OPERATION - Bathymetry, the key step
Even though the Walters Shoal seamount was discovered back in 1962 and has been well documented in existing bathymetric databases, available information is still too scarce to properly conduct the comparative research planned during the WALTERS SHOAL cruise, especially the collection of benthic samples.
Prospection within the area has already been done by several research vessels – the first of which was completed by R.V. Marion Dufresne in 1976. These research cruises provided a small number of bathymetric profiles some of which were actually located again during our cruise on board the R.V. Marion Dufresne II. Others were collected by colleagues from South Africa (Department of Environmental Affairs) when they visited the seamount in 2014. These priceless documentations guided our route through the first miles within the area. Nevertheless, the accuracy of these maps has limited use in helping organize and realize dredging and benthic trawling operations during the current two-week cruise over the seamount. Therefore, we must take on the challenge to realize within a short space of time and with the desired accuracy the mapping of the whole Walters Shoal seamount.
The R.V. Marion Dufresne has much going for it to successfully carry out this research cruise. First and foremost, its two multibeam echosounders (EM 122 “long range”, EM170 “medium range”) combined with powerful mapping software will provide in quasi real time “pictures” of the bottom topography taken vertically beneath the hull. The swath area scanned across the ship's track increased proportionally with the bottom depth. At over 600m water depth, the bottom swath is within the order of 900m, while at over 100m it is only a few hundreds of meters wide. There resides all the difficulties of being able to properly scan the top of the seamount which only lies at around 18m deep!
Clearly, a vessel the size of the R.V. Marion Dufresne II cannot wander over the summit of the seamount without knowing ahead of time if any obstacle crossing her route could damage her hull or her gondola located beneath the hull and packed with different types of sensors. Especially knowing from older documents found in the archives of the Marion Dufresne that several summits were found at depth of only a few tens of meters. The mapping can only be realized from successive passages located closer to the centre of the seamount.
This exercise could take many days before attaining a complete and accurate mapping of the area. But this is not the objective of the WALTERS SHOAL cruise. For the time being, shallow and easily accessible zones need to be found for the sampling work undergone by the divers; deeper zones which are not too rugged should be found for the benthic team to deploy their sampling gear safely. And even for the pelagic team working only along the water column, knowledge of the bottom topography is required when deploying nets and trawls.
Let us pledge that by the end of the two weeks spent by the Marion Dufresne within the area, the bathymetry of the seamount will be largely completed, at least as close as possible to its summit. The mapping, so far, already shows an impressive array of bathymetric profiles and leaves us wondering about the tormented geological history of this submerged mountain.
