A method for measuring average concentration of soil CO2 based on TDLAS technology
By horizontally burying long-path optical sampling pipelines in the soil and combining them with optical fiber transmission, the problems of insufficient representativeness and poor stability of soil CO2 measurement are solved, enabling accurate and continuous monitoring of the average concentration of soil CO2, which is suitable for long-term in-situ monitoring.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- BEIJING HUAYIRUI TECH CO LTD
- Filing Date
- 2026-02-12
- Publication Date
- 2026-06-09
AI Technical Summary
Existing methods for measuring soil CO2 concentration suffer from insufficient spatial representativeness and poor measurement stability. In particular, the traditional TDLAS technique is easily affected by moisture and particulate matter in the soil environment, leading to inaccurate measurement results.
A fiber-coupled method for measuring average soil CO2 concentration based on TDLAS technology was adopted. By combining horizontal long-path sampling with fiber optic transmission, and using horizontally buried sampling pipes and fiber optic connections, uniform infiltration and stable transmission of soil CO2 were achieved. The average CO2 concentration was then inverted using Lambert-Beer's law.
It enables accurate, continuous, and stable measurement of average soil CO2 concentration, improves the spatial representativeness and stability of the measurement, is suitable for long-term in-situ monitoring, and reduces interference with the soil environment.
Smart Images
Figure CN121855964B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of soil gas monitoring technology, specifically relating to a fiber-optic coupled method for measuring the average concentration of soil CO2 based on TDLAS technology. Background Technology
[0002] Soil CO2 is a crucial component of the ecosystem's carbon cycle, and its concentration changes directly reflect key ecological processes such as soil microbial activity and root respiration intensity. Accurate measurement of soil CO2 concentration is of great significance for studying the global carbon cycle and responses to climate change. Currently, methods for measuring soil CO2 concentration mainly include static chamber methods, dynamic chamber methods, and sensor-based single-point in-situ measurement methods, but these methods all have certain limitations.
[0003] Static and dynamic chamber methods involve collecting gas samples within the chamber for analysis, which are cumbersome, have long measurement cycles, and are difficult to implement continuously. Furthermore, the placement of the chamber can disrupt the surface soil environment, leading to measurement biases. Sensor-based single-point in-situ measurement methods typically insert the sensor probe into the soil at a specific depth. While this allows for continuous monitoring, the heterogeneity of the soil's pore structure means that single-point measurements are insufficient to represent the average CO2 concentration at the target depth, resulting in inadequate spatial representativeness.
[0004] Tunable semiconductor laser absorption spectroscopy (TDLAS) technology boasts advantages such as high measurement accuracy, fast response speed, and strong anti-interference capability, and has been widely applied in the field of gas concentration monitoring. However, current technologies applying TDLAS to soil CO2 monitoring often employ vertical sampling probes or short-path sampling devices, failing to address the issue of insufficient representativeness in single-point measurements. Furthermore, laser transmission typically utilizes direct optical paths, making it susceptible to interference from moisture and particulate matter in the soil environment, resulting in poor measurement stability. Therefore, a method capable of achieving accurate and stable measurement of the average concentration of soil CO2 is urgently needed. Summary of the Invention
[0005] To address the problems of insufficient spatial representativeness and poor measurement stability in existing soil CO2 concentration measurement methods, this invention provides a fiber-optic coupled soil CO2 average concentration measurement method based on TDLAS technology. By combining horizontal long-path sampling with fiber optic transmission, it achieves accurate, continuous, and stable measurement of the average CO2 concentration at the target soil depth.
[0006] The technical solution adopted in this invention is as follows:
[0007] This invention provides a fiber-optic coupled method for measuring the average concentration of CO2 in soil based on TDLAS technology, comprising the following steps:
[0008] Step S1, Sampling pipeline layout: At a preset target soil depth in the target soil area, a sealed sampling pipeline with an effective optical path length of L is horizontally buried; the sampling pipeline has several air vents evenly opened in its wall, and a gas permeation membrane is provided in the air vents.
[0009] Step S2, optical system setup: The TDLAS laser emitting module is optically coupled to one end of the sampling pipeline through the first optical fiber, and the optical receiver is optically coupled to the other end of the sampling pipeline through the second optical fiber, so that the TDLAS laser emitted by the TDLAS laser emitting module is transmitted to the optical receiver after passing through the first optical fiber, the sampling pipeline, and the second optical fiber in sequence.
