Text and photos: Mauro Pau, Postdoc on Hopendjupet CAGE21-4 cruise
August 6-7th, 2021.
Yesterday’s multibeam echosounder survey reveals that the seepage sites are not regularly distributed across the area, suggesting a strong geological control. Existing seismic lines show that an anticline structure may facilitate the escape of hydrocarbons from their reservoir. In order to get a more detailed picture and visualise the shallower subsurface structures, we navigate along those same lines with our chirp sub-bottom profiler switched on.
Recovering the CTD rosette.
This new data further allows us to identify sites with a sufficiently thick sediment cover to collect gravity cores. We select a number of sampling stations, both with evident seepage and virtually seepage-free as a control. Where the sediment cover is poor, we opt for box coring, which still provides plenty of material to be analysed. The analyses will include composition of the headspace gas, or interstitial gas in the sediments, as well as the pore water.
Ship’s engineer Stormer Jensen (left), CAGE researcher Henry Patton, CAGE’s new PhD student Frank Jakobsen, and IG master’s student Abidemi Akinselure await for a gravity core to be brought on deck.
At each station we also perform CTD casts to measure water mass properties and sample the water column. This data will contribute to tracking down the hydrocarbon source. One further task we aim to accomplish is to sample the oil on the sea surface, so we head towards the slicks observed in the latest satellite images. Our colleague from NPD has the right tools for the purpose, a special membrane to scoop the sea surface and a stick.
André Jensen (NPD) handles over a water sample from the CTD rosette.
Cruise leader Monica Winsborrow (left) discussing with Stormer and Henry.
Finally, before leaving the area and moving further north, we survey an additional 130 square kilometers of seabed with the multibeam echosounder, imaging several more seepage sites.
Henry Patton (left) and our NPD’s colleague André Jensen transfer on board the dinghy boat to attempt sampling an oil slick on the sea surface.