In Situ High-Precision Measurement of Deep-Sea Dissolved Methane by Quartz-Enhanced Photoacoustic and Light-Induced Thermoelastic Spectroscopy
Hao Liu, Xiang Chen, Mai Hu, Haoran Wang, Lu Yao, Zhenyu Xu, Guosheng Ma, Qiang Wang, Ruifeng Kan
Abstract
Rapid and accurate realization of in situ analysis of deep-sea dissolved gases imperative to the study of ecological geology, oil and gas resource exploration, and global climate change. Herein, we report for the first time the deep-sea dissolved methane (CH 4 ) in situ sensor based on quartz-enhanced photoacoustic and light-induced thermoelastic spectroscopy. The developed sensor system has a volume of φ120 mm × 430 mm and a power consumption of 7.6 W. The sensor, in the manner of frequency division multiplexing, is able to simultaneously measure the photoacoustic signals and light-induced thermoelastic signals, which can accurately correct laser-intensity induced influence on concentration. The spectral response of CH 4 concentration varying from 0.01 to 5% is calibrated in detail based on the pressure and temperature in the application environment. The trend of the photoacoustic signal of CH 4 at different water molecule (H 2 O) concentrations is investigated. An Allan variance analysis of several hours demonstrates a minimum detection limit of 0.21 ppm for the CH 4 spectrometer. The sensor combined with the gas–liquid separation and enrichment unit is integrated into a compact marine standalone system. Since the specifically designed photoacoustic cell has a volume of only 1.2 mL, the time response for dissolved CH 4 detection is reduced to 4 min. Furthermore, the sensor is successfully deployed in the vicinity of the “HaiMa” cold seeps at 1380 m underwater in the South China Sea, completing three consecutive days of measurements of dissolved CH 4 .