Resonant photoacoustic sensors enhanced by differential measurement and multi-pass gas cells for trace methane detection

This paper explores the reduction of background noise in a photoacoustic spectroscopy (PAS) system designed for trace methane measurements, specifically aimed at monitoring Undocumented orphaned wells (UOW). To reduce ambient noise and achieve robust field measurements, the resonant PAS sensor employs dual microphones for differential acoustic signal detection, significantly enhancing signal sensitivity and effectively suppressing flow-induced noise. The light source of the system is designed to achieve multiple reflections using two mirrors, thereby amplifying the signal tenfold after twelve reflections. Experimental results confirm the exceptional performance of the proposed system, which achieves a minimum detectable limit of 40 ppb with an averaging time of 100 seconds. The methane leak measurements were successfully carried out on-site of an abandoned natural gas well in Hillman State Park in the western Pennsylvania.