Simultaneous standoff sensing for methane and hydrogen sulfide using wavelength-modulated laser absorption spectroscopy with non-cooperative target

Abstract

Standoff detection of flammable and toxic gases simultaneously is important for leakage warning and safe production in oil and gas industries. A portable and robust single-ended laser sensor is developed to retrieve path-averaged gas concentrations of methane (CH4) and hydrogen sulfide (H2S) by collecting the backscattered light from a distant non-cooperative target. Time-division multiplexing of two semiconductor lasers with pigtails are employed to cover the absorption lines of CH4 and H2S at 1653.73 nm and 1578.13 nm, respectively. Wavelength modulation spectroscopy with 1 f normalized 2 f detection (WMS-2f/1 f) method is used for gas measurement to eliminate the change of return light intensity caused by different distances and different non cooperative targets. Kalman filter is developed as a post-processing method to reduce the output fluctuations during standoff gas measurement. The performance of the sensor is assessed, including signal noise ratio under different standoff distances and hard targets, measurement precision, time-response and the detection limit. The reported sensor allows for real-time non-contact standoff detection of CH4 and H2S, with great potential for its application in oil and gas industries.