Mapping of Methane Emissions from Naturally Occurring Marine Seeps using Imaging Spectrometry
Dar
Alexander
Roberts, Dept. of Geography, UC Santa Barbara, dar@geog.ucsb.edu
(Presenting)
Eliza
S
Bradley, Dept. of Geography, UC Santa Barbara, eliza.bradley@gmail.com
Leifer
Ira, Marine Science Institution, UC Santa Barbara, ira.leifer@bubbleology.com
Cheung
Ross, Marine Science Institution, UC Santa Barbara, publius314@gmail.com
Dennison
E
Philip, Dept. of Geography, University of Utah, dennison@geog.utah.edu
Dylan
Parenti, Dept. of Geography, UC Santa Barbara, geographos@juno.com
Methane is an extremely important green house gas that has increased significantly in pre- and post-industrial times. Strong absorptions in the Short Wave Infrared offer the potential for using imaging spectrometers, such as AVIRIS, to map methane emissions from strong sources such as marine seeps. Prior work suggests that a sensor such as AVIRIS should be able to map methane within the oceanic boundary layer at column amounts lower than 0.1g/m2 above background. Strong methane absorption regions between 2200 - 2350 nm are particularly important because of minimal interference from water vapor. To evaluate the potential of mapping methane using imaging spectrometry, we analyzed AVIRIS data acquired over the Coal Oil Point marine seep field off of the Santa Barbara coast in August 2007. These methane seeps are some of the largest in the world and represent an excellent test-bed for developing and testing remotely sensed methods for mapping methane.
Initial AVIRIS analysis focused on flight lines acquired on August 6th, 2007. A new analysis approach was developed, using the MODTRAN radiative transfer program to model radiance for background methane levels, then using spectral residuals to map methane anomalies over the seep field. A multistage approach was employed, first modeling radiance for a fixed surface albedo and water vapor amount for scene specific location and time of day. Next, reflected radiance at 2138 nm was used to estimate surface albedo at 1% increments, then used to generate albedo-specific look up tables for background methane. Modeled and measured radiance were used to map albedo-specific methane anomalies over the seep fields, calculating a single measure of methane strength as the sum of the residual between 2198 and 2350 nm. Using this approach, methane anomalies were mapped in close proximity to known sources, with plume directions consistent with in-situ measures of wind direction. Significantly, methane anomalies could be mapped with surface albedos as low as 1%.
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