AMISOR trilogy: the diesel and the dog
The first step in the transition to regular fixed wing aircraft deep field support was accomplished this season, with the AMISOR hot water drilling project trying to extend its range of operations to the far western reaches of the Amery Ice Shelf. A third borehole was planned to be drilled through ice some 400 m thick, nearly half of which was again expected to be marine (or jade) ice refrozen to the base of the shelf. If successful, production of this borehole would have completed a trio of instrumented holes spaced 50 km apart, in a line coincident with earlier surveys across the northern front of the shelf within 100 km of the calving front. Despite some weather delays, early season deployment allowed two Twin Otter aircraft to fly direct from a sea ice ski-way in front of Davis station, obviating the need for a ship visit to Sansom Island as an intermediate platform for operations.
The drill site this year was to be within viewing distance of Landon and Foley promontories, which flank Doggers Bay to the south and north respectively, where the early 1960s dog sled teams from Mawson dropped from the plateau onto the Amery. Their mechanical counterparts – snow-trac vehicles – were driven onto the ice shelf further north via Mechanics Bay. It was to be our privilege to have this opportunity for another brush with the justifiably proud history of ANARE exploration in the region. A wistful eye could even imagine a puff of vapour to the west as the collective breath of a faithful husky team mushing their way across the surface toward us. It was fated not to be the case.
During our passage south (Voyage 2, Aurora Australis) the aircrews at Davis had managed to deposit a cache of some 30 fuel drums at the proposed site, leading us to believe we had a great head start for the season. A further eight tonnes of cargo were delivered, and field personnel positioned who commenced basic camp construction, until a rude shock shook our season to its very foundations. On the final cargo flights for the day, with golden rays of sunlight slanting low across the flat expanse of the shelf, it became apparent that the entire area was riddled with crevasses. The campsite was in fact as near as possible to exact centre in the middle of a 50 m wide snow bridge covering a relic crevasse. Visible only from the air, in favourable lighting conditions, it was clear the site had to be abandoned, and furthermore, it was no longer valid to operate fixed wing aircraft in the area as sastrugi formation in the snow-pack dictated landing across the predominant crevasse direction.
As with all things ANARE, a dilemma remains as such only long enough to be photographed. Soon we had the eager Squirrel AS350B helicopter aircrews sling-loading our cargo back to the safety of the north-central shelf site drilled two years previously. To give an indication of relative carrying capacity, what had taken the Twin Otters 13 flights to deliver over a distance of some 350 km, now took the helicopters 26 flights to relocate over 50 km. Mere statistics. As usual the spirit of co-operation and willingness to tackle any task shone through, and we soon found ourselves in a position to salvage what we could from the remnants of the season.
Prior to relocation we had tried an aerial reconnaissance of the area for a safe site to proceed with the original plans. None was found in close enough vicinity to the proposed site to enable us to predict with any confidence that we would be drilling through the much sought after marine ice layer on the underside of the shelf. The western band of marine ice is believed to be a narrow one, extending longitudinally in the direction of the ice flow, and despite circling for two hours over the region, as each crevasse field petered out a new one came into view with entirely new and/or random orientation. It is recognised that several over-snow traverse trips (dog-sled, skidoo, snowtrac) have been conducted through the area in the past without major incident, although not surprisingly occasional slotting of vehicles had occurred. Many of the crevasses could be deemed safe, and luck has played its part. But in the era of fixed wing aircraft operations it would be negligent if not folly to ignore their presence and we must choose carefully any future drill sites in this region. Also with a hot water drilling operation relying on the establishment of a sub-surface well to store water for recirculation we could not guarantee that the drill would not strike a cavity at some depth below the surface and drain away the precious water supply. You have to have shovelled the 24 cubic metres of snow required for melting to fully appreciate the impact of this. There was no option but to move.
In brief, we finally managed a borehole alongside our old central site, supplementing previous scientific sampling there with a sediment core from the seabed, and a video record of the entire ice sequence through the shelf, plus footage of the sea floor. We retrieved another year's data from previous mooring sites, established a depot on the shelf at a future proposed drilling site 130 km to the south, where snow accumulation hopefully will not completely bury it, and managed to release several of the team on time for transfer to the ITASE ice core project inland of Casey. Not exactly flushed with success, but grateful once more for the hard working cooperative ethic of the many people we have the good fortune to be associated with in our endeavours.
Mike Craven, Glaciology Program, AAD & ACE CRC