JP2008203131A - Distributed optical fiber sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a distributed optical fiber sensor capable of measuring accurately a stain caused by hydraulic pressure. <P>SOLUTION: The distributed optical fiber sensor is bundled with at least one first sensing part of a hydraulic strain detecting optical fiber surrounded with a flexible material, and at least one second sensing part of a longitudinal-directional strain measuring optical fiber surrounded with a flexible material, via a slot, and those are collectively coated by a sheath. A temperature compensating optical fiber is stored in the slot under a condition not fixed to the respective sensing parts together with a tensile strength fiber. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a distributed optical fiber sensor for measuring a water pressure distribution in an underground storage tank or the like.

  At present, a typical one used for water pressure measurement is an electrical pore water pressure gauge, which includes a diaphragm attached with a gauge or a piezoelectric element. In the case of this electric type, a large number of sensors are required. On the other hand, the optical fiber sensor can detect strain due to water pressure with a minimum of one optical fiber. In an optical fiber sensor, the strain applied to the optical fiber is measured using an OTDR (Optical Time Domain Reflectometer) or BOTDR (Brillouin Optical Time Domain Reflectometer), and the strain can be converted into a water pressure to measure the water pressure. .

Conventionally, with respect to the structure of an optical fiber sensor, for example, techniques disclosed in Patent Documents 1 to 6 have been proposed.
In Patent Document 1, an optical fiber is spirally wound around the outer periphery of a pipe-shaped, bar-shaped, or string-shaped object or a groove of the object, and passes through the inside of the optical fiber due to a shape change of the optical fiber that occurs during deformation. An optical fiber sensor having a spiral three-dimensional configuration of an optical fiber that detects a change in optical transmission loss before and after the deformation of the optical fiber is disclosed.
Patent Document 2 discloses an optical fiber cable that is used for a pressure sensor that uses an optical fiber for pressure detection, and is a flexible round-shaped support that can be deformed by a pressure to be detected, and a support on the support. An optical fiber cable for a pressure sensor is disclosed.
In Patent Document 3, in an optical cable in which a fiber body is twisted or vertically attached to an optical fiber core wire having a resin coating, and a plastic jacket is applied, at least a part of the resin coating includes the jacket. An optical cable is disclosed that contacts an inner wall of the optical fiber.
In Patent Document 4, at least two strain-measuring optical fiber sensors each made of an optical fiber are arranged on the outer surface of a pipe provided with a spacer projecting outwardly at intervals in the pipe circumferential direction. In addition, the parallel measurement interval between the strain measurement optical fiber sensors is fixed along the length direction of the strain measurement optical fiber sensor, and after inserting the pipe into the borehole formed in the ground, A method for laying an optical fiber sensor is disclosed, in which grout is injected between an outer surface of a pipe and an inner surface of a borehole to bury the strain-measuring optical fiber sensor in the ground.
In Patent Document 5, a twisted pair of insulated wires formed by coating an insulator on a central conductor, an optical fiber cable, and the optical fiber cable at the center, It is characterized by comprising a cross-shaped interposition in which partition walls are arranged vertically in four directions, and the twisted wires are accommodated and disposed in an isolation space provided between the partition walls, and a jacket covering from the outer periphery of the cross-shaped interposition. An optical composite LAN cable is disclosed.
In Patent Document 6, in a slot for an optical fiber cable having a tension member in the center and a resin having a plurality of optical fiber core wire housing grooves around the tension member, the tension member is made of a metal pipe, An optical fiber cable slot in which an optical fiber for sensing is built in a pipe is disclosed.
JP 60-219503 A JP-A-6-43056 JP 9-49776 A JP 2002-156215 A JP 2003-338226 A JP 2004-94032 A

  Electric pore water pressure gauges that are widely used at present can only measure locally, and cannot measure over long distances. When measuring the water pressure around the storage tank, if it is an electric type, many boring holes must be dug and several water pressure gauges must be placed there. The electric type also has a problem in terms of durability.

