Comparison of Methods to Determine Gaseous Emissions to the Atmosphere Above Natural Marine Hydrocarbon Seeps

EGLAND, ERIC T., Department of Geological Sciences, University of California, Santa Barbara; JORDAN F. CLARK., Department of Geological Sciences, University of California, Santa Barbara; LIBE WASHBURN, Department of Geography, University of California, Santa Barbara; DEREK C. QUIGLEY, Geko-Prackla, Houston, TX; J. SCOTT HORNAFIUS, Mobil Oil Company, Clements Inn, London, United Kingdom; BRUCE P. LUYENDYK, Department of Geological Sciences, University of California, Santa Barbara; R. DAN FRANCIS, Department of Geological Sciences, California State University Long Beach

Emissions to the atmosphere from natural marine hydrocarbon seeps offshore of Coal Oil Point, Santa Barbara Channel, California have been shown to be a significant source of reactive organic gasses which contribute to the formation of tropospheric ozone. Three methods were employed to estimate emission rates: sonar surveys, atmospheric tracer experiments and automated direct capture experiments. Experiments were conducted over an area of the La Goleta seeps along the South Ellwood anticline approximately 3 kilometers offshore from Coal Oil Point.  An August 1996 calibrated 3.5 kHz sonar survey was used to estimate gas seepage emissions of 1.7 x 10^4 m^3(gas)/day.  Atmospheric tracer experiments sampled the SF6 and source gas mixture downwind from the source area.  Analysis of the methane component of the seep gas along with SF6 concentrations yielded a CH4/SF6 mixing ratio from which a seep gas emission estimate was calculated from the known release rate of the tracer gas. A buoy equipped with a gas capture device directly measured gas flux at the sea surface.  Gas flux estimates from the ongoing sonar surveys and preliminary results from the atmospheric tracer and direct capture methods show reasonable agreement in the estimation of the offshore gas emission rates to the atmosphere.

  Egland, Eric T. et al. "Comparison of methods to determine gaseous emissions to the atmosphere above natural marine hydrocarbon seeps". 1999. (1999 AAPG PACIFIC SECTION MEETING; ABSTRACTS; AAPG Bulletin; Vol. 83, no. 4; p. 686)