Gas-charged sediments in the Ría de Arousa: Short- to long-term fluctuations?

Repeated surveys at control sites in the gas accumulation fields from the Rı´a de Arousa (Galicia, NW Spain) have shown that the top of the acoustic turbidity fluctuates vertically. The vertical movement of gas fronts is more apparent in the gas fields located in the outer rı´a, where fluctuations of up to 1.5 ms (1.1 m) have been detected. At the inner rı´a, the depth to the top of the gas horizons varied between 0.5 and 1 ms (w40 to 80 cm). Short-term (diurnal) fluctuations associated with the hydrostatic pressure variations of the water column during a tidal cycle have been identified as the most likely factors to explain fluctuations of gas fields in the outer rı´a. At the inner rı´a, gas fields are affected by these short-term pressure changes superimposed on long-term temperature and salinity variations of bottom water masses. This combination produces a different pattern of variation in gas fields from the inner and the outer rı´a. Prominent pockmarks and acoustic plumes have also been monitored throughout the three seismic surveys. Many of these pockmarks exhibit active characteristics, including the observance of plumes of escaping fluids and sediment.