An Ultrahigh-Sensitivity and Wide-Range Measurement System of Dissolved CO2 for Deep-Sea Nature Gas Hydrates Exploration

A compact and high-sensitivity sensing system is required for the exploration of natural gas hydrates (NGHs) to measure dissolved CO2 in seawater. In this study, a mid-infrared (MIR) CO2-sensing system was investigated by wavelength modulation spectroscopy (WMS). The system contained an interband cascade laser (ICL) operating in MIR wavelength, an optical multipass gas cell (MPGC), and an MIR mercury cadmium telluride detector. A combination of three absorption lines was utilized to achieve measurement with a wide CO2-concentration range. Additionally, we proposed and introduced in detail a method of optimizing the operating temperature, pressure, and modulation depth. The optimum minimum detection limit (MDL) of 11 ppbv was achieved when the integration time was 145 s. In the northern South China Sea, a deep-sea test was performed, and the test results validated the satisfactory performance of the sensing system for deep-sea NGH exploration.