components.

[0015] Figure 1 shows an example of the system configuration of the present disclosure. The flood detection system of this embodiment includes a measuring device 91 and a flood detection device 92. In this embodiment, at least a portion of the optical fiber 82 is buried in an underground conduit 83.

[0016] The measuring device 91 measures the temperature change of the optical fiber 82. The water ingress detection device 92 acquires the measurement results from the measuring device 91 and determines the water ingress points of the optical fiber 82 based on the temperature change of the optical fiber 82. When the water ingress points are consecutive, the water ingress section, which is the range of the water ingress points, can be determined. In this embodiment, an example of determining a water ingress section is shown.

[0017] The measuring device 91 is a distributed acoustic sensing device, such as a C-OTDR (Coherent-OTDR), capable of measuring temperature changes at various points along the longitudinal direction of the optical fiber 82. By using DAS in the measuring device 91, the immersion detection device 92 can calculate the phase change or optical spectral shift of Rayleigh scattered light at each point along the longitudinal direction of the optical fiber 82. Since the phase change or optical spectral shift of Rayleigh scattered light changes according to the temperature of the optical fiber 82, the immersion detection device 92 can calculate the temperature change of the optical fiber 82.

[0018] Here, the phase change and optical spectral shift of Rayleigh scattered light in the optical fiber 82 change not only due to temperature changes but also due to distortion caused by changes in shape. For this reason, the measurement results from the measuring device 91 include not only the temperature change of the optical fiber 82 but also the distortion of the optical fiber 82.

[0019] When the optical spectral shift of Rayleigh scattered light is calculated from the measurement results of the measuring device 91, the physical properties of the optical fiber with respect to strain and temperature changes are expressed by the following equation. Here, the parameters are as follows: Δν: Optical spectral shift ν ０ : Probe light center frequency K ε : Coefficient for strain change Δε: Strain change K Ｔ : Coefficient for temperature change ΔT: Temperature change

[0020] Since temperature changes more slowly than strain, the strain changes and temperature changes included in the measurement results of the measuring device 91 can be separated by differences in time frequency. For example, the immersion detection device 92 performs the following procedure. Procedure S21: The immersion detection device 92 calculates the optical spectral shift of Rayleigh scattered light at each point on the optical fiber 82. Procedure S22: The immersion detection device 92 calculates the amount of shift of the optical spectral shift for each point on the optical fiber 82. Procedure S23: The immersion detection device 92 calculates the time frequency at which the amount of shift fluctuates. Procedure S24: The immersion detection device 92 removes the shift amount with a large fluctuating time frequency. For example, temperature changes are extracted from the optical spectral shift using a low-pass filter.

[0021] The immersion detection device 92 can extract the optical spectral shift due to temperature changes by performing these procedures. In procedure S12, the temperature change may also be calculated using phase changes. In this case as well, the temperature change can be extracted using a low-pass filter, similar to the optical spectral shift.

[0022] As shown in Figure 2, optical fibers 82-1, 82-2, and 82-3 were connected and measurements were taken using the measuring device 91. The 50m optical fiber 82-1 was placed in the air, the 100m optical fiber 82-2 was submerged in water, and the 100m optical fiber 82-3 was placed in the air. In this embodiment, the immersion detection device 92 used a low-pass filter with a cutoff frequency of 0.1 Hz to extract temperature changes.

[0023] Figure 3 shows an example of the measurement results of temperature change. The top of Figure 3 shows the distance and time distribution of the temperature change. The specific heat of air is 1.005 kJ / (kg·K), and the specific heat of water is 4.182 kJ / (kg·K). Therefore, as shown in the figure, the temperature change of optical fiber 82-2 is smaller than that of optical fibers 82-1 and 82-3.

[0024] Thus, at the flooded point, the temperature change along the longitudinal direction of the optical fiber 82 is smaller than that of the surrounding area when compared, or the temperature change over time at the same point along the longitudinal direction of the optical fiber 82 is smaller than that of the surrounding area when compared. Therefore, the flood detection method of this disclosure calculates the temperature change at each point along the longitudinal direction of the optical fiber 82 and determines the flooded section along the longitudinal direction of the optical fiber 82 based on the calculated temperature change.

