The French Navy’s Hydrographic and Oceanographic Service (SHOM) has just returned from Antarctica. After 20 years’ absence from the region, it carried out its second consecutive survey of the seabed in uncharted areas around Dumont d’Urville.
With a pole plunged into the icy waters of Terre Adélie, aboard a 7-metre vessel surrounded by ice, a detachment from the French Navy’s Hydrographic and Oceanographic Service (SHOM) ventured into uncharted areas from January 3rd to 15th to push back the limits of polar navigation. Now they’re back to share the experience. The SHOM hydrographers were able to carry out their mission using the Astrolabe, equipped by the French Navy and chartered by the French Polar Institute for a logistical mission in Antarctica.
Not an easy task, given the tight schedule with the multitude of scientific missions and the supply of the Dumont-d’Urville station (DDU) and indirectly that of Concordia. Space on board is limited. “You have to be very humble when you’re in charge of passengers and you see the possibility of a tabular iceberg overturning, or of calving glacier creating small tidal waves”, confided Jacques Bazin, an ice pilot in the French Navy, during a conference at the National Maritime School last June in France. The pilot described the false impression of serenity that hangs over ice-filled seascapes. “Everything that is uniformly white is in motion and we don’t realise the pressures that can be exerted 15 or 20 nautical miles away, enclosing the ice around the ship’s path,” he added.
If an iceberg blocks a waterway, the Astrolabe must still be able to reach Dumont-d’Urville, Cape Jules or Port-Martin when it approaches the Antarctic coast. Hence the importance of having a precise idea of the seabed. “The ship is interested in having data around the waterways if it has to deviate from them,” explains Julien Fitamant, head of the SHOM detachment. Its mission was therefore to open up new passage sometimes shorter, to improve safety and save time.
The weather and sea ice conditions only allowed them to work for six and a half days from their arrival at Dumont-d’Urville. While Astrolabe was alongside, her tender was launched so that the team could explore the maritime zone near the station. A deployable battery-powered light military hydrography system probed the seabed using acoustic signals.
“When there’s wind and bad weather, the Astrolabe has to set sail and can’t keep station,” explains Julien Fitamant. That’s why the French Polar Institute is planning to consolidate the mooring by 2030 or 2050 by building a permanent dock. The project is under study, but requires detailed mapping of the seabed around the shelters on Lion Island and Petrel Island. The SHOM therefore carried out the surveys. It then extended the perimeter to complete the data for the 2022-2023 season, the first campaign after more than 20 years’ hiatus in the hydrography of the region. The front of the Astrolabe glacier, however, blocked their path. “Glaciologists may be interested in this data so that they can install seismographs close to glaciers without the risk of them turning over on too steep a slope,” explains Julien Fitamant.
” There’s a probe at eight meters hanging around and the icebreaker has a draft of five meters.”
Julien Fitamant
Further north, the formation of the ice pack prevented them from going any further. But they did have access to the seabed, which had been blocked by icebergs the previous year. “The seabed is fairly smooth, but there are rapid rises from 100 to 10 metres, and from 75 to less than 4 metres,” he describes. The hydrography around DDU took them around two days. Astrolabe then set off towards Port-Martin to the east, a historic site for France where there is a monitored penguin colony and the Zélée glacier, which is of interest to ornithologists and glaciologists.
“The seabed drops quite quickly”
At Cape Jules, the tender was launched when sea conditions did not allow passengers to disembark at the point. The SHOM began crossing an uncharted area on a constant course, while the Astrolabe rounded it via known areas to the north. “In the middle, the area was free of water, but a strip of compact ice prevented any possibility of meeting up with the carrier and there was quite a lot of wind,” recalls Julien Fitamant. The transit between Cap Jules and Port-Martin went well, with three hours of sailing at four knots. The SHOM surveyed a strip of water 20 to 600 metres wide, depending on the depth. “In this area, the seabed drops quite quickly”, he notes. With the range of the deployable military hydrography system effective from 0 to 200 metres down, part of the transect was not echoed.
“If we haven’t detected anything, we can’t guarantee that there won’t be a tiny rise in the seabed, he explains. There’s no reason why there should be, but we prefer to be on the safe side; we don’t want to send boats into areas where we’re not 100% sure of the bathymetry.” SHOM is responsible for safety and navigation. Its departments are therefore preparing to improve seabed mapping at greater depths. They will meet this week to draw up a new system for future campaigns.
As Astrolabe approaches Port-Martin, the seabed is rising, but it is not well-paved: “There’s a probe at eight meters hanging around and the icebreaker has a draft of five meters,” says the SHOM engineer. What’s more, the knowledge in the vicinity is limited and once again, if ice were to obstruct the channel, it would not be able to adapt its route and would be forced to turn back. A rather perilous manoeuvre.
At Port-Martin, SHOM has surveyed an area measuring one cable by three cables (1 cable = m185.2 m). It’s a meticulous job given the surface area to be hydrographed. These bathymetric measurements are of interest to scientists using oceanographic circulation models. To continue this work of exploring the seabed, SHOM is planning to set off again in 2025-2026, “so that the French Polar Institute can anticipate our arrival”, explains the cartographer. This work has to be planned well in advance, in detail, but once on site, there are always adjustments to be made on a day-to-day basis.
Camille Lin, PolarJournal
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