[0010] Step S3, Gas Equilibrium and Spectral Acquisition: After the CO2 gas in the soil pores permeates through the vent and the gas permeation membrane into the sampling pipeline to form a stable gas environment and achieve gas equilibrium, the TDLAS laser emission module is activated to emit a laser beam tuned to the characteristic absorption line of CO2. During the transmission of the laser beam in the sampling pipeline, it interacts with the CO2 gas to generate an absorption spectrum. The optical receiver collects the transmitted spectral signal and transmits it to the data processing module.
[0011] Step S4, Average Concentration Inversion: The data processing module analyzes the collected transmission spectrum signal based on Lambert-Beer's law, calculates the integral concentration of CO2 gas in the sampling pipeline, and inverts the average CO2 concentration at the target soil depth by combining the length of the sampling pipeline.
[0012] Furthermore, the effective optical path length L of the sampling pipeline is 1m; the preset target soil depth is 0.5~2m; the sampling pipeline is made of an inert material, such as polytetrafluoroethylene or quartz glass; the pore size of the vent is 0.1~0.5mm, and the spacing between adjacent vents is 5~10cm; the gas permeation membrane is a polydimethylsiloxane membrane, which only allows gas molecules to pass through while blocking soil moisture and particulate matter.
[0013] Furthermore, when the sampling pipeline is buried, the angle between its axis and the horizontal plane does not exceed ±5°; the ports at both ends of the sampling pipeline are sealed with sealed connectors; the sealed connectors are embedded with optical fiber coupling interfaces to achieve sealed optical connection between the first optical fiber, the second optical fiber and the sampling pipeline.
[0014] Furthermore, the TDLAS laser emission module uses a DFB laser with a center wavelength of 1578nm, and the laser tuning range covers the 1578nm characteristic absorption line of CO2. The linewidth of the laser beam is less than 0.001nm. Both the first optical fiber and the second optical fiber are single-mode silica optical fibers with a core diameter of 5~10μm and a transmission loss of ≤0.2dB / km.
[0015] Furthermore, the gas equilibration time is 2-4 hours to ensure that the CO2 concentration in the sampling pipeline and the CO2 concentration in the surrounding soil pores reach dynamic equilibrium; the laser emission frequency of the TDLAS laser emission module is 10-100Hz, the sampling frequency of the optical receiver is matched with the laser emission frequency, and the collected spectral signal contains information on the change of laser intensity with wavelength.
[0016] Furthermore, the specific process of the average concentration inversion is as follows:
[0017] Step S41, Data preprocessing: Baseline correction and noise suppression are performed on the acquired transmitted spectral signals to eliminate fiber optic transmission loss and ambient light interference.
[0018] Step S42, Absorption coefficient calculation: Based on the Lambert-Beer law I(λ)=I0(λ)exp(-α(λ)CL), the absorption coefficient α(λ) of CO2 is calculated; where I(λ) is the transmitted light intensity, I0(λ) is the incident light intensity, α(λ) is the absorption coefficient of CO2 at wavelength λ, C is the CO2 concentration, and L is the effective optical path length of the sampling tube;
[0019] Step S43, Concentration Inversion: Substitute the characteristic absorption line parameters of CO2 into the absorption coefficient α(λ), and obtain the integral concentration of CO2 gas in the sampling pipeline through least squares fitting, thereby obtaining the average CO2 concentration at the target soil depth.
[0020] Furthermore, it also includes temperature and pressure correction steps:
[0021] Temperature and pressure sensors are installed near the sampling pipeline to collect soil temperature and air pressure data in real time and upload them to the data processing module. The data processing module corrects the average CO2 concentration obtained by inversion to the concentration under standard conditions. The correction formula is: C0=C×(P0T) / (PT0), where C0 is the CO2 concentration under standard conditions, P0 is the standard atmospheric pressure, T0 is the standard temperature, P is the measured air pressure, T is the measured absolute temperature, and P and T are acquired by the temperature and pressure sensors.
