A method for distributed synchronous determination of soil hydrothermal parameters using heated optical fibers

Abstract

This invention discloses a method for distributed synchronous determination of soil hydrothermal parameters using heated optical fibers. The steps are as follows: burying an optical cable in the soil to be tested to sense the temperature of the surrounding soil; briefly energizing the metal layer inside the optical cable to generate Joule heating; measuring the temperature increment of the surrounding soil and its change over time; establishing a functional relationship between soil moisture and measured thermal conductivity; determining soil moisture at different points along the optical fiber using thermal conductivity measurements at different spatial locations; and evaluating the accuracy of the optical fiber method for soil moisture determination using the root mean square error (RMSE). The beneficial effects of this invention are: starting from the inherent characteristics of optical fibers and considering the influence of fiber structure and materials, this invention establishes a thermal conductivity correction model for the optical fiber method, using the thermal probe method as a reference, thereby improving the accuracy and universality of the optical fiber method for determining soil hydrothermal parameters.

Description

Technical Field

[0001] This invention relates to the field of soil data processing and measurement, specifically a method for synchronously measuring soil hydrothermal parameters using a heated optical fiber distributed system. Background Technology

[0002] Soil thermal parameters, including thermal conductivity, thermal diffusivity, and volumetric heat capacity, influence soil temperature changes and energy transport. They interact with soil moisture content and jointly control water and heat exchange and balance within the soil. Accurate measurement of these parameters is crucial for land surface model simulation and guiding precision irrigation in farmland. Therefore, simultaneous measurement of soil thermal parameters and moisture content is essential. Currently, the thermal pulse probe method is the most favored method for measuring soil hydrothermal properties, but this method can only measure soil at a point scale. However, soil hydrothermal parameters exhibit strong spatial variability, and the measured data are not highly representative of large-area soil samples in the field. Therefore, given the inconsistency between the measurement scale (large and small scales) and the application scale (medium-scale field measurement) of soil thermal properties and moisture content, there is an urgent need to develop new technologies and methods for simultaneous measurement of soil hydrothermal parameters at the field scale. The rapid development of distributed fiber optic temperature sensing (DTS) technology provides a new approach for the simultaneous measurement of soil hydrothermal parameters at the field scale.

[0003] Distributed fiber optic temperature sensing (DTS) is a temperature sensor that measures temperature based on the Raman scattering effect and locates the sensor using time-domain reflectometry (TDAR). The temperature distribution along the fiber can be obtained by comparing the intensity ratio of the backscattered anti-Stokes light to the Stokes light intensity. The principles of fiber optic measurement of soil moisture content can be divided into two categories: passive heating fiber optic methods and active heating fiber optic methods. The passive heating fiber optic method monitors soil temperature at different depths to invert the thermal diffusivity, and then deduce the moisture content from the thermal diffusivity. The drawback of this method is the significant uncertainty in fiber depth, leading to substantial errors in the calculation of the thermal diffusivity. Therefore, the passive heating method lacks feasibility for widespread application in the field.

[0004] Active heating fiber optic sensing (AHFO-DTS) is based on the principle of thermal pulse probes. It involves briefly heating the metal layer of an optical fiber and then analyzing the temperature changes during the heating and cooling phases to deduce soil moisture content. This method allows for measurements at any time and soil depth, making it more widely applicable than passive heating fiber optic sensing. Distributed fiber optic sensing based on dual-probe thermal pulses (DPHP-DTS) can simultaneously measure soil thermal conductivity, thermal diffusivity, and specific heat capacity, and use the specific heat capacity to deduce moisture content. However, its measurement accuracy is greatly affected by the fiber spacing, limiting its application. Distributed fiber optic sensing based on single-probe thermal pulses (SPHP-DTS) requires only one buried optical fiber, avoiding the aforementioned drawbacks and gaining more popularity. This method utilizes the linear heat source theory of single-probe thermal pulses, treating a segment of the optical fiber as a single probe. The optical fiber can be considered as a series of numerous single probes, thus enabling high spatiotemporal resolution monitoring of soil moisture. This method can directly utilize the temperature change over time using the thermal pulse, such as the maximum temperature rise (ΔT). max ), cumulative temperature rise (ΔT) cum Soil moisture content can be estimated by the functional relationship between soil thermal conductivity (λ) and soil moisture content, or indirectly by measuring soil thermal conductivity (λ).

