In situ sensing of methane emissions from natural marine hydrocarbon seeps: A potential remote sensing technology

The sources and sinks of methane, an important greenhouse gas, are poorly constrained. Remote sensing techniques can significantly improve our understanding of the global methane budget. Field and laboratory studies using in situ direct and spectral measurements of methane emissions from natural marine seepage allowed assessment of the feasibility of remote sensing. Methane plume characteristics were estimated by repeated transects of an intense marine seep area with a flame ion detector (FID) and then fit with a Gaussian plume to allow calculation of methane column abundances. These showed values greater than 0.5 g m −2 to a downwind distance of 70 m. A numerical radiative-transfer calculation showed that in the spectral region between 2200 and 2340 nm, which is sensitive to methane, there is mild sensitivity to water vapor interference, and that methane levels were well above the noise equivalent delta radiance of the Airborne Visible Infrared Imaging Spectrometer (AVIRIS). During a separate field study, FIDs recorded methane concentrations at 2.2, 3.6, and 5-m height while transecting an active seep area and concurrently collecting spectra using a field spectrometer. Several plumes were identified and a peak concentration of 200 ppm was measured. The presence of methane plumes along the incident path, as determined from the FID data, was related to the presence of methane absorption features in spectra above atmospheric background, which were absent outside the plumes.