[0022] The present invention also provides a measurement system for implementing the fiber-coupled soil CO2 average concentration measurement method based on TDLAS technology, including a TDLAS laser emitting module, a first optical fiber, a sampling pipeline, a second optical fiber, an optical receiver, a data processing module, and a temperature and pressure sensor;
[0023] The sampling pipeline is horizontally buried at a predetermined depth in the soil, and the pipeline wall is provided with vent holes with gas permeation membranes; the temperature and pressure sensors are arranged near the sampling pipeline.
[0024] The TDLAS laser emitting module is connected to one end of the sampling pipeline via the first optical fiber, and the optical receiver is connected to the other end of the sampling pipeline via the second optical fiber; the data processing module is electrically connected to the optical receiver and the temperature and pressure sensor respectively.
[0025] Furthermore, it also includes a sealing and protection component; the sealing and protection component is sleeved on the outside of the sealing joint of the embedded optical fiber coupling interface at both ends of the sampling pipeline to prevent soil moisture from entering the optical fiber coupling interface; the TDLAS laser emitting module, the optical receiver and the data processing module are integrated in the ground control cabinet, and the ground control cabinet has waterproof, dustproof and electromagnetic interference protection functions.
[0026] Furthermore, the data processing module has built-in spectral analysis and concentration correction algorithms, which can process the spectral signals collected by the optical receiver in real time, output the corrected average soil CO2 concentration, and support data storage and remote transmission.
[0027] The fiber-optic coupled method for measuring average soil CO2 concentration based on TDLAS technology provided by this invention has the following advantages:
[0028] 1. Strong spatial representativeness: This invention uses a horizontally buried 1m long sampling pipeline, which achieves uniform CO2 infiltration through evenly distributed air pores. The laser beam interacts with CO2 gas within a 1m optical path, and the measurement results can accurately reflect the spatial average concentration of CO2 in the soil at the target depth, effectively solving the problem of insufficient representativeness of traditional vertical single-point measurements.
[0029] 2. High measurement stability: The use of fiber optic transmission for TDLAS lasers avoids interference from moisture and particles when the laser is directly transmitted in the soil environment, reducing laser energy loss. At the same time, the sealed design of the sampling pipeline and the setting of the gas permeation membrane further prevent soil moisture and particles from entering the optical transmission path, improving the long-term stability of the measurement system.
[0030] 3. High measurement accuracy: The 1578nm strong absorption line of CO2 is selected as the monitoring wavelength. Combined with the narrow linewidth and high tuning accuracy of TDLAS technology, it can achieve concentration measurement accuracy at the ppb level. At the same time, the accuracy of concentration inversion is further improved through baseline correction, noise suppression and temperature and pressure correction.
[0031] 4. Enables continuous in-situ monitoring: The measurement system of this invention can be buried in the soil for a long time to achieve 24-hour continuous monitoring without frequent on-site operations, reducing interference with the soil environment and making it suitable for long-term dynamic monitoring of soil CO2 concentration. Attached Figure Description
[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0033] Figure 1 The schematic diagram shows the system structure of the fiber-optic coupled soil CO2 average concentration measurement method based on TDLAS technology provided by this invention.
[0034] The components are: 1-TDLAS laser emitting module, 2-first optical fiber, 3-sampling pipeline, 4-vent, 5-gas permeation membrane, 6-sealed connector, 7-second optical fiber, 8-optical receiver, 9-data processing module, 10-temperature and pressure sensor, 11-protective box, 12-optical coupler. Detailed Implementation
[0035] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the invention.
[0036] This invention provides a fiber-optic coupled method for measuring the average concentration of soil CO2 based on TDLAS technology. It adopts a horizontal long-path sampling design, which can effectively obtain the spatial average concentration of soil CO2 at the target depth and avoid the problem of insufficient representativeness of single-point sampling. The fiber optic transmission method reduces environmental interference and improves measurement stability, making it suitable for long-term, in-situ monitoring of soil CO2 concentration.
[0037] See Figure 1 The specific steps of this invention include steps S1 to S4:
[0038] Step S1, Sampling pipeline layout: At a preset target soil depth in the target soil area, a sealed sampling pipeline with an effective optical path length of L is horizontally buried; the sampling pipeline has several air vents evenly opened in its wall, and a gas permeation membrane is provided in the air vents.
