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 (>50kg/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 2f 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 1f and 2f 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 km2 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.
ABSTRACT
We have developed a novel laser heterodyne radiometer using a fiber-coupled distributed feedback laser as the local oscillator to perform spectroscopic measurements of small molecules in the near-infrared (NIR) spectral region. Here, we demonstrate measurement of HCN and CO2 in the lab and CH4 and CO2 in the atmospheric column. In addition, we demonstrate detection of a neutral iron, Fe(I), Fraunhofer line in the spectrum of the sun, at a vacuum wavelength of 1559.252 nm, that can be used to calibrate the wavelength scale of the instrument and enable verification of proper system operation for field applications.
