AH-FBG optical cable for measuring soil moisture content and application method

Abstract

The invention discloses an AH-FBG optical cable for measuring soil moisture content and an application method. The AH-FBG optical cable comprises a sensing section and a non-sensing section. The sensing section is composed of an FBG sensor, a heating armor tube, a protection tube and a sheath from the inside to the outside, and the non-sensing section is composed of an optical fiber, an armor tube, a copper mesh and a sheath from the inside to the outside. The optical fiber runs through the AH-FBG optical cable. In the sensing section, an FBG sensor is arranged on the optical fiber to realize temperature sensing. The outer side of the FBG sensor is a heating armor tube to realize segmented heating of the FBG sensor. The sheath wraps the entire AH-FBG optical cable. Based on indoor experiments, the heating uniformity of the AH-FBG optical cable is tested, and the temperature characteristic value-moisture content calibration formula for moisture content measurement is determined. The AH-FBG optical cable designed in the present invention has good heating uniformity, and the performance of the AH-FBG optical cable for in-situ soil moisture content measurement is excellent, providing a new technical means for high-precision, quasi-distributed measurement, and automated real-time measurement of deep soil moisture content.

Description

Technical Field

[0001] The invention relates to the field of soil moisture distribution monitoring, in particular to an AH-FBG optical cable for measuring soil moisture content, and also to an application method of the AH-FBG optical cable for measuring soil moisture content. Background Art

[0002] The moisture field is a key physical field that affects soil stability. Its distribution and evolution are directly related to the occurrence and development of various geological disasters (such as landslides, debris flows, and ground collapse) and geological environmental problems (soil erosion, soil cracking, and soil salinization). Therefore, in-situ monitoring of the soil moisture field is an important prerequisite for soil geological disaster risk assessment and prediction and early warning.

[0003] Conventional moisture field monitoring technology is mostly point measurement, which is difficult to meet the requirements of large-scale spatiotemporal continuous in-situ monitoring, often resulting in the loss of moisture field data of key soil layers, hindering people's understanding of the distribution and migration laws of in-situ soil moisture field. In the past decade, the actively heated fiber-optic (AHFO) monitoring technology has broken through the limitations of traditional monitoring technology due to its unique advantages such as small size, large-scale distributed measurement, and anti-electromagnetic interference. It has developed rapidly and has become the frontier and hotspot in the current field of soil moisture field measurement. The AHFO technology buries an optical fiber with internal heating function into the soil. After power is turned on and heated, the heat is conducted and diffused to the surrounding soil. The moisture content of the soil is calculated based on the temperature change law measured by the optical fiber. Since the optical fiber temperature sensing technology is divided into distributed temperature sensing (DTS) technology and fiber Bragg grating (FBG) technology, the AHFO technology can also be divided into active heating distributed temperature sensing (AH-DTS) technology and active heating fiber Bragg grating (AH-FBG) technology. Accordingly, the current AHFO sensors can be divided into AH-DTS optical cable and AH-FBG corundum tube sensors.

[0004] The AH-DTS optical cable is long-line, soft and easy to bend, and can form a good coupling effect with the soil. According to the different heating materials encapsulated in the optical cable, the AH-DTS optical cable can be divided into carbon fiber heated sensing optical cable (CFHC) and metal-net heated cable (MNHC). The biggest difference between MNHC and CFHC is that its heating material is copper mesh, whose resistance is much smaller than that of carbon fiber. Therefore, MNHC has a long heat transfer distance and is more suitable for distributed monitoring of moisture fields in soil over long distances and large areas. The AH-DTS optical cable was buried in the in-situ soil beside the river bank to prove the reliability of AHFO technology for in-situ monitoring. In the prior art, the AH-DTS optical cable was laid in farmland at three different depths, with a total length of 750m. The calibration curve of the in-situ soil was established, and the distributed measurement of long-distance in-situ soil moisture was realized. The AH-DTS optical cable was laid in a deep borehole to realize the in-situ test of the thermal conductivity of the soil. DTS technology can realize distributed measurement, but the spatial resolution of existing DTS demodulators is low, generally 1m, and existing demodulation equipment does not support wireless remote functions, making remote automated monitoring impossible.