On the other hand, since the optical fiber sensor can measure a long distance with one optical fiber, the number of boreholes and water pressure gauges is small, and it is excellent in durability. The problem can be solved.
However, as shown in FIG. 1, in the case of an optical fiber cable having a structure in which the optical fiber 2 is simply wound around the outer peripheral surface of the core material 1, the strain measured by the optical fiber 2 is the strain in the longitudinal direction due to the weight of the cable itself It is impossible to determine whether the strain is due to water pressure or not, and the strain due to water pressure cannot be measured accurately. Also for the optical fiber sensors disclosed in Patent Documents 1 to 6, it cannot be determined whether the strain is in the longitudinal direction or the strain due to water pressure, and accurate water pressure measurement cannot be performed even when these conventional techniques are combined.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a distributed optical fiber sensor capable of accurately measuring strain due to water pressure.

  In order to achieve the above-mentioned object, the present invention provides at least one first sensing unit in which a hydraulic strain detecting optical fiber is surrounded by a soft material, and at least a longitudinal strain measuring optical fiber is surrounded by a hard material. Provided is a distributed optical fiber sensor in which one second sensing unit is bundled through a slot.

  In the distributed optical fiber sensor of the present invention, it is preferable that an optical fiber for temperature compensation is accommodated in the slot together with the tensile fiber in a state where it is not fixed to each sensing unit.

  In the distributed optical fiber sensor according to the present invention, the first sensing section includes the hydraulic strain detecting optical fiber embedded in a soft material, and the second sensing includes the longitudinal strain measuring optical fiber as a hard material. It is preferable to embed in.

  According to the distributed optical fiber sensor of the present invention, strain due to its own weight, strain due to water pressure, and strain due to temperature change can be distinguished and measured, so that strain due to water pressure can be accurately measured.

Hereinafter, embodiments of a distributed optical fiber sensor of the present invention will be described with reference to the drawings.
FIG. 2 is a cross-sectional view showing an embodiment of the distributed optical fiber sensor of the present invention. In the figure, reference numeral 10 is a distributed optical fiber sensor, 11 is a first sensing unit, 12 is a second sensing unit, 13 is a hydraulic strain detection optical fiber, 14 is a soft material, 15 is a longitudinal strain detection optical fiber, 16 is a hard material, 17 is a slot, 18 is a sheath, 19 is a temperature compensating optical fiber, and 20 is a tensile strength fiber.

  The distributed optical fiber sensor 10 of the present embodiment includes two first sensing parts 11 in which a hydraulic strain detection optical fiber 13 is embedded in a soft material 14 and a longitudinal strain measurement optical fiber 15 in a hard material 16. Two embedded second sensing parts 12 are bundled through a slot 17 and the temperature compensating optical fiber is not fixed to the sensing parts 11 and 12 together with the tensile fiber 20 at the center of the slot 17. 19 is accommodated, and these are collectively covered with a flexible sheath 18.

  The distributed optical fiber sensor 10 according to the present embodiment includes two first sensing units 11 having a fan-shaped cross section and two second sensing units 12 having a fan-shaped cross section alternately via slots 17 having a cross shape. The entire cable covered and covered with the sheath 18 is formed in a circular cross section.

  In the present embodiment, the first sensing unit 11 surrounds the hydraulic strain detection optical fiber 13 with a flexible soft material 14 having flexibility and excellent durability such as an elastomer (rubber or the like) or silicone gel, When the distributed optical fiber sensor 10 is subjected to water pressure, the soft material 14 is deformed, and a strain is applied to the water pressure strain detecting optical fiber 13.

  In the present embodiment, the second sensing unit 12 surrounds the longitudinal strain measuring optical fiber 15 with a hard material 16 such as polyamide resin, and even if the distributed optical fiber sensor 10 is subjected to water pressure, it is used for longitudinal strain measurement. The optical fiber 15 has a structure in which radial strain is not applied. As a result, the longitudinal strain measuring optical fiber 15 can measure only the strain due to its own weight without being affected by the strain in the radial direction.