[0039] For example, the effective optical path length L of the sampling pipeline is 1m; the preset target soil depth is 0.5~2m, which can cover the main areas of soil microbial activity and root respiration, ensuring the ecological significance of the measurement results; the sampling pipeline is made of inert material, such as polytetrafluoroethylene or quartz glass, to avoid adsorption or chemical reaction of CO2 gas by the pipeline material, ensuring measurement accuracy; the pore size of the vent is 0.1~0.5mm, and the spacing between adjacent vents is 5~10cm, ensuring that the evenly distributed vents can ensure that CO2 gas in the soil pores permeates evenly into the sampling pipeline; the gas permeation membrane can be a polydimethylsiloxane membrane, which has good gas permeability, allowing only gas molecules to pass through, and can effectively block soil moisture and particulate matter from entering the sampling pipeline, avoiding interference with optical transmission.
[0040] When the sampling pipeline is buried, the angle between its axis and the horizontal plane does not exceed ±5° to ensure that the laser beam can be stably transmitted in the horizontal direction within the sampling pipeline, avoiding laser beam deviation or loss due to pipeline tilt; the ports at both ends of the sampling pipeline are sealed with connectors; the sealed connectors are embedded with optical fiber coupling interfaces to achieve a sealed optical connection between the first optical fiber, the second optical fiber and the sampling pipeline, further preventing soil moisture intrusion.
[0041] Step S2, optical system setup: The TDLAS laser emitting module is optically coupled to one end of the sampling pipeline through the first optical fiber, and the optical receiver is optically coupled to the other end of the sampling pipeline through the second optical fiber, so that the TDLAS laser emitted by the TDLAS laser emitting module is transmitted to the optical receiver after passing through the first optical fiber, the sampling pipeline, and the second optical fiber in sequence.
[0042] In practical applications, the TDLAS laser emission module uses a DFB laser with a center wavelength of 1578nm, which corresponds to the strong absorption line of CO2, thus improving measurement sensitivity. The laser tuning range covers the 1578nm characteristic absorption line of CO2, and the linewidth of the laser beam is less than 0.001nm, ensuring the monochromaticity and tuning accuracy of the laser. Both the first and second optical fibers are single-mode silica fibers with a core diameter of 5~10μm and a transmission loss of ≤0.2dB / km, which can effectively reduce energy loss during laser transmission and ensure the stability of the optical signal.
[0043] Step S3, Gas Equilibrium and Spectral Acquisition: After the CO2 gas in the soil pores permeates through the vent and the gas permeation membrane into the sampling pipeline to form a stable gas environment and achieve gas equilibrium, the TDLAS laser emission module is activated to emit a laser beam tuned to the characteristic absorption line of CO2. During the transmission of the laser beam in the sampling pipeline, it interacts with the CO2 gas to generate an absorption spectrum. The optical receiver collects the transmitted spectral signal and transmits it to the data processing module.
[0044] In this step, the gas equilibration time is 2-4 hours. This time ensures that the CO2 concentration in the sampling pipeline reaches a dynamic equilibrium with the CO2 concentration in the surrounding soil pores, avoiding measurement deviations caused by insufficient equilibration. The laser emission frequency of the TDLAS laser emission module is 10-100Hz, and the sampling frequency of the optical receiver matches the laser emission frequency, enabling high-frequency continuous monitoring and capturing dynamic changes in soil CO2 concentration. The collected spectral signal contains information on the change of laser intensity with wavelength, providing sufficient data support for subsequent concentration inversion.
[0045] Step S4, Average Concentration Inversion: The data processing module analyzes the collected transmission spectrum signal based on Lambert-Beer's law, calculates the integral concentration of CO2 gas in the sampling pipeline, and inverts the average CO2 concentration at the target soil depth by combining the length of the sampling pipeline.
[0046] The specific process for the average concentration inversion is as follows:
[0047] Step S41, Data Preprocessing: Baseline correction and noise suppression are performed on the acquired transmission spectrum signal to eliminate fiber optic transmission loss and ambient light interference; specifically, baseline correction uses a polynomial fitting method to eliminate spectral baseline drift, and noise suppression uses a wavelet transform method to remove environmental noise and electronic noise to ensure the accuracy of the spectral signal.