[0005] The existing fiber optic sensor based on AH-FBG technology, namely AH-FBG corundum tube sensor, has a temperature measurement accuracy of 0.1℃ and no spatial resolution problem. The AH-FBG corundum tube sensor itself has a certain rigidity and is not easy to bend, which will bring two problems. First, although the AH-FBG corundum tube sensor is easy to drill into the in-situ soil, its shear resistance is poor and it cannot drill into the deeper soil layer (>1m); second, the AH-FBG corundum tube sensor is easy to form poor contact with the surrounding soil. These problems limit the further promotion and application of AH-FBG technology. Summary of the invention

[0006] The purpose of the present invention is to provide an AH-FBG optical cable for measuring soil moisture content and an application method. The AH-FBG optical cable has the advantages of both the AH-DTS optical cable and the AH-FBG corundum tube sensor. The heating uniformity of the AH-FBG optical cable is tested through indoor experiments, and the moisture content calibration formula of the optical cable is determined.

[0007] In order to realize the above functions, the present invention designs an AH-FBG optical cable for measuring soil moisture content, including a sensing section and a non-sensing section. In the sensing section, it is composed of four parts from the inside to the outside, namely, a FBG sensor, a heating armor tube, a protection tube and a sheath. In the non-sensing section, it is composed of four parts from the inside to the outside, namely, an optical fiber, an armor tube, a copper mesh and a sheath. The optical fiber runs through the AH-FBG optical cable. In the sensing section, an FBG sensor is arranged on the optical fiber to realize temperature sensing. The outer side of the FBG sensor is a heating armor tube to realize segmented heating of the FBG sensor. The sheath wraps the whole AH-FBG optical cable to protect the whole AH-FBG optical cable.

[0008] The FBG sensor is used as the temperature sensor of the AH-FBG optical cable. The grid area of ​​the FBG sensor is placed in the heating armor tube, where the grid area of ​​the FBG sensor is located in the center of the entire heating armor tube; the heating armor tube serves as the heat source of the FBG sensor, and its two ends are connected to the copper mesh of the non-sensing section, and the copper mesh is used to transmit current; the protection tube is placed outside the heating armor tube to protect the FBG sensor inside the heating armor tube and the connector between the heating armor tube and the copper mesh.

[0009] As a preferred technical solution of the present invention: the heating armor tube and the protective tube in the sensing section, the armor tube, the copper mesh, and the sheath in the non-sensing section are all tough.

[0010] As a preferred technical solution of the present invention: the AH-FBG optical cable includes 10 sensing segments in total, the distance between adjacent FBG sensors is 1 m, and the total resistance is 4Ω.

[0011] The present invention also provides an application method of an AH-FBG optical cable for measuring soil moisture content. Based on the AH-FBG optical cable for measuring soil moisture content, the following steps are performed to complete the in-situ measurement of the moisture content of the target soil:

[0012] Step S1: Perform heating uniformity test and calibration test on the AH-FBG optical cable;

[0013] Step S2: installing and laying the AH-FBG optical cable at the position to be monitored of the target soil body;

[0014] Step S3: collecting the temperature characteristic value of the target soil measured by the AH-FBG optical cable, and calculating the moisture content of the target soil based on the functional relationship between the temperature characteristic value of the soil and the moisture content of the soil.

