Text: Frances Cooke
This is my first cruise aboard the Kronprins Haakon. It is a smooth sailing vessel, even while we are ice breaking. It is an extremely comfortable environment to be working in, you could say a little too luxurious for a scientific expedition! I am here to work with the acoustics. This is sound sent out from the vessel as a “ping” or a “chirp” and echoed back. The acoustics provide important geological, geophysical and biological information such as sediment thickness, presence of gas, and shallow buried features. This is necessary for our coring operations. While the coring teams are braving the freezing cold temperatures on deck, waiting eagerly for the cores to come onboard, the acousticians are sitting back, with a cup of coffee, watching the data scroll across the screen from the warmth of the operations room!
We already have some impressive imagery from our multibeam EM302 system (Figure 1). We completed the multibeam surveying last night over an area known for gas seeps. While “mowing the lawn” we actively watched for seeps in the water column, so that we could go back after the survey, and take water samples. I was off-shift sleeping and missed the show. There will be plenty of other opportunities this cruise. After acquiring multibeam data we first have to remove any unwanted pings (a process hydrographic surveyors refer to as “killing dots”) before creating a bathymetric map which is the topography of the seafloor. A seafloor etched by iceberg plough marks (Figure 1), large depressions formed by slow episodic release of gases, or mounds bulging out from the seafloor, formed by sudden upward flow of sediment, are all extremely pleasing on the eye and fascinating to study!
Geophysics is about visualizing what we cannot see. From tiny bubbles in the water column, to extremely large (metre to kilometre scale) features in the sub-surface. To be able to collect this data so quickly and so seamlessly, is extremely satisfying.
The recent advances in technology allow us to run large swaths of bathymetry (typically 4.5 x the water depth); where previously, using single beam echo-sounders, survey time would have been days rather than hours. We also no longer throw dynamite off the back of the ship to create sound for our seismic data collection!
What I also enjoy about geophysics is combining multiple datasets. We are currently profiling beneath the sediment surface using our sub-bottom profiler SBP300. This is a very high resolution seismic system. In a previous cruise, conventional multi-channel 2D seismic (MCS) data were collected across the same location on a previous cruise. We are able to see much more detail using the SBP300, which makes a large difference for the purpose of coring on this cruise. The advantage of using both high resolution shallow seismic and MCS is the ability to work at different depths and geological time.
After completion of the multi-beam survey at Superstation 3, the plan was to deploy both the multi-corer and gravity core, however the multi-corer returned just 12cm of sediment, which was expected, after poor sound penetration, observed on the SBP300. It was decided not to deploy the gravity core at this site – we don’t want any “bananas” (coring instrument bent by impenetrable seafloor). We are now on our way to a more core friendly location with preferably thick overlying sediment for our next Calypso core deployment. Wish us luck!