Development of a laser heterodyne radiometer for regional methane leak detection

Abstract

We have developed and tested a laser heterodyne radiometer (LHR) for detecting methane leaks from upstream oil and gas infrastructure and landfills that uses the Sun as the signal light source, demonstrating here sensitivity sufficient to detect “super-emitter” leaks ( > 50 k g / h , 1166 slm). Tracking optics follow the Sun during its apparent daily transit across the sky, and the system collects direct absorption data and optionally the 1f and 2 f wavelength modulation spectroscopy (WMS) signals. The direct absorption data are processed in real time using a retrieval algorithm with a 5 s update rate to reveal the methane concentration versus altitude for each measurement line of sight. The 1 f and 2 f WMS signals are significantly non-intuitive because of the dramatic change in the methane lineshape as a function of pressure (altitude) but may ultimately provide useful information for leak localization. We describe herein modifications to the RF detection train and data collection system that allow faster and higher signal-to-noise ratio measurements. Preliminary results suggest that leaks giving rise to methane concentrations of the order of 500 ppm-m can be effectively detected—sensitivity similar to current satellites with more continuous temporal coverage and areal coverage of the order of 100s of k m 2 for relatively low cost. We outline a method of using an array of LHRs to localize the leak using lineshape information and tomographic reconstruction techniques that will be tested in future work.

Research on application of local oscillator power locking method to laser heterodyne radiometer

“…傅里叶变换光谱仪 EM27/SUN，测量 4000～11000 cm -1 波段光谱信息以反演大 气温室气体柱浓度，为温室气体浓度的监测提供了一种新型的便捷手段，但是 其成本较高且光谱分辨率较低 [2] 。 自 20 世纪 70 年代以来，激光外差光谱技术因其成本低、体积小、分辨率 高等优点得 到 快 速发 展 ， 已 广泛用 于臭 氧(O 3 )、水汽(H 2 O)、甲烷(CH 4 )、氨 (NH 3 )、一氧化氯(ClO)等大气温室气体柱丰度和垂直廓线的测量 [3][4][5][6][7][8][9][10][11] 。近年来， 伴随着小体积低功耗的分布反馈式半导体激光器(Distributed feedback，DFB)、 光纤器件及集成电路的发展，使近红外激光外差辐射计集成成为了可能。2012 年，E.L.Wilson 团队研制了一种用于大气柱中二氧化碳(CO 2 )的高灵敏测量的低 成本、小型化的激光外差辐射计 [12][13][14] 。最近，俄罗斯 Sergei Zenevich 等人研制 了具有 0.0013 cm -1 超高光谱分辨率的多通道激光外差光谱辐射计，用于测量大 气中 CO 2 和 CH 4 分子吸收，推进了外差辐射计集成的进一步发展 [15] 。国内安徽 光机所王晶晶等人在实验室搭建了一套可实现大气 CO 2 和 CH 4 的透过率谱测量 的全光纤近红外激光外差系统 [16] 。随后，邓昊等人搭建了一套以三个分布反馈 式半导体激光器作为外差探测的本振源的近红外激光外差辐射系统，实现了对 大气二氧化碳(CO 2 )、甲烷(CH 4 )、水汽(H 2 O)和氧气(O 2 )柱浓度的同时测量 [17] 。 最近，安光所曹振松课题组搭建了以 1.316 μm 附近的半导体激光器作为本振源 的无源光纤激光外差辐射计，并对其参数进行优化，并利用该外差辐射计实现 了大气柱中的水汽浓度测量 [18] 。外差辐射计在测量大气中气体浓度时，存在本 振光功率变化导致基线斜率的问题，为获得准确的大气透过率谱，激光器的功 率变化被提出作为参考的基线 [10] ，Weidmann 等人提出对测量数据进行多项式拟 合以确定基线，将多项式拟合系数作为参数代入反演中，从而计算出信号基线…”