[0015] As a preferred technical solution of the present invention: in step S1, the heating uniformity of the AH-FBG optical cable under the ground condition and the suspended condition is tested respectively, and the steps are as follows:

[0016] Step S1.1: pre-tension each FBG sensor measuring point in an AH-FBG optical cable and fix it on the ground with adhesive tape, so that the whole AH-FBG optical cable is in a pre-tensioned state;

[0017] Step S1.2: Lay another AH-FBG optical cable freely on the ground, and the entire AH-FBG optical cable is in a free state;

[0018] Step S1.3: The non-sensing sections of the two AH-FBG optical cables are laid in a U-shape with the sensing sections, the conductors in the AH-FBG optical cables are connected to a DC power supply, the optical fibers in the AH-FBG optical cables are connected to an FBG demodulator, and then two heating tests are performed;

[0019] Step S1.4: First heating test: Each FBG sensor measuring point in the AH-FBG optical cable is pre-tensioned by a displacement table in turn, and fixed by a fixing table. At this time, the AH-FBG optical cable is in a pre-tensioned state. Then, the AH-FBG optical cable is connected to a DC power supply and an FBG demodulator respectively to perform a heating test.

[0020] Step S1.5: The second heating test changes the state of the AH-FBG optical cable so that the optical cable is fixed between the translation stage and the fixing stage in a free state, and then a heating test is performed.

[0021] As a preferred technical solution of the present invention: the calibration test method in step S1 is as follows:

[0022] Soil samples with different water contents were placed in the calibration box, and the AH-FBG optical cable was laid out in an S shape in the calibration box. During this process, the sensing section of the AH-FBG optical cable was ensured to be completely placed inside each soil sample and in a stress-free free state.

[0023] After the AH-FBG optical cable is laid, the optical fiber and wire in the AH-FBG optical cable are connected to the FBG demodulator and DC power supply respectively, and then a heating test is performed to obtain T t -θ function relationship, T t is the temperature rise value, and θ is the volumetric moisture content of the soil.

[0024] As a preferred technical solution of the present invention: the functional relationship between the soil temperature characteristic value and the soil moisture content in step S3 is as follows:

[0025]

[0026] Where, T t is the temperature rise value, θ is the soil volume moisture content, the unit is m -3 m -3 , A, B and C are constants related to soil type and are given by T tThe fitting results of the -θ calibration test are obtained.

[0027] Beneficial effects: Compared with the prior art, the advantages of the present invention include:

[0028] 1. AH-FBG optical cable integrates the advantages of AH-FBG corundum tube sensor and AH-DTS optical cable. It has the advantages of high moisture content measurement accuracy and remote automatic data collection of AH-FBG corundum tube sensor, and the advantages of simple laying of AH-DTS optical cable and deep soil moisture content measurement by drilling.

[0029] 2. AH-FBG optical cable is the first to realize the segmented heating function. Existing AHFO sensors are all encapsulated with heating materials in the entire optical cable. The entire sensor needs to be heated during testing, while AH-FBG optical cable only needs to heat the area around the FBG measuring point in segments, which greatly saves power supply and improves the measurement efficiency of AH-FBG. Especially for remote areas without electricity, even if the solar panels are insufficiently charged, a small amount of electricity can be used to heat the AH-FBG optical cable to ensure the frequency and quality of moisture content measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a structural diagram of an AH-FBG corundum tube sensor provided according to an embodiment of the present invention;

[0031] Figure 2 Schematic diagram of the structure of an AH-FBG optical cable for measuring soil moisture content provided by an embodiment of the present invention;

[0032] Figure 3 is a graph showing the uniformity test results of an AH-FBG optical cable provided in accordance with an embodiment of the present invention;

[0033] Figure 4 is a diagram of a calibration device for an AH-FBG optical cable provided according to an embodiment of the present invention;

[0034] Figure 5 is the T of the AH-FBG optical cable provided according to the embodiment of the present invention. t -θ calibration result diagram;

[0035] Figure 6 is a diagram of the arrangement of sensors in a test pit according to an embodiment of the present invention;

[0036] Figure 7 It is a comparison chart of the measured moisture content of the AH-FBG optical cable and the measured moisture content of the neutron tube provided according to an embodiment of the present invention. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0038] The existing AH-FBG corundum tube sensor structure reference Figure 1 In order to overcome the shortcomings of the existing AH-FBG corundum tube sensor, the AH-FBG optical cable designed by the present invention adopts a whole-through heating armor tube, and then the copper mesh connects the heating armor tube around the FBG sensor, which can ensure that only the surrounding area of ​​the FBG sensor can be heated. It can ensure both heating uniformity (the whole armor tube is through) and heating efficiency (only the armor tube around the FBG sensor is used for heating). Segmented heating can save electricity and is also convenient for on-site power supply. Compared with the rigid structure of the AH-FBG corundum tube sensor, the AH-FBG optical cable is flexible, easy to couple with the soil, and undergo coordinated deformation. In addition, the AH-FBG is in the shape of a cable, which is easy to be laid in the deep soil through drilling, so as to realize the quasi-distributed measurement of the moisture content of the deep soil.