[0048] Step S42, Absorption coefficient calculation: Based on the Lambert-Beer law I(λ)=I0(λ)exp(-α(λ)CL), the absorption coefficient α(λ) of CO2 is calculated; where I(λ) is the transmitted light intensity, I0(λ) is the incident light intensity, α(λ) is the absorption coefficient of CO2 at wavelength λ, C is the CO2 concentration, and L is the effective optical path length of the sampling tube; therefore, by measuring the incident light intensity and the transmitted light intensity, the absorption coefficient α(λ) of CO2 is calculated.
[0049] Step S43, Concentration Inversion: Substitute the standard parameters of the 1578nm characteristic absorption line of CO2 (such as absorption line intensity, half width, etc.) into the absorption coefficient α(λ), and obtain the integral concentration of CO2 in the sampling pipeline through the least squares fitting method. Since the CO2 concentration in the sampling pipeline is the average concentration of CO2 in the soil pores at the target depth, the average CO2 concentration at the target soil depth is obtained directly.
[0050] Furthermore, the present invention may also include a temperature and pressure correction step:
[0051] Temperature and pressure sensors are installed near the sampling pipeline to collect soil temperature and air pressure data in real time and upload them to the data processing module. Since the gas concentration is greatly affected by temperature and air pressure, the data processing module corrects the average CO2 concentration obtained by inversion to the concentration under standard conditions. The correction formula is: C0=C×(P0T) / (PT0), where C0 is the CO2 concentration under standard conditions, P0 is the standard atmospheric pressure (101325Pa), T0 is the standard temperature (273.15K), P is the measured air pressure, and T is the measured absolute temperature. This correction step can improve the comparability and accuracy of the measurement results.
[0052] The present invention also provides a measurement system for implementing the fiber-coupled soil CO2 average concentration measurement method based on TDLAS technology, including a TDLAS laser emitting module, a first optical fiber, a sampling pipeline, a second optical fiber, an optical receiver, a data processing module, and a temperature and pressure sensor;
[0053] The sampling pipeline is horizontally buried at a predetermined depth in the soil, and the pipeline wall is provided with vent holes with gas permeation membranes; the temperature and pressure sensors are arranged near the sampling pipeline.
[0054] The TDLAS laser emitting module is connected to one end of the sampling pipeline via the first optical fiber, and the optical receiver is connected to the other end of the sampling pipeline via the second optical fiber; the data processing module is electrically connected to the optical receiver and the temperature and pressure sensor respectively.
[0055] The present invention also includes a sealing and protection component; the sealing and protection component is sleeved on the outside of the sealing joint of the embedded optical fiber coupling interface at both ends of the sampling pipeline to prevent soil moisture from entering the optical fiber coupling interface; the TDLAS laser emitting module, the optical receiver and the data processing module are integrated in the ground control cabinet, and the ground control cabinet has waterproof, dustproof and electromagnetic interference protection functions.
[0056] The data processing module has built-in spectral analysis and concentration correction algorithms, which can process the spectral signals collected by the optical receiver in real time, output the corrected average soil CO2 concentration, and support data storage and remote transmission.
[0057] The following is a specific example:
[0058] Step S1, Sampling pipeline layout: Select a forest ecosystem as the target area, with a preset soil monitoring depth of 1m; use a sampling pipeline made of polytetrafluoroethylene (PTFE), 1m in length, with an outer diameter of 20mm and an inner diameter of 18mm; evenly open ventilation holes on the pipe wall of the sampling pipeline, with a hole diameter of 0.3mm and a spacing of 8cm between adjacent ventilation holes, and attach a polydimethylsiloxane gas permeation membrane inside each ventilation hole; dig a horizontal tunnel with a depth of 1m in the target area, place the sampling pipeline horizontally in the tunnel, ensuring that the angle between the axis of the sampling pipeline and the horizontal plane is 0°, and then backfill and compact with soil; the ports at both ends of the sampling pipeline are sealed with stainless steel sealing joints, with fiber optic coupling interfaces embedded in the sealing joints.
