High-resolution mapping of shallow gas accumulations and gas seeps in San Simón Bay (Ría de Vigo, NW Spain). Some quantitative data |
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Authors: | Jorge Iglesias Soledad García-Gil |
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Institution: | 1. Departamento de Xeociencias Mari?as e Ordenación do Territorio, Universidade de Vigo, 36200, Vigo, Spain
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Abstract: | San Simón Bay in the innermost part of the Ría de Vigo is characterized by an abundance of very shallow gas accumulations
and methane seeps. During the expeditions of April–June–September 2004 within the Spanish-funded Gs2G project, detailed very
high-resolution seismic and field investigations were carried out to study the shallow gas and the seeps. Direct gas fluxes
also were measured from bubble streams. For the first time, the surface area and gas front depth of a shallow gas field has
been mapped and quantified in the inner bay of Ría de Vigo. This field overlaps spatially with the distribution of Holocene
mud within the bay. Seismic data show 3.6 km2 affected by acoustic turbidity but this surface can be extended up to 9.5 km2 of San Simón’s muddy subtidal area. Mounded turbidity superimposed on the main gas field has been mapped and characterized
as anthropogenically (mussel rafts) mediated gas accumulations. Different acoustic anomalies have been identified and interpreted
as being due to gas escapes from the present seabed sediment. The very high resolution of the seismic data makes it possible
to identify a new type of seep, here named ‘acoustic smoke.’ A direct relationship can be observed between the gas front of
accumulations and escape features, both acoustic seeps and pockmarks. The methane flux has been estimated from the subtidal
environment in San Simón based on detected acoustic targets and direct measurements of current bubble flow. The total estimated
methane flux from the seabed into the water column ranges from 10.1 to 48.8 t/year, and into the atmosphere from 7.0 to 34.2 t/year.
The intertidal San Simón environment is also actively venting methane, as indicated by the presence of bubbling during high
tide and white patches of Beggiatoa sp. |
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