[0039] An AH-FBG optical cable for measuring soil moisture content provided by an embodiment of the present invention is referred to Figure 2 , including a sensing section and a non-sensing section. In the sensing section, it is composed of four parts from the inside to the outside, namely, a FBG (Fiber Bragg grating, FBG) sensor, a heating armor tube, a protection tube and a sheath. In the non-sensing section, it is composed of four parts from the inside to the outside, namely, an optical fiber, an armor tube, a copper mesh and a sheath. The optical fiber runs through the AH-FBG optical cable. In the sensing section, an FBG sensor is configured on the optical fiber to realize temperature sensing. The outer side of the FBG sensor is a heating armor tube to realize segmented heating of the FBG sensor. The sheath wraps the entire AH-FBG optical cable to protect the entire AH-FBG optical cable.

[0040] In one embodiment, the total length of the AH-FBG optical cable is 9.3 m, the diameter of the sensing section is 5 mm, the diameter of the non-sensing section is 3 mm, the AH-FBG optical cable includes 10 sensing sections, the distance between adjacent FBG sensors is 1 m, and the total resistance is 4Ω.

[0041] The FBG sensor is used as the temperature sensor of the AH-FBG optical cable. The grid area of ​​the FBG sensor is 1 mm long and is placed in the heating armor tube, where the grid area of ​​the FBG sensor is located in the center of the entire heating armor tube. The heating armor tube is 15 mm long. The heating armor tube serves as the heat source of the FBG sensor and has a resistance of 4Ω / m. Both ends of the heating armor tube are connected to the copper mesh of the non-sensing section. Since the resistance of the copper mesh is much smaller than that of the heating armor tube and can be ignored, the copper mesh only plays the role of transmitting current. The protective tube is placed outside the heating armor tube to protect the FBG sensor inside the heating armor tube and the connector between the heating armor tube and the copper mesh.

[0042] The heating armor tube and protective tube in the sensing section, the armor tube, copper mesh, and sheath in the non-sensing section are all tough. The non-sensing section only plays the role of transmitting current and light waves. AH-FBG sensor optical cable can be set with multiple sensor sections according to actual needs to form a quasi-distributed sensor optical cable.

[0043] The embodiment of the present invention further provides an application method of the AH-FBG optical cable for measuring soil moisture content. Based on the AH-FBG optical cable for measuring soil moisture content, the following steps are performed to complete the in-situ measurement of the moisture content of the target soil:

[0044] Step S1: Perform heating uniformity test and calibration test on the AH-FBG optical cable;

[0045] In step S1, the heating uniformity of the AH-FBG optical cable is tested under the ground-attached condition and the suspended condition respectively, and the steps are as follows:

[0046] Step S1.1: pre-tension each FBG sensor measuring point in an AH-FBG optical cable and fix it on the ground with adhesive tape, so that the whole AH-FBG optical cable is in a pre-tensioned state;

[0047] Step S1.2: Lay another AH-FBG optical cable freely on the ground, and the entire AH-FBG optical cable is in a free state;

[0048] Step S1.3: The non-sensing sections of the two AH-FBG optical cables are laid in a U-shape with the sensing sections, the conductors in the AH-FBG optical cables are connected to a DC power supply, the optical fibers in the AH-FBG optical cables are connected to an FBG demodulator, and then two heating tests are performed;

[0049] Step S1.4: First heating test: Each FBG sensor measuring point in the AH-FBG optical cable is pre-tensioned by a displacement table in turn, and fixed by a fixing table. At this time, the AH-FBG optical cable is in a pre-tensioned state. Then, the AH-FBG optical cable is connected to a DC power supply and an FBG demodulator respectively to perform a heating test.