[0025] The standard deviation, calculated as an indicator of the degree of temperature change over time, is shown below in Figure 3. The air-filled section and the submerged section are separated at a standard deviation of around 0.1 K. Thus, it can be seen that by using the standard deviation calculation results, it is possible to determine whether or not the optical fiber 82 is submerged in water.

[0026] Figure 4 shows an example of the configuration of the flood detection device 92. The flood detection device 92 includes a temperature change extraction function 21, an index calculation function 22, and a flood determination function 23. The flood detection device 92 performs the flood detection method of this disclosure.

[0027] Figure 5 shows an example of an embodiment of the flood detection method of the present disclosure. In this embodiment of the flood detection method, the measuring device 91 measures the DAS in the optical fiber 82 (S11), the flood detection device 92 removes the strain component and extracts the temperature change (S12), the flood detection device 92 calculates an index of temperature fluctuation at each point in the optical fiber 82 (S13), and the flood detection device 92 determines the flooded section in the optical fiber 82 based on the index (S14).

[0028] Here, the extraction of temperature changes in step S12 can employ any method capable of removing distortion components. For example, a low-pass filter that cuts out frequency components due to distortion and allows the frequency components of temperature changes to pass through, or a filter that cuts out wavenumbers due to distortion and allows the wavenumbers of temperature changes to pass through, can be used.

[0029] The index calculated in procedure S13 can be any index that can evaluate the degree of temperature change over time, for example, the standard deviation.

[0030] As an example of how to determine the flooded area in procedure S14, at least one of the following areas can be used: (i) an area where the temperature fluctuation index is small compared to the surrounding area is a flooded area; (ii) an area within the same area where the index value is small compared to previously obtained values.

[0031] This disclosure utilizes the fact that the specific heat of water is greater than that of air, and that temperature changes in water are more stable than temperature changes in air, to determine flooded areas. Therefore, by performing temperature sensing during times when temperatures are prone to change, such as from sunrise to midday and from sunset to night, it becomes easier to measure the difference in temperature changes between air and water.

[0032] (Second Embodiment) In the first embodiment, time frequency was used to remove distortion and extract temperature changes in steps S23 and S24, but spatial frequency may be used instead of time frequency. In this case, the spatial frequency at which the shift amount fluctuates is calculated in step S23, and the small shift amount of the fluctuating spatial frequency is removed in step S24. For example, the optical spectral shift can be used to extract temperature changes using a high-pass filter. This makes it possible to extract the optical spectral shift due to temperature changes. When calculating temperature changes using phase changes, temperature changes can also be extracted using a high-pass filter in the same way as with optical spectral shifts.

[0033] (Other Embodiments) The flood detection device 92 of this disclosure can also be implemented using a computer and a program, and the program can be recorded on a recording medium or provided via a network.

[0034] 21: Temperature change extraction function 22: Index calculation function 23: Flood detection function 82, 82-1, 82-2, 82-3: Optical fiber 83: Underground conduit 91: Measurement device 92: Flood detection device

Claims

1. A flood detection device that calculates temperature changes at various points along the longitudinal direction of an optical fiber and determines flooded locations along the longitudinal direction of the optical fiber based on the calculated temperature changes.

2. The flood detection device according to claim 1, wherein a point where the temperature change in the longitudinal direction of the optical fiber is smaller than that of the surrounding area when compared, or a point where the temperature change over time in the longitudinal direction of the optical fiber is smaller than that of the surrounding area when compared, is determined to be the flooded point.

3. A flood detection system comprising: a distributed acoustic sensing device capable of measuring temperature changes at various points along the longitudinal direction of an optical fiber; and a flood detection device according to claim 1 or 2 for acquiring the measurement results from the distributed acoustic sensing device.

4. A flood detection method comprising: a flood detection device calculating temperature changes at various points along the longitudinal direction of an optical fiber; and the flood detection device determining the flooded section along the longitudinal direction of the optical fiber based on the calculated temperature changes.

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