Variability of a natural hydrocarbon seep and its connection to the ocean surface

Abstract

Natural hydrocarbon seeps are ubiquitous along continental margins. Despite their significance, we lack a basic understanding of the long-term temporal variability of seep dynamics, including bubble size, rise velocity, composition, and upwelling and entrainment processes. The shortcoming makes it difficult to constrain the global estimates of oil and gas entering the marine environment. Here we report on a multi-method approach based on optical, acoustic, satellite remote sensing, and simulations, to connect the characteristics of a hydrocarbon seep in the Gulf of Mexico to its footprint on the sea surface. Using an in-situ camera, bubble dynamics at the source were measured every 6 h over 153 days and the integrated total hydrocarbon release volume was estimated as 53 m 3. the vertical velocity was acoustically measured at 20 m above bed (mab) and found to be approximately 40% less than the dispersed-phase at the source, indicating that the measured values are reflecting the plume continuous-phase flow. Numerical simulations predict that the oily bubbles with diameters larger than 8 mm reach the surface with a small footprint, i.e. forming an oil slick origin, deflection of which with wind and surface current leads to the formation of an oil slick on the surface. Nineteen SAR images are used to estimate the oil seepage rate from GC600 for 2017 giving an average discharge of 14.4 cm 3 /s. Natural hydrocarbon seeps have been reported from a broad range of oceanographic settings from the coast to the deep ocean over a wide variety of geological environments that affect the biosphere, the hydrosphere, and the atmosphere 1. Beside the anthropogenic sources, hydrocarbon seeps are the single most important natural source of oil and methane (CH 4) that enter the ocean 2. Best estimates indicate that 3 to 48 Tg/y of geo-CH 4 is released from marine seeps alone 3,4. Recent global estimates of crude oil seepage range from 0.2 to 2 Mt with a best estimate of 0.6 Mt that accounts for 47% of the global estimate. The large uncertainties in estimates of hydrocarbons released from offshore marine sources are due to a wide range of factors including size, rise velocity, contamination by surfactants such as gas hydrates, composition, upwelling and entrainment effects, and interaction of gaseous bubbles and oil droplets with the local ocean environment that vary constantly with time. To quantify the bubble/droplet size and rise velocity and the corresponding emission rate of hydrocarbons from the seafloor, investigators have primarily relied on snapshot acoustic 5-9 or optical 10-13 measurements. Based on these measurements, significant changes in hydrocarbon seepage and bubble venting have been inferred to occur over intervals of months to years 14. Sufficiently long time series of seepage characteristics rarely exist to conclusively address the day-today variability of bubble size and rise velocity or the spatial changes in vent location over an extended period. Time variation in seep emissions is of particular interest. It impl...

Volumetric Mapping of Methane Concentrations at the Bush Hill Hydrocarbon Seep, Gulf of Mexico

“…These flux variations can include times when no appreciable methane bubbles are released at all. The release point on the seafloor can also shift location on time scales of days (Razaz et al, 2020).…”

Monitoring oil spill in Norilsk, Russia using satellite data

“…Box: 2713, Doha, Qatar. 2 Department of Climate Change, State Hydrological Institute, St. Petersburg, Russia. 3 Department of Applied Geology, National Institute of Technology, Raipur, India.…”

“…The oil spill detection and tracking through satellite sensors have remarkable advances in utilizing the visible, shortwave to thermal infrared (optical) and the microwave radar bands. Surface identification and mapping of an oil spill are essential to evaluate the potential spread and float from the source to the adjacent areas or endpoints 1,2 . The ability to characterize the interactions of incident oil spills is critical for detecting the spill in both the optical and radar bands.…”

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Technological Developments Since the Deepwater Horizon Oil Spill

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