[0050] Step S1.5: The second heating test changes the state of the AH-FBG optical cable so that the optical cable is fixed between the translation stage and the fixing stage in a free state, and then a heating test is performed.

[0051] Temperature characteristic value of AH-FBG optical cable after heating (T t ) Distribution reference Figure 3 It can be seen that the T of each FBG on the AH-FBG cable is t The distribution is very uniform. The T tThe standard deviation (σ) is around 0.3℃, which shows that the uniformity of the AH-FBG optical cable is very good. It is worth noting that the T t The average values ​​are different. In general, whether it is ground-mounted or suspended, the T t Both are greater than the T of free optical cable t This is because the pre-stretched optical cable will be restricted by the fixed point during the thermal expansion process, so that the FBG cannot be fully extended, and the measured wavelength change will be small. t In addition, if the pre-tensioned or free state of the optical cable is determined, the T of the suspended AH-FBG optical cable t It will be higher than the T of the ground-level optical cable. t This is because the ground is more likely to transfer heat than the air, and the T t In summary, the uniformity of AH-FBG optical cable is good, but its T t There are differences. In practical applications, the moisture content of AH-FBG optical cable should be calibrated according to the actual installation environment.

[0052] The calibration test method in step S1 is as follows:

[0053] Soil samples with different water contents were placed in the calibration box, and the AH-FBG optical cable was laid out in an S shape in the calibration box. During this process, the sensing section of the AH-FBG optical cable was ensured to be completely placed inside each soil sample and in a stress-free free state.

[0054] After the AH-FBG optical cable is laid, the optical fiber and wire in the AH-FBG optical cable are connected to the FBG demodulator and DC power supply respectively, and then a heating test is performed to obtain T t -θ function relationship, T t is the temperature rise value, and θ is the volumetric moisture content of the soil.

[0055] The actual application environment of AH-FBG optical cable is in-situ soil, and it needs to be laid through drilling, at which time the optical cable is in a free state. Therefore, when conducting a moisture content calibration test on the AH-FBG optical cable, it should be ensured that the AH-FBG sensing optical cable is surrounded by the actual soil to be tested, and the optical cable should be in a free state without pre-tension.

[0056] The temperature rise uniformity of the 10 FBG measuring points on the AH-FBG optical cable is good, but its overall length is large. If the calibration device of the traditional AH-FBG sensor is used, it will be difficult. In order to facilitate the indoor calibration test, a corresponding calibration device is proposed. The device can accommodate 10 soil samples with different moisture contents at the same time. During calibration, the AH-FBG optical cable is arranged in an S shape in the calibration box. During this process, the sensing section of the optical cable is ensured to be completely placed inside each soil sample, and the sensing section is in a stress-free free state. The advantage of this calibration device is that 10 calibration data can be obtained by heating once, which greatly simplifies the calibration process.

[0057] Figure 4 The moisture content values ​​of the 10 soil samples set up in this test and the layout of the AH-FBG optical cable are displayed. After the AH-FBG optical cable is laid, the optical fiber and wire in the optical cable are connected to the FBG demodulator and DC power supply respectively, and then a heating test is carried out. The heating voltage is 24V and the heating time is 20min. It should be noted that the local loess used in the in-situ test was used this time. The dry density of the soil is the average dry density of the in-situ soil (1.4g / cm 3 ).

[0058] It should be noted that the heating parameters of the AH-FBG optical cable under ground-attached conditions and suspended conditions are exactly the same.

[0059] Step S2: installing and laying the AH-FBG optical cable at the position to be monitored of the target soil body;

[0060] Step S3: collecting the temperature characteristic value of the target soil measured by the AH-FBG optical cable, and calculating the moisture content of the target soil based on the functional relationship between the temperature characteristic value of the soil and the moisture content of the soil.