[0005] Because the characteristics of optical fibers and probes differ significantly, the optical fiber method does not satisfy the assumption of an infinite linear heat source. This leads to relatively large errors in the current optical fiber method for measuring soil thermal conductivity and soil moisture, and precise simultaneous measurement of the two methods has not yet been achieved. This invention compares and verifies the measurements of soil hydrothermal parameters using the heated optical fiber method and the probe method, analyzes the underlying reasons for the differences between the two methods, and clarifies the accuracy of each method in measuring soil hydrothermal parameters. Furthermore, this invention establishes a thermal conductivity correction model for the heated optical fiber method, thereby improving the accuracy of soil hydrothermal parameter measurement using this method. Summary of the Invention

[0006] The purpose of this invention is to provide a method for distributed synchronous measurement of soil hydrothermal parameters using heated optical fibers. This method is of great significance for optimizing the optical fiber method and improving the accuracy of soil hydrothermal parameter measurement. It is also of great value for monitoring environmental factors such as soil hydrothermal parameters in ecosystems at the kilometer scale.

[0007] The technical solution adopted in this invention is as follows: A method for determining soil hydrothermal parameters using a heated optical fiber method, comprising the following steps:

[0008] Step S11: Bury the optical cable in the soil to be tested. The optical cable includes an insulating sheath, a metal layer and an optical fiber from the outside to the inside. The insulating sheath wraps the metal layer and the optical fiber. The metal layer wraps the optical fiber. The optical fiber is used to sense the temperature around the soil to be tested.

[0009] Step S12: Briefly apply electricity to the metal layer inside the optical cable to generate Joule heating. The heating power is obtained by the resistance R of the metal layer and the voltage U recorded in real time, and the calculation formula is Q = U. 2 / R; Q is the heating power per unit length of the metal layer;

[0010] Step S13: Use the optical signal demodulation module DTS to collect the temperature increment ΔT and its change with time t around the soil at different spatial points along the optical fiber in step S11; calculate the thermal conductivity λ of the soil at different spatial points along the optical fiber using formula (1). FO ;

[0011]

[0012] Where ΔT is the temperature increment around the soil being tested, measured at different spatial points along the optical fiber; λ FO The thermal conductivity of the soil to be measured is given by the following values ​​at different spatial points along the optical fiber: t is time; t0 is the heating time; t′ is the calibration time.

[0013] Step S14: Soil moisture probes (TDRs) are installed near different spatial locations of the optical fiber in the soil to be tested. The soil moisture probes (TDRs) are used to monitor the soil moisture θ near the optical fiber. The soil moisture θ at different spatial locations of the optical fiber is compared with the thermal conductivity λ of the soil to be tested at different spatial locations along the optical fiber. FO By fitting the Lu Sen model using formula (2), the relationship between soil moisture θ and the measured soil thermal conductivity λ at different spatial points along the optical fiber is established. FO The functional relationship;

[0014]

[0015] Where, λ sat , λ dry These represent the saturated thermal conductivity and dry thermal conductivity of the soil to be measured, respectively; exp is the exponential function; α is the shape index; θ sat The saturated water content of the soil to be tested;

[0016] Step S15: Using soil moisture θ and the measured soil thermal conductivity λ at different spatial points along the optical fiber... FO The soil moisture θ is obtained from the functional relationship between the soil moisture and the soil moisture content.

[0017] Step S16: The root mean square error (RMSE) is used to evaluate the accuracy of soil moisture θ measurement by the fiber optic method. The root mean square error (RMSE) is the square root of the ratio of the square of the deviation between the soil moisture probe TDR observation value and the soil moisture θ measurement value by the fiber optic method to the number of observations (n). The smaller the root mean square error (RMSE), the higher the measurement accuracy. The calculation of the root mean square error (RMSE) is as shown in formula (3):

[0018]

[0019] Where RMSE is the root mean square error, X obs,i X is the moisture observation value of the i-th soil moisture probe TDR; pre,i θ is the soil moisture value measured by the i-th fiber optic method, and n is the number of observations.

