CN117074458A - Method for synchronously measuring soil hydrothermal parameters by heating optical fibers in distributed mode - Google Patents
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Abstract
The application discloses a method for synchronously measuring soil hydrothermal parameters by heating optical fibers in a distributed mode, which comprises the following steps: the optical cable is buried in the soil to be detected and used for sensing the temperature of surrounding soil; electrifying a metal layer in the optical cable for a short time to generate Joule heat; measuring the temperature increment of surrounding soil and the change of the surrounding soil with time; establishing a functional relation between soil moisture and the measured thermal conductivity; measuring soil moisture at different points along the optical fiber by using the thermal conductivities measured at different spatial points of the optical fiber; the soil moisture measurement accuracy of the optical fiber method was evaluated by using root mean square error RMSE. The beneficial effects of the application are as follows: according to the application, from the characteristics of the optical fiber, the influence of the optical fiber structure and the material is considered, the thermal conductivity correction model of the optical fiber method is established by taking the thermal probe method as a reference, and the accuracy and universality of the optical fiber method for measuring the soil hydrothermal parameters are improved.
Description
Technical Field
The application relates to the field of soil data processing and measurement, in particular to a method for synchronously measuring soil hydrothermal parameters by heating optical fibers in a distributed mode.
Background
Soil thermal parameters including thermal conductivity, thermal diffusivity and volumetric heat capacity affect the temperature change and energy migration of the soil, and interact with the water content of the soil and jointly control the water heat exchange and balance in the soil. Accurate measurement of hydrothermal parameters is important for simulation of land models and guidance of accurate irrigation of farmlands. Therefore, the synchronous measurement of the thermal parameters and the water content of the soil is very critical. The current heat pulse probe method is the most favored method for measuring the soil hydrothermal property, but the method can only measure the soil mass with a point scale. However, the soil hydrothermal parameters have strong spatial variability, and the measured data are not strong representative of large-area soil in the field. Therefore, in view of the current situation that the thermal properties and the water content of the soil are inconsistent in the measurement scale (large scale and small scale) and the application scale (field mesoscale), development of a novel technology and a novel method capable of synchronously measuring the soil hydrothermal parameters in the field scale are needed. The rapid development of distributed optical fiber temperature sensing technology (DTS) provides a new idea for synchronous measurement of soil hydrothermal parameters in field scale.
The distributed optical fiber temperature sensing technology DTS is a temperature sensor which is used for measuring temperature based on the Raman scattering effect and is positioned by an optical time domain reflection technology, and the temperature distribution along the optical fiber can be obtained through the back scattering anti-Stokes light and Stokes light intensity ratio. The principle of measuring the water content of soil by using optical fibers can be divided into two types: a passive heating optical fiber method and an active heating optical fiber method. The passive heating optical fiber method is characterized in that the thermal diffusivity is inverted by monitoring the soil temperatures of different soil depths, and then the water content is estimated by the thermal diffusivity, so that the defect of the method is that the optical fiber depth has larger uncertainty, and the solution of the thermal diffusivity has larger error. Therefore, the passive heating method lacks feasibility in field application and popularization.
The active heating optical fiber method (AHFO-DTS) is based on the principle of heat pulse probe, and is to heat the metal layer of optical fiber in short time by heating and analyzing the temperature in heating and cooling stagesThe change of the degree can be used for deducing the water content of soil, and the soil depth can be measured at any time and at any time, so that the method is more widely applied compared with a passive heating optical fiber method. The distributed optical fiber sensing method (DPHP-DTS) based on double-probe heat pulse can measure the thermal conductivity, the thermal diffusivity and the specific heat capacity of soil at the same time, and calculate the water content by utilizing the specific heat capacity, but the measurement accuracy is greatly influenced by the distance between the optical fibers, so that the application of the method is limited. The single probe thermal pulse distributed optical fiber sensing method (SPHP-DTS) only needs to embed 1 optical fiber, so that the defects are avoided, and the single probe thermal pulse distributed optical fiber sensing method is more favored. The method utilizes the theory of single-probe heat pulse linear heat source, a certain section of the optical fiber is regarded as a single probe, and the optical fiber can be regarded as formed by serially connecting innumerable single probes, so that the high space-time resolution monitoring of soil moisture can be realized. The method can directly utilize the time-dependent information of the temperature of the heat pulse, such as the maximum temperature rise (delta T) max ) Cumulative temperature increase value (DeltaT) cum ) The water content is deduced by a functional relation with the soil water content, and the soil water content can be deduced by indirectly measuring the soil heat conductivity (lambda).