[0059] Step S2, Optical System Setup: The TDLAS laser emitting module uses a DFB laser with a center wavelength of 1578nm, a laser tuning range of 1577.8~1578.2nm, and a laser linewidth of 0.0008nm; both the first and second optical fibers are single-mode silica fibers with a core diameter of 8μm and a transmission loss of 0.15dB / km; one end of the first optical fiber is connected to the laser output end of the TDLAS laser emitting module, and the other end of the first optical fiber is optically coupled to one end of the sampling pipeline through the fiber optic coupling interface of the sealed connector; one end of the second optical fiber is optically coupled to the other end of the sampling pipeline through the fiber optic coupling interface of the sealed connector, and the other end of the second optical fiber is connected to the signal input end of the optical receiver; the optical receiver uses an InGaAs photodetector with a sampling frequency of 50Hz; the data processing module uses an industrial control computer, which is electrically connected to the optical receiver and the temperature and pressure sensor respectively; the temperature and pressure sensor is placed on the outer wall of the bottom middle of the sampling tube for real-time acquisition of soil temperature and air pressure data.
[0060] Step S3, Gas Balance and Spectral Acquisition: After the system is deployed, allow it to stand for 3 hours to allow CO2 gas in the soil pores to permeate through the vents and gas permeation membrane into the sampling pipeline, forming a stable gas environment; activate the TDLAS laser emission module to emit a laser beam tuned to the CO2 1578nm characteristic absorption line at a frequency of 50Hz; the laser beam is transmitted to one end of the sampling pipeline via the first optical fiber, and after traveling horizontally for 1m within the sampling pipeline, it is transmitted to the optical receiver via the second optical fiber; the optical receiver acquires the transmitted spectral signal and transmits it to the data processing module.
[0061] Step S4, average concentration inversion:
[0062] Step S41: The data processing module preprocesses the acquired transmission spectrum signal and performs baseline correction using cubic polynomial fitting.
[0063] The spectral signal can be decomposed into three parts: I(λ) = Isig(λ) + Ibase(λ) + Inoise(λ)
[0064] Where: Isig(λ): characteristic absorption signal of the target substance (such as CO2); Ibase(λ): baseline drift signal (slowly changing smooth curve); Inoise(λ): random noise.
[0065] Ibase(λ) can be expressed as a cubic polynomial: Ibase(λ) = ;
[0066] Where λ is the wavelength, and a0, a1, a2, a3 are polynomial coefficients. Choosing a cubic polynomial can accurately fit the nonlinear drift of the spectral baseline. Polynomial fitting process:
[0067] 1. Select the baseline region: In the transmission spectrum, select the wavelength range where there is no absorption peak of the target substance (i.e., the "flat region", where the signal only contains the baseline and noise).
[0068] 2. Fitting the baseline curve: For the wavelength-signal data points in the baseline region, fit a 3rd degree polynomial using the least squares method to obtain the baseline function Ibase(λ).
[0069] 3. Baseline correction calculation: Subtract the fitted baseline curve from the original spectral signal to obtain the pure absorption signal after drift elimination: Icorrected(λ) = Iraw(λ) - Ibase(λ)
[0070] Step S42, then noise suppression is performed using db4 wavelet transform:
[0071] 1. The baseline-corrected spectral signal Icorrected(λ) is decomposed into N-level multi-scale components using the db4 wavelet basis to obtain one approximate component AN and N detail components D1, D2, ..., DN.
[0072] 2. Thresholding: Set a noise threshold to filter high-frequency detail components Di: Detail components with amplitudes below the threshold are identified as noise and set to zero; detail components with amplitudes above the threshold are identified as signal details and their amplitudes are retained. Note: The amplitude range is determined using the minimax threshold method.
[0073] 3. Signal Reconstruction: Reconstructing the processed approximate components AN and detail components. Perform inverse wavelet transform to obtain the denoised spectral signal Idenoised(λ).
[0074] Baseline drift caused by systematic errors is eliminated through cubic polynomial baseline correction. Then, db4 wavelet transform is used for denoising to eliminate high-frequency noise caused by random errors, improving the signal-to-noise ratio and obtaining a clean spectral signal. Next, based on the Lambert-Beer law I(λ)=I0(λ)exp(-α(λ)CL), the absorption coefficient α(λ) of CO2 is calculated. The standard parameters of the CO2 1578nm characteristic absorption line (absorption line intensity S=4.5×10⁻²) are then used. 0 Substituting the absorption coefficient α(λ) into the half-width γ (cm⁻¹ / (molecule·cm⁻²) and the half-width γ=0.002nm), the integral concentration of CO2 in the sampling pipeline is obtained by least squares fitting. Combined with the sampling pipeline length of 1m, the average concentration of CO2 at the target soil depth is obtained by inversion. Finally, based on the temperature and pressure data collected by the temperature and pressure sensor, the CO2 concentration obtained by inversion is corrected to the concentration under standard conditions by the correction formula C0=C×(P0T) / (PT0).
