Turbine tent measurements of marine hydrocarbon seeps on subhourly timescales

Abstract

Three turbine seep‐tents simultaneously measured marine seep gas fluxes with high time resolution (0.2 s) at multiple locations. Tents were inverted polyvinyl cones, 2‐m diameter, 1‐m tall, and weighted on their lower skirt edges. Rising gas bubbles induce vertical fluid motions, which were measured by laboratory‐calibrated turbines in chimneys on top of the tents. Initial deployment was at an active seep area in the Coal Oil Point seep field, in the Santa Barbara Channel, California. The three tents simultaneously collected data for continuous time periods of 2 hours in both the morning and afternoon. Seabed temperature and pressure were acquired every 3 s over the same time periods as the flux measurements from a conductivity temperature depth, CTD, mounted on one tent. Results strongly suggest that oceanic swell had a significant forcing effect on the flux at a subhourly timescale. There was an inverse relationship between effect of swell height on the flux and flux. Swells from 1 to 4 m height and periodicities of 7 and 12 s caused variations of ∼1% to 4% from the average flux. Proposed mechanisms to explain the observations are diffusion with surrounding sediments, termed gas charging, swell induced changes in fracture size, termed fracture forcing, and swell induced vent activation/deactivation, termed pore activation. On the basis of the seep frequency response, we propose pore activation was dominant.