Because of the large difference in fiber characteristics from the probe, the fiber optic approach does not meet the assumption of an infinite linear heat source. This results in larger errors in the current fiber optic method for measuring the thermal conductivity and moisture content of the soil, and the two have not been accurately and synchronously measured. According to the application, by comparing and verifying the soil hydrothermal parameters measured by the heating optical fiber method and the probe method, the intrinsic cause of the measurement difference of the heating optical fiber method and the probe method is analyzed, and the accuracy effect of the measurement of the soil hydrothermal parameters by the heating optical fiber method and the probe method is clarified. In addition, the application also establishes a thermal conductivity correction model of the heating optical fiber method, thereby improving the soil hydrothermal parameter measurement precision of the heating optical fiber method.
Disclosure of Invention
The application aims to provide a method for synchronously measuring soil hydrothermal parameters in a heating optical fiber distributed mode, which has important significance for optimizing an optical fiber method in the future and improving the measurement accuracy of the soil hydrothermal parameters and has important value for kilometer scale monitoring of environmental factors such as soil hydrothermal in an ecological system.
The technical scheme adopted by the application is as follows: a method for measuring soil hydrothermal parameters by a heating optical fiber method realizes synchronous measurement of soil hydrothermal parameters by the heating optical fiber method, and comprises the following steps:
step S11: the optical cable is buried in the soil to be tested, 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 for sensing the temperature around the soil to be tested;
step S12: the metal layer in the optical cable is electrified for a short time to generate Joule heat, the heating power is obtained through the resistor 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 of the unit length of the metal layer;
step S13: measuring the temperature increment delta T around the soil to be measured and the change of the temperature increment delta T along the different space points of the optical fiber in the step S11 by using a light signal demodulation module DTS; calculating according to a formula (1) to obtain the thermal conductivity lambda of the soil to be measured at different spatial points along the optical fiber FO ；
Wherein DeltaT is the temperature increment around the soil to be measured, which is measured along different spatial points of the optical fiber; lambda (lambda) FO The thermal conductivity of the soil to be measured is different spatial point positions along the optical fiber; t is time; t is t 0 Is the heating time; t' is the correction time;
step S14: embedding soil moisture probes TDR near different spatial points of optical fibers in soil to be detected, wherein the soil moisture probes TDR are used for monitoring soil moisture theta near the optical fibers, and combining the soil moisture theta of the different spatial points of the optical fibers with the thermal conductivity lambda of the soil to be detected along the different spatial points of the optical fibers FO The soil thermal conductivity lambda of the soil to be measured of different spatial points along the optical fiber is established by fitting the Lu Sen model of the formula (2) FO Is a function of (a);
wherein lambda is sat 、λ dry Respectively measuring the saturation heat conductivity and the drying heat conductivity of the soil to be measured; exp is an exponential function; alpha is a shape index; θ sat The saturated water content of the soil to be measured;
step S15: soil thermal conductivity lambda to be measured through soil moisture theta and different spatial points along optical fiber FO Obtaining soil moisture theta according to the functional relation of the water content;
step S16: the measurement accuracy of the soil moisture theta of the optical fiber method is evaluated by adopting a root mean square error RMSE, wherein the root mean square error RMSE is the square root of the ratio of the square sum observation times n of the deviation of the moisture observed value of the soil moisture probe TDR and the measured value of the soil moisture theta of the optical fiber method, the smaller the root mean square error RMSE is, the higher the measurement accuracy is, and the calculation of the root mean square error RMSE is as shown in a formula (3):
wherein RMSE is root mean square error, X obs,i Is the moisture observed value of the ith soil moisture probe TDR; x is X pre,i The measurement value of the soil moisture θ by the ith fiber method is defined, and n is the number of observations.