[0020] This invention employs another approach: a method for improving the accuracy of soil hydrothermal parameter determination using the heated fiber optic method. The method comprises the following steps:

[0021] Step S21: Bury the thermal probe near the optical fiber. The thermal probe includes a wire with a built-in resistor R. HP The heating probe consists of a resistance wire and a sensing probe with a built-in thermistor, the built-in resistor being R. HP The resistance wire generates Joule heat after being energized for 15 seconds, and the sensing probe is used to measure the temperature increment change ΔT. HP The temperature increment ΔT of the sensing probe was collected using a CR1000 data acquisition unit. HP Data is collected once per second as time t changes; simultaneously, the resistance R is recorded using a data acquisition device CR1000. HP The voltage U across the resistance wire HP Calculate the heating power Q' of the thermal probe using the formula Q' = U. HP 2 / R HP ;

[0022] Step S22: Obtain the soil thermal diffusivity k and soil volumetric heat capacity ρc by fitting the radial heat conduction equation. The radial heat conduction equation is shown in Equation (4) and Equation (5).

[0023]

[0024]

[0025] Where, ΔT HP1 ΔT HP2 These represent the temperature increments during the heating and cooling phases, respectively, in °C; Q' is the heating power of the thermal probe; k is the soil thermal diffusivity; -E i (-x) is the exponential integral function; r is the distance between the induction probe and the heating probe, in meters; t is time, t0 is the heating time, and ΔT HP1 In ΔT, t satisfies 0 < t ≤ t0; HP2 In this context, t satisfies t≥t0; the unit is s; Q'=q' / ρc, where q' is the heat input per unit time per unit length of the thermal probe, and ρc is the volumetric heat capacity of the soil;

[0026] Step S23: Using formula λ HP =k*ρc to calculate the soil thermal conductivity λ measured by the thermal probe. HP ;

[0027] Step S24: Using the probe method as the true value, evaluate the thermal conductivity λ of the soil at different spatial points along the optical fiber through the root mean square error (RMSE). FO The accuracy of the measurement;

[0028] Step S25: Obtain the thermal conductivity λ of the soil at different spatial points along the optical fiber using a linear fitting method. FO The correction model is Equation (6);

[0029] λ HP =a*λ FO +b (6)

[0030] Where a and b are parameters related to the characteristics of the optical fiber;

[0031] Step S26: Measure the thermal conductivity λ of the soil at different spatial points along the optical fiber. FO Substituting into the correction model (6), we obtain the new soil thermal conductivity λ determined by the fiber optic method. FO ′;

[0032] Step S27: Soil thermal conductivity λ determined by the new fiber optic method FO Substitute the θ′ into the Lu Sen model in step S14 to calculate the new soil moisture θ′ determined by the fiber optic method.

[0033] Step S28: Evaluate the accuracy of soil hydrothermal parameters measured by the fiber optic method after soil thermal conductivity correction by the root mean square error (RMSE).

[0034] The beneficial effects of this invention are as follows: Starting from the inherent characteristics of optical fibers and considering the influence of fiber structure and materials, this invention establishes a thermal conductivity correction model for the optical fiber method, using the thermal probe method as a reference, thereby improving the accuracy and universality of the optical fiber method for determining soil hydrothermal parameters. This invention can achieve synchronous, real-time, in-situ measurement of soil hydrothermal parameters at the centimeter to kilometer scale in the field through distributed optical fiber temperature sensing technology. This is of great significance for future optimization of the optical fiber method and improvement of the accuracy of soil hydrothermal parameter measurement, and has important value for kilometer-scale monitoring of environmental factors such as soil hydrothermal parameters in ecosystems. Attached Figure Description

[0035] Figure 1 This is a flowchart illustrating the structure of the present invention.

[0036] Figure 2 This is a graph showing the relationship between soil thermal conductivity and moisture content as determined by the fiber optic method and probe method of this invention.

[0037] Figure 3This is a graph showing the relationship between thermal conductivity measured by the fiber optic method and the probe method of this invention.

