Development Of CO₂ Gas Detection System For Earthquake Monitoring

Every year,tens of millions of people in China suffer loss of life and property due to natural disasters.Earthquakes account for more than 50%of the losses.In order to ensure the safety of people's life and property,it is an urgent problem to find a method to accurately analyze earthquake precursors.By studying the abnormal changes before earthquakes through gas geochemical measurements,the relationship between them and the occurrence of earthquakes can be effectively analyzed,and prediction opinions can be given.Due to the degassing phenomenon in the core and mantle,the change of in-situ stress in the source area on the eve of the earthquake leads to changes in the crustal faults,providing or blocking the overflow channels of underground fluids,resulting in changes in the concentration of escaped gases in the fault zone.Therefore,the surface concentration of CO₂ near the fault zone before the earthquake will be abnormal,and the high-precision measurement of CO₂ gas near the earthquake zone can provide important help for the analysis of earthquake precursors.In this paper,the basic principle of Tunable Diode Laser Absorption Spectroscopy(TDLAS)is analyzed,and the concentration of CO₂ gas to be measured is measured and calculated by using the characteristics of different types of gases with different absorption degrees of light intensity,that is,the selectivity of gas absorption.In order to accurately measure the trace amount of CO₂ and avoid overlapping with the absorption spectrum of water vapor,the CO₂ absorption spectrum with a wavenumber of 4978.202 cm^-1 was selected in this paper.Theoretical derivation for TDLAS technology and wavelength modulation technology(WMS)is carried out,and the model simulation is carried out through MATLAB.On the basis of theoretical derivation and simulation experimental results,the research and development of the hardware of CO₂ gas detection system is realized,which are:the electrical subsystems including main control module,multi-pass cell temperature,pressure control and other modules;the optical paths and related parts including lasers,multi-pass cells,etc.;the gas path design part.The research and development process and device selection considerations of each part are introduced in detail,the circuit required by the system and the related structure of the optical part are designed,and key parameters in the TDLAS-WMS method are determined.Make the system obtain the ideal second harmonic signal for gas concentration calculation,and obtain higher system accuracy and precision.Aiming at the analysis and theoretical derivation of the existing noise in the system,in order to suppress the detector noise and interference fringes,two main noises that affect the measurement results of the system,the Kalman-Wavelet analysis algorithm is used.Experiments show that the system combined with this algorithm has a certain improvement in measurement accuracy and stability,and method detection limit reaches 5.2 ppmv at the optimal integration time of 61.24 s.In order to verify the field applicability of the instrument,this paper used the CO₂detection system to conduct field use tests at Jilin Fengman Seismic Station on April22 and 23,2021.After analyzing the measured CO₂ results,it can be concluded that the repeatability of the continuous sampling results after each injection is high,the system is in good working condition,and the instrument can complete the field measurement work.