[0061] FBG sensor is a kind of optical fiber technology that can reflect specific wavelengths by writing gratings on optical fibers. When broadband incident light enters the optical fiber, FBG will reflect light of specific wavelengths. The central wavelength (λ B ) is linearly related to strain and temperature:

[0062]

[0063] In the formula, Δλ B is the change in FBG wavelength (nm); ε is the axial strain of the optical fiber; ΔT is the change in temperature (℃); P e is the elastic-optical coefficient of the optical fiber; α is the thermal expansion coefficient of the optical fiber (℃ -1 );ξ is the thermal-optical coefficient of the optical fiber (℃ -1 ).

[0064] It can be seen from the above formula that when FBG is in a stress-free relaxation state, its wavelength change is linearly related to temperature. Based on this principle, FBG can realize single sensing of temperature. On this basis, the heating material and FBG are encapsulated in a certain material at the same time, and an active heating fiber Bragg (AH-FBG) sensor is made, which can be used as both a heat source and a temperature sensor.

[0065] AH-FBG sensors usually have a large aspect ratio, so the fiber optic sensor buried in the soil can be regarded as an infinitely long cylindrical heat source. When the fiber optic sensor is powered on and heated, its thermal response process can be described as:

[0066]

[0067] Where, T t is the temperature rise value (℃), also known as the temperature characteristic value, T(t) is the sensor temperature corresponding to the heating time t (℃), T0 is the initial ambient temperature (℃), Q is the heating power per unit length (W / m), λ is the thermal conductivity of the soil (W / (m K)), R is the thermal resistance between the AHFO sensor and the soil wall per unit length (mK / W), and K is the thermal diffusion coefficient of the soil (m 2 / s), a is the outer diameter of the sensor (m), and c is a constant whose value is 1.7811.

[0068] From the above formula, we can see that T t It is well known that λ has a unique corresponding relationship with the soil moisture content (θ). Therefore, T t There is a monotonic and unique functional relationship between it and θ, which is the T of the AH-FBG sensor. t -θ calibration curve. The T t The -θ function relationship is and Konrad model:

[0069]

[0070] Where, T t is the temperature rise value, θ is the soil volume moisture content, the unit is m -3 ·m -3 , A, B and C are constants related to soil type and are given by T t The fitting results of the -θ calibration test are obtained.

[0071] Therefore, as long as the T of the AH-FBG sensor is established t -θ calibration curve, that is, the measured T tIt should be noted that the calibration test of the AH-FBG sensor can be indoor calibration or on-site calibration, which can be selected according to the actual situation.

[0072] Figure 5 Demonstrating the T of AH-FBG optical cable t -θ calibration results. It can be seen that the fitting effect of the calibration results is very good, and the correlation coefficient is as high as 0.995, which indirectly proves the good heating uniformity of the AH-FBG optical cable again. t The -θ calibration curve is:

[0073]

[0074] The following is an application example of the present invention:

[0075] 1. Monitoring plan

[0076] The in-situ test site is a test pit with a diameter of 1 m and a depth of 29.56 m, which is located in a flat bare soil area of ​​Ganguyi Loess Plateau in Yan'an (36°47′22.69″N, 109°49′05″E). Figure 6 This is the layout plan of the sensors in the test pit. Two AH-FBG optical cables and a neutron probe contact tube are arranged in the test pit. The AH-FBG optical cables are consistent with the optical cables used in the above calibration test. There are 10 FBG measuring points on each AH-FBG optical cable, and the adjacent measuring points are 1m apart. In the test pit where both optical cables are laid out in a free state, the first FBG measuring points of the two AH-FBG optical cables are located at 0.1m and 13.1m underground respectively. It should be noted that the etching quality of the FBG at the 18.1m measuring point is not good, and the reflection signal at this point is weak, so this FBG measuring point is not considered in the actual analysis. The neutron probe contact tube runs through the borehole, which can provide a test channel for the neutron probe to measure the soil moisture content.