[0038] Figure 4 This is a graph showing the accuracy of moisture content determination using the fiber optic method and probe method of the present invention. Detailed Implementation

[0039] like Figure 1 The diagram shown is a flowchart of the invention. The invention involves deploying optical fibers in the soil and placing soil moisture sensors and thermal probes around the fibers. Electric heating is applied, and temperature data is recorded using a Digital Transducer (DTS). The thermal conductivity λ of the soil at different spatial points along the optical fiber is calculated using a formula. FO Soil moisture θ is continuously monitored using a soil moisture sensor. A Lu-Sen model is used to establish the relationship between soil moisture θ and the thermal conductivity λ of the soil at different spatial points along the optical fiber. FO The relationship was investigated. A self-made thermal probe was connected to a CR1000 data acquisition system, and the temperature change of the thermal probe was recorded using the CR1000. The soil thermal conductivity λ measured by the thermal probe was calculated using the heat conduction equation. HP A calibration model λ for determining soil thermal conductivity using the fiber optic method was established. HP =a*λ FO +b. The new soil thermal conductivity λ measured by the fiber optic method was obtained through a calibration model. FO The soil thermal conductivity λ was determined using a new fiber optic method. FO Substituting θ into the Lusen model yields a new and more accurate soil moisture θ′.

[0040] This invention works and is implemented as follows: a method for determining soil hydrothermal parameters using a heated optical fiber method, which utilizes a heated optical fiber method to achieve simultaneous determination of soil hydrothermal parameters, including the following steps:

[0041] Step S11: Bury the optical cable in the soil to be tested. The optical cable includes an insulating sheath, a metal layer and an optical fiber from the outside to the inside. The insulating sheath wraps the metal layer and the optical fiber. The metal layer wraps the optical fiber. The optical fiber is used to sense the temperature around the soil to be tested.

[0042] Step S12: Briefly apply electricity to the metal layer inside the optical cable to generate Joule heating. The heating power is obtained by the resistance R of the metal layer and the voltage U recorded in real time, and the calculation formula is Q = U. 2 / R; Q is the heating power per unit length of the metal layer;

[0043] Step S13: Use the optical signal demodulation module DTS to collect the temperature increment ΔT and its change with time t around the soil under test measured at different spatial points along the optical fiber in step S11; calculate the thermal conductivity λ of the soil under test at different spatial points along the optical fiber using formula (1). FO ;

[0044]

[0045] Where ΔT is the temperature increment around the soil being tested, measured at different spatial points along the optical fiber; λ FO The thermal conductivity of the soil to be measured is given by the following values ​​at different spatial points along the optical fiber: t is time; t0 is the heating time; t′ is the calibration time.

[0046] Step S14: Soil moisture probes (TDRs) are installed near different spatial locations of the optical fiber in the soil to be tested. The soil moisture probes (TDRs) are used to monitor the soil moisture θ near the optical fiber. The soil moisture θ at different spatial locations of the optical fiber is compared with the thermal conductivity λ of the soil to be tested at different spatial locations along the optical fiber. FO By fitting the Lu Sen model using formula (2), the relationship between soil moisture θ and the measured soil thermal conductivity λ at different spatial points along the optical fiber is established. FO The functional relationship;

[0047]

[0048] Where, λ sat , λ dry These represent the saturated thermal conductivity and dry thermal conductivity of the soil to be measured, respectively; exp is the exponential function; α is the shape index; θ sat The saturated water content of the soil to be tested;

[0049] Step S15: Using soil moisture θ and the measured soil thermal conductivity λ at different spatial points along the optical fiber... FO The soil moisture θ is obtained from the functional relationship between the soil moisture and the soil moisture content.

[0050] Step S16: The root mean square error (RMSE) is used to evaluate the accuracy of soil moisture θ measurement by the fiber optic method. The root mean square error (RMSE) is the square root of the ratio of the square of the deviation between the soil moisture probe TDR observation value and the soil moisture θ measurement value by the fiber optic method to the number of observations (n). The smaller the root mean square error (RMSE), the higher the measurement accuracy. The calculation of the root mean square error (RMSE) is as shown in formula (3):

[0051]

[0052] Where RMSE is the root mean square error, X obs,i X is the moisture observation value of the i-th soil moisture probe TDR; pre,i θ is the soil moisture value measured by the i-th fiber optic method, and n is the number of observations.

[0053] This invention employs another approach: a method for improving the accuracy of soil hydrothermal parameter determination using the heated fiber optic method. The method comprises the following steps:

[0054] Step S21: Bury the thermal probe near the optical fiber. The thermal probe includes a wire with a built-in resistor R. HP The heating probe consists of a resistance wire and a sensing probe with a built-in thermistor, the built-in resistor being R. HP The resistance wire generates Joule heat after being energized for 15 seconds, and the sensing probe is used to measure the temperature increment change ΔT. HP The temperature increment ΔT of the sensing probe was collected using a CR1000 data acquisition unit. HP Data is collected once per second as time t changes; simultaneously, the resistance R is recorded using a data acquisition device CR1000. HP The voltage U across the resistance wire HP Calculate the heating power Q' of the thermal probe using the formula Q' = U. HP 2 / R HP ;

[0055] Step S22: Obtain the soil thermal diffusivity k and soil volumetric heat capacity ρc by fitting the radial heat conduction equation. The radial heat conduction equation is shown in Equation (4) and Equation (5).

