Offshore emission data are available mainly from the US (offshore of California and the Gulf of Mexico), the North Sea, the Black Sea, Spain, Denmark, Taiwan and Japan. However, in many cases the data only refer to gas outputs from the seafloor to the water column, and not to the fraction entering the atmosphere. Submarine seeps can release 103-106 tonnes yr-1 of gas over a 105km2 area (Judd et al, 1997). The flux of individual seepage or groups of bubble streams may reach several tonnes per year (Hornafius et al, 1999; Judd, 2004). One of the main problems in extrapolating regionally and globally is the uncertainty associated with the actual area of active seepage.
Submarine gas fluxes are generally estimated on the basis of geophysical images (echo-sounders, seismic, sub-bottom profilers and side-scan sonar records) and bubble parameterization (the size of the bubble plume and single bubbles), sometimes associated with geochemical seawater analysis (for example Judd et al, 1997). Recent studies have proposed using remote sensing techniques based on airborne visible/infrared imaging spectrometry (Leifer et al, 2006).
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