The application adopts another scheme, a method for measuring soil hydrothermal parameters by a heating optical fiber method to improve the measuring precision, and the method for improving the measuring precision of the soil hydrothermal parameters by the heating optical fiber method comprises the following steps:
step S21: embedding a thermal probe near the optical fiber, the thermal probe including a built-in resistor R HP Heating probe of resistance wire and sensing probe with built-in thermistor, wherein the built-in resistor is R HP Is energized for 15s to generate Joule heat, and the sensing probe is used for measuring the temperature increment change delta T HP The method comprises the steps of carrying out a first treatment on the surface of the Acquisition of temperature delta change deltat of sensing probe by data acquisition unit CR1000 HP Acquiring 1 data per second as a function of time t; at the same time, the data acquisition device CR1000 is used for recording the resistance as R HP Voltage U across the resistance wire of (2) HP The heating power Q 'of the thermal probe is calculated, and the calculation formula is 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 a radial heat conduction equation, wherein the radial heat conduction equation is shown as formula (4) and formula (5);
wherein DeltaT HP1 、ΔT HP2 The temperature increment changes of the heating stage and the cooling stage are respectively shown in the unit of DEG C; q' is the heating power of the thermal probe; k is soil thermal diffusivity; -E i (-x) is an exponential integral function; r is the distance between the induction probe and the heating probe, and the unit is m; t is time, t 0 For heating time DeltaT HP1 T in (b) satisfies t being more than 0 and less than or equal to t 0 ；ΔT HP2 T in (b) satisfies t.gtoreq.t 0 The method comprises the steps of carrying out a first treatment on the surface of the The unit is s; q ' =q '/ρc, Q ' is the heat input per unit length of the thermal probe per unit time, ρc is the volumetric heat capacity of the soil;
step S23: by the formula lambda HP Calculation of =k×ρc to obtain soil thermal conductivity λ measured by thermal probe HP ；
Step S24: the soil thermal conductivity lambda to be measured at different spatial points along the optical fiber is evaluated by using a probe method as a true value and Root Mean Square Error (RMSE) FO The measurement accuracy of (2);
step S25: obtaining the thermal conductivity lambda of the soil to be measured at different spatial points along the optical fiber by a linear fitting method FO The correction model of (2) is formula (6);
λ HP ＝a*λ FO +b (6)
wherein a and b are parameters related to the characteristics of the optical fiber;
step S26: soil thermal conductivity lambda to be measured at different spatial points along optical fiber FO Substituting into correction model (6) to obtain new optical fiber methodMeasured soil thermal conductivity lambda FO ′；
Step S27: soil thermal conductivity lambda measured by novel optical fiber method FO ' substituting the soil moisture theta ' into the Lu Sen model in the step S14, and calculating to obtain the soil moisture theta ' measured by a new optical fiber method;
step S28: and evaluating the measurement accuracy of the soil hydrothermal parameters of the optical fiber method after the soil thermal conductivity correction through the Root Mean Square Error (RMSE).
The beneficial effects of the application are as follows: according to the application, from the characteristics of the optical fiber, the influence of the optical fiber structure and the material is considered, the thermal conductivity correction model of the optical fiber method is established by taking the thermal probe method as a reference, and the accuracy and universality of the optical fiber method for measuring the soil hydrothermal parameters are improved. According to the application, soil hydrothermal parameters of a field scale centimeter to kilometer can be synchronously measured in real time in situ by a distributed optical fiber temperature sensing technology, so that the method has important significance for optimizing an optical fiber method in the future and improving the measurement accuracy of the soil hydrothermal parameters, and has important value for kilometer scale monitoring of environmental factors such as soil hydrothermal in an ecological system.
Drawings
Fig. 1 is a structural flow chart of the present application.
FIG. 2 is a graph showing the relationship between the thermal conductivity and the water content of soil measured by the optical fiber method and the probe method of the present application.
FIG. 3 is a graph showing the relationship between the thermal conductivity measured by the optical fiber method and the probe method according to the present application.
FIG. 4 is a graph showing the accuracy of the water content measured by the optical fiber method and the probe method according to the present application.
Detailed Description
As shown in fig. 1, which is a structural flow chart of the present application, the present application lays optical fibers in soil and lays soil moisture sensors and thermal probes around the optical fibers. Electrifying and heating, recording temperature data by using a light signal Demodulator (DTS), and calculating the thermal conductivity lambda of soil to be detected at different spatial points along the optical fiber by using a formula FO The method comprises the steps of carrying out a first treatment on the surface of the Soil moisture theta is continuously monitored through a soil moisture sensor, and soil moisture theta and soil thermal conductivity lambda to be measured along different spatial points of the optical fiber are established through a Lu Sen model FO Is a relationship of (3). Connecting self-heating probe with data acquisition CR1000, adopting data acquisition CR1000 record temperature change of the thermal probe, calculate soil thermal conductivity lambda measured by the thermal probe through thermal conduction equation HP Establishing a correction model lambda for measuring soil heat conductivity by using an optical fiber method HP ＝a*λ FO +b. Obtaining new soil thermal conductivity lambda measured by optical fiber method through correction model FO ' soil thermal conductivity lambda measured by a novel optical fiber method FO By substituting the model Lusen, a new and more accurate soil moisture θ' can be obtained.