[0077] The installation and layout process of AH-FBG optical cable is divided into four steps. First, use cable ties to fix the sensor on the steel strand, and fix the front end of the steel strand and the guide hammer. It should be noted that in order to prevent the FBG measuring point from being affected by stress, use cable ties to fix both ends of the measuring point and ensure that the entire FBG sensing section is in a relaxed state; then, slowly lower the AH-FBG optical cable into the test pit; then, keep the steel strand taut and fix it to the bracket; finally, backfill the sieved in-situ loess in layers, and control the dry density of the backfill soil to the average dry density of the in-situ loess, that is, 1.4g / cm 3 .

[0078] After being laid, the two AH-FBG optical cables are connected in parallel to the FBG wireless demodulator in the in-situ monitoring station. The FBG wireless demodulator can collect data in real time and transmit the data to the client through 4G / 5G signals. All types of equipment in the in-situ monitoring station are powered by solar energy. In order to ensure stable contact between the AH-FBG optical cable and the backfill soil, monitoring will begin two months after the test pit is backfilled. The monitoring period is from February 3, 2020 to February 3, 2022. The AH-FBG optical cable is heated and tested once a day, with a heating voltage of 24V and a heating time of 20min, which is exactly the same as the parameters used in the calibration test.

[0079] 2. Accuracy of measurement results

[0080] The measured temperature characteristic value (T t ) is calculated to obtain the measured soil moisture content (θ). Figure 7 The figure shows the comparison between the measured moisture content of the AH-FBG optical cable and the measured moisture content of the neutron tube. It can be seen that the data points are all around the 1:1 line, indicating that the moisture content measurement results of the two methods are basically consistent. It should be noted that most of the data points are distributed in the upper part of the 1:1 line, indicating that the measured moisture content of the AH-FBG optical cable is mostly higher than that of the neutron probe. This may be due to the difference between the soil used in the AH-FBG optical cable calibration test and the in-situ soil structure. The in-situ backfill loess was tested after being stable for a period of time. The soil structure is more stable than that of the calibration test soil. Therefore, the T of the AH-FBG optical cable measured on site is t It will be lower than the calibration test, so the measured moisture content will be slightly higher. If the moisture content measured by the neutron probe is the true value, the moisture content measurement error of the AH-FBG optical cable is 0.023cm 3 / cm 3 , indicating that the difference between the calibration test and the on-site soil structure has little effect on the moisture content measurement results. Therefore, the moisture content results of the AH-FBG optical cable are reliable and accurate.

[0081] In summary, the AH-FBG optical cable integrates the advantages of various sensors, so it is suitable for both indoor and in-situ tests. The number of FBG measuring points of the AH-FBG optical cable can be freely set according to actual needs, so its length can also be freely set. Therefore, the AH-FBG optical cable is also suitable for indoor tests with small soil volumes. When the AH-FBG optical cable is applied to the in-situ soil, the optical cable has good flexibility and is easy to couple with the in-situ soil. Therefore, when applied in-situ, it only needs to control the backfill quality to ensure good contact between the AH-FBG optical cable and the in-situ soil, thereby ensuring the accuracy of the moisture content measurement of the AH-FBG optical cable.

[0082] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the present invention is not limited to the above embodiments, and various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of the present invention.

Claims

1. An AH-FBG optical cable for measuring soil moisture content, characterized in that: It includes a sensing section and a non-sensing section. The sensing section is composed of four parts from the inside to the outside, namely, a FBG sensor, a heating armor tube, a protection tube and a sheath. The non-sensing section is composed of four parts from the inside to the outside, namely, an optical fiber, an armor tube, a copper mesh and a sheath. The optical fiber runs through the AH-FBG optical cable. In the sensing section, an FBG sensor is arranged on the optical fiber to realize temperature sensing. The outer side of the FBG sensor is a heating armor tube to realize segmented heating of the FBG sensor. The sheath wraps the entire AH-FBG optical cable to protect the entire AH-FBG optical cable. The FBG sensor is used as the temperature sensor of the AH-FBG optical cable. The grid area of ​​the FBG sensor is placed in the heating armor tube, where the grid area of ​​the FBG sensor is located in the center of the entire heating armor tube; the heating armor tube serves as the heat source of the FBG sensor, and its two ends are connected to the copper mesh of the non-sensing section, and the copper mesh is used to transmit current; the protection tube is placed outside the heating armor tube to protect the FBG sensor inside the heating armor tube and the connector between the heating armor tube and the copper mesh.

