Ninth international conference on - Marum

Abstracts of posters 85
area were fit to a Gaussian plume. This created a 3D model of the plume from which methane column
abundances greater than 0.5 g/m 2 Were estimated for an area covering ~1000 m 2 . Radiative-transfer simulations
with these column abundances indicated AVIRIS could map methane within the seep-field oceanic boundary
layer.
Fig. 1 A. Contour map of atmospheric methane plume from an intense area of seepage. B. Methane column
abundances from Gaussian plume model fit to data.
AVIRIS data were collected over the COP seep field in August 2008, and methane anomalies were successfully
mapped. To correct for albedo variations, the 2138 nm band (largely insensitive to H2O, CO2, and CH4
absorption) was compared between AVIRIS data and a model atmosphere calculation for the appropriate solar
and view geometry (MODTRAN 4.3, surface albedo 10%). Residuals were calculated from the AVIRIS data and
the best fit Modtran simulation from an albedo-specific lookup table. Signal to noise was improved by
integrating the residuals over several bands sensitive to methane to derive a single metric. Distinct methane
plumes were identified for the largest intense, mega seeps (10 6 cm 3 /day from direct flux buoy measurements)
down to seeps many orders of magnitude weaker. There was good spatial agreement between the location of
atmospheric plumes and sonar-mapped bubble plumes.
Fig. 2 Color density -slice image of the integral of residuals between 2200 and 2350 nm for the albedo-specific
model (for all pixels with albedos greater than 5%). Residual spectra are shown for several areas that
demonstrated strong methane signatures and several areas that did not. Contours shown are for sonar-mapped
bubble plume data.
Preliminary surveys used Foxboro, OVA-88 FIDs and measured THC in the air above the seeps and assumed all
THC was methane. These instruments also had a slow response time. These limitations were overcome for
ground validation of aerial surveys with a four-channel FID gas chromatograph configured as a dual mudlogger
and modified for operation on small (7-m) boats. The system provides periodic speciation of the THC. Surveys
showed that atmospheric plumes in the seep field can be locally enhanced in some areas with higher
hydrocarbons – namely ethane and propane. Plumes contained butane and pentane as well.