The application works and implements in this way, a method for measuring soil hydro-thermal parameters by heating optical fiber method, which realizes the synchronous measurement of soil hydro-thermal parameters by heating optical fiber method, includes the following steps:
step S11: the optical cable is buried in the soil to be tested, 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 for sensing the temperature around the soil to be tested;
step S12: the metal layer in the optical cable is electrified for a short time to generate Joule heat, the heating power is obtained through the resistor 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 of the unit length of the metal layer;
step S13: acquiring the temperature increment delta T and the change of time T around the soil to be detected, which are measured along different spatial point positions of the optical fiber in the step S11, by using a light signal demodulation module DTS; calculating according to a formula (1) to obtain the thermal conductivity lambda of the soil to be measured at different spatial points along the optical fiber FO ；
Wherein DeltaT is the temperature increment around the soil to be measured, which is measured along different spatial points of the optical fiber; lambda (lambda) FO The thermal conductivity of the soil to be measured is different spatial point positions along the optical fiber; t is time; t is t 0 Is the heating time; t' is the correction time;
step S14: embedding soil moisture probes TDR near different spatial points of optical fibers in soil to be detected, wherein the soil moisture probes TDR are used forMonitoring soil moisture theta near the optical fiber, and combining the soil moisture theta of different spatial points of the optical fiber with the thermal conductivity lambda of the soil to be measured along the different spatial points of the optical fiber FO The soil thermal conductivity lambda of the soil to be measured of different spatial points along the optical fiber is established by fitting the Lu Sen model of the formula (2) FO Is a function of (a);
wherein lambda is sat 、λ dry Respectively measuring the saturation heat conductivity and the drying heat conductivity of the soil to be measured; exp is an exponential function; alpha is a shape index; θ sat The saturated water content of the soil to be measured;
step S15: soil thermal conductivity lambda to be measured through soil moisture theta and different spatial points along optical fiber FO Obtaining soil moisture theta according to the functional relation of the water content;
step S16: the measurement accuracy of the soil moisture theta of the optical fiber method is evaluated by adopting a root mean square error RMSE, wherein the root mean square error RMSE is the square root of the ratio of the square sum observation times n of the deviation of the moisture observed value of the soil moisture probe TDR and the measured value of the soil moisture theta of the optical fiber method, the smaller the root mean square error RMSE is, the higher the measurement accuracy is, and the calculation of the root mean square error RMSE is as shown in a formula (3):
wherein RMSE is root mean square error, X obs,i Is the moisture observed value of the ith soil moisture probe TDR; x is X pre,i The measurement value of the soil moisture θ by the ith fiber method is defined, and n is the number of observations.