[0056]

[0057]

[0058] Where, ΔT HP1 ΔT HP2 These represent the temperature increments during the heating and cooling phases, respectively, in °C; Q' is the heating power of the thermal probe; k is the soil thermal diffusivity; -E i (-x) is the exponential integral function; r is the distance between the induction probe and the heating probe, in meters; t is time, t0 is the heating time, and ΔT HP1 In ΔT, t satisfies 0 < t ≤ t0; HP2 In this context, t satisfies t≥t0; the unit is s; Q'=q' / ρc, where q' is the heat input per unit time per unit length of the thermal probe, and ρc is the volumetric heat capacity of the soil;

[0059] Step S23: Using formula λ HP =k*ρc to calculate the soil thermal conductivity λ measured by the thermal probe. HP ;

[0060] Step S24: Using the probe method as the true value, evaluate the thermal conductivity λ of the soil at different spatial points along the optical fiber through the root mean square error (RMSE). FO The accuracy of the measurement;

[0061] Step S25: Obtain the thermal conductivity λ of the soil at different spatial points along the optical fiber using a linear fitting method. FOThe correction model is Equation (6);

[0062] λ HP =a*λ FO +b (6)

[0063] Where a and b are parameters related to the characteristics of the optical fiber;

[0064] Step S26: Measure the thermal conductivity λ of the soil at different spatial points along the optical fiber. FO Substituting into the correction model (6), we obtain the new soil thermal conductivity λ determined by the fiber optic method. FO ′;

[0065] Step S27: Soil thermal conductivity λ determined by the new fiber optic method FO Substitute the θ′ into the Lu Sen model in step S14 to calculate the new soil moisture θ′ determined by the fiber optic method.

[0066] Step S28: Evaluate the accuracy of soil hydrothermal parameters measured by the fiber optic method after soil thermal conductivity correction by the root mean square error (RMSE).

[0067] This invention conducted a comparative verification experiment on the measurement of soil hydrothermal parameters using the heated fiber optic method and the probe method, revealing the differences between the thermal pulse probe method and the heated fiber optic method in monitoring soil hydrothermal parameters. The soil thermal conductivity measured by the fiber optic method was corrected by the thermal probe method, thereby improving the accuracy of soil moisture measurement and providing theoretical evidence for the simultaneous measurement of soil hydrothermal parameters using the heated fiber optic method.

[0068] (1) Fiber optic hydrothermal measurement test;

[0069] The hydrothermal parameters of soils with different textures were measured using a thermal pulse probe and a heated fiber optic method. A 4m long soil trough was divided into five 0.8m long compartments by a steel plate. Four soil samples of different textures were filled into four compartments respectively, with the soil being compacted in multiple layers. The experiment first tested air-dried soil, then fully saturated soil. After the saturated soil test, the drainage holes at the bottom of the trough were opened to monitor the soil hydrothermal parameters during the process from unsaturated to dry. An infrared lamp was suspended above the trough to increase the temperature and accelerate soil evaporation, thereby speeding up the rate of soil water reduction and shortening the experimental cycle. To avoid the influence of the infrared lamp heating the soil on the thermal pulse, the infrared lamp was turned off before each thermal pulse to allow the soil temperature to reach ambient temperature before measurement began. The fiber optic heating lasted for 10 minutes each time, with a heating power of 5 W·m. -1 The TDR315 soil moisture sensor (Acallima, USA) was deployed near the optical fiber. The TDR315 had been calibrated using a drying method before use, and its moisture content measurement accuracy was approximately 0.03 m. 3 ·m -3During fiber optic heating, temperature changes induced by the fiber optic cable are recorded via DTS, with temperature data recorded every 5 seconds. Soil moisture content is continuously monitored using a TDR315, with soil moisture data collected every 5 minutes using a data acquisition system (CR1000 model, Campbell Scientific, Logan, USA).