[0075] The measurement results of this embodiment were verified by using a traditional single-point sensor and the method of the present invention to simultaneously measure at the same soil depth for 72 hours. The results showed that the relative error between the average CO2 concentration measured by the method of the present invention and the traditional single-point measurement results was ±3.2%, and the measurement data of the method of the present invention had less fluctuation (the standard deviation was 40% lower than that of the control group), indicating that the method of the present invention has higher measurement accuracy and stability.
[0076] This invention discloses a fiber-optic coupled method for measuring the average concentration of soil CO2 based on TDLAS technology. The method involves horizontally burying a 1-meter-long sealed sampling pipe at a predetermined depth in the soil. The pipe's permeable pores and gas permeation membrane achieve a dynamic balance between CO2 in the soil pores and the gas within the pipe. An optical fiber couples a TDLAS laser emission module to one end of the sampling pipe, while an optical receiver is coupled to the other end via the same optical fiber. This creates a 1-meter optical path for the TDLAS laser within the pipe, where it absorbs the CO2 gas. The optical receiver collects the transmission spectrum signal, and the average CO2 concentration within the sampling pipe is obtained using the Lambert-Beer law. After temperature and pressure correction, the soil CO2 concentration under standard conditions is obtained. This invention employs a horizontal long-path sampling design, effectively acquiring the spatial average concentration of soil CO2 at the target depth, avoiding the representativeness issues of single-point sampling. The fiber optic transmission method reduces environmental interference and improves measurement stability, making it suitable for long-term, in-situ soil CO2 concentration monitoring.
[0077] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.
Claims
1. A fiber-optic coupled method for measuring the average concentration of CO2 in soil based on TDLAS technology, characterized in that, Includes the following steps: Step S1, Sampling pipeline layout: At a preset target soil depth in the target soil area, a sealed sampling pipeline with an effective optical path length of L is horizontally buried; the sampling pipeline has several air vents evenly opened in its wall, and a gas permeation membrane is provided in the air vents. Step S2, optical system setup: The TDLAS laser emitting module is optically coupled to one end of the sampling pipeline through the first optical fiber, and the optical receiver is optically coupled to the other end of the sampling pipeline through the second optical fiber, so that the TDLAS laser emitted by the TDLAS laser emitting module is transmitted to the optical receiver after passing through the first optical fiber, the sampling pipeline, and the second optical fiber in sequence. Step S3, Gas Equilibrium and Spectral Acquisition: After the CO2 gas in the soil pores permeates through the vent and the gas permeation membrane into the sampling pipeline to form a stable gas environment and achieve gas equilibrium, the TDLAS laser emission module is activated to emit a laser beam tuned to the characteristic absorption line of CO2. During the transmission of the laser beam in the sampling pipeline, it interacts with the CO2 gas to generate an absorption spectrum. The optical receiver collects the transmitted spectral signal and transmits it to the data processing module. Step S4, Average Concentration Inversion: The data processing module analyzes the collected transmission spectrum signal based on Lambert-Beer's law, calculates the integral concentration of CO2 gas in the sampling pipeline, and inverts the average CO2 concentration at the target soil depth by combining the length of the sampling pipeline.
2. The fiber-optic coupled method for measuring average soil CO2 concentration based on TDLAS technology according to claim 1, characterized in that, The effective optical path length L of the sampling pipeline is 1m; the preset target soil depth is 0.5~2m; the sampling pipeline is made of an inert material, namely polytetrafluoroethylene or quartz glass; the pore size of the vent is 0.1~0.5mm, and the spacing between adjacent vents is 5~10cm; the gas permeation membrane is a polydimethylsiloxane membrane, which only allows gas molecules to pass through and blocks soil moisture and particulate matter.
3. The fiber-optic coupled method for measuring average soil CO2 concentration based on TDLAS technology according to claim 1, characterized in that, When the sampling pipeline is buried, the angle between its axis and the horizontal plane shall not exceed ±5°; the ports at both ends of the sampling pipeline shall be sealed with sealed connectors; the sealed connectors shall have embedded fiber optic coupling interfaces to achieve sealed optical connection between the first optical fiber, the second optical fiber and the sampling pipeline.
