A method for measuring soil-sensor contact thermal resistance and correcting soil moisture content

Abstract

The present invention discloses a soil-sensor contact thermal resistance measurement and soil moisture content correction method, which determines the dry density ρ of the soil to be measured by in-situ sampling. d and mass moisture content w, and use the thermal response curve to obtain T t The slope k and intercept d of the ‑ln(t) line are used to calculate the thermal diffusion coefficient K of the soil to be tested, and the contact thermal resistance R of the sensor is obtained based on the measured parameters and the calculation results. The effective temperature characteristic value T is obtained by subtracting QR from the temperature characteristic value measured in situ. te , substitute it into T t The calibration formula of ‑w can obtain accurate moisture content and obtain the moisture content calculation formula considering contact thermal resistance. The method of the present invention can significantly reduce the moisture content measurement error of the active heating optical fiber sensor through correction, enhance the accuracy of measurement, and is applicable to all kinds of soils.

Description

Technical Field

[0001] The invention relates to the fields of geotechnical engineering and geological engineering, and in particular to a method for measuring soil-sensor contact thermal resistance and correcting soil moisture content. Background Art

[0002] Water content is an important basic physical index of soil. Its distribution and evolution directly affect the hydraulic and mechanical properties of soil, and further affect the stability of soil. Therefore, accurately obtaining the water content of soil is an important prerequisite for scientific research in various fields (such as geotechnical, engineering geology, hydrology, soil and environment).

[0003] At present, soil moisture measurement technology can be divided into two categories: destructive measurement and non-destructive measurement. The destructive measurement technology is drying and weighing. This method is fast and simple, but it requires the collection of a certain amount of samples during measurement, which requires a lot of manpower and material resources, and will destroy the original structure of the soil; non-destructive measurement mainly includes non-contact measurement methods based on remote sensing (RS) and contact measurement methods based on time domain reflectometry (TDR) and frequency domain reflectometry (FDR). Non-contact measurement methods can achieve large-scale soil moisture measurement, but their accuracy and spatial resolution are poor, and they cannot obtain the moisture content distribution deep in the soil. Contact measurement methods can accurately obtain moisture content changes, but these methods are all point measurement methods, which are difficult to achieve large-scale distributed measurement of soil moisture.

[0004] At present, the soil moisture measurement technology is gradually developing in the direction of miniaturization, refinement, intelligence and long-term development. In the past decade, the moisture measurement technology based on the active heating optical fiber (AHFO) method has the unique advantages of small size, distribution, real-time measurement and corrosion resistance, and has gradually become the forefront and hot spot of soil moisture measurement technology research. AHFO technology is to make optical fiber and heating material into a sensor with both active heating function and temperature measurement. During measurement, the AHFO sensor buried in the soil is both a heat source and a temperature sensor. It measures the temperature change along the optical cable itself to invert the moisture content of the surrounding soil. Therefore, in the in-situ application of AHFO technology, it is inevitable to involve the layout and installation of AHFO sensors. If the contact quality between the soil and the sensor is not good, it will have a great impact on the moisture measurement results.

[0005] However, current research on the in-situ application of AHFO technology has neglected the key factor of the contact quality between soil and sensor. When various sensors are used in indoor experimental research, good contact between soil and sensor is easy to control; but in in-situ tests, due to the complex field conditions, the installation and layout process of the sensor is difficult to accurately control, and poor contact between the sensor and the soil will inevitably occur. Especially for the three types of sensors, AH-FBG corundum tube sensor, CFHC measuring tube and MNHC measuring tube, because they are in the shape of a circular tube, the sensor itself has a certain rigidity, is not easy to bend, and is more likely to form poor contact with the surrounding soil. Therefore, it is of urgent practical significance to explore and correct the influence of soil-AHFO sensor contact quality on the moisture content measurement results. Summary of the invention

[0006] The purpose of the present invention is to provide a method for measuring soil-sensor contact thermal resistance and correcting soil moisture content, which overcomes the disadvantage that the soil-sensor contact quality affects the moisture content measurement result, corrects the moisture content measurement result, and improves the moisture content measurement accuracy of the AHFO method.