[0070] (2) Probe hydrothermal measurement test;

[0071] Probe Fabrication: A custom-made dual probe was used in the experiment. One probe was for temperature measurement, and the other was for heating. Both probes were stainless steel tubes, 40 mm long and 2.8 m in diameter, with a probe spacing of approximately 6 mm. A thermistor (10K3MCD1, Betatherm Corp., Shrewbury MA) was installed in the middle of the temperature probe for temperature measurement. A resistance wire was installed inside the heating probe to release Joule heat to the measuring medium. The interior of both probes was filled with epoxy resin (Omegaengineering, Stamford, CT), a high thermal conductivity insulating material, to fix the heating wire or thermistor. The probes were inserted into hollow PTFE cylindrical bases and sealed with epoxy resin.

[0072] Probe Setup: The thermal pulse probe measurement device includes a data acquisition unit (powered by DC power), a heating circuit (relay, heating wire), and a sensing circuit (thermistor, 1Ω resistor). The data acquisition unit controls the circuit and records temperature data through a program, while the relay acts as a switch for the circuit, controlling the heating wire (86Ω·m resistor). -1 When the soil medium is heated by electricity, the data acquisition instrument records the voltage generated across a 1Ω resistor, thus obtaining the current flowing through the heating wire, and finally calculating the heating amount. The probe is positioned opposite the TDR (Transmission Controlled Refrigerator) of the fiber optic accessory. Simultaneously with the fiber heating, the probe performs a 15s pulse heating (45W·m). -1 Temperature data is collected once per second using the CR1000.

[0073] (3) Soil sampling;

[0074] Regarding the soil samples, the samples were taken from sandy soil from the Weihe River in Yangling, clay loam 1 from farmland in Yangling, and clay loam 2 and 3 from apple orchards in Changwu and Qingjian. The soil samples were air-dried and ground through a 2mm sieve for later use. The volume fraction of soil particle size distribution is shown in Table 1. Except for the sandy soil, the other three soil types were clay loam.

[0075] Table 1. Soil particle size distribution volume fraction (%)

[0076]

[0077] (4) DTS temperature measurement principle;

[0078] Regarding the temperature measurement principle of distributed fiber optic temperature sensing technology, after a pulsed pump light of a certain energy is injected into the optical fiber, photons undergo inelastic collisions with fiber molecules, generating two backscattered Raman beams. Stokes scattered light, with wavelengths longer than the incident light, is unaffected by temperature, while anti-Stokes scattered light, with wavelengths shorter than the incident light, exhibits a strong temperature dependence. Therefore, the temperature can be calculated based on the intensity ratio of the Stokes and anti-Stokes beams, and the location can be determined by the travel time of the Stokes scattered light, thus obtaining the distributed temperature measurement value. The intensity ratio R(z) of the two backscattered beams can be expressed as:

[0079]

[0080] Where C represents parameters related to the wavelength and frequency of the emitted light, the backscattered Raman light, the photon detector of the instrument, and the operating conditions of the DTS; Δα represents the difference in backscattered light loss between Stokes and anti-Stokes; γ = ΔE / k * ΔE is the difference in molecular energy states driving Raman scattering (J); k * Boltzmann constant (J·K) -1 );

[0081] Therefore, as long as C, Δα, and γ are calibrated based on the known reference temperatures of one or more segments of the optical fiber, the temperature value of any point z on the optical fiber can be obtained:

[0082]

[0083] The position z on the optical fiber where this temperature value is located is half the distance the light travels.

[0084]

[0085] In the formula, c is the speed of light in a vacuum (m·s). -1 v is the refractive index of the fiber cladding; t * The time (s) required for light to travel forward and backward.

[0086] Characteristics of the differences in accuracy between the fiber optic method and the probe method for determining soil thermal conductivity.