4. The fiber-optic coupled method for measuring average soil CO2 concentration based on TDLAS technology according to claim 1, characterized in that, The TDLAS laser emission module uses a DFB laser with a center wavelength of 1578nm, and the laser tuning range covers the 1578nm characteristic absorption line of CO2. The linewidth of the laser beam is less than 0.001nm. Both the first and second optical fibers are single-mode silica optical fibers with a core diameter of 5~10μm and a transmission loss of ≤0.2dB / km.
5. The fiber-optic coupled method for measuring average soil CO2 concentration based on TDLAS technology according to claim 1, characterized in that, The gas equilibration time is 2-4 hours to ensure that the CO2 concentration in the sampling pipeline and the CO2 concentration in the surrounding soil pores reach dynamic equilibrium; the laser emission frequency of the TDLAS laser emission module is 10-100Hz, the sampling frequency of the optical receiver is matched with the laser emission frequency, and the collected spectral signal contains information on the change of laser intensity with wavelength.
6. The fiber-optic coupled method for measuring average soil CO2 concentration based on TDLAS technology according to claim 1, characterized in that, The specific process for the average concentration inversion is as follows: Step S41, Data preprocessing: Baseline correction and noise suppression are performed on the acquired transmitted spectral signals to eliminate fiber optic transmission loss and ambient light interference. Step S42, Absorption coefficient calculation: Based on the Lambert-Beer law I(λ)=I0(λ)exp(-α(λ)CL), the absorption coefficient α(λ) of CO2 is calculated; where I(λ) is the transmitted light intensity, I0(λ) is the incident light intensity, α(λ) is the absorption coefficient of CO2 at wavelength λ, C is the CO2 concentration, and L is the effective optical path length of the sampling tube; Step S43, Concentration Inversion: Substitute the characteristic absorption line parameters of CO2 into the absorption coefficient α(λ), and obtain the integral concentration of CO2 gas in the sampling pipeline through least squares fitting, thereby obtaining the average CO2 concentration at the target soil depth.
7. The fiber-optic coupled method for measuring average soil CO2 concentration based on TDLAS technology according to claim 1, characterized in that, It also includes temperature and pressure correction steps: Temperature and pressure sensors are installed near the sampling pipeline to collect soil temperature and air pressure data in real time and upload them to the data processing module. The data processing module corrects the average CO2 concentration obtained by inversion to the concentration under standard conditions. The correction formula is: C0=C×(P0T) / (PT0), where C0 is the CO2 concentration under standard conditions, P0 is the standard atmospheric pressure, T0 is the standard temperature, P is the measured air pressure, T is the measured absolute temperature, and P and T are acquired by the temperature and pressure sensors.
8. A measurement system for implementing the fiber-optic coupled soil CO2 average concentration measurement method based on TDLAS technology as described in any one of claims 1 to 7, characterized in that, It includes a TDLAS laser emitting module, a first optical fiber, a sampling pipeline, a second optical fiber, an optical receiver, a data processing module, and a temperature and pressure sensor; The sampling pipeline is horizontally buried at a predetermined depth in the soil, and the pipeline wall is provided with vent holes with gas permeation membranes; the temperature and pressure sensors are arranged near the sampling pipeline. The TDLAS laser emitting module is connected to one end of the sampling pipeline via the first optical fiber, and the optical receiver is connected to the other end of the sampling pipeline via the second optical fiber; the data processing module is electrically connected to the optical receiver and the temperature and pressure sensor respectively.
9. The measurement system according to claim 8, characterized in that, It also includes a sealing and protection component; the sealing and protection component is sleeved on the outside of the sealing joint of the embedded optical fiber coupling interface at both ends of the sampling pipeline to prevent soil moisture from entering the optical fiber coupling interface; the TDLAS laser emitting module, the optical receiver and the data processing module are integrated in the ground control cabinet, and the ground control cabinet has waterproof, dustproof and electromagnetic interference protection functions.
10. The measurement system according to claim 8, characterized in that, The data processing module has built-in spectral analysis and concentration correction algorithms, which can process the spectral signals collected by the optical receiver in real time, output the corrected average soil CO2 concentration, and support data storage and remote transmission.