[0007] In order to achieve the above functions, the present invention designs a soil-sensor contact thermal resistance measurement and soil moisture content correction method. For the soil to be measured, the following steps S1 to S5 are performed to calculate the moisture content of the soil to be measured considering the contact thermal resistance:

[0008] Step S1: Take in-situ samples of the soil to be tested and measure the dry density ρ of the soil sample. d and mass moisture content w;

[0009] Step S2: Based on the dry density ρ of the soil sample d and mass moisture content w, calculate the thermal diffusion coefficient K of the soil to be tested;

[0010] Step S3: bury the active heating type optical fiber sensor in the soil, power it on and heat it for a certain period of time, and obtain the temperature characteristic value T measured by the optical fiber sensor. t , and the temperature characteristic value T t The temperature rise curve between the heating time t and the heating time t is obtained by fitting. t -The slope k and intercept d of the ln(t) line;

[0011] Step S4: Based on the dry density ρ of the soil sample measured in step S1 d and mass moisture content w, according to the thermal diffusion coefficient K and T of the soil to be tested calculated in step S2 and step S3 t The slope k and intercept d of the -ln(t) straight line are used to calculate the contact thermal resistance R of the optical fiber sensor;

[0012] Step S5: Based on the temperature characteristic value T measured by the optical fiber sensort , calculate and obtain the effective temperature characteristic value T te , T te Substitute into T t -w calibration formula is used to obtain the corrected mass moisture content w, and the calculation of the moisture content of the soil to be tested considering the contact thermal resistance is completed.

[0013] Beneficial effects: Compared with the prior art, the advantages of the present invention include:

[0014] 1. The present invention adopts a soil-sensor contact thermal resistance measurement and soil moisture content correction method based on active heating FBG, which overcomes the disadvantage of the influence of soil-sensor contact quality on the moisture content measurement result, corrects the moisture content measurement result, and improves the moisture content measurement accuracy of the AHFO method;

[0015] 2. The present invention has accuracy and applicability for measuring the moisture content of various in-situ soil bodies;

[0016] 3. The present invention requires few parameters, and only needs to correct the soil dry density and mass moisture content at the depth, and the process of determining thermal resistance and correcting moisture content is simple;

[0017] 4. The invention is economical and safe, easy to operate, has strong anti-interference ability, and reliable and effective accuracy;

[0018] 5. The present invention helps to improve the accuracy of AHFO technology in in-situ soil moisture content measurement and promotes the promotion and application of this technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A test device for measuring soil-sensor contact thermal resistance and correcting soil moisture content according to an embodiment of the present invention;

[0020] Figure 1 In: 1. Computer; 2. FBG demodulator; 3. DC power supply; 4. AH-FBG corundum tube sensor; 5. Soil sample;

[0021] Figure 2 It is a flow chart of a method for measuring soil-sensor contact thermal resistance and correcting soil moisture content provided by an embodiment of the present invention;

[0022] Figure 3 is the measured temperature characteristic value T provided by the embodiment of the present invention t Dry density ρ d ;

[0023] Figure 4 The soil-sensor contact thermal resistance R according to the embodiment of the present invention varies with the dry density ρ d ;

[0024] Figure 5 is the effective temperature characteristic value T provided according to the embodiment of the present invention te Dry density ρ d ;

[0025] Figure 6 is the temperature characteristic value T before and after considering the contact thermal resistance R provided by the embodiment of the present invention t / T te and dry density ρ d The fitting result diagram of ;

[0026] Figure 7 is the contact thermal resistance R determined after correction according to an embodiment of the present invention and the moisture content θ calculated after considering the thermal resistance;

[0027] Figure 8 is the measurement error of the soil volume moisture content θ measured by the AH-FBG method considering thermal resistance correction provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0028] 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.