The application adopts another scheme, a method for measuring soil hydrothermal parameters by a heating optical fiber method to improve the measuring precision, and the method for improving the measuring precision of the soil hydrothermal parameters by the heating optical fiber method comprises the following steps:
step S21: embedding a thermal probe near the optical fiber, the thermal probe including a built-in resistor R HP Heating probe of resistance wire and sensing probe with built-in thermistor, wherein the built-in resistor is R HP Is energized for 15s to generate Joule heat, and the sensing probe is used for measuring the temperature increment change delta T HP The method comprises the steps of carrying out a first treatment on the surface of the Acquisition of temperature delta change deltat of sensing probe by data acquisition unit CR1000 HP Acquiring 1 data per second as a function of time t; at the same time, the data acquisition device CR1000 is used for recording the resistance as R HP Voltage U across the resistance wire of (2) HP The heating power Q 'of the thermal probe is calculated, and the calculation formula is 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 a radial heat conduction equation, wherein the radial heat conduction equation is shown as formula (4) and formula (5);
wherein DeltaT HP1 、ΔT HP2 The temperature increment changes of the heating stage and the cooling stage are respectively shown in the unit of DEG C; q' is the heating power of the thermal probe; k is soil thermal diffusivity; -E i (-x) is an exponential integral function; r is the distance between the induction probe and the heating probe, and the unit is m; t is time, t 0 For heating time DeltaT HP1 T in (b) satisfies t being more than 0 and less than or equal to t 0 ；ΔT HP2 T in (b) satisfies t.gtoreq.t 0 The method comprises the steps of carrying out a first treatment on the surface of the The unit is s; q ' =q '/ρc, Q ' is the heat input per unit length of the thermal probe per unit time, ρc is the volumetric heat capacity of the soil;
step S23: by the formula lambda HP Calculation of =k×ρc to obtain soil thermal conductivity λ measured by thermal probe HP ；
Step S24: the soil thermal conductivity lambda to be measured at different spatial points along the optical fiber is evaluated by using a probe method as a true value and Root Mean Square Error (RMSE) FO The measurement accuracy of (2);
step S25: obtaining the thermal conductivity lambda of the soil to be measured at different spatial points along the optical fiber by a linear fitting method FO The correction model of (2) is formula (6);
λ HP ＝a*λ FO +b (6)
wherein a and b are parameters related to the characteristics of the optical fiber;
step S26: soil thermal conductivity lambda to be measured at different spatial points along optical fiber FO Substituting into the correction model (6) to obtain the new soil thermal conductivity lambda measured by the optical fiber method FO ′；
Step S27: soil thermal conductivity lambda measured by novel optical fiber method FO ' substituting the soil moisture theta ' into the Lu Sen model in the step S14, and calculating to obtain the soil moisture theta ' measured by a new optical fiber method;
step S28: and evaluating the measurement accuracy of the soil hydrothermal parameters of the optical fiber method after the soil thermal conductivity correction through the Root Mean Square Error (RMSE).
According to the application, a comparison verification test for measuring the soil hydrothermal parameters by the heating optical fiber method and the probe method is carried out, the difference of the soil hydrothermal parameters monitored by the heating pulse probe method and the heating optical fiber method is revealed, and the soil thermal conductivity measured by the optical fiber method is corrected by the heating probe method, so that the soil moisture measurement precision is improved, and theoretical evidence is further provided for synchronously measuring the soil hydrothermal parameters by the heating optical fiber method.
(1) Performing optical fiber hydrothermal measurement test;
the soil hydrothermal parameters of different lands are measured by using a heat pulse probe and a heating optical fiber method. The 4m long soil tank is equally divided into 5 compartments with the length of 0.8m by a steel plate, 4 soil samples are respectively filled into the 4 compartments, and the soil is compacted and filled in layers for a plurality of times. The test is to test the air-dried soil first, then to fully saturate the soil and to test the saturated soil. After the saturated soil test is completed, the drain hole at the bottom of the soil tank is opened, and the soil hydrothermal parameters from unsaturated to drying process are monitored. The infrared lamp is hung above the soil tank, and the soil evaporation is accelerated by increasing the temperature, so that the soil water reduction rate is accelerated, and the test period is shortened. To avoid the influence of the infrared bulb on the heat pulse caused by the soil heating, the infrared lamp is turned off before the heat pulse starts to make the soil temperatureThe measurement was started when the ambient temperature was reached. The optical fiber is heated for 10min each time, and the heating power is 5 W.m -1 . The soil moisture sensor TDR315 (Acalima Co., USA) is arranged near the optical fiber, the TDR315 is calibrated by a drying method before use, and the water content measuring precision is about 0.03m 3 ·m -3 . When the optical fiber is heated, the temperature change sensed by the optical fiber is recorded through the DTS, and the temperature data is recorded every 5 seconds. Soil moisture was continuously monitored by TDR315 and 1soil moisture data was collected every 5min using data acquisition (CR 1000 model Campbell Scientific, logan, USA).
(2) A probe hydrothermal assay;
and (3) manufacturing a probe: the test is self-made double probes, one of the double probes is a temperature measuring probe, the other is a heating probe, the 2 probes are stainless steel pipes with the length of 40mm and the diameter of 2.8m, and the probe spacing is about 6mm. A thermistor (10K3MCD1,Betatherm Corp. Shrewbury MA) is mounted in the middle of the temperature probe for measuring temperature, and a resistance wire is mounted in the heating needle for releasing Joule heat to the measuring medium. The inside of the probe is filled with insulating material epoxy resin filled grease (Omega engineering, stamford, CT) with high heat conductivity for fixing the heating wire or the thermistor, and the probe is inserted into a hollow tetrafluoro cylinder base and sealed and fixed by epoxy resin.
