Shallow gas features in incised-valley fills (Rı́a de Vigo, NW Spain): a case study

Areas with gas accumulations and gas seeps, where gas escapes from the seabed to the water column have been mapped in the fill of a submarine incised valley, the Rı́a de Vigo. The various gas features have been classified into four types according to their specific seismic signatures: (1) acoustic blanket, (2) acoustic curtains, (3) acoustic columns, and (4) acoustic turbidity. At the same time, three types of gas escapes features have been distinguished: (1) acoustic plumes, (2) cloudy turbidity, and (3) pockmarks. Calculations indicate mean densities of 1.7 acoustic plumes km−2 and 1.6 pockmarks km−2. Estimations of the gas fluxes towards the atmosphere range from 144.37 to 4134.9 t yr−1. It is concluded that sedimentary facies is the main factor determining whether gas accumulates or seeps, and also determines the specific type of accumulation or seep at each location.