[0029] The embodiment of the present invention provides a method for measuring soil-sensor contact thermal resistance and correcting soil moisture content. The test device is referred to Figure 1 The AH-FBG corundum tube sensor 4 is inserted into the soil sample 5 to form a soil sample with different soil-sensor contact thermal resistance. The cable and optical cable of the AH-FBG corundum tube are respectively connected to the FBG demodulator 2 and the DC power supply 3. The FBG demodulator 2 is connected to the computer 1, and the demodulation software of the computer 1 controls and records the data.

[0030] Reference Figure 2 , for the soil to be tested, execute the following steps S1 to S5 to calculate the moisture content of the soil to be tested considering the contact thermal resistance:

[0031] Step S1: Take in-situ samples of the soil to be tested and measure the dry density ρ of the soil sample. d and mass moisture content w;

[0032] Step S2: Based on the dry density ρ of the soil sample d and mass moisture content w, calculate the thermal diffusion coefficient K of the soil to be tested;

[0033] The thermal diffusion coefficient K of the soil to be tested is calculated as follows:

[0034]

[0035] Where C is the volume heat capacity of soil, in J / (m 3 K), ρ d is the dry density of soil, in kg / m 3 , ρ s is the density of soil particles, in kg / m 3 , ρ w is the density of water in kg / m 3 , C s is the volume heat capacity of soil particles, in J / (m 3 K), whose value is the specific heat capacity of soil particles c ps and ρ s The product of ps The unit is J / (kg K), C w is the volume heat capacity of water, in J / (m 3 K)), whose value is 4.2×10 6 J / (m 3 K).

[0036] Step S3: bury the active heating type (AHFO) optical fiber sensor in the soil, regard the optical fiber sensor as an infinitely long cylindrical heat source, and according to the principle of cylindrical heat source, heat it for a certain period of time to obtain the temperature characteristic value T measured by the optical fiber sensor. t , and the temperature characteristic value T t The temperature rise curve between the heating time t and the heating time t is obtained by fitting. t -The slope k and intercept d of the ln(t) line;

[0037] The specific steps of step S3 are as follows:

[0038] Step S3.1: bury the active heating type optical fiber sensor in the soil, power it on and heat it for a certain period of time, and obtain the temperature characteristic value T measured by the optical fiber sensor t As follows:

[0039]

[0040] Where, T t is the temperature characteristic value, that is, the temperature rise value, in °C, T(t) is the sensor temperature corresponding to the heating time t, in °C, T0 is the initial ambient temperature, in °C, Q is the heating power per unit length, in W / m, λ is the thermal conductivity of the soil, in W / (m K), R is the thermal resistance between the optical fiber sensor and the soil wall per unit length, in m K / W, K is the thermal diffusion coefficient of the soil, in m 2 / s, a is the outer diameter of the optical fiber sensor, in m, c = 1.7811 = exp(γ) and γ = 0.5772, γ is the Euler constant;

[0041] Step S3.2: Set the temperature characteristic value T t The expression is simplified to the following formula:

[0042]

[0043] When t>>a 2 / K, the temperature characteristic value T t There is a linear relationship between ln(t), and the fitted T t The slope and intercept of the -ln(t) line are calculated as follows:

[0044]

[0045]

[0046] Where k is T t -The slope of the ln(t) line, d is T t -The intercept of the ln(t) line.

[0047] Step S4: Based on the dry density ρ of the soil sample measured in step S1 d and mass moisture content w, according to the thermal diffusion coefficient K and T of the soil to be tested calculated in step S2 and step S3 t The slope k and intercept d of the -ln(t) straight line are used to calculate the contact thermal resistance R of the optical fiber sensor;

[0048] The expression of the contact thermal resistance R of the optical fiber sensor is as follows:

[0049]

[0050] Where k is T t -The slope of the ln(t) line, d is T t -ln(t) is the intercept of the straight line, Q is the heating power per unit length, K is the thermal diffusion coefficient of the soil to be measured, a is the outer diameter of the optical fiber sensor, and c = 1.7811.

