Retrieval techniques for airborne imaging of methane concentrations using high spatial and moderate spectral resolution: application to AVIRIS

Abstract

Two quantitative retrieval techniques were evaluated to estimate methane (CH₄) enhancement in concentrated plumes using high spatial and moderate spectral resolution data from the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). An Iterative Maximum a Posteriori Differential Optical Absorption Spectroscopy (IMAP-DOAS) algorithm performed well for an ocean scene containing natural CH₄ emissions from the Coal Oil Point (COP) seep field near Santa Barbara, California. IMAP-DOAS retrieval precision errors are expected to equal between 0.31 to 0.61 ppm CH₄ over the lowest atmospheric layer (height up to 1.04 km), corresponding to about a 30 to 60 ppm error for a 10 m thick plume. However, IMAP-DOAS results for a terrestrial scene were adveresly influenced by the underlying landcover. A hybrid approach using Singular Value Decomposition (SVD) was particularly effective for terrestrial surfaces because it could better account for spectral variability in surface reflectance. Using this approach, a CH₄ plume was observed immediately downwind of two hydrocarbon storage tanks at the Inglewood Oil Field in Los Angeles, California, with a maximum near surface enhancement of 8.45 ppm above background. At COP, the distinct plume had a maximum enhancement of 2.85 ppm CH₄ above background and was consistent with known seep locations and local wind direction. A sensitivity analysis also indicates CH₄ sensitivity should be more than doubled for the next generation AVIRIS sensor (AVIRISng) due to improved spectral resolution and sampling. AVIRIS-like sensors offer the potential to better constrain emissions on local and regional scales, including sources of increasing concern like industrial point source emissions and fugitive CH₄ from the oil and gas industry

Reduced Methane Emissions from Santa Barbara Marine Seeps

“…In contrast to this study, much larger column enhancements for June 2008 were found via remote sensing and at a distance of about 100 m from the source of approximately 10%-20% total column above background [14]. Taking into account the weaker wind speed of about 2.5 m·s −1 [14] in contrast to 4-5 m·s −1 for this work and that measurements from this campaign were further away from the source where the plume is broader (about 500 m instead of 100 m), equivalent column enhancements to be expected are about 1.0%-2.0%, which is similar to the plume simulations using the emissions from Hornafius et al [4].…”

“…Larger concentrations in 1994-1995 [4] and particularly in 2008 [14] and decreased concentrations in 2014 as found here are in agreement with the trend in ground-based in situ measurements taken at West Campus Station that were additionally analyzed for this study. The seasonal variability with reduced emissions in summer of about a factor of 2-3 relative to the mean can only partly explain the observed low emissions from the airborne experiment.…”

“…Taking into account the weaker wind speed of about 2.5 m·s −1 [14] in contrast to 4-5 m·s −1 for this work and that measurements from this campaign were further away from the source where the plume is broader (about 500 m instead of 100 m), equivalent column enhancements to be expected are about 1.0%-2.0%, which is similar to the plume simulations using the emissions from Hornafius et al [4]. This is significantly more than what is derived from these campaign data, both remote sensing and in situ.…”

“…Hydrocarbon release mechanisms and emissions have been investigated using various methodologies, including sonar [4,6], in situ methane and other hydrocarbon measurements [5,[7][8][9], as well as underwater flow meters and optical bubble counters [6,10,11]. Airborne hyperspectral imaging spectroscopy has been used to map CH 4 plumes from a range of the larger seeps [12][13][14]. Figure 1 displays the main seep area visualized by sonar return measurements from 2005 [6].…”

Airborne methane remote measurements reveal heavy-tail flux distribution in Four Corners region

“…Recent studies have shown that both can retrieve methane quantitatively using methane absorption features in the shortwave infrared around 2.3 μm [AVIRIS-NG (7,8)] and in the thermal infrared around 7.65 μm [HyTES (ref. 9 or refs.…”