  In the present embodiment, a structure in which two first sensing parts 11 having a cross-sectional fan shape and two second sensing parts 12 having a cross-sectional fan shape are bundled through a cross-shaped slot 17 is illustrated. The shape of the slot 17 and the shape of each sensing unit are not limited to this example, and various changes can be made. For example, the shape of the slot 17 suffices if the first sensing unit 11 and the second sensing unit 12 can be separated, and need not be a cross-shaped slot. Only the soft material 14 may be dug, and the other portions may be constituted by slots. Further, the number of optical fibers is not limited to four. With this structure, it is possible to measure the water pressure by taking into account the strain caused by the water pressure and its own weight.

  The distributed optical fiber sensor 10 of the present embodiment is laid to measure the water pressure distribution in an underground storage tank or the like. When water pressure is applied, the soft material 14 of the first sensing unit 11 is deformed, and light for detecting hydraulic strain. Strain is applied to the fiber 13. The strain applied to the optical strain detecting optical fiber 13 is measured using an OTDR (Optical Time Domain Reflectometer) or a BOTDR (Brillouin Optical Time Domain Reflectometer).

  Since the strain measured by the hydraulic strain detection optical fiber 13 is a strain due to the water pressure + a strain due to the sensor's own weight + a strain due to a temperature change, the strain measured by the hydraulic strain detection optical fiber 13 is By subtracting the strain due to the sensor's own weight and the strain due to temperature change, the strain due to water pressure can be accurately measured.

  In the second sensing unit 12, the longitudinal strain measurement optical fiber 15 embedded in the hard material 16 is not subjected to radial strain due to water pressure, but is subject to longitudinal strain due to the sensor's own weight. Therefore, the strain due to the sensor's own weight can be detected by measuring the strain of the optical fiber 15 for longitudinal strain measurement using OTDR or BOTDR.

  At the center of the cross-shaped slot 17, the temperature compensating optical fiber 19 is housed loosely together with the tensile strength fiber 20. The temperature compensating optical fiber 19 measures a change in temperature by Raman scattered light. If strain is applied to the temperature compensating optical fiber 19, the temperature cannot be measured accurately. Therefore, the temperature compensating optical fiber 19 is covered with a tensile fiber such as Kevlar so that the strain is not applied. 11 and 12 are not fixed. By measuring the strain of the temperature compensating optical fiber 19, the strain due to the temperature change can be detected.

  As described above, according to the distributed optical fiber sensor 10 of the present embodiment, the strain due to the water pressure, the strain due to the weight of the sensor, and the strain due to the temperature change can be distinguished and measured. By subtracting the strain due to the weight of the sensor and the strain due to temperature change from the strain, the strain due to water pressure can be accurately measured.

  The distributed optical fiber sensor 10 of the present invention can be applied not only to water pressure measurement but also to measurement of pressure changes of other fluids such as hydraulic pressure.

It is a side view explaining an example of the prior art of an optical fiber sensor. It is sectional drawing which shows one Embodiment of the distributed optical fiber sensor which concerns on this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Distributed optical fiber sensor, 11 ... 1st sensing part, 12 ... 2nd sensing part, 13 ... Optical fiber for hydraulic-strain detection, 14 ... Soft material, 15 ... Optical fiber for longitudinal direction strain detection, 16 ... Hard material , 17 ... slot, 18 ... sheath, 19 ... temperature compensating optical fiber, 20 ... tensile strength fiber.

Claims (3)

  At least one first sensing unit that surrounds the optical fiber for detecting hydraulic strain with a soft material, and at least one second sensing unit that surrounds the optical fiber for measuring longitudinal strain with a hard material, A distributed optical fiber sensor characterized by being bundled through.   2. The distributed optical fiber sensor according to claim 1, wherein a temperature compensating optical fiber is accommodated in the slot together with a tensile strength fiber without being fixed to each sensing unit.   The first sensing unit includes the hydraulic strain detecting optical fiber embedded in a soft material, and the second sensing includes the longitudinal strain measuring optical fiber embedded in a hard material. The distributed optical fiber sensor according to claim 1 or 2.

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