[0051] Step S5: Based on the temperature characteristic value T measured by the optical fiber sensor t , calculate and obtain the effective temperature characteristic value T te , T te Substitute into T t -w calibration formula is used to obtain the corrected mass moisture content w, and the calculation of the moisture content of the soil to be tested considering the contact thermal resistance is completed.

[0052] By using the temperature characteristic value T measured by the optical fiber sensor t Subtract QR to obtain the effective temperature characteristic value T te , the expression is as follows:

[0053] T te =T t -QR

[0054] Where, T te is the effective temperature characteristic value, in °C, T t is the measured temperature characteristic value, in °C, R is the soil-sensor contact thermal resistance per unit length, in m K / W, and Q is the heating power per unit length, in W / m.

[0055] Temperature characteristic value T t The calibration model of the mass moisture content w of the soil to be tested considering the contact thermal resistance is as follows:

[0056]

[0057] Where, T t is the temperature characteristic value, w is the mass moisture content of the soil to be tested considering the contact thermal resistance, the unit is g / g, A, B, and C are all constants, and T t -θ fitting results are determined, where:

[0058] The soil volume moisture content θ and mass moisture content w have the following relationship:

[0059]

[0060] In the formula, ρ d is the dry density of soil, in g / cm 3 , ρ w is the density of water in g / cm 3 ;

[0061] Temperature characteristic value T t The calibration model of the volume moisture content θ of the soil to be tested considering the contact thermal resistance is as follows:

[0062]

[0063] Where, T t is the temperature characteristic value, in °C; θ is the volumetric water content of the soil, in cm 3 / cm 3 ; A1, B1, C1 are all constants, through T t -θ fitting results are determined; are constants, through T t -θ fitting results are determined;

[0064] When measuring the moisture content in situ, the dry density at different depths often changes. Therefore, based on the calibration model, a dual-parameter model that considers both mass moisture content and dry density is proposed ( and Konrad correction model), which is expressed as:

[0065]

[0066] Where A2, B2 and C2 are temperature characteristic values ​​T t With mass moisture content w, dry density ρ d The fitting parameters of .

[0067] The specific steps of obtaining the calculation formula of the moisture content of the soil to be tested considering the contact thermal resistance in step S5 are as follows:

[0068] Step S5.1: For For the Konrad calibration model, the calculation formula for the moisture content of the soil to be tested considering the contact thermal resistance is expressed as follows:

[0069]

[0070] Where w is the mass moisture content of the soil to be tested considering the contact thermal resistance, in g / g, T te is the effective temperature characteristic value, T t is the temperature characteristic value measured by the optical fiber sensor, R is the contact thermal resistance of the soil-sensor per unit length, Q is the heating power per unit length, and A, B, and C are all constants;

[0071] Step S5.2: For T t For the calibration formula in the form of -θ, the calculation formula for the moisture content of the soil to be tested considering the contact thermal resistance is expressed as follows:

[0072]

[0073] Where θ is the volume moisture content of the soil to be tested considering the contact thermal resistance, in cm 3 / cm 3 ; A1, B1, C1 are all constants;

[0074] Step S5.3: For For the Konrad calibration correction model, the moisture content of the soil to be tested considering the contact thermal resistance is expressed as follows:

[0075]

[0076] In the formula, ρ d is the dry density of the soil sample; A2, B2, C2 are the temperature characteristic values ​​T t , mass moisture content w, dry density ρd The fitting parameters of .

[0077] The following is an application example of a soil-sensor contact thermal resistance measurement and soil moisture content correction method designed by the present invention:

[0078] Embodiment 1:

[0079] The method and device of the present invention are applied to carry out indoor experiments on measuring the moisture content of different soil bodies.