And (3) probe layout: the heat pulse probe measuring device comprises a data acquisition instrument (powered by a direct-current power supply), a heating circuit (a relay and a heating wire), and an induction circuit (a thermistor and a resistor 1 omega). The data acquisition instrument uses a program control circuit and records temperature data, and a relay as a switch of the circuit, and uses a heating wire (resistance 86 Ω·m -1 ) The soil medium is electrified and heated, the data acquisition instrument can record the voltage generated at the two ends of the 1 omega resistor, so that the current flowing through the heating wire is obtained, and finally the heating quantity can be obtained through calculation. The probe is disposed on the opposite side of the TDR of the fiber attachment. The probe was pulsed for 15s at the same time as the optical fiber was heated (power 45 W.m -1 ) Temperature data were acquired 1 time per second by CR 1000.
(3) Collecting a soil sample;
regarding soil samples, soil samples were from sandy soil (Sand) of Yang Lingwei river, the roquette soil of Yang Ling farmland (Clay loam 1), clay loam of Changwu and qing apple woods (Clay loam2, clay loam 3). And (5) grinding the soil sample after air drying, and sieving with a 2mm sieve for later use. The particle size distribution volume fractions of the soil particles are shown in table 1, and the other 3 kinds of soil except sand are clay loam.
TABLE 1soil particle size distribution volume fraction (%) Table 1Soil particle size distribution volume fraction (%)
(4) DTS temperature measurement principle;
regarding the temperature measurement principle of the distributed optical fiber temperature sensing technology, after pulse pump light with certain energy is injected into an optical fiber, photons and optical fiber molecules collide inelastically to generate 2 beams of back Raman scattered light. Wherein, stokes scattered light with a wavelength greater than that of the incident light is not affected by temperature, and anti-Stokes scattered light with a wavelength less than that of the incident light has a strong temperature dependence. Thus, the temperature can be calculated from the light intensity ratio of Stokes to anti-Stokes, and the distributed temperature measurement can be obtained by locating the travel time of Stokes scattered light. The intensity ratio R (z) of the two back-scattered light beams can be expressed as:
wherein, C is a parameter related to the wavelength and frequency of the emitted light, the back-scattered light, the photon detector of the instrument and the operation condition of the DTS; Δα is the difference in stokes and anti-stokes back-scattered light losses; gamma = Δe/k * Δe is the difference in molecular energy states (J) driving raman scattering; k (k) * Is Boltzmann constant (J.K) -1 )；
Therefore, the temperature value of any point z on the optical fiber can be obtained by only marking C, delta alpha and gamma according to a certain section or a certain sections of known reference temperatures on the optical fiber:
the position z of the temperature value on the optical fiber is half of the optical walking path:
wherein c is the speed of light in vacuum (m.s -1 ) The method comprises the steps of carrying out a first treatment on the surface of the v is the refractive index of the fiber cladding; t is t * The time(s) required for light to propagate forward and backward.
The difference in precision of the thermal conductivity of the earth is measured with respect to the optical fiber method and the probe method.
The optical fiber probe method has a difference in the effect of measuring accuracy of moisture for different soil types (fig. 4). For sand, the optical fiber method and the probe method have higher precision for measuring the sand in the whole moisture range. For clay loam, in the low water content range (less than 0.35m 3 ·m -3 ) The optical fiber method has better measurement accuracy. However, in the high water content range, the scatter of the observed value and the predicted value deviates from 1: line 1, which indicates that the measurement error in the high water content range is large. This is because the thermal conductivity gradually decreases as the soil moisture increases in sensitivity. When the water content is high, moisture around the optical fiber tends to migrate due to the temperature gradient, and the moisture measurement error increases. Furthermore, it was found that the thermal conductivity method (λ FO ) Is significantly less accurate than the probe-based thermal conductivity method (lambda HP ) (FIG. 4). Lambda for moisture determination of sandy soil and 3 clay loams FO Law ratio lambda HP The measurement accuracy of the method is respectively lower than 0.01, 0.04, 0.05 and 0.03m 3 ·m -3 . Thus, based on optical fiber lambda FO The method has larger soil moisture measurement error, and the main reason is that the optical fiber method has larger thermal conductivity error, which influences the functional relation between the soil moisture content and the thermal conductivity, so that the soil moisture measurement error is increased. According to the application, the thermal conductivity measured by the optical fiber method is required to be corrected by taking the thermal probe method as a reference, so that the accuracy of measuring the soil moisture by the optical fiber method is improved. It can be seen from the present application that the probe is based onThe thermal conductivity method for measuring the water of different soil types has higher precision, a plurality of students establish a thermal conductivity lambda (theta) model at present, and model parameters have physical significance, thus providing a model foundation for the water measurement of the optical fiber method. Therefore, in the case of accurate thermal conductivity measurement, the optical fiber-based thermal conductivity method has great potential for measuring soil moisture.
