In-situ Spectroscopic Gas Analyzer For Dissolved CH₄,CO₂ And ¹³CH₄ In Water

Methane(CH4)and carbon dioxide(CO2)gas dissolved in water are widely involved in the substance exchange in nature.The concentration distribution of these gases often contains the distribution information of energy minerals and organic matter in the nearby environment.Dissolved CH4 and CO2 has a close relationship with the growth of animals and plants in the water environment,and they also have a direct connection with the gas environment through the process of water-gas exchange.Therefore,the measurement of dissolved gas concentration in water is an important method for research on atmospheric science,geological science and ecological science.Off-axis integrated cavity output spectroscopy has the advantages of fast response time,high sensitivity and small volume.It is the frontier development direction of current in-situ detection technology of dissolved gas in water.The thesis focuses on the in-situ dissolved CH4,CO2 and δ13CH4 detection technology based on off-axis integrated cavity output spectroscopy,and the development of a high-precision in-situ detection system for dissolved gases that can work continuously in the high pressure and low temperature environment under the deep sea.The thesis introduces the principles of spectroscopy and signal processing techniques applied in the system.Based on these techniques,a set of system design schemes are proposed.In these schemes,a high-definition optical resonant cavity with a mirror reflectivity of up to 99.999%is used,and the effective optical path length of the cavity is about 20 km.Under the control of the temperature and pressure control unit in the system,the temperature and pressure of the gas in the optical cavity can be kept stable,so that the line shape of the spectrum does not change during the measurement.The self-developed spectroscopic measurement unit can measure the absorption lines of multiple different gases at different wavelength,with using an optical switch controlled by an embedded computer software,so as to automatically complete the concentration measurements of multiple gases.The spectral measurement unit has the characteristics of low power consumption and small volume.The unit can also fine-tune the laser modulation current waveform,which realizes the correction of the position shift of the absorption line during the measurement process,and reduces the error in absorption measurement due to the position shift.The current noise of the laser drive current source is less than 10 nA,and the current source current noise density at 1 Hz is about 10-9 A·Hz-1/2.The low noise has a minor effect on the laser output laser wavelength and power.After the spectral signal is collected,the embedded computer uses clipping,filtering,fitting and other methods to process the signal.Using the Trust Region Reflection algorithm,each parameter of the spectral signal fitting is limited to a reasonable range,which speeds up the fitting process and reduces the possibility of fitting failure.The Kalman filtering method applied to the measured results of gas concentration further improves the measurement accuracy.After the system is established,calibration and experimental verification are essential.Sample gases of different concentrations are input to the system for calibration.The measurement results show that the area of the absorption line measured by the system has a nonlinear relationship with the gas concentration,while the absorption coefficient at the center of the line has a linear relationship with the gas concentration.After calculation,the detection limits(1σ)of methane and carbon dioxide are 0.56 ppbv and 0.62 ppmv.The δ13CH4 measurement result shows that the output of the system has a fluctuation of ±2‰,and after Kalman filtering,the fluctuation is reduced to ±0.5‰.Using the developed in-situ detection system,an field test experiment was conducted in the Shenhu area of South China Sea.At a depth of 500 m to 700 m,the system measured strong fluctuations in the concentration of dissolved methane and carbon dioxide in the water.According to the analysis of the local geology and mineral conditions,this phenomenon is due to the abundant mineral resources in the sea area.The gas released by the cold springs and gas hydrate will cause the abnormal concentration of dissolved gas in the seawater,which is detected by the system.The results of this field experiment show that the system can be used to detect dissolved methane and carbon dioxide in the deep sea under harsh conditions.Thus,this research provides an effective way for further exploration of deep-sea gas hydrate.