[0080] Step 1: Prepare two different soil samples. The specific steps are as follows:

[0081] (1) Two different soil samples were selected: silt and a sand-kaolin mixture with a mass ratio of 9:1;

[0082] (2) Prepare soil samples with moisture contents of 0.08 g / g, 0.12 g / g and 0.16 g / g;

[0083] (3) The soil samples with the same moisture content prepared in step (2) were respectively filled into a 20 cm × 15 cm × 20 cm test box (a small circular hole with a diameter of 1 cm was opened on one surface of the 20 cm × 20 cm box) in layers with different dry densities. The dry density range was 1.1-1.6 g / cm 3 , the gradient is 0.1g / cm 3 ;

[0084] Step 2: Slowly insert the AH-FBG corundum tube sensor with a length of 15 cm and a diameter of 6 mm into the soil sample of the test box configured in step 1 along the small circular hole on the test box;

[0085] Step 3: Use heating power of 35W / m2 and heating time of 20min for 1.1g / cm 3 The 0.08g / g silt, 0.12g / g silt, 0.12g / g silt, 0.08g / g sand-kaolin mixture, 0.12g / g sand-kaolin mixture, and 0.16g / g sand-kaolin mixture were heated and tested; then the 6 soil samples at the next dry density were tested until all the dry densities were measured and the following were obtained: Figure 3 The measured temperature characteristic value T t Dry density ρ d The changing trend of Figure 3 The soil sample in the middle left picture is silt, and the soil sample in the right picture is a sand-kaolin mixture;

[0086] Step 4: Calculate the soil-sensor contact thermal resistance R with dry density ρ d The trend of change, see Figure 4 , Figure 4The soil sample on the left is silt, and the soil sample on the right is a sand-kaolin mixture; the effective temperature characteristic value T te Dry density d The specific steps are as follows:

[0087] The expression for calculating the contact thermal resistance (R) of the sensor (cable) is as follows:

[0088]

[0089] By using the in situ measured T t Subtract QR to obtain the effective temperature characteristic value T te , the expression is as follows:

[0090] T te =T t -QR

[0091] Where, T te is the effective temperature characteristic value (℃), T t is the measured temperature characteristic value (℃), R is the contact thermal resistance of soil-sensor per unit length (m K / W); the effective temperature characteristic value T te Dry density ρ d Reference to the trend of change Figure 5 , Figure 5 The soil sample in the middle left picture is silt, and the soil sample in the right picture is a sand-kaolin mixture.

[0092] Step 5: Get Figure 6 The temperature characteristic value T before and after considering the contact thermal resistance R is shown t / T te and dry density ρ d The fitting results are as follows: Figure 6 (a) is silt without considering R; Figure 6 (b) is silt soil considering R; Figure 6 (c) is a sand-kaolin mixture without considering R; Figure 6 (d) is the sand-kaolin mixture considering R. As can be seen from the figure, for silt, the fitting effect after considering R is R 2 The fitting effect and RMSE of sand-kaolin mixture are significantly improved compared with those before consideration. For the sand-kaolin mixture, the fitting effect and RMSE before and after considering R are not much different, which is mainly because the T t P d The response sensitivity is low, but it is obvious that after considering R T t and ρ d The regularity and correlation between them are stronger.

[0093] Embodiment 2:

[0094] The method of the present invention was applied to carry out an in-situ field test, and the test site was selected on a loess ridge in Ganguyi Town, Yan'an City, Shaanxi Province.

[0095] Step 1: Install and lay out the AH-FBG sensor. The specific steps are as follows:

[0096] (1) The sensor is fixed on the steel strand with a cable tie. Considering that the AH-FBG sensor has poor shear strength and is not resistant to bending, in order to prevent the AH-FBG sensor from being damaged during the backfilling process, a groove is cut in the pit wall to provide a safe installation space for the sensor.

[0097] (2) Fix the stainless steel cap of the AH-FBG corundum tube sensor with a special U-shaped nail to fix the sensor in the groove. It should be noted that the lead optical cable should be kept at a distance of more than 10 cm from the AH-FBG corundum tube sensor to prevent it from affecting the heating test of the AH-FBG corundum tube sensor.

[0098] (3) The in-situ loess was crushed and sieved, and then backfilled in layers at a constant dry density (1.4 g / cm3). However, since it is difficult for the sensor in the trench to come into close contact with the backfill soil, this installation method will inevitably generate contact thermal resistance between the soil and the sensor.