The fiber optic method determines the error source of the hydrothermal parameter.
The error sources for measuring the hydrothermal parameters by the optical fiber method mainly comprise the following 4 points, wherein the difference between the structure and the material of the optical fiber and the thermal probe is large, the assumption of infinite linear heat source is not met, and the analysis solution of a heat conduction equation is directly utilized to have errors; secondly, there are multiple interface thermal contact resistances (fiber core-air layer-metal layer-sheath-soil) between the optical fiber and the soil, so that the thermal contact resistance between the optical fiber and the soil is far greater than that between the probe, and therefore the measurement accuracy is greatly affected; in addition, the temperature measurement precision and the sensitivity of the optical fiber to the temperature are far lower than those of the probe, so that certain fluctuation exists in the temperature rise values in the heating and cooling processes, and noise interference is increased; finally, the fiber optic method requires a heat source to input more heat and extends the heating time, producing a higher temperature rise than the probe to improve the signal to noise ratio, and therefore moisture around the fiber may migrate under the influence of the temperature gradient, which may lead to overestimation of the thermal conductivity. Therefore, when the thermal conductivity of the soil is measured by using the optical fiber method, the optical fiber with simple structure, small diameter and good thermal conductivity of the sheath can be selected to be close to a linear heat source, so that the thermal conductivity measurement precision is improved. In addition, when the soil moisture is measured by the optical fiber method, on the premise of accurately measuring the thermal conductivity, the influence of the surrounding moisture migration caused by the too high temperature of the optical fiber is also considered. Note also that, in the case where the measurement environment changes greatly, the measurement error of the soil hydrothermal parameter increases.
The above description is only of the preferred embodiments of the present application and is not intended to limit the present application, but various modifications and variations can be made to the present application by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.
Claims (2)
1. A method for synchronously measuring soil hydrothermal parameters in a distributed mode by heating optical fibers is characterized in that the soil hydrothermal parameters are synchronously measured by using a heating optical fiber method: the method comprises the following steps:
step S11: the optical cable is buried in the soil to be tested, 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 for sensing the temperature around the soil to be tested;
step S12: the metal layer in the optical cable is electrified for a short time to generate Joule heat, the heating power is obtained through the resistor 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 of the unit length of the metal layer;
step S13: measuring the temperature increment delta T around the soil to be measured and the change of the temperature increment delta T along the different space points of the optical fiber in the step S11 by using a light signal demodulation module DTS; calculating according to a formula (1) to obtain the thermal conductivity lambda of the soil to be measured at different spatial points along the optical fiber FO ；
Wherein DeltaT is the temperature increment around the soil to be measured, which is measured along different spatial points of the optical fiber; lambda (lambda) FO The thermal conductivity of the soil to be measured is different spatial point positions along the optical fiber; t is time; t is t 0 Is the heating time; t' is the correction time;
step S14: embedding soil moisture probes TDR near different spatial points of optical fibers in soil to be detected, wherein the soil moisture probes TDR are used for monitoring soil moisture theta near the optical fibers, and combining the soil moisture theta of the different spatial points of the optical fibers with the thermal conductivity lambda of the soil to be detected along the different spatial points of the optical fibers FO The soil thermal conductivity lambda of the soil to be measured of different spatial points along the optical fiber is established by fitting the Lu Sen model of the formula (2) FO Is a function of (a);
wherein lambda is sat 、λ dry Respectively measuring the saturation heat conductivity and the drying heat conductivity of the soil to be measured; exp is an exponential function; alpha is a shape index; θ sat The saturated water content of the soil to be measured;
step S15: soil thermal conductivity lambda to be measured through soil moisture theta and different spatial points along optical fiber FO Obtaining soil moisture theta according to the functional relation of the water content;
step S16: the measurement accuracy of the soil moisture theta of the optical fiber method is evaluated by adopting a root mean square error RMSE, wherein the root mean square error RMSE is the square root of the ratio of the square sum observation times n of the deviation of the moisture observed value of the soil moisture probe TDR and the measured value of the soil moisture theta of the optical fiber method, the smaller the root mean square error RMSE is, the higher the measurement accuracy is, and the calculation of the root mean square error RMSE is as shown in a formula (3):
wherein RMSE is root mean square error, X obs,i Is the moisture observed value of the ith soil moisture probe TDR; x is X pre,i The measurement value of the soil moisture θ by the ith fiber method is defined, and n is the number of observations.