2. The AH-FBG optical cable for measuring soil moisture content according to claim 1, characterized in that: The heating armor tube and the protection tube in the sensing section, the armor tube, the copper mesh and the sheath in the non-sensing section are all tough.

3. The AH-FBG optical cable for measuring soil moisture content according to claim 1, characterized in that: The AH-FBG optical cable contains a total of 10 sensing segments, the distance between adjacent FBG sensors is 1m, and the total resistance is 4Ω.

4. An application method of AH-FBG optical cable for measuring soil moisture content, characterized in that: Based on the AH-FBG optical cable for measuring soil moisture content according to any one of claims 1 to 3, the following steps are performed to complete the in-situ measurement of the moisture content of the target soil: Step S1: Perform heating uniformity test and calibration test on the AH-FBG optical cable; Step S2: installing and laying the AH-FBG optical cable at the position to be monitored of the target soil body; Step S3: collecting the temperature characteristic value of the target soil measured by the AH-FBG optical cable, and calculating the moisture content of the target soil based on the functional relationship between the temperature characteristic value of the soil and the moisture content of the soil.

5. The application method of the AH-FBG optical cable for measuring soil moisture content according to claim 4, characterized in that: In step S1, the heating uniformity of the AH-FBG optical cable under the same heating parameters is tested under the ground-attached condition and the suspended condition, and the steps are as follows: Step S1.1: pre-tension each FBG sensor measuring point in an AH-FBG optical cable and fix it on the ground with adhesive tape, so that the whole AH-FBG optical cable is in a pre-tensioned state; Step S1.2: Lay another AH-FBG optical cable freely on the ground, and the entire AH-FBG optical cable is in a free state; Step S1.3: The non-sensing sections of the two AH-FBG optical cables are laid in a U-shape with the sensing sections, the conductors in the AH-FBG optical cables are connected to a DC power supply, the optical fibers in the AH-FBG optical cables are connected to an FBG demodulator, and then two heating tests are performed; Step S1.4: First heating test: Each FBG sensor measuring point in the AH-FBG optical cable is pre-tensioned by a displacement table in turn, and fixed by a fixing table. At this time, the AH-FBG optical cable is in a pre-tensioned state. Then, the AH-FBG optical cable is connected to a DC power supply and an FBG demodulator respectively to perform a heating test. Step S1.5: The second heating test changes the state of the AH-FBG optical cable so that the optical cable is fixed between the translation stage and the fixing stage in a free state, and then a heating test is performed.

6. The application method of the AH-FBG optical cable for measuring soil moisture content according to claim 4, characterized in that: The calibration test method in step S1 is as follows: Soil samples with different water contents were placed in the calibration box, and the AH-FBG optical cable was laid out in an S shape in the calibration box. During this process, the sensing section of the AH-FBG optical cable was ensured to be completely placed inside each soil sample and in a stress-free free state. After the AH-FBG optical cable is laid, the optical fiber and wire in the AH-FBG optical cable are connected to the FBG demodulator and DC power supply respectively, and then a heating test is performed to obtain T t -θ function relationship, T t is the temperature rise value, and θ is the volumetric moisture content of the soil.

7. The application method of the AH-FBG optical cable for measuring soil moisture content according to claim 4, characterized in that: The functional relationship between the soil temperature characteristic value and the soil moisture content described in step S3 is as follows: Where, T t is the temperature rise value, θ is the soil volume moisture content, the unit is m -3 ·m -3 , A, B and C are constants related to soil type and are given by T t The fitting results of the -θ calibration test are obtained.

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