[0099] Step 2: All deployed AH-FBG sensors are connected to the soil moisture field in-situ monitoring station, and the FBG wireless demodulator collects data in real time and transmits it to the client. The monitoring period is from March 2021 to February 2022, during which heating tests are performed once a day. The heating power of the AH-FBG sensor is 35W / m, and the heating time is 20min. The neutron method is manually collected every two months, and the collection depths are 0.42m, 0.59m, 1.06m, and 1.26m.

[0100] Step 3: Consider the influence of soil-sensor contact thermal resistance and calibrate the soil moisture content. The specific steps are as follows:

[0101] (1) The dry density values ​​at the AH-FBG corundum tube sensor locations in the test pit fluctuate slightly, with an average value of 1.40 g / cm 3 ;

[0102] (2) Take soil samples from the on-site test pit for indoor calibration experiments. The moisture content of the soil samples is set from dry to saturated. The heating parameters used in the test are the same as those on site.

[0103] (3)Use and Konrad calibration model, and compare the calibration results with the measured values. t The distribution range is 19.95-50.28℃, which is much larger than the Tt The distribution range of the temperature is (14.36-27.47°C), indicating that contact thermal resistance is generated between the soil and the sensor. Therefore, the soil moisture content is calculated by using a soil-sensor contact thermal resistance measurement and soil moisture content correction method based on active heating FBG proposed in the present invention.

[0104] (4) Figure 7 The contact thermal resistance determined after correction and the moisture content calculated after considering the thermal resistance can be seen. The moisture content calculated after considering the thermal resistance is within the theoretical range, which preliminarily illustrates the feasibility of the present invention.

[0105] Step 4: Take the water content measurement result of the neutron scattering method as the true value to evaluate the water content measurement accuracy of the AH-FBG method and obtain Figure 8 Considering the measurement error of the soil volume moisture content θ measured by the AH-FBG method with thermal resistance correction, it can be seen that the consistency of the two methods is very high, the data points are distributed near the 1:1 line, and the error is only 0.011cm 3 / cm 3 , which fully illustrates the reliability of the soil-sensor contact thermal resistance measurement and soil moisture content correction method based on active heating FBG proposed in the present invention.

[0106] 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. A method for measuring soil-sensor contact thermal resistance and correcting soil moisture content, characterized in that: For the soil to be tested, the following steps S1 to S5 are performed to calculate the moisture content of the soil to be tested taking into account the contact thermal resistance: Step S1: Take in-situ samples of the soil to be tested and measure the dry density ρ of the soil sample. d and mass moisture content w; Step S2: Based on the dry density ρ of the soil sample d and mass moisture content w, calculate the thermal diffusion coefficient K of the soil to be tested; Step S3: bury the active heating type optical fiber sensor in the soil, power it on and heat it for a certain period of time, and obtain the temperature characteristic value T measured by the optical fiber sensor. t , and the temperature characteristic value T t The heating curve between the heating time t and the heating time t is obtained by fitting. t -The slope k and intercept d of the ln(t) line; Step S4: Based on the dry density ρ of the soil sample measured in step S1 d and mass moisture content w, according to the thermal diffusion coefficients K and T of the soil to be tested calculated in steps S2 and S3 t The slope k and intercept d of the -ln(t) straight line are used to calculate the contact thermal resistance R of the optical fiber sensor; Step S5: Based on the temperature characteristic value T measured by the optical fiber sensor t , calculate and obtain the effective temperature characteristic value T te , T te Substitute into T t -w calibration formula, obtain the corrected mass moisture content w, and complete the calculation of the moisture content of the soil to be tested considering the contact thermal resistance; In step S5, the temperature characteristic value T measured by the optical fiber sensor is t Subtract QR to obtain the effective temperature characteristic value T te , the expression is as follows: T te =T t -QR Where, T te is the effective temperature characteristic value, in °C, T t is the measured temperature characteristic value, in °C, R is the soil-sensor contact thermal resistance per unit length, in m K / W, and Q is the heating power per unit length, in W / m.

