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Victoria University Antarctic Research Expedition Science and Logistics Reports 1979-80: VUWAE 24

Surveillance of the Sea Ice (BAS)

Surveillance of the Sea Ice (BAS)

Four bench marks number BM1 to BM4 were established on a line bearing 300° and situated at 25, 50, 100 and 200 metres respectively from the rig (Fig. 3). These, together with a point on the Science Hut 10 m from the rig and a point on the rig itself were levelled throughout the drilling operation. Until October 19 a Kern DkM1 theodolite was used, thereafter a Wild level; observational error is estimated at less than ±5 mm. Differential vertical movements of all stations relative to BM3 are plotted in Figure 4 which shows that BM1 and BM2 descended rapidly with respect to BM3 in the period October 10 to 19 when weight was being concentrated at the rig. There followed a 10 day period during which a small but significant rebound occurred. This was greatest and most pronounced at BM2, the farthest station from the rig measured other than the datum bench mark BM3. After October 28 BM1 & 2 and the Rig show a consistant subsidence relative to BM3 while BM4 had risen, Which is taken to indicate that BM3 in fact descended relative to sea level and that the zone of subsidence due to loading at the rig exceeded 200 m in diameter. The total subsidence of the rig relative to BM4 in the period October 28 to November 21 was 65 mm.

Late in the drilling operation when snow had ablated from the sea ice surface a number of old healed cracks were discovered in a region 40 m from the rig. In contrast to the experience at DVDP 15 no relative movement was observed on any existing cracks and no new cracks in the vicinity of the rig or camp were observed during the entire operation.

As at DVDP 15 differential slackness on the guy ropes of the drill mast occurred. The slack guys were on the side of the mast loaded by the main lifting pulley. The amount of slackness increased with load and was attributed to deformation of the mast.

Ice thickness was monitored and is graphed in Figure 4. On September 19 when the site was first visited the ice was 1.98 m thick. The ice thickness increased to 2.33 m by November 14 after which there was a slight decline to 2.28 m at the termination of drilling. All thicknesses were measured in a small region about 10 m from the rig. During the bathymetric survey of the drillsite, ice thickness was measured in 16 holes and found to be the same to within 20 mm over the entire area.

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Figure 4 Behaviour of the sea ice at MSSTS 1 during the drilling

Figure 4 Behaviour of the sea ice at MSSTS 1 during the drilling

  • A - vertical movement of the sea ice
  • B - vertical in ice thickness