Monitoring of natural oil seepage in the Lower Congo Basin using SAR observations

Romain Jatiault (1) , Damien Dhont (2) , Lies Loncke (1) , Dominique Dubucq (2)

1 CEFREM - Centre de Formation et de Recherche sur les Environnements Méditérranéens
2 Total Energies

Synthetic Aperture Radar (SAR) is a spaceborne tool allowing near real-time imagery over large ground footprints of areas of hundreds of kilometres wide. SAR also allows persistent observations of the process of oil discharges, benefiting from (1) day and night observations, (2) independence of cloud cover and (3) high revisiting frequencies. The interpretation of 82 SAR scenes over the Lower Congo Basin for the 1994-2012 period allowed the recognition of 1400 natural seepage slicks associated with 102 individual seep sites. We acquired an additional SAR data set with short revisit time over a selected prolific area. The data set consisted in 22 SAR scenes acquired over a period 10 days, meaning a maximum revisiting period of 12 hours between subsequent SAR acquisitions. The short term approach shows that seepage slicks were detected with wind speed from 1.5 and 6.5 m/s (2.91 to 12.63 knots). Both long-term and short-term monitorings evidence that seepage events are intermittent, with the occurrence rate of oil release ranging from 5 to 80%. Short-term monitoring shows that the seepage pace is independent between seep sites suggesting contrasted controlling factors. It also shows that the residence time of seepage slicks is systematically shorter than 12 hours, which is far below reported values. Understanding the seepage slicks residence time is essential to provide an accurate estimation of the fluid flow and to compute the volumetric outflow. The integration of in-situ mooring points measuring the current velocity 10 m below the sea surface, in addition to local wind field and slick length, allowed us to estimate more accurately the residence time of oil slicks at the sea surface before vanishing. The results show that the sea surface residence time of oil slicks is limited to a few hours, with a median value of 3h15min. These new residence time estimations allowed us to propose a quantification of the regional oil output in the Lower Congo Basin, estimated at 4381 m3/year. This area may therefore be considered as the world’s third biggest oil-supplying province from natural leakages.

Mots clés: Synthetic aperture radar; Remote sensing; Satellite; Lower Congo Basin; Oil slick; Natural seepage; Monitoring; Seepage pace; Oil; Residence time; Expelled volumes; Synthetic aperture radar

Domaines: Océanographie; Ingénierie de l'environnement; Traitement du signal et de l'image [eess.SP]

Romain Jatiault, Damien Dhont, Lies Loncke, Dominique Dubucq. Monitoring of natural oil seepage in the Lower Congo Basin using SAR observations. Remote Sensing of Environment, 2017, 191, pp.258 - 272. ⟨10.1016/j.rse.2017.01.031⟩. ⟨hal-04498632⟩