2. A method for measuring soil-sensor contact thermal resistance and correcting soil moisture content according to claim 1, characterized in that: In step S2, the thermal diffusion coefficient K of the soil to be tested is calculated as follows: Where C is the volume heat capacity of soil, in J / (m 3 K), λ is the thermal conductivity of the soil, Q is the heating power per unit length of the active heating optical fiber sensor; ρ d is the dry density of soil, in kg / m 3 , ρ s is the density of soil particles, in kg / m 3 , ρ w is the density of water in kg / m 3 , C s is the volume heat capacity of soil particles, in J / (m 3 K), whose value is the specific heat capacity of soil particles c ps and ρ s The product of ps The unit is J / (kg K), C w is the volume heat capacity of water, in J / (m 3 K)), whose value is 4.2×10 6 J / (m 3 K).

3. The method for measuring soil-sensor contact thermal resistance and correcting soil moisture content according to claim 1, characterized in that: The specific steps of step S3 are as follows: Step S3.1: bury the active heating type optical fiber sensor in the soil, power it on and heat it for a certain period of time, and obtain the temperature characteristic value T measured by the optical fiber sensor t As follows: Where, T t is the temperature characteristic value, in °C, T(t) is the sensor temperature corresponding to the heating time t, in °C, T0 is the initial ambient temperature, in °C, Q is the heating power per unit length, in W / m, λ is the thermal conductivity of the soil, in W / (m K), R is the thermal resistance between the optical fiber sensor and the soil wall per unit length, in m K / W, K is the thermal diffusion coefficient of the soil, in m 2 / s, a is the outer diameter of the optical fiber sensor, in m, c = 1.7811 = exp(γ) and γ = 0.5772, γ is the Euler constant; Step S3.2: Set the temperature characteristic value T t The expression is simplified to the following formula: When t>>a 2 / K, the temperature characteristic value T t There is a linear relationship between ln(t), and the fitted T t The slope and intercept of the -ln(t) line are calculated as follows: Where k is T t -The slope of the ln(t) line, d is T t -The intercept of the ln(t) line.

4. The method for measuring soil-sensor contact thermal resistance and correcting soil moisture content according to claim 1, characterized in that: The expression of the contact thermal resistance R of the optical fiber sensor in step S4 is as follows: Where k is T t -The slope of the ln(t) line, d is T t -ln(t) is the intercept of the straight line, Q is the heating power per unit length, K is the thermal diffusion coefficient of the soil to be measured, a is the outer diameter of the optical fiber sensor, and c = 1.7811.

5. The method for measuring soil-sensor contact thermal resistance and correcting soil moisture content according to claim 1, characterized in that: The specific steps of obtaining the calculation formula of the moisture content of the soil to be tested considering the contact thermal resistance in step S5 are as follows: Step S5.1: For For the Konrad calibration model, the calculation formula for the moisture content of the soil to be tested considering the contact thermal resistance is expressed as follows: Where w is the mass moisture content of the soil to be tested considering the contact thermal resistance, in g / g, T te is the effective temperature characteristic value, T t is the temperature characteristic value measured by the optical fiber sensor, R is the contact thermal resistance of the soil-sensor per unit length, Q is the heating power per unit length, and A, B, and C are all constants; Step S5.2: For T t For the calibration formula in the form of -θ, the calculation formula for the moisture content of the soil to be tested considering the contact thermal resistance is expressed as follows: Where θ is the volume moisture content of the soil to be tested considering the contact thermal resistance, in cm 3 / cm 3 ; A1, B1, C1 are all constants; Step S5.3: For For the Konrad calibration correction model, the moisture content of the soil to be tested considering the contact thermal resistance is expressed as follows: In the formula, ρ d is the dry density of the soil sample, A2, B2, C2 are the temperature characteristic values ​​T t , mass moisture content w, dry density ρ d The fitting parameters of .

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