[0087] The accuracy of fiber optic probe methods for moisture measurement varies depending on the soil type. Figure 4 For sandy soils, the fiber optic method and probe method offer high accuracy for determining moisture content across the entire range. For clay loam, the accuracy is lower in the low moisture content range (less than 0.35 m). 3 ·m -3The fiber optic method exhibits good measurement accuracy. However, in the high moisture content range, the scatter plots of observed and predicted values ​​deviate from the 1:1 line, indicating a larger measurement error in this range. This is because the sensitivity of thermal conductivity gradually decreases with increasing soil moisture. Furthermore, at higher moisture contents, water around the optical fiber easily migrates under the influence of temperature gradients, leading to increased measurement errors. In addition, the study found that the fiber optic-based thermal conductivity method (λ...)... FO The measurement accuracy of λ is significantly lower than that of the probe-based thermal conductivity method. HP ()( Figure 4 For the moisture determination of sandy soil and three types of clay loam, λ FO Fabry-Perot (λ) HP The measurement accuracies of the methods were 0.01, 0.04, 0.05, and 0.03 m lower, respectively. 3 ·m -3 Therefore, based on optical fiber λ FO The large error in soil moisture measurement using the fiber optic method is mainly due to the significant error in the thermal conductivity measurement, which affects the functional relationship between soil moisture content and thermal conductivity, leading to increased measurement error. This invention demonstrates the necessity of first correcting the thermal conductivity measured by the fiber optic method using a thermal probe method as a reference, thereby improving the accuracy of soil moisture measurement using the fiber optic method. This invention also shows that the probe-based thermal conductivity method has high accuracy in measuring moisture in different soil types. Currently, many researchers have established thermal conductivity λ(θ) models, and the model parameters have physical meaning, providing a model basis for moisture measurement using the fiber optic method. Therefore, given accurate thermal conductivity measurement, the fiber optic-based thermal conductivity method for soil moisture measurement has great potential.

[0088] Sources of error in the determination of hydrothermal parameters using the fiber optic method.

[0089] The main sources of error in the fiber optic method for determining hydrothermal parameters are as follows: First, the structure and materials of the optical fiber differ significantly from those of a thermal probe, violating the assumption of an infinite linear heat source, thus introducing errors when directly applying the analytical solution of the heat conduction equation. Second, multiple interface thermal resistances exist between the optical fiber and the soil (core-air layer-metal layer-sheath-soil), resulting in a much higher contact thermal resistance between the fiber and soil than that of a probe, significantly impacting measurement accuracy. Third, the temperature measurement accuracy and sensitivity of the optical fiber are much lower than those of the probe, leading to fluctuations in temperature rise during heating and cooling, increasing noise interference. Finally, the fiber optic method requires a larger heat input and longer heating time to generate a higher temperature rise than the probe, improving the signal-to-noise ratio. Therefore, moisture around the fiber may migrate under the influence of the temperature gradient, potentially leading to an overestimation of thermal conductivity. Therefore, when using the fiber optic method to determine soil thermal conductivity, optical fibers with simple structures, small diameters, and good sheath thermal conductivity should be selected to approximate a linear heat source, improving measurement accuracy. Furthermore, when measuring soil moisture using the fiber optic method, the impact of excessively high fiber temperature causing surrounding moisture migration should be considered, while ensuring accurate thermal conductivity measurement. It should also be noted that significant changes in the measuring environment can increase the measurement error of soil hydrothermal parameters.

