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From the Seabed to the Sea Surface:
Measurements and Implications of Hydrocarbon Seepage in the Santa Barbara Channel
By
Ira Leifer, Jordan Clark, and Bruce Luyendyk
Marine hydrocarbon seeps are a potential source of
atmospheric methane, and hydrocarbon pollution and energy input to the marine
ecosystem. In addition, they are ideal for studying the oil-gas pathway from the
ocean bed to the surface. This is applicable to predicting seabed location from
surface oil slicks and predicting the fate of an underwater oil spill. Bubbles released by the surface
rise to the surface, exchanging gas with the ocean and releasing their contents
to the atmosphere. Furthermore, dissolved seep gas may transfer into the
atmosphere, while some that enters is reduced by microbial action. The fate of
hydrocarbon seep gas depends upon several parameters. Sensitivity studies show
important sensitivities to seep depth, bubble
size-flux distribution, dissolved
gas concentration, and upwelling flows from the rising bubbles. Given a complete
data set, numerical model validation is possible, and such a model can predict
the seep gas fate for seeps spanning a wide range of conditions. Unfortunately,
most of the important parameters and ambient conditions are currently
unavailable in the literature.
Our group has been working to acquire a complete data
set for a single seep area, Shane Seep
located in the Santa Barbara Channel Coal OBI Point seepage field, a few
kilometers from UCB. A bubble
measurement system was designed, tested, and
deployed to measure
bubble
size flux distributions at the seabed and sea
surface. Seepage at the site spans a wide range from extremely active mud
volcanoes to single
bubble
trains. Thus video surveys were conducted at the site
to understand spatial seepage variations. Aqueous alkane concentrations in the
seep flow at the sea bed and surface and in the
bubble
gas were measured ,as was
the upwelling fluid velocities. These measurements were compared with numerical
model calculations.