2. The method for synchronously measuring soil hydrothermal parameters by heating optical fibers in a distributed mode according to claim 1 is characterized by comprising the following steps: the method for improving the measurement accuracy of the soil hydrothermal parameters by the heating optical fiber method comprises the following steps of:
step S21: embedding a thermal probe near the optical fiber, the thermal probe including a built-in resistor R HP Heating probe of resistance wire and sensing probe with built-in thermistor, wherein the built-in resistor is R HP Is energized for 15s to generate Joule heat, and the sensing probe is used for measuring the temperature increment change delta T HP The method comprises the steps of carrying out a first treatment on the surface of the Acquisition of temperature delta change deltat of sensing probe by data acquisition unit CR1000 HP Variation with time t1 data acquisition per second; at the same time, the data acquisition device CR1000 is used for recording the resistance as R HP Voltage U across the resistance wire of (2) HP The heating power Q 'of the thermal probe is calculated, and the calculation formula is 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 a radial heat conduction equation, wherein the radial heat conduction equation is shown as formula (4) and formula (5);
wherein DeltaT HP1 、ΔT HP2 The temperature increment changes of the heating stage and the cooling stage are respectively shown in the unit of DEG C; q' is the heating power of the thermal probe; k is soil thermal diffusivity; -E i (-x) is an exponential integral function; r is the distance between the induction probe and the heating probe, and the unit is m; t is time, t 0 For heating time DeltaT HP1 T in (b) satisfies t being more than 0 and less than or equal to t 0 ；ΔT HP2 T in (b) satisfies t.gtoreq.t 0 The method comprises the steps of carrying out a first treatment on the surface of the The unit is s; q ' =q '/ρc, Q ' is the heat input per unit length of the thermal probe per unit time, ρc is the volumetric heat capacity of the soil;
step S23: by the formula lambda HP Calculation of =k×ρc to obtain soil thermal conductivity λ measured by thermal probe HP ；
Step S24: the soil thermal conductivity lambda to be measured at different spatial points along the optical fiber is evaluated by using a probe method as a true value and Root Mean Square Error (RMSE) FO The measurement accuracy of (2);
step S25: obtaining the thermal conductivity lambda of the soil to be measured at different spatial points along the optical fiber by a linear fitting method FO The correction model of (2) is formula (6);
λ HP ＝a*λ FO +b (6)
wherein a and b are parameters related to the characteristics of the optical fiber;
step S26: soil thermal conductivity lambda to be measured at different spatial points along optical fiber FO Substituting into the correction model (6) to obtain the new soil thermal conductivity lambda measured by the optical fiber method FO ′；
Step S27: soil thermal conductivity lambda measured by novel optical fiber method FO ' substituting the soil moisture theta ' into the Lu Sen model in the step S14, and calculating to obtain the soil moisture theta ' measured by a new optical fiber method;
step S28: and evaluating the measurement accuracy of the soil hydrothermal parameters of the optical fiber method after the soil thermal conductivity correction through the Root Mean Square Error (RMSE).
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| CN118090821A (en) * | 2024-04-26 | 2024-05-28 | 江西省水利科学院(江西省大坝安全管理中心、江西省水资源管理中心) | Method for measuring soil evaporation by using distributed optical fibers |
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| CN118090821A (en) * | 2024-04-26 | 2024-05-28 | 江西省水利科学院(江西省大坝安全管理中心、江西省水资源管理中心) | Method for measuring soil evaporation by using distributed optical fibers |
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