Temporal Variations of a Natural Hydrocarbon Seep Using a Deep-Sea Camera System

Abstract

Two video time-lapse cameras (VTLCs) were deployed by a remotely operated underwater vehicle (ROV) to observe the temporal and spatial variability of a natural hydrocarbon seep at 1180 m depth in the Green Canyon 600 lease block, Gulf of Mexico. The VTLCs were positioned approximately 60 and 90 cm away from the vent, each recording 15 s video bursts at 30 frames per second, illuminated by a 2000 lumen (lm) LED lamp. One camera functioned for 2 weeks; the second camera recorded 568 video bursts at 6 h intervals from 3 September 2017 to 2 February 2018 (153 days). Over the campaign period, seepage from three vents along a 10 cm cluster shifted toward a new fault line with up to nine intermittent individual vents shifting along 20 cm. We developed a semisupervised algorithm using Mathematica and ImageJ routines to resolve the rise velocity and size of individual bubbles. The algorithm was applied to the last 30 frames of each video burst. Bubble characteristics were also analyzed in the videos recorded by the ROV camera. Processing VTLC records yielded a bubble size distribution comparable (5% deviation) to the ROV camera, while the rise velocities were found to be 12% smaller than the ROV data. Hydrocarbon flux estimated from VTLC data was also compared favorably (2% difference) with synoptic physical collections of hydrocarbons into an ROV-held funnel. The long-term measurements indicate that bubble rise velocity was weakly correlated to the discharge rate as well as to the cross-flow velocity.