Development And Application Of Mid-infrared In-situ System For Carbon Dioxide Detection Near Seabed

The subject of this thesis originates from the Chinese National Key Research and Development Program named:High-precision detecting technology for hydrates exploration in the offshore seabed(No.2016YFC0303900).Dissolved carbon dioxide(CO₂)in the ocean is one of the main carriers of the earth's carbon cycle,which is closely related to the global climate and environment condition due to its persistent exchange with the atmosphere.Besides,the distribution information of CO₂ content and carbon isotopic characteristics in seawater has important guiding significance for research of marine biological and chemical processes.It can promote the development of marine ecological environment monitoring,seabed sedimentary resource exploration,and other scientific fields.Due to the progress of marine exploration in recent years,in-situ quantitative detection of dissolved gases in seawater based on geochemical methods has become a key breakthrough direction in marine science.Corresponding detection techniques require characteristics including high-precision,multi-parameter analysis,fast response,and long-term continuous measurement.The application scenario tends to be the deep-water area near seabed.Tunable diode laser absorption spectroscopy(TDLAS)technique is a well-developed trace gas sensing method in nowadays.Compared with the existing in-situ dissolved gas detecting technology,TDLAS has the advantages of simple system structure,good molecular selectivity,fast response,and so on.Moreover,molecular absorption lines in the mid-infrared waveband can make the detection obtain a high precision of ppbv(parts per billion in volume)level.Combined with highly efficient gas-liquid separation device,instrument based on TDLAS technique has great potential in marine application for in-situ gas detection.In this paper,a mid-infrared laser wavelength modulation spectroscopy(WMS)based sensor system is developed,which is assisted by polymer degassing film for high-precision in-situ measurement of dissolved CO₂ content and carbon isotope abundance(?¹³CO₂)in deep sea.The research work is of great significance in the field of marine geochemical exploration.In view of the key problems existing in the deployment and application of the instrument in deep sea,important technical research and optimization of the sensor system are carried out in detail.Based on a group of CO₂ isotopic absorption lines near4319 nm,an interband cascade laser(ICL)is used as the laser source.Combined with a multi-pass cell(MPC),a compact free-space optical path structure with a minimized size is designed,and solutions for the background absorption of high-concentration CO₂in the atmosphere are proposed.That makes the integration of the system can be miniaturized,and the detection is immune to interference of the external gaseous environment.A dissolved gas sampling and analyzing system based on a membrane degassing device is developed,which has the ability of high-precision real-time controlling of gas ambient temperature and pressure.Facing different gas source conditions,a low-pressure controlling mechanism for the gas cell with two operating modes is proposed to improve the application ability of the system towards different environments.The target pressure is set at 40 Torr to perform independent extractions of different molecular spectral lines.The main controller circuit of the system with miniaturization and low power consumption is developed based on a digital signal processor(DSP)chip.Employing it as the controlling core,a line division multiplexing spectral signal processing scheme for wide range CO₂ concentration measurement and simultaneous analysis of?¹³CO₂ is proposed,and the automatic workflow program of the system under water is designed.A data monitoring and instrument controlling Lab VIEW platform acting as the upper computer is developed.The RS-485 remote communication protocol between the instrument and the upper computer is established.For deep-sea practical application,two generations of sensor prototypes are successively designed and integrated.The second-generation prototype for marine trial reached the maximum utilization of the internal space through precise mechanical design.It has an overall size of 836×175×150 mm³.Based on the line division multiplexing scheme,the accurate CO₂ concentration measuring range of the developed sensor is 0-500 ppmv(parts per million in volume).And the detection limit reaches 0.72 ppbv under an original data output interval of 2 s,which is almost the highest spectrum detecting precision of TDLAS technique.In the CO₂ concentration range of 50-400 ppmv,?¹³CO₂ analysis can be carried out with relatively high accuracy.For different sample concentration levels,the detection sensitivity of?¹³CO₂ differs.The best?¹³CO₂ measuring sensitivity of the instrument is characterized to be 0.769‰at an averaging time of 50 s via experimental assessment in detail.Due to the development and optimization of the gas sampling and analyzing system,response time of the dynamic gas analysis mode and the static gas analysis mode with pulse injection are both less than 1min,which are 30 s and 47.5 s respectively.The static gas analysis mode not only makes the sensor operate normally in the case of insufficient gas source,but also further expands the measurement range of CO₂concentration,which can also be adaptively adjusted by the self-developed software program.The integrated sensor prototype is deployed in the Shenhu area of the South China Sea through the towing of scientific research vessel,mainly for the exploration of submarine natural gas hydrate mineral resources.The deployment is carried out in deep-sea environment with a depth of 2000 m.Dissolved CO₂ concentration and?¹³CO₂value are measured in real-time during the diving process of the sensor.In the whole process of the test,the sensor performs well and uploads real-time measurement data,which marks the first successful application of mid-infrared TDLAS gas sensing technology in deep-sea in-situ gas detection.Compared with the existing foreign commercial instruments in corresponding research field,the developed CO₂ sensor is on the advanced level in several main performance indicators.It shows that the instrument has the ability of dissolved gas in-situ detection in marine with high-precision,fast response,and multi-parameter analysis characteristics,which exhibits an extraordinary prospect of this research.The innovations of this thesis are as follows:1.In order to reduce the volume of the instrument and realize the application in deep-sea environment,a multi degree of freedom adjusting platform for ICL is developed.A compact linear optical structure for mid-infrared free-space light is designed,which simplifies the optical system to the greatest extent and improves the mechanical stability.The miniaturized integration of the instrument is realized.2.To avoid the interference of high-concentration CO₂ in the atmosphere on the detection of low-concentration CO₂ samples in the low-pressure gas cell,the background harmonic component in the second harmonic(2f)spectral signal is extracted and deducted by a polynomial fitting method.The scheme for wavelength modulation depth optimization under the atmospheric background absorption condition is proposed,which increases the detection precision.3.Aiming at the problem of insufficient gas sample caused by different gas-liquid separation efficiency and dissolved gas volume in deep-sea environment,a dual-mode low-pressure controlling mechanism for the gas cell is proposed.Based on the traditional PID(Proportional-Integral-Derivative)dynamic pressure controlling mode,a new scheme with pulsed sampling and static gas analyzing characteristic is designed.It can adapt to the application scenario of insufficient separated gas in deep-sea environment.And the measuring range of CO₂ concentration is increased via introducing carrier gas.4.A sensor for in-situ detection of dissolved gas in deep sea is developed,which obtains a low CO₂ detection limit of ppbv level and a?¹³CO₂ analysis precision of less than 1‰.Compared with the existing international instruments for detecting dissolved CO₂ in seawater,these performance indicators are at the advanced level.An experiment is carried out in the Shenhu area of the South China Sea with a depth of 2000 m using the developed instrument.This is the first time that mid-infrared TDLAS gas sensor is applied to deep-sea in-situ gas detection.