UC Santa Barbara

Two large steel tents (each 30 m by 30 m), open at the bottom to the seafloor, capture ∼16,800 m3 d-1 (594 MCF) of primarily methane from a large natural hydrocarbon seep, occurring a kilometer offshore in 67 m of water. The gas is piped to shore where it is metered and processed. The seep flow rate was monitored hourly for 9 months. Our results show that the tidal forcing causes the flow rate to vary by 4-7% around the mean. These results are the first quantitative documentation of the effect of tides on natural gas seepage in relatively deep water. Time series analyses of the 9 month record clearly show four principal tidal components with periods of 12.0, 12.4, 23.9, and 25.8 hours. High tide correlates with reduced flow, and low tide correlates with increased flow. The correlation indicates that each meter increase of sea height results in a decrease of 10-15 m3 hr-1 or 1.5-2.2% of the hourly flow rate. The observed changes are best accounted for by a pore activation model, whereby gas is released from small pores at low pressures but is inhibited at higher pressure. Pressure-dependent gas solubility changes are a less likely cause of flow variation. Our study implies that sea level differences, on a tidal timescale, can significantly change the gas seepage rate from sediments. Lower sea level in the last hundred thousand years would presumably allow higher gas loss from the sediment, assuming sufficient gas present, because of reduced hydrostatic pressure at the sediment-sea interface. The magnitude of this long-term change cannot be extrapolated from our tidal data. Copyright 2001 by the American Geophysical Union.