[0090] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for heating optical fiber distributed synchronous measurement of soil water and heat parameters, which utilizes a heating optical fiber method to realize synchronous measurement of soil water and heat parameters, characterized in that: Comprising the following steps: Step S11: Burying the optical cable in the soil to be measured, the optical cable comprises an insulating sheath, a metal layer and an optical fiber from outside to inside, the insulating sheath wraps the metal layer and the optical fiber, the metal layer wraps the optical fiber, and the optical fiber is used to sense the temperature around the soil to be measured; Step S12: short-time energization of the metal layer within the optical cable to generate Joule heat, the heating power being obtained by the resistance R of the metal layer and the voltage U recorded in real time, the calculation formula being Q=U 2 / R; Q is the heating power per unit length of the metal layer; Step S13: Collect the temperature increment ΔT around the soil to be measured at different spatial points along the optical fiber and the change with time t by using the optical signal demodulation module DTS; the thermal conductivity λ of the soil to be measured at different spatial points along the optical fiber is calculated by formula (1) FO ; （1）； wherein, is the temperature increment of the soil to be measured around the different spatial points of the optical fiber; λ FO is the thermal conductivity of the soil to be measured along the different spatial points of the optical fiber; t is time; t0 is the heating time; is the correction time; Step S14: soil moisture probes TDR are buried near different spatial points of the optical fiber in the soil to be measured, and the soil moisture probes TDR are used to monitor the soil moisture θ near the optical fiber, and the soil moisture θ at different spatial points of the optical fiber is correlated with the soil thermal conductivity λ to be measured along the optical fiber at different spatial points of the optical fiber. FO The function relationship between the soil moisture θ and the soil thermal conductivity λ to be measured along the optical fiber at different spatial points of the optical fiber is established by fitting the Lu Sen model of formula (2). FO The function relationship between the soil moisture θ and the soil thermal conductivity λ to be measured along the optical fiber at different spatial points of the optical fiber is established by fitting the Lu Sen model of formula (2). （2）； wherein λ sat , λ dry are the saturated and dry thermal conductivity of the soil under test, respectively; is an exponential function; α is a shape index; θ sat is the saturated moisture content of the soil under test; Step S15: obtaining the soil moisture θ by the function relationship between the soil moisture θ and the soil thermal conductivity λ of different spatial points along the optical fiber FO ; Step S16: Using the root mean square error RMSE to evaluate the measurement accuracy of the soil moisture θ of the optical fiber method, the root mean square error RMSE is the square root of the square sum of the deviation of the moisture observation value of the soil moisture probe TDR and the soil moisture θ measured value of the optical fiber method divided by the observation number n, the smaller the root mean square error RMSE, the higher the measurement accuracy, and the calculation of the root mean square error RMSE is as formula (3): （3）； wherein RMSE is the root mean square error, X obs,i is the moisture observation value of the i-th soil moisture probe TDR; X pre,i is the soil moisture measurement value of the i-th optical fiber method θ, and n is the number of observations. The measurement accuracy of the soil water and heat parameters of the improved heating optical fiber method, comprising the following steps: Step S21: Bury the thermal probe near the optical fiber. The thermal probe includes a wire with a built-in resistor R. HP The heating probe consists of a resistance wire and a sensing probe with a built-in thermistor, the built-in resistor being R. HP The resistance wire generates Joule heat after being energized for 15 seconds, and the sensing probe is used to measure the temperature increment change ΔT. HP The temperature increment ΔT of the sensing probe was collected using a CR1000 data acquisition unit. HP Data is collected once per second as time t changes; simultaneously, the resistance R is recorded using a data acquisition device CR1000. HP The voltage U across the resistance wire HP Calculate the heating power Q' of the thermal probe using the formula Q'=U HP 2 / R HP ; Step S22: Obtaining the soil thermal diffusivity k and the volumetric heat capacity ρc of the soil by fitting the radial heat conduction equation, the radial heat conduction equation is as formula (4) and formula (5); （4）； （5）； wherein ΔT HP1 , ΔT HP2 are the temperature increment changes in the heating stage and the cooling stage, respectively, in units of °C; Q' is the heating power of the heat probe; k is the soil thermal diffusivity; -E i (-x) is an exponential integral function; r is the distance between the induction probe and the heating probe, in units of m; t is time, t0 is the heating time, ΔT HP1 t satisfies 0 < t ≤ t0; ΔT HP2 t satisfies t ≥ t0; units of s; Q' = q' / ρc, q' is the heat input per unit time per unit length of the heat probe, and ρc is the volumetric heat capacity of the soil; Step S23: Calculate the soil thermal conductivity λ by the formula HP ;​ Step S24: Taking the probe method as the true value, the determination accuracy of the soil thermal conductivity λ of different spatial points along the optical fiber is evaluated by the root mean square error RMSE FO . Step S25: Obtain the correction model of the soil thermal conductivity λ along different spatial points of the optical fiber by linear fitting, and the correction model is formula (6). FO Step S25: Obtain the correction model of the soil thermal conductivity λ along different spatial points of the optical fiber by linear fitting, and the correction model is formula (6). （6）； Wherein a and b are parameters related to the characteristics of the optical fiber; Step S26: the soil thermal conductivity λ to be measured along different spatial points of the optical fiber is obtained FO Substitute the correction model (6) to obtain the new soil thermal conductivity λ measured by the optical fiber method FO ´; Step S27: New optical fiber method measured soil thermal conductivity λ FO Substitute the Lu Sen model in step S14, and calculate the new optical fiber method measured soil moisture θ´; Step S28: Using the root mean square error RMSE to evaluate the measurement accuracy of the soil water and heat parameters of the optical fiber method after the correction of the soil thermal conductivity.

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